Seminar (FY2016)

February/6/2019 (Wed.) 10:00-12:00 @ Room 431
Andre Amend
''Quasi-free-standing Graphene on SiC(0001) and Graphene on SiC(000-1) ''
Epitaxial graphene grown on SiC offers a lot of potential to study various graphene systems. On the Si-face of hexagonal SiC strong substrate coupling determines the alignment of grown graphene, thus it forms commensurate stacks. On this face, graphene can be decoupled from the substrate by hydrogen intercalation, yielding “quasi-free-standing” graphene. On the C-face, on the other hand, due to the weak substrate coupling, newly grown graphene layers can have many different alignments, thus epitaxial graphene stacks grown on this face posses rotational disorder between layers. The relative rotation of layers removes the inter layer symmetry breaking (that is present in AB stacked graphene) and gives rise to additional van Hove singularities. Both QFS graphene on the Si-face and graphene grown on the C-face will be the topic of this colloquium.

November/14/2018 (Wed.) 10:00-12:00 @ Room 431
Takamasa Suzuki
''Magnetic Property of monolayer Helium-3''
Helium-3 absorbed on graphite is ideal fermion 2D-system. Monolayer of Helium-3 shows various kinds of phases depending on areal density. I will introduce several spin susceptibility and NMR experiments on each phase of 3He/HD/HD/graphite and 3He/4He/graphite.

November/07/2018 (Wed.) 10:00-12:00 @ Room 431
Jun Usami
''The density problems of monolayers of He4 ~layer promotion, KT transition~ ''
Monolayer of helium has attracted much interest since it has not only a rich phase diagram but also the superfluidity. In the second layers of He4 on graphite substrate, the quantum liquid crystal phase existing between the liquid and solid phase was predicted by the heat capacity measurement. The superfluidity was also observed from the torsional oscillator experiment around that phase. However the density dependence of the superfluid response were in consistent among those previous observation by several groups. This ambiguity of density scale is because the inhomogeneities of graphite substrate.
I summarize those inconsistencies in the critical densities in this colloquium. The way of comparing the superfluid response given in higher densities with the torsional oscillator measurement with KT theory will be explained, where the relation among the signal size, the critical temperature and the density is focused. Finally, I talk about the unusual third sound observed in the monolayers of He4 on hydrogen.

November/6/2018 (Tue.) 15:30-17:00 @ Seminar room on 3rd floor, CRC
Dr. Christpher Bauerle
''Manipulating single electrons on the fly using a sound wave?''
Surface acoustic waves (SAW) provide a promising platform to realize quantum optics experiments with electrons at the single particle level. Pioneering single-shot experiments have shown SAW-assisted electron transport between spatially separated quantum dots over a distance of 4 μm with an efficiency of about 92% [1,2]. More recently we have also been able to transfer spin information [3], demonstrating the potential of this technique for coherent long distant transfer between two static qubits.
Here we go an important step further. We couple two quantum channels by a tunnel barrier along a region of 2 μm. At the end of each channel a quantum dot is placed serving as single-electron source and detector, respectively. We demonstrate single-electron transport over a distance of 22 μm with efficiencies above 99%. Changing the energy detuning in the coupling region we can partition the electron on-demand into two paths. By gradually changing the barrier height we additionally observe tunnel oscillations of the probability that the electron ends up at the upper or the lower detector quantum dot. This finding demonstrates coherent manipulation of the electron quantum state on the fly [4].
Our results pave the way for the implementation of a solid-state flying qubit having high relevance in fundamental research and quantum information technology.
[1] S. Hermelin et al., Nature 477, 435 (2011).
[2] R. P. G. McNeil et al., Nature 477, 439 (2011).
[3] B. Bertrand et al. Nature Nanotechnology 11, 672 (2016).
[4] S. Takada, H. Edlbauer et al, to be published.

October/24/2018 (Wed.) 10:00-12:00 @ Seminar room on 3rd floor, CRC
Shota Yamazaki
''Blackbody radiator in 3He-B''
The dilute gas of excitations from 3He-B in the temperature range below about 0.2Tc can be regarded as the ideal medium for a quasiparticle experiment because of their very long mean free paths. A blackbody radiator is composed of two vibrating wire resonators(VWRs) and a box with a small hole, and it can be used as a quasiparticle detector and source.
In my colloquium, I explain the experiments about (i)calibration of blackbody radiator, (ii)the blackbody as source, (iii) the beam spread experiment with the blackbody as detector.

October/10/2018 (Wed.) 10:00-12:00 @ Seminar room on 3rd floor, CRC
Shohei Takimoto
''quantized vortex tangle and knot theory''
Vortex is important phenomena both in classical and quantum fluid. And vortex lines are tangled in turbulence. Knot theory is one of the tools to analyze the vortex tangle because tangled vortex lines form knots and links. I will explain knot theory briefly and how to apply knot theory to the vortex tangle.

July/11/2018 (Wed.) 10:00-12:00 @ Room 431
Andre E. B. Amend
''Spin-Orbit Coupling in Graphene''
Single layer pristine graphene is conventionally thought of as a zero-gap semiconductor, where, without doping, low-energy excitations posses a linear dispersion relation. However, even in this idealized case a band-gap exists theoretically that is induced by the atomic spin-orbit interaction, but the gap size is usually so small that it is neglected. Nevertheless, spin-orbit interaction in graphene lifts the degeneracy between spin up and down states at the two Dirac points and enables the control of the spin by external electric fields via the Rashba effect. It also introduces additional spin scattering mechanisms, the commonly low spin-orbit coupling in graphene thus makes possible longer spin coherence lengths that are desireable in spintronic devices. Aside from external fields, the spin-orbit coupling can also be strongly influenced by adsorbates, defects and curvature. In this presentation some theoretical background on the spin-orbit coupling in graphene as well as experiments that measure and modify it will be presented.

July/04/2018 (Wed.) 10:00-12:00 @ Seminar room on 3rd floor, CRC
Shota Yamazaki
''The theory and the experiments of two-dimensional topological insulator HgTe/CdTe''
In the quantum Hall system, the quantized hall conductivity is explained by TKNN number, and the system can be regarded as a kind of topological insulators in which the time-reversal symmetry is broken. The quantum spin Hall system has time-reversal symmetry because of the strong spin-orbit interaction, and the Z2 invariant is important for classifying the topological property. In my colloquium , first I will introduce the time-reversal operator, Kramers pair, and Z2 invariant. Then I explain the two-dimensional topological insulator HgTe/CdTe through the tight binding calculation and the experimental results.

June/13/2018 (Wed.) 10:00-12:00 @ Room 431
Dr. Tomohiro Matsui
''Quasi-Free-standing Graphene''
We are recently studying the H-plasma etching of graphene epitaxially grown on SiC(0001). For this sample preparation, one can expect not only the formation of the zigzag edges as in the case of graphite surfaces, but also the realization of so-called quasi-free-standing graphene (QFG), which is a graphene layer decoupled from the SiC substrate. In this colloquium, I want to show the properties of such QFG and its related amterials.

May/30/2018 (Wed.) 10:00-12:00 @ Room 431
Jun Usami
''Two dimensional melting -property of hexatic phase-''
The two dimensional melting is explained by KTHNY theory. KTHNY theory predicts the continuous phase transition solid to liquid via hexatic phase, which has quasi-long-range bond orientational order. There are several real systems for two dimensional system. To my knowledge, there is not the real two dimensional system completely consistent with KTHNY theory or other theories.
The two dimensional hexatic phase is first observed in liquid crystal systems. I show the properties of such systems and the relation to the theory.

May/09/2018 (Wed.) 10:00-12:00 @ Room 431
Takamasa Suzuki
''Frustrated magnetism of YbMgGaO4''
I will introduce one of QSL material, YbMgGaO4.
YbMgGaO4 is triangular-lattice material and have very high frustration.(even higher than some kagome-lattice material) and this leads to peculiar nature.(example: specific heat)
I will introduce several experiments and theories of YbMgGaO4 tomorrow.

April/25/2018 (Wed.) 10:00-12:00 @ Seminar room on 3rd floor, CRC
Kensuke Yoshida
近年注目を集めているトポロジカル物質の理論的基礎について発表します. 量子系の断熱的変化を考えることによって出てくる幾何学的位相の導入から,量子ホール効果の説明まで行う予定です. 基礎となる量子力学をできるだけ厳密に紹介したいので数式多めになると思います.

April/11/2018 (Wed.) 10:00-12:00 @ Seminar room on 3rd floor, CRC
Shohei Takimoto
''Development of a compact and continuous nuclear demagnetization refrigerator''
A copper nuclear demagnetization refrigerator(NDR)is a method to reach sub-mK temperature. However, it requires a large superconducting magnet. And it is also a one-shot refrigerator. To solve these problems, we are developing a compact and continuous NDR with two PrNi5 nuclear stages.
I designed a superconducting magnet with a FeCoV shield and simulated the distribution of heat and magnetic field. In this colloquium, the details of simulation and its results are discussed.

February/07/2018 (Wed.) 10:00-12:00 @ Room 431
Andre E. B. Amend
''Experimental Indications of Magnetic Polarization and Electron Confinement on Graphene Nano-ribbons''
I will introduce one of QSL material, YbMgGaO4.
Zigzag-type graphene edges host electronic edge-localized states. The high density of states at the edges opens the possibility of magnetic polarization of the spins at the edges. On zigzag edge ribbons with nano-scale width, the ground state is expected to be spin-polarized, due to the interaction of the edge-states along opposite edges. Anti-ferromagnetic edge interaction is expected to be most stable, and cause a spin-gap in the LDOS with one or two peaks on either side. In addition, to the edge-states, the nano-ribbons can feature carrier confinement, where the borders of the valence and conduction bands may also give rise to density of state peaks. In this colloquium the theoretical calculations and possible experimental results on different substrates of spin-split states and electron confinement on graphene nano-ribbons will be reviewed.

February/05/2018 (Wed.) 10:00-12:00 @ Seminar room on 3rd floor, CRC
Shota Yamazaki
''Mobility of ions trapped below a surface of 3He-B''
A copper nuclear demagnetization refrigerator(NDR)is a method to reach sub-mK temperature. However, it requires a large superconducting magnet. And it is also a one-shot refrigerator. To solve these problems, we are developing a compact and continuous NDR with two PrNi5 nuclear stages.
I designed a superconducting magnet with a FeCoV shield and simulated the distribution of heat and magnetic field. In this colloquium, the details of simulation and its results are discussed.The surface Andreev bound states (SABSs) are formed at the surfaces of 3He-B. On my colloquium, I explain the experiment to detect SABSs from the mobility of the impurity trapped below a free surface of 3He-B. First, I show the mobility data of positive and negative ions.The data don't depend on the depth from the surface, which indicate that the contribution from SABSs can be ignored. Second, I introduce the scattering theory of the Majorana fermions in the SABSs. The theory suggests that the transport cross section increases as the depth deeper. The theory quantitatively reproduces the mobility, and we conclude that the mobility experiment succeeded to detect the SABSs.

December/25/2017 (Wed.) 10:00-12:00 @ Room 431
Takamasa Suzuki
''Triangular-lattice quantum spin liquid''
Due to strong magnetic frustration, several materials which have triangular lattice don't show magnetic ordering until very low temperature. Tommorrow,I introduce basic theory for spin liquid at first.(maybe mean field approach to spin liquid,and so on) Next,I review theoretical papers which discuss the possibility of spin liquid on triangular-lattice. Finally, I review several experimental results of triangular-lattice.

December/06/2017 (Wed.) 10:00-12:00 @ Room 431
Dr. Tomohiro Matsui
''Lifting of the four-fold degeneracy of graphene''
Graphene is fourfold degenerated reflecting its peculiar honeycomb structure. This fourfold degeneracy can be lifted by breaking the honeycomb symmetry; for example by introduce substrate potential or by applying high magnetic field to enhance the electron-electron interaction. In this colloquium, I want to show some experimental observations of the lifting of the four-fold degeneracy of graphene. Direct observation of Landau quantization by use of the scanning tunneling spectroscopy can clearly show the lifting. In addition, the observation of the Landau level on the surface layer can be used to characterize graphene itself, such as number of layers.

November/20/2017 (Wed.) 10:00-12:00 @ Seminar room on 3rd floor, CRC
Kensuke Yoshida
''Andreev Reflection and Surface Andreev bound state''
I will introduce about Andreev reflection and Andreev bound state with this colloquium. Andreev reflection was theoretically clarified by Andreev in 1964. This theory explains the increase in thermal resistance at the NS junction. The same phenomenon is generally observed in the region where the density of state changes spatially. In connection with this, I will also introduce the state that appears in the surface state of p-wave superconductors. This state is called the Andreeff bound state and also appears on the surface of the superfluid helium three B phase. At the end of the presentation, I will also describe the prospect of further experiments on this surface state.

November/08/2017 (Wed.) 10:00-12:00 @ Room 431
Masahiro Kamada
''Heat Capacity of 3He monolayer on HD plated graphite''
Helium three (3He) monoatomic layer adsorbed on graphite surface is an ideal strongly correlated fermion system with S = 1/2. Especially the second layer 3He shows interesting properties such as the quantum spin liquid state. We measured heat capacity of 3He monolayer on graphite preplated with a bilayer of HD, and found quantum phase diagram of this system is qualitatively different from that of the second layer 3He on graphite.
Recently I reanalyzed our heat capacity data based on new calibration for thermometers and reconsidered amorphous contribution. This analysis requires modification about the phase diagram in the vicinity of solidification. In this colloquium, the detail of the reanalysis and the resulting heat capacity data are discussed.

November/06/2017 (Wed.) 10:00-12:00 @ Room 431
Kazuma Kita
''Fabrication of Zigzag edges on Graphite/Exfoliated Graphene and its Electrical transport properties''
Graphene nanostructure with zigzag edges is one of the most interesting materials for diverse electronic and spintronic devices. To obtain zigzag edges hydrogen plasma etching is commonly used because it allows precise, top-down fabrication and tailoring of graphene nanostructures. Recently, a lot of evidence for patterning of graphene with zigzag edges by hydrogen-plasma treatment has been reported. In this colloquium, first, the detail of the mechanism of hydrogen plasma etching is discussed and compared with the experimental results. Second, there is a possibility to observe zigzag edges studying the Raman spectrum and electrical transport properties. Therefore, how to distinguish zigzag edges based on the Raman spectrum and scattering behavior of electrons in graphene is discussed.

October/30/2017 (Wed.) 10:00-12:00 @ Room 431
Katsuyoshi Ogawa
''NMR measurements of 3He adsorbed on graphite''
In solid 3He, the higher order cyclic exchanges such as three- or four-particle exchange are favored over the simple interchange of two atoms, because of a strong hard core repulsive potential between atoms. Exchange of an even number of particles is AFM, while that of odd number is FM. The existence of two competing interactions makes the system intrinsically frustrated, leading to various peculiar magnetic properties both in bulk and in adsorbed 3He. Recent studies of solid 3He films adsorbed on graphite have proved to provide a ruly two-dimensional frustrated S=1/2 quantum spin system. In this Colloquium, I will introduce some kinds of NMR measurement of the frustrated two-dimensional system and theoretical interpretation. In latter part, I discuss what is expected in NMR measurement of 3He on HD bilayer system from earlier experiments.

October/18/2017 (Wed.) 10:00-12:00 @ Room 431
Jun Usami
''vortex in superflow -phase slip and Josephson effect-''
Superfluid is named because of zero viscosity of helium. However superfluid looks to has viscosity due to quantized vortex. In toroidal experiment, vortex line can be detected one by one. In thin aperture experiments, phase slip can be seen as the flow increase to reach the critical velocity. When the size of aperture is comparable to the healing length, Josephson effect appears as same as superconductor in both 4He and 3He system. In this colloquium, I introduce those experiments and theoretical explanation.

July/11/2017 (Wed.) 10:00-12:00 @ Room 431
Katsuyoshi Ogawa
''Anisotropic superconductivity state in Sr2RuO4''
Following the discovery of high-Tc superconductivity, cuprates, a large number of related compounds have been found that are superconducting at relatively high temperatures. The superconductivity of Sr2RuO4 was discovered in 1994 and has attracted much attention. Not only because it has the same structure as La2-xSrxCuO4 of high-Tc cuprates but also because the experimental data are strongly suggestive of a triplet order parameter being different from conventional s-wave superconductivity and high-Tc cuprates. In this colloquium, I will introduce some unconventional properties of Sr2RuO4 superconductivity and discuss the possible p-wave order-parameter symmetry of Sr2RuO4 by introducing the concept of the d-vector.

July/6/2017 (Thu.) 13:30-14:30 @ Room 445
Dr. Aron Beekman
''Dislocation-mediated melting to quantum liquid crystals''
While liquid crystals can be viewed as rotational symmetry breaking of a liquid, such order can also be obtained by translational symmetry restoration in a solid. This Kosterlitz-Thouless dislocation-mediated melting process was shown to give rise to the so-called hexatic liquid crystal phase by Nelson, Halperin and Young in 1978-1979. Here we consider the analogous zero-temperature quantum phase transition from a two-dimensional bosonic solid to a quantum liquid crystal by condensation of dislocations. We employ a duality mapping where the phonons of the solid are represented by gauge bosons ("stress photons") that mediate the elastic interactions between dislocations. In the liquid crystal, transverse shear phonons undergo a dual "stress Meissner effect" and can no longer propagate long-range shear forces.
In this talk I shall focus on the phenomenology of such quantum hexatic phases, mostly in the form of the spectrum of low-lying excitations. Here we find two gapless modes: the longitudinal phonon (sound) but with a reduced velocity, and a Nambu-Goldstone mode due to rotational symmetry breaking. There are also two gapped modes coming from the gapped shear phonons and from the dislocation condensate itself. I will also discuss the limitations of this idealized model and the possible modifications needed when applying it to real systems.
Reference: Physics Reports 683, 1-110 (2017), arXiv:1603.04254

June/28/2017 (Wed.) 10:35-12:00 @ Room 431
Kazuma Kita
''Transport Properties in Graphene and Multilayer Graphene''
Graphene is a model system for the study of electrons confined to a strictly two-dimensional layer and a large number of electronic phenomena have been demonstrated in graphene, from the fractional quantum Hall effect to superconductivity. In this colloquium I will present (1)general transport properties such like temperature dependence and layer number dependence, (2)scattering properties due to defects, (3) some characteristic oscillation in magnetic filed which is related to our experiments.

May/31/2017 (Wed.) 10:00-12:00 @ Room 431
Dr. Tomohiro Matsui
''How Robust Graphene Edge Magnetism is ?''
It is widely believed for graphene nanoribbon (GNR) with zigzag edges that the spins are polarized ferromagnetically along the edges and anti-ferroomagnetically interacting between the edges in its ground state. However, in principle, long-range order is prohibited in one dimensions. One dimensional spin ordering is observed only with a help of large anisotropy (which is not expected for graphene because of its small spin-orbit interaction) and the substrate. In addition, such a picture is for ideal system and the realistic situation can be dependent on the environment. Here, in this colloquium, I will introduce some theoretical considerations of graphene edge magnetism with fluctuations, charge doping and the substrate. Please note that there are plenty of theoretical studies in this field and what I will show are just a tiny part of them. I just hope they can be good starting points for further considerations.

May/22/2017 (Mon.) 10:00-12:00 @ Room 431
Andre Amend
''STS study of Graphene Edges Fabricated by Hydrogen Plasma Etching''
Imbalance between the sublattice sites in a bipartite system such, as graphene near Dirac point energy, induces zero-energy modes. In the presence of sublattice interaction these modes can persist and thus electrical localized states can be measured on disordered real graphene. The local density of state (LDOS) depends on the disorder type. The extended Zigzag edge state is of particular interest since it may support magnetic polarization. The electrical states produced by single vacancies and Zigzag edges, as well as methods to produce Zigzag edges, are reviewed. An analysis of our STM/S measurements of Zigzag edges and Zigzag graphene nano-ribbons (GNR) on graphene, fabricated by Hydrogen-plasma etching, is presented. The Zigzag edge spectra show the characteristic low-energy peak in the LDOS and a pseudo-gap surrounding the peak, which may stem from the edge state degeneracy. The GNRs also hosted the edge state, but no evidence of a LDOS double-peak was observed, which is expected for spin-polarization.

Feb/20/2017 (Mon) 14:00-15:00 @ Room431
Kazuyuki Matsumoto
Applied physics and chemistry-based department, Muroran Institute of Technology
''Scaling Theory for Nuclear Magnetism of Adsorbed Solid 3He of Two-dimensions''
 グラファイトなどの基板上に吸着させた3Heは2次元”固体“を形成し核磁性を 示す。その研究は1980年代に始まったが、その様子が分かってきたのはつい最近のこ とである。理論的には2次元三角格子反強磁性ハイゼンベルク模型が提案されてきた が、最近の超低温領域実験によって、こうした簡単な模型では説明できないデータが 蓄積されつつある。具体的には、磁化率はT-1/3、比熱はT2/3の温度依存性を 示すことが明らかになりつつある。これらは、2次元三角格子反強磁性ハイゼンベル ク模型では説明できない。そこで、この講演では2次元三角格子反強磁性ランダムハ イゼンベルク模型を提案したい。実際、この模型を解析することによって、磁化率、 比熱の温度依存性が説明できそうなのである。解析方法としては、バットとリーに よって拡張された数値的スケーリングを用いてみた。実際に計算を実行してみると磁 化率および比熱の温度依存性は、計算条件の詳細にはよらないことが分かった。そし て、ランダムネスが十分につよい時にはさきほどの指数、すなわち-1/3と2/3と矛盾 のない数値が得られた。このことから2次元三角格子反強磁性ランダムハイゼンベル ク模型がこの系に適用できる可能性は十分に高い。さらに、これらの結果をサポート するある強相関系におけるスケーリング理論も紹介したい。ただし、問題にしている 2次元”固体“のような整合相で、はたして相互作用がランダムであるかどうかは確 かに疑問の残るところである。この点については今後さらなる考察が必要となるであ ろう。

Feb/8/2017 (Wed) 10:00-12:00 @ CRC 3F seminar room
Katsuyoshi Ogawa
''Multiple spin exchange in three- and two-dimensional solid 3He''
Three-dimensional solid helium 3 forms bcc structure under 3 < P < 10 MPa. The nuclear spin system in this solid has an unusual magnetic phase diagram which cannot be explained by the Hamiltonian including only nearest neighbor interactions. However, the phase diagram can be explained by the Hamiltonian including multiple spin exchange (MSE) proposed by Thouless in 1965. In MSE, exchanges among the even number of atoms favor anti-ferromagnetic interaction. On the other hand, those among the odd number of atoms favor ferromagnetic one. The competition among them make the system magnetically frustrated. When the dimensionality is reduced from three to two, the frustration is enhanced. In two-dimensional triangular lattice an intriguing ground state, quantum spin liquid (QSL), is expected with MSE. The existence of QSL is also indicated from experimental studies on helium 3 monoatomic layer adsorbed on graphite expected to form triangular lattice.

Dec/15/2016 (Thu) 16:00-17:00 @ Room445
Prof. Keith E. Gubbins
Dept. of Chemical & Biomolecular Engn., North Carolina State University, USA
''Confinement-Induced High Pressure Phases in Nanopoers: Can the Pressure be in the Megabar Range?''
There is an abundance of anecdotal evidence that nanophases adsorbed within nanoporous materials can exhibit high pressures as a result of the confinement1,2. For example, phase changes and chemical reactions that only occur at high pressures in the bulk phase occur in the confined phase at bulk phase pressures that are orders of magnitude lower. The pressure in the pore is a second order tensor, and for simple pore geometries has both a normal pressure component (normal to the walls) and one or more tangential components (parallel to the walls).
For simple fluids in pores that are up to a few nanometers in width, molecular simulations show that both the normal and tangential pressures can be locally very high (thousands or tens of thousands of bars) in the pore, even though the bulk phase in equilibrium with the pore is at a pressure of one bar or less. The cause of these high in-pore pressures will be discussed, and where possible comparison with experimental results will be made3.
When the molecules in the confined nanophase react with each other chemically it may be possible to achieve even higher tangential pressures, in the megabar range. Evidence for this is provided by recent experiments on sulfur (an insulator at ambient conditions) in narrow single-walled carbon nanotubes, carried out by Kaneko and coworkers4. They find that the sulfur atoms within the pore covalently bond to form a one-dimensional phase that is metallic. In the bulk phase sulfur forms a metallic phase only at pressures above 95 GPa. In our recent molecular dynamics simulations of this system we find that the sulfur atoms are covalently bonded in the pore and that they experience tangential pressures in excess of 100 GPa as a result of the strong confinement5.
1. Y. Long, J.C. Palmer, B. Coasne, M. Sliwinska -Bartkowiak and K.E. Gubbins, "Pressure enhancement in carbon nanopores": A major confinement effectモ, Physical Chemistry Chemical Physics, 13, 17163 (2011).
2. Y. Long, J.C. Palmer, B. Coasne, M. Sliwinska- Bartkowiak, G. Jackson, E.A. Muller and K.E. Gubbins, "On the Molecular Origin of High Pressure Effects in Nanoconfinement: Effects of Surface Chemistry and Roughness", Journal of Chemical Physics, 139, 144701 (2013).
3. M. Sliwinska-Bartkowiak, H. Drozdowski, M. Kempinski, M. Jazdzewska, Y. Long, J.C. Palmer and K.E. Gubbins, "Structural Analysis of the Behavior of Water Adsorbed in Activated Carbon Fibers", Physical Chemistry Chemical Physics, 14, 7145 (2012).
4. Y. Fujimori, A. Morelos-Gomez, Z. Zhu, et al., "Conducting Linear Chains of Sulphur Inside Carbon Nanotubes", Nature Comm., 4, 2162 (2013).
5. K.E. Gubbins, C.A Addington and J.M. Mansell, to be published.

Dec/14/2016 (Wed) 10:00-12:00 @ Room431
Tomohiro Matsui
''Magnetic Property in Graphite''
Since magnetic order is observed for pi-electrons in non-metallic organic polymer-based materials in 1991, possible magnetism in carbon-based structure are also attracting great interest. In fact, ferromagnetic and superconducting magnetization hysteresis loops were observed in HOPG in a broad temperature range. However, the details of the hysteresis depend on the sample, sample heat treatment and on the direction of the applied field. The origin of magnetic order in such a light element is only poorly understood. In this colloquium, I want to overview the ferromagnetic properties observed in graphite.

Nov/29/2016 (Tue) 10:00-12:00 @ Room431
Sachiko Nakamura
''Mass transport measurement of 4He''
To consider alternatives to torsional-oscillator experiments for detection of superfluidity in monolayer 4He, recent mass transport measurements on solid 4He will be reviewed. Previous studies on 4He films related to forthcoming mass transport measurements will be introduced as well.

Oct/12/2016 (Wed) 10:00-12:00 @ Room431
Andre Amend
''Graphene Zig-Zag direction Edge States''
Due to the theoretical bipartite Hamiltonian of graphene it is expected that sublattice vacancy imbalances give rise to localized zero-energy states. Such localized states increase the electrical conductivity near the defects and may produce a magnetic moment in some cases. The graphene Zig-Zag edge has attracted much interest since it features a large sublattice imbalance and thus supports localized edge-states. Our recent STS studies of edges in Zig-Zag direction have probed the edge-state localization and have revealed features beyond the single or double (polarized edge) peak spectrum which is expected for Zig-Zag edges. The observed edge-states may be the result of Zig-Zag direction edges other than the pure Zig-Zag edge.

Sep/21/2016 (Wed) 10:00-12:00 @ CRC 3F seminar room
Ryuji Nakamura
''Kitaev Spin Liquids''
The Kitaev model is one of the few exactly solvable models in physics. In recent years, this model has attracted both experimental and theoretical physicists’ attentions.A. Kitaev proved that the ground state of the model has both gapless and gapped quantum spin liquid phases.
In this colloquium, I will show you the novel spin liquid behaviors of the Kitaev model. This model is thought have a relevance for undiscovered Majorana fermions, so I will discuss the relationship with our experimental results.

Sep/12/2016 (Mon) 10:00-11:30 @ Room431
Kazuma Kita
''Electronic transport properties in graphene and graphene nanoribbon''
Graphene samples can have a very high carrier mobility, allowing to observe a ballistic transport, the fractional quantum Hall effect or quantum interference in bulk graphene. Another interesting properties can be seen in graphene-based antidot lattice with lattice periods down to 50 nm. They have pronounced commensurability features stemming from ballistic orbits. Finally, I will show that the effects of edge and bulk disorder on the conductance of graphene nanoribbons comparing zigzag edge with armchair edge.

Jun/30/2016 (Thu) 13:30-14:30 @ Room431
Prof. Yasumasa Takano
Dept. of Physics, Univ. of Florida
''Spin-1/2 Heisenberg antiferromagnet in one dimension''
The one-dimensional spin-1/2 Heisenberg model is one of the few exactly solvable non-trivial models in physics. When the interaction is antiferromagnetic, the ground state of this model is a Tomonaga-Luttinger liquid, in which dynamic and static properties are inextricably linked. Low-energy excitations are spinons instead of bosonic magnons, with a unique gapless dispersion. These and other properties of the model have been extensively studied since the pioneering work by Bethe, published in 1931. This seminar describes our recent experiment that puts some of the theoretical predictions to tests.

Jun/13/2016 (Mon) 10:00-11:30 @ CRC 3F seminar room
Ryuji Nakamura
''The possibility of two dimensional superfluid 3He''
The superfluidity is a one of the states of matter, which behaves like a fluid without viscosity. For superfluid 3He, the symmetry of Cooper pairs is an odd parity (p-wave). We could describe the mechanism of the superfluid 3He by using the Legget equations. On the other hand, for 2D superfluid 3He, the symmetry of Cooper is considered to have a density dependence by some theoretical physicists. In this colloquium, I show two theories for 2D and 3D super fluidity and compare them. I will talk about a new NMR experiment to seek 2D 3He superfluidity.

Jun/6/2016 (Mon) 10:00-11:30 @ Room431
Tomohiro Matsui
''Graphene Antidot Lattices''
Introducing a periodic array of holes, i.e., an antidot lattice, in a graphene has been suggested as a route towards the band gap opening in otherwise zero-gap semiconductor. However, so far, most studies in this field are restricted to experiments for circular antidots and theoretical discussions without spin polarization. Recently we have succeeded to control the shape of antidot between circular and hexagonal, in other words, between irregular edges and zigzag edges, by changing the temperature during hydrogen plasma etching of graphene/graphite. This technique is potentially able to extend the non-magnetic properties of graphene antidot lattices (GALs) further to spin dependent properties. In this colloquium, I want to overview the electronic properties of GALs and to provide a base to consider the electronic and magnetic properties of graphene with either circular or hexagonal nanopits.

Apr/25/2016 (Mon) 10:00-11:30 @ Room431
Andre Amend
''Zero-energy states in disordered graphene''
Any real honeycomb graphene lattice is subject to different types disorder, which can change the system's electrical properties. Defects and disorder can often be modeled as vacancy configurations in the hexagonal lattice, such as single vacancies or edges. For low energy excitations the system can be described by a bipartite Hamiltonian. It is shown that for for all types of vacancy configurations, the number of induced zero-energy states has a lower bound which is equal to the local site imbalance between the graphene sublattices. These induced states were found to be (quasi-) localized, depending on the disorder configuration. The localization of such states for some vacancy configurations is reviewed.

Apr/18/2016 (Mon) 10:00-11:30 @ Room431
Kazuma Kita
''Quantum transport in GaN/AlGaN two-dimensional electron system under periodic potentials induced by GaN nanocolums''
We are trying to detect the influence of electronic state in GaN nanocolumns array through the measurement of the magneto-resistance in GaN/AlGaN 2D-electron gas underneath the nanocolumns.The magneto-resistance shows characteristics oscillations which are the Shubnikov-de Haas oscillations and the Aharonov-Bohm oscillations.

Katsuyoshi Ogawa
''Relation between symmetric property of the gap energy and thermodynamic quantities'' In BCS theory, the expectation value of Cooper pairs is adopted as order parameter. It is assumed in the theory that Cooper pairs are isotropic. In this work, an anisotropic Cooper pair like line node (order parameter is 0 on equator) is considered. Thermodynamic quantities are derived from the gap equation and the Hamiltonian numerically. As a result, the temperature dependence of thermodynamic quantities assuming anisotropic Cooper pairs are different from that assuming isotropic Cooper pairs. For example, specific heat became proportional to the third power of temperature at very low temperatures in the line node case.

Apr/11/2016 (Mon) 10:00-11:30 @ CRC 3F seminar room
Masahiro Kamada
''Domain wall in adsorbed monoatomic layer''

Seminar (FY2015)

Mar/7/2016 (Mon) 10:00-11:30 @ Room431
Sachiko Nakamura
''Localization of Quantum Molecules''

Feb/1/2016 (Mon) 10:00-11:30 @ Room431
Kazuki Nakayama
''Tomonaga Luttinger Liquid in CNT network''

Dec/7/2015 (Mon) 10:00-11:30 @ Room431
Tomohiro Matsui
''From Graphene to Graphite: Band Structure and Electronic States in Magnetic Fields.''
Graphene is known as a zero-gap semi-conductor with a linear Dirac type energy dispersion. On the other hand, graphite which is a multi-layer graphene coupled with van der Waals interaction shows a semi-metallic behavior. It is interesting to investigate, for both fundamental and industrial reasons, how the electronic structure changes as a function of the number of graphene layers and how many layers are necessary to obtain the graphite behavior. Indeed, it has been studied that the electronic properties of multi-layer graphene are sensitive to the stacking geometry and the next-nearest-layer couplings, etc. In this colloquium, I am planning to overview the band structure and Landau level differences depending on the number of graphene layers and stacking geometry. If possible, I will also discuss the effect of the edges to them.

Nov/16/2015 (Mon) 10:00-11:30 @ Room431
Masahiro Kamada
''helium-three adsorbed on heterogeneous surfaces''
Adsorbed helium-three films are ideal systems to study strongly correlated Fermions. Especially, helium-three adsorbed on atomically flat surfaces of the graphite substrate show interesting nuclear magnetism. However, there are some effects from helium-three solid adsorbed on inhomogeneities of the substrate. In this colloquium, previous studies on helium-three solid adsorbed on rough surfaces is reviewed, and then helium-three on inhomogeneities of the graphite substrate is discussed.

Nov/2/2015 (Mon) 10:00-11:30 @ Room431
Hideki Sato
''Hydrogen-Plasma etching of graphite surfaces and their STM/S studies''
There are two types of edges for graphene: zigzag edge and armchair edge. It is known that localized states exist at zigzag edge. In addition, it is expected that spin-polarized states exist at zigzag edge and it is attracting attention both from the viewpoint of fundamental physics and electronic device applications. But, it is not easy to obtain zigzag edges controlled in an atomic scale selectively. In this colloquium, I will introduce hydrogen plasma etching as one way of obtaining zigzag edge. In particular, I will introduce the details of the hexagonal pit formation process.

Oct/19/2015 (Mon) 10:00-11:30 @ Room431
Andre Amend
''Introduction of the Hong Kong University of Science and Technology and Prisoner's Dilemma Games with adapting players.''
Introduction of myself and my undergraduate university in Hong Kong. Introduction of the Prisoner's Dilemma game. The evolution of players with certain initial characteristics that repeatedly interact with other players through the Prisoner's Dilemma game while adapting their strategy was studied. While interacting the players can change their strategy according to a partial imitation rule depending on their success, which is measured by the payoff players gain through the Prisoner’s Dilemma game. The tendencies that the players show in their strategy adaption under different conditions was investigated. Some interesting features of the results could be explained heuristically: in the repeated interaction between an adaptive cheater against a patient nice player in a noisy environment, a minimum amount of cooperation of the cheater towards the nice player in the Prisoner's Dilemma was observed at a specific noise level.

Oct/5/2015 (Mon) 10:00-11:30 @ Room431
Ryuji Nakamura
''The Ground States of Quantum Spin System''
The quantum spin system is a localized spin system which has strong quantum effects. In low dimensional system, we can observe curious physical phenomena, but it is difficult to solve many-body problem of the spins exactly.
In this colloquium, I will focus on the ground states and the elementary excitations of quantum spin systems in low dimensions (d= 1,2). The various ground states and elementary excitations will teach us important clues to understand frustrated magnetism.

Jul/12/2015 (Mon) 10:00-11:30 @ CRC 3F seminar room
Sachiko Nakamura
''4He Third Sound Studies using Graphite Substrate''
To see if the third-sound measurement techniques are useful to evaluate Pyrolytic Graphite Sheet (PGS) as a substrate to investigate superfluidity in 4He films adsorbed on graphite, previous experimental studies using other sorts of graphite will be reviewed.

Jul/6/2015 (Mon) 10:00-12:00 @ CRC 3F seminar room
Ryo Toda
''Operation and Optimization of the Continuous Demagnetization Refrigerator''
10 mK以下の温度を得るための方法として核スピンの断熱消磁冷却(NDR)がある。 断熱消磁サイクルは原理的にシングルショットであるが、複数の独立に制御可能 な断熱消磁ユニットを組み合わせれば、連続的に冷却能力を得ることが可能であ る。これは電子スピンを用いた断熱消磁冷凍機(最低温度50 mK)おいては実現 されている。本コロキウムでは、既存の連続断熱消磁冷凍機の運転方法、最適化 についてレビューし、これを核断熱消磁冷凍機に適用することについて議論する。

Jun/29/2015 (Mon) 10:00-11:30 @ CRC 3F seminar room
Takenori Fujii
''Evidence of Anisotropic Impurity Scattering in Iron Pnictide :Measurement of Anisotropy in the Thermopower of Ba(Fe1-xCox)2As2''
強相関電子系においては、通常の金属や半導体では無視し得る電子間の相互作用が重要となり、複雑な電子相が現れる。 そこでは、スピン、電荷、軌道の自由度が絡み合い、様々な秩序が競合、共存し、その結果、高温超伝導や、 巨大磁気抵抗のような特異な物性が表れると考えられる。 我々は、近年鉄系超伝導体において報告されている、自発的に回転対称性を破るネマティック相という電子状態を 熱輸送特性の測定から観測し、超伝導との競合・共存関係を調べている。 鉄系超伝導体Ba(Fe1-xCox)2As2では、反強磁性転移及び構造転移温度より遥かに高温において、正方晶にもかかわらず 抵抗率に面内異方性が観測されており、ネマティック秩序との関係が議論されている。 抵抗率の異方性の起源としては、①電子状態の異方性(スピンもしくは軌道秩序によって再構築されたフェルミ面の 異方性)、②散乱時間の異方性(Coによる異方的な不純物散乱)、の2つが考えられるが、未だ結論は出ていない。 今回我々はこの異常な異方性の起源を調べるために、熱起電力の面内異方性を測定した。 その結果、反強磁性転移温度以上においては、熱起電力に異方性は見られず、転移温度以下で明瞭に異方性が見られた。 熱起電力は基本的に散乱時間に依存しない物理量であるため、抵抗率に見られる転移温度以上の異方性は、 異方的散乱時間によるものと考えられる。 コロキウムでは、以上の結果を踏まえて、反強磁性転移温度以上の電子・スピン状態と超伝導の関係について議論する。

Jun/22/2015 (Mon) 10:00-11:30 @ Room431
Tomohiro Matsui
''Edge states in condensed matter physics''
The edge states in 2D systems include a challenging problem in solid state physics. For instance, edge states are predicted to give rise to a novel type of magnetic ordering and may lead to the realization of novel spintronic devices. They are also known to play an important role in quantum hall systems and topological insulators. In this colloquium, I will focus on the edge states in condensed matter physics and introduce how such edge states are observed by scanning tunneling microscopy and spectroscopy technique.

Jun/8/2015 (Mon) 10:00-11:30 @ Room431
Hideki Sato
''Modulated structure appearing around the edges''
金属中に不純物や欠陥が存在すると、ポテンシャルからの影響を受けた自由電子によって、その周辺には変調構造が生じる。 この変調構造の例として、電子定在波やフリーデル振動などがよく知られている。

May/25/2015 (Mon) 10:00-12:00 @ CRC 3F seminar room
Masahiro Kamada
''Doped Mott Insulator and 2D 3He''
Mott insulators are what should be metal under band theory, but are insulators due to electron-electron interaction. Career doping into Mott insulators show various phenomenon such as high-Tc superconductivity. In this colloquium, basics of doped Mott insulator are reviewed, and liquid-solid phase transition of 2D 3He on grahpite is discussed in view of hole-doped Mott localized state.

May/11/2015 (Mon) 10:00-12:00 @ Room431
Kazuki Nakayama
''Basic properties of carbon nanotubes and how to measure them''
Many believes that the discovery of carbon nanotubes(CNT) is attributed to Sumio Iijima in 1991. Nowadays, CNT is common material even among those who are not familiar with science.
In this colloquium, basics of CNT and structures made of CNT are explained. Then, the techniques for measuring properties of CNT are reviewed, mainly focusing on elecrical transport measurements.

Apr/6/2015 (Mon) 10:00-11:30 @ Room431
Ryuji Nakamura
''Exact two-magnon states in a multiple-spin exchange model on a zigzag ladder''
Solid helium-3 is known as a quantum solid, which exhibits exotic behavior at ultra-low temperatures. The magnetic properties of solid helium-3 are well described by a multiple-spin exchange model, which includes both Heisenberg and ring-exchange interactions among nuclear spins. As a simple model of helium-3 absorbed on graphite, we consider a quantum spin-1/2 model on a zigzag ladder with competing Heisenberg (J) and 4-spin cyclic exchange (K) interactions. We show that one-magnon states are strictly localized when K=-J/2. Then we find in this particular case that the exact two-magnon dispersion can be obtained analytically. We also present numerical results for the ground state in a magnetic field.

Seminar (FY2014)

Feb/23/2015 (Mon) 10:00-11:30 @ Room431
Tomohiro Matsui
''Graphite and Graphene Surfaces reacted with Hydrogen''
The step edges of graphite / graphene terminated by unknown materials, are expected to be refreshed and re-terminated by hydrogen by exposing graphite / graphene surfaces to atomic hydrogen at elevated temperatures. However, such edge termination cannot be an only reaction. Etched pits and step edges are found to be created additionally by atomic hydrogen. Moreover, it is reported that the sp2 bonding of graphite can be broken to form tetrahedral amorphous structures at the edges. In this colloquium, such reactions of graphite / graphene surfaces against hydrogen and resultant electronic properties will be discussed.

Feb/19/2015 (Thu) 10:00-11:30 @ CRC 3F seminar room
Masahiro Kamada
''Heat Capacity of 3He of Bilayer HD Preplated Graphite''
グラファイト基板上の吸着第2層目の2次元3He整合固体相(C2相)をはじめとし た2次元三角格子をもつ量子スピン液体候補物質が盛んに研究されており、これ らの物質の多くが低温で温度に 比例する比熱を示している。
しかし、最近我々が行った2層HDをプレ コートしたグラファイト上^3 Heの比熱 測定では従来の報告とは異なり温度の2/3乗に比例する比熱が観測された。この 特異な温度依存性 はスピノン或いはマヨラナフェルミオンといった素励起の理 論で理解できる可能性があり、先行研究の帯磁率測定の結果と合わせて議論する。

Feb/10/2015 (Tue) 13:30-15:00 @ CRC 3F seminar room
Ryo Toda
''Development of a Compact and Continuous Sub-mK Refrigerator''
既存の一般的な希釈冷凍機に搭載して、1 mK以下までの 温度を連続的に発生させられる小型の核断熱消磁冷凍システムの 開発を進めている。まず、一般に使われている核断熱消磁冷凍に ついて紹介した後、本システムの特徴である小型化、 連続冷凍の方法およびその設計について議論したい。

Dec/22/2014 (Mon) 10:00-11:30 @ Room431
Sachiko Nakamura
''XRD Studies of Graphite Sheets''

Dec/8/2014 (Mon) 10:00-11:30 @ Room431
Kazuki Nakayama
''Properties and utilization of suspended graphene''
Suspended graphene is commonly used technique to look into the intrinsic properties of graphene. Not merely to show high quality transport properties, spatially-isolated graphene enables variety of experiments.
In this colloquium, previous experiments utilizing suspended graphene are reviewed, focusing mainly on its unique features and differences with non-suspended graphene.

Oct/27/2014 (Mon) 10:00-11:30 @ Room431
Hideki Sato
''An anisotropic etching of graphene/graphite by hydrogen-plasma''
水素プラズマによって、グラフェン・グラファイトに異方性エッチングを施した 研究を紹介する。 このようにエッチングされたエッジは、主にジグザグエッジになると報告されて いる。 また、この方法によって得られたエッジやグラフェンナノリボンについて、ラマ ン分光法による測定や電気的特性を測定した研究も紹介する。

Jul/14/2014 (Mon) 10:00-11:30 @ Room431
Masahiro Kamada
''Isotope Effect on 2D Hydrogen on Graphite''

Jul/7/2014 (Mon) 10:00-11:30 @ Room431
Sachiko Nakamura
''Preferential Adsorption of Helium Isotopes''
Because of its smaller zero-point energy, it is to be expected that helium-4 (4He) will be preferentially adsorbed over helium-3 (3He) next to the substrate. Previous measurements directly measured isotope ratio in films adsorbed in equilibrium with a bulk liquid mixture and with a vapor. In film-liquid equilibrium such adsorption is successfully confirmed. However, the isotope ratio of whole films is independent of the film thickness in film-vapor equilibrium, possibly because of too thick films or too high temperatures to obtain high enough vapor pressures. Direct measurements might be too difficult for film-vapor equilibrium. Our recent heat-capacity measurements on bilayer mixture films adsorbed on ZYX graphite show an extra heat capacity which is not seen in isotopically pure bilayers. Energetically the films should comprise a 3He layer (2nd layer) on a layer of 4He (1st layer). I suggest a simple partition function of inter-layer swap of helium isotopes and calculated the heat capacity due to the swap between the 1st layer 3He and 2nd layer 4He. The model includes only one fitting parameter, swap energy, but it describes the extra heat capacities quite well.

Jun/2/2014 (Mon) 10:00-11:30 @ Room431
Tomohiro Matsui
''Experimental Studies of Graphene Edge State''
Peculiar edge states on Graphene and the magnetism expected for the zigzag structure were discussed in the last colloquium of mine. However, in these discussions, the edges show an "ideal" structure keeping honeycomb symmetry of Graphene. In reality, variety of elements, such as N, O, H, Fe and Si, are know to attach to the edge, and/or the edge may also be reconstructed. In this colloquium, I want to clarify the edge structure and introduce some studies under, more ore less, controlled conditions.

Apr/28/2014 (Mon) 10:00-11:30 @ Room431
Kazuki Nakayama
''Characterizing graphene with Raman spectroscopy''
Raman spectroscopy is a tool that enables understanding the electronic and vibrational properties of a system nondestructively by means of the inelastic scattering of light. It is widely used to characterize or evaluate nanocarbons including graphene. In this colloquium, scattering mechanisms responsible for the characteristic Raman peaks seen on graphene are reviewed. Then, Raman spectrum of the damaged graphene samples we obtained (accidentally) are considered.

Apr/14/2014 (Mon) 10:00-11:00 @ Room431
Hideki Sato

Seminar (FY2013)

Mar/19/2014 (Wed) 14:00-15:00 @ Room431
Dr. Yosuke Matsumoto
Institute for Solid State Physics, The University of Tokyo
''Novel quantum criticality in the valence fluctuating heavy-fermions''
   “Novel quantum phases formed in the vicinity of a quantum critical point (QCP)” is one of the most important topics in condensed matter physics on strongly correlated electrons. Archetypical examples are found in the heavy-fermion (HF) intermetallics, for which, owing to its relatively small energy scale, we can fine tune the ground state of the systems by controlling magnetic field, pressure and chemical doping etc. Indeed, unconventional superconductivity and non-Fermi liquid behavior have been found in the vicinity of the magnetic QCP of the several compounds such as CeIn3, CeRhIn5, CeCu2Si2 and have been studied extensively over the past few decades [1].
   So far, all the quantum critical (QC) materials in the HF intermetallics are known to have an almost integral valence which stabilizes the local moments considered essential for the criticality. In contrast, departures from integral valence associated with valence fluctuations are thought to promote screening of local moments, suppressing critical phenomena. On the other hand, the first Yb-based HF superconductor β-YbAlB4 provides a unique example of a QC in the mixed valent compounds [2-4]. In addition, the QC cannot be explained by spin fluctuation and emerges without tuning any control parameter, suggesting realization of a novel metallic phase [4].
   Here I review the novel quantum criticality in β-YbAlB4. I will further discuss the future scope of the studies on QC materials in HF systems introducing a few more examples. Possible extensions of these studies into the ultra-low temperatures will be also discussed.
[1] H. Löhneysen, A. Rosch, M. Vojta, P. Wölfle, Rev. Mod. Phys. 79, 1015 (2007), and references therein.
[2] S. Nakatsuji, K.Kuga, Y. Machida, T. Tayama, T. Sakakibara , Y. Karaki, H. Ishimoto, S. Yonezawa, Y. Maeno, E. Pearson, L.Balicas, H. Lee, Z. Fisk, Nature Phys. 4, 603-607 (2008).
[3] K. Kuga, Y. Karaki, Y. Matsumoto, Y. Machida, and S. Nakatsuji, Phys. Rev. Lett. 101, 137004 (2008).

Mar/10/2014 (Mon) 13:30-15:30 @ Room431
Takayuki Nakajima

Mar/6/2014 (Thu) 14:30-15:30 @ Room445
Dr. Satoshi Murakawa
Faculty of Science and Technology, Keio University
『トポロジカル超流動の表面状態 -超流動3He-B相の表面アンドレーエフ束縛状態とマヨラナ状態-』
 多くの超流動体・超伝導体の中から我々は超流動ヘリウム3(3He)に着目している。超流動3Heはp波スピン3重項超流動体であるため、複数の相が存在することが知られ、そのバルクの性質は詳細に研究されてきた。その中でもバルクでは等方的なギャップ2Δを持つB相と呼ばれる相は、トポロジカル超流動体であることが理論的に示されている[1]。また超流動3Heは不純物が極端に少ないため、表面状態の研究を行う試料として理想的である。バルク‐エッジ対応から予想される表面状態は、超流動3He-B相では表面アンドレーエフ束縛状態(Surface Andreev bound states; SABS )として現れるが、それがマヨラナフェルミ粒子であるとの指摘がされた。SABSは壁での準粒子散乱の境界条件である鏡面度Sに大きく依存し、特に鏡面散乱極限(S = 1)では、SABSバンドの幅Δ*が広がりギャップレスになり、エネルギーに比例する表面状態密度が現れ、表面にマヨラナコーンが存在することが示されている[2]。
 また、鏡面度が拡散的散乱条件ではない(S > 0)とき、新たなピークがZの温度依存性に現れ、そのピークはSの増加にともない成長した。この低温ピークはSABSのゼロエネルギー状態が減少したことから生じると理論計算から示され、その成長はマヨラナコーンへと漸近する振る舞いと見なせる。これは鏡面散乱極限(S = 1)の極限で現れると予測されているマヨラナコーンの存在を強く支持する実験結果である[5,6]。
[1] A. P. Schnyder et al., Phys. Rev. B 78, 195125 (2008).
[2] Y. Nagato et al., J. Low Temp. Phys. 149, 294 (2007).
[3] Y. Aoki et al., Phys. Rev. Lett. 95, 075301 (2005); M. Saitoh et al., Phys. Rev. B 74, 220505(R) (2006).
[4] Y. Wada et al., Phys. Rev. B 78, 214516 (2008).
[5] S. Murakawa et al., Phys. Rev. Lett. 103, 155301 (2009)
[6] S. Murakawa et al., J. Phys. Soc. Jpn. 80, 013602 (2011).

Jan/27/2014 (Mon) 13:30-14:30 @ Room445
Prof. Yoonseok Lee
Dept. of Physics, University of Florida, USA
''MEMS-based Probe for the Study of Quantum Fluids''
   One of the most distinct properties of unconventional superfluids/superconductors is the extreme fragility of Cooper pairs against any types of impurity or disorder. The surface scattering in these systems readily breaks Cooper pairs and consequently induces quasi- particle bound states near the surface within the coherence length, often called Andreev surface bound states.

   This generic nature of the unconventional order parameter near the boundaries becomes prominent in a film. This property combined with the exotic symmetries in the superfluid phases of 3He with p-wave spin triplet pairing is at the heart of many fascinating phenomena in confined geometry. However, it has been an experimental challenge to form thin films and to make sensitive measurements on them.

   We have developed a Micro-Electro-Mechanical System (MEMS) based probe for this purpose. Using a commercial micro-machining process, MEMS oscillators were designed, manufactured, and characterized. Each device consists of a pair of parallel plates with a well-defined gap in which a film forms when immersed in liquid. The mobile plate is suspended above the fixed plate (substrate) by four springs. This geometry allows to study the properties of the surrounding liquid through the resonant behavior of the mobile plate. We will discuss the design and the operation of the device, and will present results obtained using these devices in air, liquid 3He [1] and also in liquid 4He [2] in a wide range of temperature down to submillikelvin range. Our work demonstrates great potential of the device in a wide range of experiments in quantum fluids.

[1] M. Gonzalez et al., Rev. Sci. Instr. 84, 025003 (2013).
[2] M. Gonzalez et al., J. Low Temp. Phys. 171, 200 (2013).

Jan/14/2014 (Tue) 11:00-12:00 @ Room431
Prof. Markus Morgenstern
II. Institute of Physics and JARA-FIT, RWTH Aachen, Germany
''Probing Graphene on the Nanoscale by Scanning Tunneling Microcopy''
We use ultra-high-vacuum scanning tunneling microscopy at low temperature (6 K) to probe basic features of graphene relevant to mechanical and quantum mechanical applications.
Firstly, I present wave function mapping in graphene quantum dots deposited on Ir(111) [1]. The quantum dots are confined exclusively by zig-zag edges. However, edge states are absent due to an exchange interaction of the π-bands of graphene with the dz2 surface states of the Ir(111) [2]. The exchange interaction gets continuously smaller away from the edges, which leads to weak confinement of the quantum dot states being decisive for a rather regular wave function shape observed experimentally. The wave functions are additionally influenced by the penetration of an sp-like surface state of Ir(111) into graphene [1]. Concerning the edge state, DFT calculations reveal that H-terminated graphene nanoribbons on Au(111) should exhibit magnetic edge states [3].
Secondly, the mechanical properties of graphene flakes deposited by the scotch tape method are probed by STM. The graphene is partly not conformal to the substrate, but provides areas which are freely suspended [4]. The valleys of this corrugation can be further lifted by the van-der Waals forces and the electrostatic forces of the tip of the STM [5]. During continuous lifting, we often observe a breaking of atomic symmetry, i.e. the atomic structure switches reproducibly between a hexagonal and a triangular appearance.

[1] D. Subramaniam et al., Phys. Rev. Lett. 108 (2012) 046801.
[2] Y. Li et al., Adv. Materials, 25, 1967 (2013)
[3] Y. Li et al., arXiv:1210.2876.
[4] V. Geringer et al., Phys. Rev. Lett. 102 (2009) 076102.
[5] T. Mashoff et al., Nano. Lett. 10 (2010) 461.

Dec/5/2013 (Thu) 11:00-12:00 @ Room445
Prof. Eunseong Kim
KAIST, Korea
''How is the torsional oscillator response connected with shear modulus of solid helium?''
Supersolidity, the appearance of viscousless flow in solids, was first indicated in 4He torsional oscillator (TO) experiments [1]. Despite many efforts to uncover the mechanism of supersolidity since the first observation of non-classical rotational inertia (NCRI), the microscopic origin and physical interpretation of the observed phenomena has been under considerable debate. Recently, the TO response was considerably suppressed when the effect of shear modulus on TO was removed, suggesting alternative non-superfluid explanations responsible for the TO response [2]. On the other hand, quantum statistics dependence of the TO response [3], heat capacity signatures [4], and rotation effects on the TO response [5] are conflicting with non-supersolid explanations. In this talk, I will present simultaneous measurement of the TO response and shear modulus to understand the fundamental connection between two phenomena.

[1] E. Kim and M. H. W. Chan, Nature 427, 225 (2004).
[2] D. Y. Kim and M. H. W. Chan, Phys. Rev. Lett. 109, 155301 (2012).
[3] J. T. West et. al, Nat. Phys. 5, 598 (2009).
[4] X. Lin, A. C. Clark, and M. H. W. Chan, Nature, 449, 1025 (2007).
[5] H. Choi et. al, Science 330, 1512 (2010).

Dec/2/2013 (Mon) 10:00-12:00 @ Room431
Kazuki Nakayama
''Understanding the oxidative etching in graphite/graphene''
Pit formation by oxidative etching in graphite has long been studied since 1960s. Nowadays, the etching technique is utilized as a tool for the fabrication of graphite/graphene nanostructures.
In this colloquium, the experiments concerning environmental dependence of oxidative etching and their mechanism are reviewed. Based on that, the way to optimize the etching parameters to obtain desired etch pits is discussed.

Nov/18/2013 (Mon) 10:00-12:00 @ Room431
Yuya Kubota
''Development of a Torsional Oscillator for Studies of Superfluid Responses in Monolayers of Helium Four''
Recent heat capacity measurements for the second layer of helium four adsorbed on ZYX graphite clearly show the existence of a commensurate phase (C2) at a density C2 = 19.7 nm^-2 in between a low density liquid phase and a high density incommensurate phase. In addition, previous torsional oscillator measurements of the second layer of helium four by three different groups using Grafoil show frequency shifts below 300 mK at densities near C2 suggesting unusual superfluidity. However, the observed reentrant density variations of superfluid response are rather different each other, and the shifts are too small to convince the supersolidity.
In this colloquium, we report details of experimental setup and preliminary results of our new torsional oscillator measurement for studies of superfluid responses in monolayers of helium four.

Nov/11/2013 (Mon) 10:00-12:00 @ Room431
Masahiro Kamada

Oct/28/2013 (Mon) 10:00-12:00 @ Room431
Tomohiro Matsui
''Magnetic Properties of Zigzag edge in Graphene''
The understanding of the edge state is one of the central issues in graphene research, since the edge contribution becomes larger in nano-devices as nano-ribbons and nano-islands. It is especially the case when the edge is constructed in zigzag structure because spin polarized localized state is expected around Fermi energy in zigzag edges as had been discussed in last colloquium of mine. In this colloquium, I want to focus on some structures with zigzag edges such as zigzag nano-ribbons, zigzag islands and zigzag antidots, and review their electronic properties obtained in transport measurements and Raman spectroscopies.

Sep/9/2013 (Mon) 10:00-12:00 @ Room431
Takayuki Nakajima
測定原理、特徴とともに実際どのように使われているのか、実験データとともに紹介する 予定である。熱スイッチも主に3つあり(超伝導熱スイッチ、ガスーギャップ式、機械的熱スイッチ) 特徴、実験論文も紹介し、今回自身の研究で用いるインジウムの超伝導熱スイッチについても考察する予定である。

Jul/30/2013 (Tue) 14:00-15:00 @ Room431
Prof. Dr. Christian Enss
Universitat Heidelberg
Kirchhoff-Institut fur Physik
''New Frontiers at Ultralow Temperatures: Micro Calorimetry and Noise Thermometry''
In recent years important advances have been made in astro/particle physics that have deepened our understanding of the nature of the universe and the fundamental properties of matter. Much of this experimental progress has been made possible by technological developments in others fields like optics, computers, semiconductors, etc. In this vein, the emerging technology of cryogenic particle detection is now poised to make major contributions to a variety of experiments in astrophysics, atomic physics and material science. We will discuss the status of development of metallic magnetic calorimeters and their applications. In addition, we present a new concept for precision thermometry at ultralow temperatures based on magnetic flux noise.

Jul/22/2013 (Mon) 10:00-12:00 @ Room431
Kazuki Nakayama

Jul/8/2013 (Mon) 10:00-12:00 @ Room431
Tomohiro Matsui
''Peculiar Electronic States at Graphene Edges''
It is a central question how graphene behaves when it is patterned at nanometer scale with different edge geometries. The graphene nanoribbon (GNR), a narrow strip of graphene, is a good playground to study the electronic states at graphene edges and their effect to the ribbon. And there are many predictions for GNRs to exhibit a wide range of behavior, including tunable energy gaps and the presence of one-dimensional (1D) edge states with unusual magnetic structure. In this colloquium, I want to overview the peculiar electronic states of graphene edges, based on the STM/S point of view.

Jun/10/2013 (Mon) 12:30-14:30 @ Room431
Sachiko Nakamura
''Phase diagram of the second layer of 4He on graphite''
I will show the coverage-temperature phase diagram of the second helium-4 layer adsorbed on ZYX graphite, based on our thermodynamical studies.
Our coverage scale might include absolute errors, as our experiment is not a diffraction study, however, we obtained the coverage scale with a small relative error using the 1/3 commensurate solid phase of the first layer of helium or nitrogen, and we believe, the same batch of ZYX graphite behaves similarly in determination of the coverage scale.
Therefore, for our on-going measurement of the superfluid responce using ZYX graphite, to be discussed in the next talk, the phase diagram of 4He on the same substrate is very important.
I will also review the heat capacity and the vapor pressure of the second layer of 4He adsorbed on ZYX graphite and related isotope systems, namely, bilayer 3He, and 3He on a layer of 4He. Substitutions of a few isotope particles by other groups will be dealt with briefly.

Yuya Kubota
''Search for Possible Supersolidity in 2D Solid 4He by Torsional Oscillator Measurement''
Recent heat capacity measurements for the second layer of 4He adsorbed on graphite clearly show the existence of a commensurate phase (C2) at a density C2 = 19.7 nm−2. The C2 phase is the lowest density quantum solid ever found with substantially fast exchanges of atoms and vacancies. Therefore, it is a hopeful candidate for the novel supersolid phase. Previous torsional oscillator measurements of the second layer of 4He by three different groups using Grafoil show frequency shifts below 300 mK at densities near C2 suggesting unusual superfluidity. However, the observed reentrant density variations of superfluid response are rather different each other, and the shifts are too small to convince the supersolidity. This is presumably because of large uncertainties in their density scales and poor connectivity of platelet boundaries in Grafoil. Here, we report details of experimental setup and preliminary results of our new torsional oscillator measurement down to 10 mK. The oscillator is made from coin silver containing ZYX substrate which is an exfoliated graphite with much larger platelet size than Grafoil.

May/27/2013 (Mon) 10:00-12:00 @ Room431
Masahiro Kamada

Apr/15/2013 (Mon) 10:00-12:00 @ Room431
Sachiko Nakamura
''Possible Substrates for our Future Adsorption Study''

Seminar (FY2012)

Dec/28/2013 (Thu) 10:00-12:00 @ Room445
Tomohiro Matsui
''STM/S Studies of Rare Gas Atoms on Graphite''
Rare gas atoms on various surfaces are regarded as two-dimensional (2D) model systems in surface science due to their closed-shell electronic structure and weak interaction with substrates and among themselves. These systems provided ideal testing grounds for 2D adsorbate phases and phase transitions as well as for growth mechanism of physisorbed films. Among several techniques, STM/S is a powerful tool to study such systems since it is possible to obtain real-space images of the rare gas adlayer structure from submonolayer to monolayer coverage. However, it had not been well cleared yet why rare-gas atoms are visible in the STM. In principle, an adsorbate is visible when it modifies the LDOS near /E/_F, but it is unlikely the case for the rare gas atoms. In this colloquium, I want to review the experimental examples of the rare gas observations and discuss about their mechanisms.

Feb/21/2013 (Thu) 10:00-12:00 @ Room445
Kouta Matsui

Feb/14/2013 (Thu) 10:00-12:00 @ Room445
Daisuke Sato

Dec/20/2012 (Thu) 10:00-12:00 @ Room445
Masahiro Kamada
''The edge state and the magnetic heat capacity of graphite substrate''
While bulk graphite is diamagnetic, the possibility of graphite being ferromagnet at the edge is indicated.
In this colloquium, I eestimate internal magnetic field of graphite substrate used for making 2 demensional 3He commensurate phase from the calculated magnetic heat capacity assuming ferromagnetic edge state and measured addendum heat capacity data.

Dec/10/2012 (Mon) 13:30-15:30 @ Room431
Yuya Kubota
''Zero-point vacancy and it's contribution to supersolid''
Since Andreev and Lifshitz proposed the existence of zero-point vacancy (ZPV) and the possibility of its BEC, ZPV has been focused on in research on supersolid. Its contribution is also expected in the 2D He system which is our topic.
In this colloquium, I will review ZPV and its work at the densities near the 4/7 phase of 2D He adsorbed on graphite.

Nov/15/2012 (Thu) 10:00-12:00 @ Room445
Kazuki Nakayama
''Oxygen adsorption on graphene''
Graphene's reaction to the exposure in various gases has been investigated, one reason of which is its possible application to the gas sensor.
In this colloquium, I am going to review the oxygen adsorption on graphene, and try comparing the data from previous researches and my measurement on the change in the electronic properties.

Nov/8/2012 (Thu) 10:00-12:00 @ Room445
Sachiko Nakamura

Sep/5/2012 (Wed) 10:00-11:30 @ Room445
Kamada Masahiro
''Studies on Quantum Spin Liquid''
My research theme is studying the ground state of 2 dimensional 3He in the 4/7 phase, which is expected to be the gapless spin liquid state.
In this colloquium, I will talk about the idea of Resonating Valence Bond (RVB), some systems show the possibility to be spin liquid state, and our study on 4/7 phase of 3He.

Jul/5/2012 (Thu) 16:30-18:00 @ Room431
Kouta Matsui
''LEED Calculations for Complex Structures''
The dynamical LEED calculation has a difficulty in calculating complex structures because of the computational complexity grows with the square or cube of the number of atoms in the unit cell.
Since adsorbed 2D He on graphite has complex structures depending on its density, only 1st layer commensurate solid can be calculated with the simple lattice model.
In this colloquium, I will show necessary approximation methods for such complex structures.

Jun/28/2012 (Thu) 16:30-18:00 @ Room431
Tomohiro Matsui
''SiC graphene -STM/S studies ant its modification by adsorbates-''
Various strategies exist for preparing graphene samples, for example popular micromechanical exfoliation of graphite, high temperature decomposition of polar surface of SiC crystal, and chemical vapor deposition on metallic substrate. In each case, however, there are unavoidable influences from the substrate to graphene, which modifies the genuine electronic properties of graphene. Here I want to focus on the electronic properties of the graphene grown on SiC(0001) and its appearance in STM/S measurement. In addition, I want to show some examples of the effect of adsorbates on SiC-graphene and our experimental results of Kr deposited on SiC-graphene.

Jun/21/2012 (Thu) 16:30-18:00 @ Room431
Sachiko Nakamura
''4/7 Phase in the 2nd Layer Helium-4''
I argue with our recent heat capacity and vapor pressure result of 4/7 phase in the 2nd layer helium-4 on ZYX graphite, caring especially about its critical behavior and substep in adsorption isotherm.
First, I compare our system to previous studies of 1/3 phase in the 1st layer helium-4 and the 2nd layer helium-4 with preplated krypton on graphite, in terms of q-state Potts model. Then, I show more complex system of 1/3 phase in the 1st layer nytrogen and consider what substep shows in adsorption isotherm.
In heat capacity, they already show various behaviors but 4/7 phase shows another behavior. I tell imaginable models of the behavior.

Jun/7/2012 (Thu) 14:30-15:30 @ Room414
''Probing chirality of superfluid 3He-A''
In this talk, we report the first detection of chirality of superfluid 3He-A using a novel effect, intrinsic Magnus effect, experienced by a traveling impurity [1,2]. When an impurity is traveling in a plane perpendicular to the l vector, the impurity experiences the intrinsic Magnus force in the direction perpendicular to both its velocity and the l vector, as a result of the skew scattering of quasiparticles by the impurity. We investigated the intrinsic Magnus effect for charged impurities, electron bubbles, trapped below the free surface of superfluid 3He at a depth ~30 nm by transport of electron bubbles along the surface. The free surface aligns the l vector uniformly normal to the surface. We observed the current associated with the intrinsic Magnus effect in the direction transverse to the electric field. Two behaviors in the transverse current are observed, which are equal in magnitude but opposite in sign, and these two correspond to the l vector pointing upward (+l) and downward (-l). After repeating many cooling runs from normal statewe find that the ratio between +l and -l emerging just below Tc is asymmetric about the direction of magnetic field.
[1] R. H. Salmelin et al., Phys. Rev. Lett. 63, 868 (1989).
[2] R. H. Salmelin and M. M. Salomaa, Phys. Rev. B 41, 4142 (1990).

Jun/4/2012 (Mon) 10:00-10:45 @ Room445
Osamu ISHIKAWA (Osaka City University, Graduate School of Science Mathematics and Physics)
''Experimental Study on Intrinsic Angular Momentum in Superfluid 3He-A Phase''
NMR study has been done to investigate intrinsic angular momentum of Cooper pairs as a macroscopic quantity. Recent experiments using a rotating nuclear demagnetization cryostat at ISSP shows a novel result related to this problem. A sample cell is composed of one long cylinder with 0.1mm in diameter, in which there exists a special texture, so-called Mermin-Ho texture. Texture is a spacial pattern of order parameter describing superfluid 3He. A small change of this texture by rotation is detectable by NMR and gives us an information about macroscopic quantity of intrinsic angular momentum.

May/31/2012 (Thu) 17:00-18:00 @ Room431
Yuya Kubota
''Torsional oscillator studies for supersolid He''
I am preparing for torsional oscillator study. I will first introduce some studies of supersolid and describe principle of torsional oscillator. Moreover, I'm going to talk about previous studies of torsional oscillator and report our experimental design.

Apr/26/2012 (Thu) 16:30-18:00 @ Room431
Kazuki Nakayama
''A certain conclusion of verification experiments for gap opening of rare-gas/graphene system''
I will first describe current procedure of graphene sample preparation for transport measurement, mainly focusing on "Indium nanosoldering" method. Then, I will report a certain conclusion of verification experiments for gap opening of rare-gas/graphene system, so far obtained in current experimental setup.

Seminar (FY2011)

Feb/21/2012 (Tue) 16:30-18:00 @ Room431
Tomohiro MATSUI
''STM/S studies of Indium nanoparticles on graphite and SiC-graphene exposed to Kr atoms''
I will overview our ULT-STM activities in 2011 in this colloquium. As a first part, I will talk about Indium nanoparticles on a surface of graphite. In this measurement, unexpectedly large and deep superconducting gap with small coherence peaks were observed suggesting the localization and/or confinement of Cooper pairs in superconducting particles. STM/S studies of epitaxial graphene on SiC substrate exposed to Kr atoms would be presented as a second part. The analysis is still on-going but I hope I can show some hints for future experiments.

Dec/13/2011 (Tue) 15:00-16:15 @ Room431
Yu. M. Bunkov (Institut Neel, Grenoble, France)
''Magnon Bose-Einstein condensation''
Superfluid 3He can be considered as a quantum vacuum carrying various types of quasiparticles and topological defects. The structure of this system shows many similarities to that of our Universe. It can act as a model system for the study of many types of general physics experiments, which are difficult or even impossible in Cosmology, Atomic or Nuclear physics. There is a complete analogy between the Bose-Einstein condensation of atomic gases and the Bose-Einstein condensation of magnons in superfluid 3He. Five different states of magnon condensation have been found; the homogeneously precessing domain (HPD) in 3He-B; the persistent signal, which is formed by a Q-ball in 3He-B at very low temperatures; coherent precession with fractional magnetization in 3He-B and coherent precession of magnetization in 3He-A and 3He-B in a squeezed aerogel [1]. All these cases are examples of the Bose-Einstein condensation of magnons with the interaction potential provided by specific spin-orbit coupling. The BEC phenomenon in the gas of magnons is readily accessible owing to the possibility of modifying the spin-orbit coupling. In some cases the BEC of magnons corresponds to almost 100% condensation.
In Fig.1 the frequency and phase distribution of BEC radiation is demonstrated. The accuracy of phase and frequency measurements is mixed due to the analogy with the uncertainty. Fig 2 shows the BEC radiation broadening, which is about 0.2 Hz, the 4000 time smaller then it should be due to the inhomogeneity of magnetic field. The broadening of magnetic field on the sample corresponds to an 800 Hz!
Finally the magnon BEC state was found on a 55Mn nuclear in antiferromagnetic CsMnF3 [3].
[1] Yuriy M. Bunkov and Grigoriy Volovik, J. Phys.: Condens. Matter 22 164210 (2010)
[2] Ю. М. Буньков, УФН, 180, 884; Physics–Uspekhi, 53:8, 848–853(2010)
[3] Yu.M. Bunkov, E.M. Alakshin, R.R. Gazizulin, A.V. Klochkov, V.V. Kuzmin, T.R. Safin, M.S. Tagirov. « Discovery of the Classical Bose–Einstein Condensation of Magnons in Solid Antiferromagnets » JETP Letters, 94, 68–72 (2011)
[*] PDF File

Dec/8/2011 (Thu) 17:00-19:00 @ Room431
Kazuki Nakayama
''Gap opening mechanism of graphene by adatoms''
The unique properties of graphene are attracting a variety of research interests. Among them, gap opening of graphene is an important step for future applications. In this colloquium, I will describe a theoretical prediction which says the symmetry breaking by adatoms on graphene would result in the gap opening. In addition, I want to introduce current status of our experiment to confirm the prediction.

Nov/1/2011 (Tue) 16:30-18:30 @ Room431
Kimiaki Naruse
''Helium molecules, 3He on 4He films''

Oct/18/2011 (Tue) 16:30-18:30 @ Room431
Jan Raphael Bindel
''UHV-AFM/STM at 300mK and 14T''
Investigations of partial electrically conducting samples on an isolating substrate, like exfoliated graphene on SiO2 or micro-structured graphene, with scanning tunneling microscopy (STM) lead to several challenges due to the limitation of STM to conducting surfaces. It gets even more challenging if the field of interest leads to ULT-STM, e.g. Superconductivity, Kondo-physics, Wigner-crystallization. I will present the development of an UHV-System for combined atomic force microscopy (AFM) and STM using a tuning fork. It is designed for temperatures down to 300mK and magnetic fields up to 14T. Additionally, it permits direct optical access to the microscope inside the cryostat. I will concentrate on the design of the AFM/STM and the Cryostat, which enables measurement periods of 100h at 300mK.

Oct/4/2011 (Tue) 16:30-18:30 @ Room431
Sachiko Nakamura
''Heats of Adsorption''
In order to know the heat capacity of the 2nd-layer Helium above 2 K, some corrections for desorption are needed. I will introduce how to derive the heat capacity of adsorbed films from the measured heat capacity with adsorption isotherms.

Jul/27/2011 (Mon) 13:00-15:00 @ Room431
Kimiaki Naruse
''New phase formation on solid surfaces''

Jul/25/2011 (Mon) 16:30-18:30 @ Room431
Hideto Takei

Jul/08/2011 (Fri) 10:00-12:00 @ Room431
Kouta Matsui

Jul/01/2011 (Fri) 10:00-12:00 @ Room431
Tomohiro Matsui
''Band Gap Opening in Graphene''
Graphene exhibits a number of exotic electronic properties, such as unconventional integer quantum Hall effect, ultrahigh electron mobility, electron-hole symmetry, and ballistic transport even at room temperature. While many other properties of graphene are very promising for nanoelectronics, its zero-gap semiconductor nature is detrimental, since it prevents the pinch off of charge current as requested in conventional electronic devices. Thus, the opening and tuning band-gap of graphene are especially crucial to the practical application of graphene to electronic devices. Roughly speaking, there are two ways to lift the degeneracy of the two bands crossing at the Dirac points. One is to hybridize the electronic states at K and K', which requires breaking of the translational symmetry. The other is to break the equivalence between the A and B sublattice, which does not require any translation symmetry breaking. In this colloquium, I want to introduce some techniques to open and tune the band-gap of graphene, especially the way to functionalize the graphene by adsorbing atoms.

Jun/17/2011 (Fri) 10:00-12:00 @ Room431
Daisuke Sato
''Spin relaxation studies of dilute 3He systems''

May/27/2011 (Fri) 10:15-12:00 @ Room431
Sachiko Nakamura
''Finite-size Scaling of 4He at the Superfluid Transition''

Apr/08/2011 (Fri) 10:00-12:00 @ Room431
Kazuki Nakayama
''Phase measurement in self assembled InAs quantum dot josephson junctions in SQUID''

Seminar (FY2010)

Feb/25/2011 (Fri) 10:00-12:00 @ Room431
Kimiaki NARUSE
''Nuclear magnetic resonance methods in two dimensional helium 3 system''

Feb/04/2010 (Fri) 10:00-12:00 @ Room431
Tomohiro MATSUI
''STS Observation of Dirac Fermion Topologically appeared on Graphite''
Since the experimental success to fabricate Graphene, the uniquecharacters of Dirac fermion (DF) start to attract a big interest. Inaddition to Graphene, a typical material with massless DF, some othermaterials are also paid many attentions recently because of theirtopologically appeared DFs. Graphite is a well known quasitwo-dimensional (2D) material and one of the good candidates of suchtopological Dirac fermion (TDF). Here, we observed the TDF on thesurface of graphite with scanning tunneling microscopy and spectroscopy(STM/S) in magnetic fields. The observed surface density of states (DOS)show many peaks which are originated from the Landau levels on graphitesurfaces. The field (B) dependencies of the peaks are usually verycomplicated reflecting the irregular structure of graphite in zdirection with many stacking faults. However, the detailed analysissuggests that there certainly are peaks which have the √nB dependencelike massless DF in graphene. In this talk, I will show bothexperimentally and theoretically that massless DF can be localized onthe surface of graphite, in general.

Dec/10/2010 (Fri) 10:00-12:00 @ Room431
Sadashige MATSUO

Dec/3/2010 (Fri) 10:00-12:00 @ Room431

Nov/26/2010 (Fri) 10:00-12:00 @ Room431

Nov/09/2010 (Tue) 14:00-16:00 @ Room431
Masashi MORISHITA (Tsukuba University)

Nov/05/2010 (Fri) 10:00-12:00 @ Room431
''Definition of the Surface Density in Adsorption Study on Graphite Substrate''

Nov/02/2010 (Thu) 13:00-14:00 @ Room445
Ya Yang (University of Science and Technology Beijing)
''SPM Studies of Single ZnO Nanomaterials''
We, for the first time, investigated the transverse electrical and electromechanical properties of single ZnO nanomaterials by using a scanning probe microscope [AFM, C-AFM, EFM, STM]. The measurement method is usually based on a single ZnO nanomaterial lying down on a metal substrate, as shown in Fig. 1. The conductive atomic force microscope tip was used as the top electrode. These investigations are very important because the carrier path through single transverse nanomaterials may be only several tens of nanometers. The physical mechanisms of the observed experimental results were discussed in detail. Based on these transverse physical properties, we have demonstrated several typical transverse nanodevices, such as hybrid diodes, piezoelectric switches, piezoelectric field effect transistors, and so on. Moreover, the service safety of the single ZnO nanomaterials and nanodevices was also characterized by SPM. These results support the applications of ZnO nanomaterials as the electrical and electromechanical nanodevices.
[*] Figure: PDF file

Oct/20/2010 (Wed) 11:00-12:00 @ Room414
Ryo TODA (Kyoto University)

Oct/04/2010 (Fri) 10:00-12:00 @ Room431
Daisuke SATO
''Physics of 2D Fermi fluid''

Sep/02/2010 (Thr) 16:00-18:00 @ Room414
Sander Otte (Delft University of Technology, The Netherlands & NIST, USA)
''The Graphene Landau Quartet Unveiled: an ultra-high resolution tunneling spectroscopy study''
With its focus shifting more and more from topography to spectroscopy,the STM has evolved into an incredibly versatile tool that can map boththe spatial and energy domains of an electronic quantum system. Whereasthe spatial resolution depends mostly on the stability of the tip-samplejunction, the energy resolution of tunneling spectroscopy measurementshas been limited by the temperature of the microscope. In this talk Iwill describe the design and development of a recently completeddilution refrigerator cooled STM system, in which tip and sample can bekept at a temperature of 13 mK in magnetic fields up to 15 T whilemaintaining picometer stability at the junction. The unparalleled energyresolution of this instrument is demonstrated very well by aspectroscopic study of Landau levels in epitaxial graphene: a perfecttwo-dimensional electron system. In this experiment the predictedfourfold degeneracy of the Landau levels is broken and their splittingas a function of the magnetic field measured for the first time. Inaddition, intriguing partial filling of the sublevels is observed,yielding access to promising electron correlation effects.

Aug/03/2010 (Tue) 14:00-16:00 @ Room431
Pedram Roushan (Princeton University)
''Visualizing Topological Surface States by Scanning Tunneling Microscopy''
Topological insulators are a new class of insulators in which a bulk gap for electronic excitations is generated because of strong spin-orbit coupling. These materials are distinguished from ordinary insulators by the presence of gapless metallic surface states, resembling chiral edge modes in quantum Hall systems, but with unconventional spin textures. We have performed scanning tunneling microscopy and spectroscopic studies of topological surface states on a range of different compounds Bi1-xSbx, Sb, Bi2Te3, MnxBi2-xTe3. I will describe how these experiments illustrate the importance of the spin-texture of these novel states for their scattering and quantum confinement. Experiments demonstrate that these states are protected from backscattering between opposite spin states due to their chiral spin textures [1]. Furthermore, we have measured the transmission and reflection probabilities of topological surface states of Sb through naturally occurring crystalline steps separating atomic terraces. In contrast to non-topological Shockley surface states of metals (such as those found on copper, silver and gold), which are either reflected or absorbed by atomic steps, we show that topological surface states of Sb penetrate such barriers with high probability [2]. Finally, I will also discuss the modification of the spin texture and the possibility of localization of these surface states in the presence of magnetic scattering centers.
[1] Roushan et al. Nature 460, 1106 (2009).
[2] J. Seo et al. Nature 466, 343 (2010).

Jul/16/2010 (Fri) 10:00-12:00 @ Room431
Tomohiro MATSUI
''Three Months Stay at NIST''
I have been in CNST/NIST (Center for Nanoscale Science and Technology/National Institute of Standards and Technology) as a CNST Visiting Fellow in the CNST Electron Physics Group for three months supported by the Excellent Young Researcher Oversea Visit Program (Japan Society for the Promotion of Science), where I have joined the research of Dr. Joseph A. Stroscio concering Graphene and other novel electronic properties obtained with scanning tunneling microscopy and spectroscopy (STM/STS). In this colloquium, I want to introduce some STM/STS studies about Graphene in NIST and show some pictures of other groups in eastern US.

Jul/09/2010 (Fri) 10:00-12:00 @ Room431

Jun/25/2010 (Fri) 10:00-12:00 @ Room431
Sadashige MATSUO

Jun/18/2010 (Fri) 10:00-12:00 @ Room431
Daisuke SATO
''Physics of quantum spin liquid''

Jun/4/2010 (Fri) 10:00-12:00 @ Room431
''Surface Triggering''

May/17/2010 (Mon) 10:00-12:00
Naoki F. Kawai

Apr/23/2010 (Fri) 16:00-18:00 @ Room431
Hideto TAKEI

Kimiaki NARUSE
''Vortex-pair annihilation''

Apr/9/2010 (Fri) 16:00-18:00 @ Room431
Sadashige MATSUO

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Seminar (FY2009)

Feb/18/2010 (Tue) 10:00-12:00 @ Room431

Dec/18/2009 (Fri) 10:00-12:00 @ Room431
"Graphene edge properties and its control"

Dec/2/2009 (Wed) 10:00-12:00 @ Room414

Nov/13/2009 (Fri) 10:00-12:00 @ Room431
''Surface Effect to the 2D Helium on Graphite''

Oct/30/2009 (Fri) 10:00-12:00 @ Room431
Daisuke SATO
''Substrate heterogeneity effects of monolayer 3He on Graphite''

Sep/17/2009 (Thu) 10:00-12:00 @ Room431
Sadashige MATSUO

Sep/7/2009 (Mon) 14:00-16:00 @ Room431

Jul/6/2009 (Mon) 14:00-16:00 @ Room431
Tomohiro MATSUI
''Superconductivity in nanometer scale''

Jun/22/2009 (Mon) 14:00-16:00 @ Room431
Daisuke Sato
''Heat capacity studies of monolayer 3He adsorbed on graphite''

Jun/8/2009 (Mon) 14:00-16:00 @ Room431
Naoki F. Kawai
''Recent topics on graphene''

May/25/2009 (Mon) 14:00-16:00 @ Room431
Sachiko Nakamura
''Two-dimensional Rare Gas Solids on Graphite''

Apr/27/2009 (Mon) 14:00-16:00 @ Room431
Kazunari Kawabata
''Kosteritz-Thouless theory''

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Seminar (FY2008)

Mar/3/2009 (Tue) 14:00- @ Room431
Dr. Yuki Sato (Dept. of Physics, UC Berkeley, USA)
''Superfluid 4He Matter Wave Interferometers . Physics and Applications''

Dec/16/2008 (Tue) 16:30- @ Room367
Shinji Watanabe (School of Engineering, The University of Tokyo)
''Two Dimensional Quantum Spin Liquid: Universal character of Helium-3 and electrons''

Dec/15/2008 (Mon) 10:00-12:00 @ Room431
Naoki F. Kawai
''ARPES and STM studies of epitaxial graphene on SiC''

Dec/8/2008 (Mon) 16:00-17:00 @ Room431
Dr. Joseph A. Stroscio
''Electronic Structure of Epitaxial Graphene on SiC''

Dec/5/2008 (Fri) 16:00-17:00 @ Room431
Daisuke Sato
''Quantum phase diagram of the second layer 3He adsorbed on graphite''

Nov/17/2008 (Mon) 10:00-12:00 @ Room431
Shintaro Takayoshi
''NMR measurements of the spin-spin relaxation time (T2) for 2D solid 3He at densities around 4/7 localized phase''

Nov/10/2008 (Mon) 10:00-12:00 @ Room431
Kazuyoshi Obata

Nov/7/2008 (Fri) 17:00-18:30 @ Room414
Dr. Sebastian Loth (IBM Almaden Research Center)
''Inside an operating diode: electric field effects at individual dopant atoms''

Oct/27/2008 (Mon) 11:00-12:15 @ Room431
Dr. Cyrus F. Hirjibehedin (University College London, UK)
''Spin-Excitation Spectroscopy: Magnetic Interactions at the Atomic Scale''

Sep/29/2008 (Mon) 10:00-12:00 @ Room431
Sachiko Nakamura
''Heat capacity measurements and superconductivity/superfluidity''

Sep/8/2008 (Mon) 10:00-12:00 @ Room431
Kazunari Kawabata
''Physical properties of layered thin superconductor''

Jun/30/2008 (Mon) 10:00-12:00 @ Room431
Masashi Morishita (Graduate School of Pure and Applied Sciences, University of Tsukuba)
''Magnetic field dependence of the heat capacity for solid 3He in low areal densities''

Jun/23/2008 (Mon) 10:00-12:00 @ Room431
Naoki F. Kawai
''Epitaxial Graphene''

Jun/2/2008 (Mon) 10:00-12:00 @ Room431
Daisuke Sato
''Two-dimensional 3He-4He mixture''

May/19/2008 (Mon) 10:00-12:00 @ Room431
Tomohiro Matsui
''STM/STS Studies of Graphene''

May/2/2008 (Mon) 10:00-12:00 @ Room431
Kazuyoshi Obata
''Magnetism of 2D solid 3He in higher areal densities''

Apr/21/2008 (Mon) 10:00-12:00 @ Room431
Shintaro Takayoshi
''Nuclear Magnetic Relaxation of 2D 3He''

Apr/14/2008 (Mon) 10:00-12:00 @ Room431
Kazunari Kawabata
''Quantum electrodinamic derivation of the Lamb shift''
Sachiko Nakamura
''The construction of a diaphragm pressure gauge which works at low temperature''

Apr/17/2008 (Thu) 16:30-17:30 @ Room431
Erik Henriksen (Columbia University, USA)
''Cyclotron resonance in graphene''

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Seminar (FY2007 former)


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