RIKEN's CMM


2019 2018 2017 2016 2015 2014 2013 2012 2011 2010 2009 2008 2018.04.03 Annual Report 3（Oshima、Fujiyama、Cui） 2018.03.20 JPSJ Presentation Practice 2 2018.03.13 JPSJ Presentation Practice 1 2018.03.06 Annual Report 1（Kim、Tajima、Lee） 2018.02.27 Annual Report 1（Isono、Kawasugi） 2018.02.20 Literature Survey 2018.02.06 "ダイマーモット系の磁性と誘電性の理論" Dr. T. Naka, (Assistant professor, Waseda University) 分子性固体は、複雑な結晶構造とは裏腹に明快な電子構造を持つことから、強相関系のモデル物質として注目されてきた。しかしながら近年、典型的なモット絶縁体とされる分子性固体において、分子内の電子自由度や水素結合中のプロトン自由度といった非自明な内部自由度の存在が相次いで指摘された。発端となったのは、分子ダイマー構造を持つモット絶縁体κ型ET塩における誘電異常の発見である。これはダイマー内部で片方の分子に電子が局在する電荷自由度に起因する可能性が指摘され、多くの議論を呼んだ。この電荷自由度は、誘電性だけでなく量子スピン液体や超伝導といったこれまで単一軌道系の観点から考察されてきた性質にも深く関わると予想されるため、κ型ET塩をはじめとした分子性のモット絶縁体の多くを、多軌道・多自由度系として再考察する必要性が生じた。我々はET系をはじめとしたダイマーモット系の磁性と誘電性に対するダイマー内自由度の働きを包括的に調べたので、その結果を報告する。 2018.01.30 "α-(BEDT-TTF)₂I₃の電荷秩序およびゼロギャップ状態に対する電子相関効果" Dr. Y. Tanaka, (Assistant professor, Chuo University) 圧力下においてディラック電子系(ゼロギャップ状態)の性質を示す有機導体α-(BEDT-TTF)₂I₃について、その電子状態を変分モンテカルロ法と自己無撞着RPAにより調べた結果を報告する。まず、低圧下で現れる電荷秩序相では、電子相関効果によりスピンが非磁性になることや、スピン相関長がほとんど圧力依存性を持たないことを示し、最近のNMRによる実験結果との比較を議論する。また、静水圧と一軸圧縮の二つの加圧方法における圧力効果の違いを紹介する。特に、静水圧下でのゼロギャップ状態の出現に対して、電子相関効果が重要な役割を果たす可能性について議論する。 2018.01.16 Literature Survey 2017.12.19 Midterm Report 3（Cui, Kawasugi） 2017.12.12 Midterm Report 2（Oshima, Fujiyama, Isono） 2017.11.28 Midterm Report 1（Kim, Tajima, Lee） 2017.11.21 Literature Survey 2017.11.14 Literature Survey 2017.11.07 "Importance of intradimer charge degree of freedom in κ-(ET)₂X: Mott transition, charge order, and superconductivity" Dr. H. Watanabe（Assistant professor, Waseda University） In the family of molecular conductor κ-(ET)₂X, superconductivity (SC) is observed next to the so-called dimer-Mott insulating state. Although κ-(ET)₂X is a 3/4-filled electron system, the 1/2-filled Hubbard (dimer) model has been widely used to describe them, and the analogy with high-Tc cuprates is often discussed. The model is based on the fact that the two ET molecules facing each other (dimer) are strongly hybridized and each pair can be regarded as one lattice site. While this “dimer approximation” successfully describes many aspects of the metal-insulator (MI) transition, it is controversial whether it can describe the SC and spin-liquid phases in κ-(ET)₂X. On the other hand, recent studies suggest that the charge degree of freedom within dimers, which is discarded in the dimer approximation, plays important roles in various emergent phenomena. To investigate the MI transition and phase competition in κ-(ET)₂X, we study the 3/4-filled extended Hubbard model using the variational Monte Carlo method. We find various phases in the ground state phase diagram including SC, which is enhanced near the charge-order instability. The pairing symmetry is an “extended-s+d_x²-y²”-wave type and different from the one expected from the dimer approximation. Our results suggest the importance of the charge degree of freedom within dimers and the intersite Coulomb interactions. We also study the ground state phase diagram by changing the electron filling and discuss the SC from both aspects of bandwidth and filling control MI transitions. 2017.10.31 Midterm Report (Cui, Kawasugi, Lee) 2017.10.24 Midterm Report (Fujiyama, Oshima, Isono) 2017.10.17 "Self-assembled bilayer molecular metals (CNB-EDT-TTF)₄X; The role of C-N…H interactions in a new prototype of 2D Molecular Conductors", Dr. Manuel Almeida (Professor, Instituto Superior Tecnico, Universidade de Lisboa) The electron donor BEDT-TTF and related symmetrical derivatives have been at the origin of a large number of 2D electronic systems presenting a large diversity of ground states that have been intensively studied during the last years. In this presentation it will be shown how dissymmetrical ET derivatives like cyanobenzene-ethylenedithio-tetrathiafulvalene (CNB-EDT-TTF), thanks to a combination of C-N…H hydrogen bond assisted dimeric interactions can be at the origin of a new type of 2D systems, (CNB-EDT-TTF)4X, (X= ClO₄^-, PF₆^-, I₃^-, …) which are characterized by a unique bilayer structure of the donors. This novel type of salts exhibit 2D metallic properties with special characteristics derived both from the unusual stoichiometry and the weak interaction between paired layers. These properties are related to a high band filling, large electronic effective masses and quasi degenerated 2D Fermi surfaces, as predicted by band structure calculations and in agreement with electron transport measurements in single crystals. Depending on the donor packing pattern some of these 2D metals can present superconductivity at temperatures close to 3 K. The formation of the bilayers by self-assembly is however critically dependent on the counter anions and growth conditions. The rich variety of polymorphism and physical properties that has been recently observed in these (CNB-EDT-TTF)₄X salts will be also presented in detail. 2017.10.10 Midterm Report (Kim, Tajima) 2017.10.03 Literature Survey 2017.09.19 Practice for JPS/ISCOM 2017.09.12 Practice for JPS/ISCOM 2017.09.05 Literature Survey 2017.07.25 Literature Survey 2017.07.18 Literature Survey 2017.07.11 Midterm Report（Fujiyama、Kawasugi） 2017.07.04 Midterm Report（Oshima、Kim、Lee） 2017.06.27 Midterm Report（Cui、Isono） 2017.06.20 "有機超伝導体の超伝導対称性に関する熱的研究" Mr. S. Imajo（D3, Nakazawa Laboratory, Graduate School of Science, Osaka University）超伝導状態では電子が引力によってクーパー対と呼ばれる電子対を形成している。従来型超伝導ではその引力の起源が電子ー格子相互作用だとわかっているが、有機物における超伝導をはじめ、多くの超伝導ではその引力の詳細な起源が分かっていない。その起源を知るためには引力相互作用を強く反映した超伝導ギャップ関数の決定が重要であるが、有機超伝導体においては実験的難しさからあまり研究は進んでいない。本研究では状態密度を反映した電子熱容量を測定することで低エネルギー励起構造を検出し、ギャップ関数について議論している。またギャップ関数の正確な決定には波動関数の軌道だけではなく位相に関する情報も必要なため、磁場中角度分解熱容量測定という手法によってギャップ関数の異方性も検出し、正確なギャップ関数の決定を行っている。セミナーでは高感度熱測定手法の開発やdimer-Mott系有機超伝導体の測定結果などを紹介する。 2017.06.13 "Microscopic Investigations of Solid / Liquid Interfaces Using Electrochemical Scanning Probe Microscopy" Dr. Y. Yokota（Research Scientist, Serface and Interface Science Laboratory, RIKEN） Since Volta invented the voltaic pile late in the eighteenth century, electrochemistry has been one of the most important fields of study not only for academia but for industry. For instance, some parts of thermodynamics and kinetics chapters in the textbook of physical chemistry were developed by electrochemistry. By the virtue of the thermodynamics, there is a general consensus concerning the fundamental aspects of electrolyte / electrode interfaces [1]. Although classical models can explain the electrochemical measurements to some extent, the macroscopic theory and measurements do not provide molecular scale pictures of the electrolyte / electrode interfaces. In this seminar, microscopic structures of ionic liquid / solid interfaces revealed by electrochemical AFM (EC-AFM) will be discussed with respect to the device performance of electric double layer transistors [2]. Then, ongoing studies of electrochemical interfaces using (EC)-STM, related to Pioneering Project, will also be presented. 2017.05.30 Literature Survey Report 2017.05.23 Literature Survey Report 2017.05.09 "Peculiar Paramagnetic Phase Adjacent to Superconducting Phase in Lambda-(BEDT-STF)₂GaCl₄" Dr. T. Minamidate（Laboratory of electronic properties of low-dimensional conductors, Department of physics, School of science, Hokkaido University　Postdoctoral Researcher） The organic quasi-two-dimensional conductor lambda-(BETS)₂GaCl₄ shows the superconducting (SC) phase below Tc=5.5 K. To clarify the adjacent phase of the SC phase in lambda-(BETS)₂GaCl₄ and discuss the origin of this phase, we have measured the electrical resistivity under pressure and the magnetic properties at ambient pressure in lambda-(STF)₂GaCl₄ (STF = bis(ethylenedithio)diselenadithiafulvalene), which was expected to be placed at the negative pressure region of the lambda-(STF)₂GaCl₄. At first, we found the SC phase above 1.2GPa. We also found that the ground state of the STF salt at ambient pressure is the insulating phase without any magnetic ordering. Accordingly, the phase neighboring the SC phase is the non-AF-ordered paramagnetic insulating (PMI) phase. Remarkably, in this PMI phase, the temperature dependence of the spin susceptibility obeys to the two-dimensional AF Heisenberg model with the regular triangle lattice. This is the typical behavior of the spin-liquid system. In the seminar, to explain the peculiar spin-liquid-like behavior in lambda-(STF)₂GaCl₄, we discuss the relation between the charge disproportionation and the geometry of the spin-interaction. 2017.05.02 2016 Annual Report and 2017 Annual Plan 2017.04.25 2016 Annual Report and 2017 Annual Plan 2017.04.18 2016 Annual Report and 2017 Annual Plan 2017.04.13 "Symmetry Breaking Characteristic of Tight Dimerization in Antiferromagnetic Molecular Solid" Dr. T. Yamamoto（Associate professor, Department of Chemistry, Faculty of Science, Ehime University） Antiferromagnetic state of β'-Me₄P[Pd(dmit)₂]₂ has been known as the obtuse and isosceles triangular lattice consisting of tight dimers with S = 1/2. Vibrational spectroscopy focused on electrical densities of C=C double bonds revealed symmetry breakings in lattice and molecule, which are discussed from the viewpoint of molecular orbitals specific to the tight dimerization.