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2019.03.26 年次報告
2019.03.12 JPS予聴会
2019.03.05 JPS予聴会
2019.02.26 文献調査
2019.02.19 "Counterion Polarity-Induced Novel Electronic States in Organic Conductors" 圷 広樹准教授（大阪大学理学研究科化学専攻 中沢研究室）
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2019.02.12 "Exciton Condensation: History and Recent Progress" 渡部 洋博士（早稲田大学）
Exciton condensation (excitonic insulator) is one of the interesting quantum phenomena in strongly-correlated electron system. The exciton is a bound electron-hole pair mediated by the Coulomb interaction. When the binding energy of an exciton exceeds the energy gap between the electron and hole bands, the system has an instability towards exciton condensation. Although exciton condensation was theoretically predicted in low carrier-density system such as semimetal or semiconductor in 1960's, it has not been identified experimentally so far. This is partially because the excitons are neutral quasiparticles and difficult to detect. In addition, the CDW state is naturally induced by exciton condensation and makes it difficult to distinguish the pure CDW state.
In this seminar, I will give a brief introduction of the theory of exciton condensation and discuss the possibility of exciton condensation in real materials, showing recent progress in several candidates such as 1T-TiSe2, Ta2NiSe5, and α-(BEDT-TTF)2I3.
2019.01.22 文献調査
2019.01.15 四半期報告2（大島、南舘、上辺）
2019.01.08 四半期報告1（崔、川椙、藤山）
2018.12.18 Hadi final report and discussion
2018.12.11 "Signatures of Quantum Dipole Liquid in an Organic Mott Insulator" Dr. Natalia Drichko (Associate Research Professor, Department of Physics and Astronomy, Johns Hopkins University, USA)
Mott insulators are commonly pictured with electrons localized on lattice sites. Their low-energy physics involves spins only. Recent theoretical work suggests that in molecular systems a new on-site charge degree of freedom can emerge. On a frustrated lattice with charge-spin coupling it would result in a new quantum spin liquid state [1-3].
We experimentally demonstrate a presence of this fluctuating charge degree of freedom in a molecule-based Mott insulator κ-(BEDT-TTF)2Hg(SCN)2Br. When electrons localize on a triangular lattice of molecular dimers of this compound at temperatures below 100 K, they form electric dipoles which do not order at low temperatures and fluctuate, resulting in a so-called quantum dipole liquid state. A frequency of dipole fluctuations of 40 cm-1 is detected experimentally in our Raman spectroscopy experiments through an observation of a related collective mode. We show that this spectroscopic response of a quantum dipole liquid is qualitatively different from a response of molecular Mott insulators with no on-site charge degree of freedom. The Raman spectra of the latter show two-magnon excitations at frequencies below 500 cm-1 expected for an S=1/2 antiferromagnet on a triangular lattice with J≈250 K. Our results can be a key to understanding of organic triangular lattice spin liquid candidates.
References
[1] C. Hotta. 2010 Phys. Rev. B 82, 241104.
[2] M. Naka, S. Ishihara. 2016 Phys. Rev. B 93, 195114.
[3] N. Y. Yao, M. P. Zaletel, D. M. Stamper-Kurn, A. Vishwanath. 2018 Nature Physics.
2018.12.04 Defence training for Kim
2018.11.27 Defence training for Lee
2018.11.20 文献調査
2018.11.13 文献調査
2018.11.06 "Excitonic Interaction in Dirac Electronic System" 藤山 茂樹
Excitonic interaction is an attractive interaction between an electron and a hole with an energy of the electrostatic Coulomb repulsion. This excitonic interaction had been argued to modulate electronic dispersions and open a gap for charge excitations. The proposed phase diagram of excitonic comdensated state 50 years ago includes a semimetal-semiconducting crossover as a function of electron-hole interaction. Recently the excitonic insulator is recognized to examine BCS-BEC crossover and several chalcogenides like Ta2NiSe5 and 1T-TiSe2 attract much attention.
α-(BEDT-TTF)2I3 under pressure is well accepted to realize massless Dirac electron-like dispersion relation and the bands for electrons and holes touch at the Dirac point, which satisfies a condition of excitonic interaction. In this seminar, I would like to discuss possible effects by excitonic interaction to the Dirac organic conductor and show indications of excitonic BCS-BEC crossover.
2018.10.30 "Recent Topics on Single Component Molecular Conductors" 加藤 礼三
Single component molecular conductors belong to the multiple-band system where both HOMO and LUMO bands that originate from the same molecule are located near the Fermi level. Metal dithiolene complexes are characterized by a small energy gap between HOMO and LUMO and suitable for the formation of the single component molecular conductors.
In addition, single component molecular conductors with the crossing band structure can provide Dirac electron systems where the Dirac points are located at the Fermi level.
In this seminar, I will talk about the Dirac cone formation and topological properties of single component molecular conductors based on metal dithiolene complexes.
2018.10.23 "Towards Highly Conducting Metal bis(dithiolene) Complexes" (Hachem Hadi)
2018.10.16 中間報告3（大島、藤山）
2018.10.09 中間報告2（崔、川椙、南舘）
2018.10.02 中間報告1（李、金、上辺）
2018.09.25 文献調査
2018.09.18 文献調査
2018.09.04 物理学会発表練習2
2018.08.28 物理学会発表練習1
2018.07.24 四半期報告3
2018.07.17 四半期報告2
2018.07.10 四半期報告1
2018.06.26 文献調査
2018.06.19 文献調査
2018.06.15 at 14:00 "NH…S型相互作用がある[Pd(dmit)2]2塩での金属状態の実現" 上田 康平博士（東京理科大学理工学部物理学科 助教）
Cation [Pd(dmit)2]2(dmit = C3S5)は，強束縛近似の下で，ハーフフィルドのバンド構造が期待されるが，ほとんどの場合モット絶縁体としてふるまう。Pyridinium 誘導体や1-Azabicyclo[2.2.2]octanium (ABCOH)，1,4-diazabicyclo[2.2.2]octanium (DABCOH)といった，プロトン性のアンモニウムをカチオンとするいくつかの塩は，常圧で金属的電気伝導を示すことがわかった。これらには，①カチオン－アニオン間にNH…S型の水素結合がある，②カチオン配向が無秩序である，という共通点がある。一方，類似骨格をもち，カチオン－アニオン間にNH…S型の水素結合がない塩のいくつかは，常圧でモット絶縁体であった。このことから，NH…S型の水素結合の存在が，モット絶縁体を不安定化させた可能性を検討している。セミナーでは、これらNH…S型水素結合が存在する試料および，類似骨格でNH…S型水素結合が無いカチオン塩の，電気伝導，偏光顕微反射率（600 ～ 15000 cm-1），の結果を中心に話す予定である。
2018.06.05 "Kitaev Honeycomb Model and New Quantum Liquid Material H3LiIr2O6 as a Candidate" 北川 健太郎博士（東京大学理学系研究科・物理学専攻 ）
キタエフハニカム模型[1]は、二次元以上でありながら量子液体としての厳密解が得られる模型であること、その後物質系での展開が提唱された[2]ことから近年話題を集めている。この系は、三方向にそれぞれ異方的なイジング相互作用を仮定していたため実現が困難であったが、計算上は元々の1/2スピンの自由度を２種類のマヨラナフェルミオンに分解して簡単に解くことが出来る。遍歴マヨラナ粒子がディラック分散をもつこと、局在マヨラナ粒子がトポロジカル量子計算に利用可能な点も注目されている。
この模型はスピン―軌道相互作用が強いIr4+やRu3+などを辺共有八面体配位に置くとJeff=1/2擬スピン間の超交換相互作用として実現可能であることが導かれる[2]。ただし、これまでの候補物質は全て磁気秩序を示してきた。我々はIr化合物の探索から、H3LiIr2O6において実験的に自発的対称性の破れが少なくとも50 mKまで起きないことを確認し、量子スピン液体状態を発見したと結論した[3]。他物質と同様、非キタエフ型相互作用が卓越しているにも関わらず、ハニカム系として初の量子液体である。NMR緩和率、比熱、磁化を通じて観測された温度励起は欠陥に支配されていると推測され、純粋なキタエフスピン液体から予想されるものとは異なっていたが、ディラック分散的フェルミ粒子状態密度モデルで記述出来る点は一致しているとも言える。セミナーでは、キタエフ物質の基礎と、今後の展開も絡めて報告したい。
References
1. A. Kitaev, Annals of Physics 321, 2 (2006).
2. G. Jackeli and G. Khaliullin, PRL 102, 017205 (2009).
3. KK, TT, et al., Nature 554, 341 (2018) .
2018.05.29 "Spin Dynamics in Antiferromagnetic Phase of λ-(BEDT-STF)2FeCl4" 南舘 孝亮
Quasi-two-dimensional organic conductor λ-(BETS)2FeCl4 is known as the first organic field-induced-superconductor. This material shows the rich phase diagram due to the interaction between the π-spin on the donor layer and the local d-spin on the FeCl4 anion. Especially in the antiferromagnetic(AF) state below TAF = 8.3 K, the spin interaction is complicated and which spin-system is orderly is still open issue.
To elucidate the spin dynamics in the AF state in the &lambda-type salt, we focused on &lambda-(BEDT-STF)2FeCl4, whose donor molecules are little smaller than that of the BETS salt and tried the band width control. We performed the magnetic susceptibility and 1H-NMR measurements. Magnetic susceptibility arising from the d-spin system shows an anisotropy at low temperature and its temperature dependence for the external field parallel to the c axis is described as a broad peak structure at 8 K. On the other hand, a sharp peak in the temperature dependence of 1/T1 associated with the antiferromagnetic (AF) transition is observed at TAF = 16 K, together with the drastic splitting of the NMR spectrum. The relation between the static susceptibility and the splitting of the NMR shift suggests the existence of the relatively strong d-d AF interaction. These results can be explained by the model considering the AF-coupled d-spin system in the AF long-range-ordered π-spin system. We find that the AF phases in &lambda-type salts can be universally explained by this model.
In the presentation, I will talk about the details of the above research results and the comparison with the another organic π-d system which newly found during the research.
2018.05.22 "The Quest for Molecular Spin-Ladders" Dr. R. A. L. Silva（Universidade de Lisboa）
Spin-ladders are quantum magnetic systems composed of interacting chains which attracted a large interest due to unusual magnetic properties critically dependent on the number of legs. Since the first report of an organic spin-ladder (DT-TTF)2 [Au(mnt)2] by a team in our laboratory [1] several attempts were made to enlarge this type of systems by selective modification of donors and acceptors. The success of these attempts has been so far restricted to very small modifications which preserve the structure with pairs of magnetic chains [2] that nevertheless led to the largest family of closely related molecular spin-ladder systems.
In this presentation recent developments to enlarge this family of spin-ladders will be described. These include two new compounds with the electron donor α-DT-TTF, (α-DT-TTF)2 [Au(mnt)2] and (α-DT-TTF)2 [Au(i-mnt)2] which due to disorder in the thiophenic rings of the donors present properties of weakly disordered spin-ladder systems [3-4]. With the cobalt analogue it is obtained (α-DT-TTF)2 [Co(mnt)2] with a novel type of ladder structure though without the magnetic spin-ladder behaviour due to the strong coupling between the disordered anion and donor chains [4]. A similar structure was found in the (DT-TTF)2 [Cu(dcdmp)2] salt [5].
The nature of the spin carrying units in these systems, previously subject to speculations, was enlightened by the resolution of the superstructure of (DT-TTF)2[Cu(mnt)2] below the transition at 235 K, using synchrotron radiation. A charge ordering scheme of alternating neutral (diamagnetic) and ionic (paramagnetic) donor units along the stacking axis was found rather than a bond ordering scheme [4].
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2018.05.15 "Studies on Triphenylamine-Based Functional Materials" 上辺 将士
To date, electronic and optoelectronic devices using organic materials as organic light-emitting diodes (OLEDs), organic photovoltaic devices (OPVs), organic field effect transistors (OFETs) and so on, have recently received a great deal of attention. The devices using organic materials are attractive because they can take advantage of organic materials such as lightweight, low cost, and flexibility. Particularly, π-electron systems are pivotal compounds in the organic electronic devices. In organic electronics, triphenylamine derivatives which are embedded nitrogen atom into π-electron systems have been regarded as very important hole-transport materials from the viewpoints of fundamental physical organic chemistry because they have low oxidation potential and can generate the relatively highly stable oxidized species by electrochemical and chemical oxidation.
We have studied triphenylamine-based acene materials, optical properties of triphenylamine-based materials and intramolecular electron transfer mechanisms of triphenylamine-based materials. These molecules have their unique electronic properties originating from their structures. We have newly prepared these materials by synthetic method and investigated the electronic properties of these molecules by means of UV-vis-NIR and photoluminescence spectroscopic, electrochemical, X-ray crystallographic, ESR, and magnetic susceptibility measurements as well as DFT calculations. In this presentation, these results will be reported.
2018.05.01 文献調査（南舘、上辺）
2018.04.24 文献調査（崔、大島、磯野、川椙）
2018.04.17 文献調査（加藤、 Kim、Lee、藤山）
2018.04.10 年間計画
2018.04.03 年度報告会3（大島、藤山、崔）
