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セミナー (2016年度)

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
松元和幸 氏
室蘭工業大学 応用理化学系応用物理
 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 @ 低温センター3階セミナー室
小川 勝之
''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
松井 朋裕
''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
中村 祥子
''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 @ 低温センター3階セミナー室
中村 龍司
''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
喜田 和馬
''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
高野 安正 教授
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 @ 低温センター3階セミナー室
中村 龍司
''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
松井 朋裕
''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
喜田 和馬
''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.

小川 勝之
''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 @ 低温センター3階セミナー室
鎌田 雅博
''Domain wall in adsorbed monoatomic layer''

セミナー (2015年度)

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

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

Dec/7/2015 (Mon) 10:00-11:30 @ Room431
松井 朋裕
''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
鎌田 雅博
''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
佐藤 秀樹
''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
中村 龍司
''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 @ 低温センター3階セミナー室
中村 祥子
''4He Third Sound Studies using Graphite Substrate''
グラファイト基板を用いたヘリウム4薄膜の第三音波測定に ついて概観し、現在、我々が、ヘリウム4薄膜の超流動測定における 新しいグラファイト基板として着目している熱分解黒鉛シート(PGS)の 評価に第三音波測定が有意義であるかどうかを考察する。

Jul/6/2015 (Mon) 10:00-12:00 @ 低温センター3階セミナー室
戸田 亮
''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 @ 低温センター3階セミナー室
藤井 武則
''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
松井 朋裕
''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
佐藤 秀樹
''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 @ 低温センター3階セミナー室
鎌田 雅博
''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
中山 和貴
''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
中村 龍司
''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.

セミナー (2014年度)

Feb/23/2015 (Mon) 10:00-11:30 @ Room431
松井 朋裕
''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 @ 低温センター3階セミナー室
鎌田 雅博
''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 @ 低温センター3階セミナー室
戸田 亮
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
中村 祥子
''XRD Studies of Graphite Sheets''

Dec/8/2014 (Mon) 10:00-11:30 @ Room431
中山 和貴
''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
佐藤 秀樹
''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
鎌田 雅博
''Isotope Effect on 2D Hydrogen on Graphite''

Jul/7/2014 (Mon) 10:00-11:30 @ Room431
中村 祥子
''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
松井 朋裕
''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
中山 和貴
''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
佐藤 秀樹

セミナー (2013年度)

Mar/19/2014 (Wed) 14:00-15:00 @ Room431
松本 洋介 氏
東京大学 物性研究所
''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
仲嶋 孝友起

Mar/6/2014 (Thu) 14:30-15:30 @ Room445
村川 智 氏
慶応義塾大学 理工学部
『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
中山 和貴
''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
久保田 雄也
''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
鎌田 雅博

Oct/28/2013 (Mon) 10:00-12:00 @ Room431
松井 朋裕
''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
仲嶋 孝友起
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
中山 和貴

Jul/8/2013 (Mon) 10:00-12:00 @ Room431
松井 朋裕
''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
中村 祥子
''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.

久保田 雄也
''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
鎌田 雅博

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

セミナー (2012年度)

Feb/28/2013 (Thu) 10:00-12:00 @ Room445
松井 朋裕
『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
松井 幸太

Feb/14/2013 (Thu) 10:00-12:00 @ Room445
佐藤 大輔

Dec/20/2012 (Thu) 10:00-12:00 @ Room445
鎌田 雅博
今回のコロキウムでは我々が2次元3He整合相の吸着基盤として用いているグラファイト基盤に対して、エッジ状態の強磁性を仮定して求めた磁気比熱と実際に測定されたアデンダ比熱から基盤の内部磁場 を見積もる。

Dec/10/2012 (Mon) 13:30-15:30 @ Room431
久保田 雄也
『zero-point vacancyとその超固体への寄与』
AndreevとLifshitzによってzero-point vacancy(ZPV)の存在およびそれがBECを起こす可能性があると提唱されて以来、超固体の研究においてZPVが注目されてきた。我々が測定する2次元ヘリウムにおいてもその寄与が期待される。

Nov/15/2012 (Thu) 10:00-12:00 @ Room445
中山 和貴

Nov/8/2012 (Thu) 10:00-12:00 @ Room445
中村 祥子
これまで、第2層目のヘリウム4の実験結果のみ紹介してきたので、本コロキ ウムでは、最近取得した第1層目ヘリウム4の熱容量データと、第1層目が ヘリウム4で第2層目がヘリウム3のときのデータについて紹介したい。
第1層目のヘリウム4は、全てやり尽されているというのが通説であったが、 整合固体相の融解比熱の密度依存性について、顕著な非対称性が今回新たに 観測された。
第2層目のヘリウム3においては、第1層目がヘリウム4であることによる 効果だけでなく、おそらく基板の質に依存したと期待される変化が観測されて いる。

Sep/5/2012 (Wed) 10:00-11:30 @ Room445
鎌田 雅博
''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''

Jun/28/2012 (Thu) 16:30-18:00 @ Room431
松井 朋裕
''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
中村 祥子

Jun/7/2012 (Thu) 14:30-15:30 @ Room414
池上 弘樹 博士 (理研)
''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
石川 修六 教授 (大阪市立大学 大学院理学研究科 数物専攻)
''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
久保田 雄也

Apr/26/2012 (Thu) 16:30-18:00 @ Room431
中山 和貴

セミナー (2011年度)

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)
[*] 図はこちらのPDFファイルを参照

Dec/8/2011 (Thu) 17:00-19:00 @ Room431
中山 和貴

Nov/1/2011 (Tue) 16:30-18:30 @ Room431
成瀬 公暁

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
中村 祥子
『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
成瀬 公暁
『New phase formation on solid surfaces』
 量子性の強い3Heや4Heが2次元平面上で自己凝縮するか否かは数多くの議論がなされてきた。我々の実験結果ではpuddle相の存在を示すγ値のキンクが見られている。しかし、 3Heの純粋な二次元系については、自己凝縮しないことを示す理論計算や実験結果が大勢を占めているのが現状である。

Jul/25/2011 (Mon) 16:30-18:30 @ Room431
武井 英人
 基板上に作成した擬2次元、あるいはナノスケールのアイランド状の超伝導については、超伝導の次元性やサイズ効果、または基板への 超伝導の染み出しなど興味深い物理がある。

Jul/08/2011 (Fri) 10:00-12:00 @ Room431
松井 幸太
 LEEDの動力学解析では、運動学モデルでは解析できないような複雑な表面の構造を解析できる一方、 X線などと異なり散乱電子同士の干渉が非常に大きくその取り扱いは複雑である。

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
佐藤 大輔
 具体的には、3次元固体4He中にて準粒子として存在する希薄な3Heについての研究、3次元における 希薄な気体3Heについての研究結果を紹介し、最後に我々の研究しているグラファイト上2次元3Heの 低密度領域におけるNMRの結果を議論する予定である。

May/27/2011 (Fri) 10:15-12:00 @ Room431
中村 祥子
 有限サイズスケーリングは、臨界現象を調べる際に、有限系での計算結果を無限系に外挿する手段として一般的な方法である。 ヘリウム4超流動転移は、無限系とみなせる系における臨界現象の性質がよくわかっている上に、 マクロにクリーンな系が実現できるため、有限サイズスケーリングを実験的に検証できる希有な系である。 本コロキウムでは、スケーリングの概念と、ヘリウム4超流動転移における有限サイズスケーリングの実験の紹介を行いたい。

Apr/08/2011 (Fri) 10:00-12:00 @ Room431
中山 和貴

セミナー (2010年度)

Feb/25/2011 (Fri) 10:00-12:00 @ Room431
成瀬 公暁

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
松尾 貞茂
 まず、薄膜超伝導系の特徴的な現象であるKT transitionを取り上げて、referenceとして参考にしている先行研究(超伝導体で修飾したグラフェン系のKT transitionなど)をいくつか紹介し、その後に今得られている我々の実験結果をまとめて報告する予定である。

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

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

Nov/09/2010 (Tue) 14:00-16:00 @ Room431
森下 将史(筑波大学大学院 数理物質科学研究科)

Nov/05/2010 (Fri) 10:00-12:00 @ Room431
中村 祥子
 炭素系物質において一般的な比表面積の決定方法と細孔形状の解析法を紹介する。 また、過去の結果で我々がよく参照する吸着実験について、比表面積の決定方法と、物理量に特異性が観察された面密度の論文上での値をまとめ、今後の実験方針の指針としたい。

Nov/02/2010 (Thu) 13:00-14:00 @ Room445
Ya Yang (北京科技大学 材料科学与工程学院)
『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.
[*] 図はこちらのPDFファイルを参照

Oct/20/2010 (Wed) 11:00-12:00 @ Room414
戸田 亮(京都大学大学院理学研究科特定研究員GCOE)
近年注目を集めている4He超流動固体現象は、系に含まれる3He不純物の量に大きく影響されることが知られている。 京大低温グループではこの現象の解明を目指し、捩れ振り子測定と同時に微量3Heに対するNMR測定を行ってきた。 セミナーでは実験結果を紹介するとともに、4He超流動固体現象への3He不純物の関わり方について議論を行う。

Oct/04/2010 (Fri) 10:00-12:00 @ Room431
佐藤 大輔

Sep/02/2010 (Thu) 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
松井 幸太

Jun/25/2010 (Fri) 10:00-12:00 @ Room431
松尾 貞茂

Jun/18/2010 (Fri) 10:00-12:00 @ Room431
佐藤 大輔
 本コロキウムでは、Andersonが提案したResonating Valence Bondについての説明を行った後、ギャップレスの量子スピン液体が実現していると考えられている幾つかの物質を示し、それらについての理論の一部を紹介する。
[1] P.W. Anderson, Mater. Res. Bull. 8, 153 (1973).

Jun/4/2010 (Fri) 10:00-12:00 @ Room431
中村 祥子
 超流動ヘリウム3のA相からB相への相転移では、核生成に大きなエネルギー障壁が存在し、明瞭な過冷却状態が観測される。 しかし、観測される過冷却状態は、ほとんど相転移が実現しないほど安定であろうという予想に比べると、ずっと不安定である。 そのため、宇宙線によるエネルギー供与等、単純な計算では想定していない効果が、この相転移を励起していることが確実視されている。
 今回は、A-B転移に対する表面の効果を研究した Matt O'Keefe 氏の学位論文を中心に紹介する。

May/17/2010 (Mon) 10:00-12:00 @ Room431
河合 直樹

Apr/23/2010 (Fri) 16:00-18:00 @ Room431
武井 英人
 BiCo酸化物Bi2Sr2Co2Oy(BC-2202)は高い熱起電力をもつ物質として注目を集め数多くの研究がなされてきた。構造においては、BC-2202は岩塩構造であるBi2Sr2O4層(絶縁層)と八面体であるCoO2層(伝導層)という二つの構造の違う層がクーロン力で結合している。周期性の違う構造が積み重なったmisfit構造をもち、modulationも確認されている。Biに対してPbを10%置換すると構造相転移がおこる。その際、electron diffractionの結果からmodulationが消えることがわかっている。

成瀬 公暁

Apr/9/2010 (Fri) 16:00-18:00 @ Room431
松尾 貞茂

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セミナー (2009年度)

Feb/18/2010 (Tue) 10:00-12:00 @ Room431
松井 幸太
 前回のコロキウムでは一層だけ吸着した二次元水素系の電子線散乱による構造解析について扱ったが、今回はより我々の目的に近い二層の重水素系に対する散乱実験の結 果について詳しく見ていく。また、これらの実験では輻射と電子線による水素の脱離を抑えるために様々な工夫がなされているが、水素の脱離に関しては分子の自由度に由来する光の吸収や電子線に よるイオン化の影響などが報告されている。我々の行おうとしているヘリウム系との比較材料としてこれらの結果をいくつか紹介する。

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

Dec/2/2009 (Wed) 10:00-12:00 @ Room414
川端 一成
 本コロキウムでは、Micromechanical Cleavage法により作成されたBSCCO超薄膜試料の輸送特性測定に関する研究の成果を報告する。本研究において観測された、試料を薄膜化することによる超伝導転移温度の低下と、二次元系での特殊な相転移として知られているKosterlitz-Thouless転移の特徴を示すデータを中心に紹介し、この他に、先行研究の紹介や試料作成法、厚さの評価法など、研究の全体的な内 容についても話す予定である。

Nov/13/2009 (Fri) 10:00-12:00 @ Room431
中村 祥子
『グラファイト上 2次元ヘリウムに対する基板の効果』
 CorbozらのPIMC計算[1]の結果は、グラファイト上2次元ヘリウムについて、一層目の実験結果をよく説明している。そして、さらに粒子を増やして行って二層目に達した後のふるまいについても、続けて計算を行っている。 彼らは、4/7相が生じていると提案されてきた密度において生じているのは不整合相固体相であり、今までのPIMC計算で見られてきた4/7相は、一層目の零点振動を無視した結果として得られる虚像に過ぎないと結論付けている。そして、比熱の実験で見られているピーク構造は、この不整合固体相由来の何かでは、としている。 比熱の実験からは、この結果を検証することができないのだろうか。 整合相の存在が広く認められている一層目の1/3相における比熱測定[2]を調べ、検討したい。
[1] Corboz et al. PRB 78, 245414(2008)
[2] Campbell et al. PRB 32, 2861(1985) etc.

Oct/30/2009 (Fri) 10:00-12:00 @ Room431
佐藤 大輔
 グラファイト上に吸着したヘリウムは理想的な2次元粒子系として振る舞うことが知られている。しかし、一方で、グラファイトの結晶表面は有限サイズの結晶子のあつまりで表わされ、その大きさは、我々の用いているGrafoilにおいては~10 nm×10 nm程度と十分に大きいとは言えない。その為に、熱容量や帯磁率の測定結果には基板の不均一部分の影響が現われてくることがある。本コロキウムでは、これまでの我々の測定結果や、他グループの結果から、不均一性の影響がどのような形で現われてきているかを理解し、我々がこれまで求めてきた2次元ヘリウム3の量子相図のより深い解釈を目指す。

Sep/17/2009 (Thu) 10:00-12:00 @ Room431
松尾 貞茂

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

Jul/6/2009 (Mon) 14:00-16:00 @ Room431
Tomohiro MATSUI
『Superconductivity in nanometer scale』
 There are a lot of interesting problems in low dimensional and in "very small" superconductors. When the size of the system reaches to the coherence length ξ, they exhibit new aspects that cannot be seen in bulk superconductors. In this colloquium, I want to discuss about superconductivity in such nanometer size superconductors including superconducting films, islands and random network of Josephson junctions in FOMP (film of oxidized metal particles). Superconducting junctions which can be fabricated with STM will also be discussed, if time permitted. The language of the talk will be switched depending on the audience.

Jun/22/2009 (Mon) 14:00-16:00 @ Room431
佐藤 大輔

Jun/8/2009 (Mon) 14:00-16:00 @ Room431
河合 直樹
 前回のコロキウムではSiC上にエピタキシャルに合成されたグラフェンの話題を紹介した。そこではグラフェンが基板との相互作用から受ける影響の大きさが問題となっていた。一方、劈開法により絶縁体基板上に作成されたグラフェンは、試料サイズは小さいものの基板との相互作用が小さくグラフェン自身の性質が最もよく現れる系として盛んに研究が進められている。本コロキウムでは特にKlein tunnelingとedge stateというトピックを取り上げ、その理論的背景と最近の研究の進展を紹介したい。

May/25/2009 (Mon) 14:00-16:00 @ Room431
中村 祥子
 グラファイト上2次元希ガス結晶を対象に、KTHNY(Kosterlitz-Thouless-Halperin-Nelson-Young) theoryとdomain wall 構造の理論を紹介し、それに適合する実験結果を参照した。

Apr/27/2009 (Mon) 14:00-16:00 @ Room431
川端 一成
『Kosteritz-Thouless theory』
 固体表面に吸着したHe-4薄膜などの二次元系における相転移では渦が本質的な役割を担う。 この渦の統計力学をくりこみ群の方法を用いて具体的に取り扱ったものが、Kosterlitz-Thouless 理論(KT理論)である。 本コロキウムでは、磁性体の転移点近傍での臨界現象(ゆらぎの発散)を例にとり、相関関数の定義、くりこみ変換の 考え方を紹介し、これらを二次元系に適用することで、KT理論の基本的な部分の導出を確認する。 また、残りの時間でKT理論を検証したいくつかの実験を紹介する予定である。

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セミナー (2008年度以前)


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