Measurements of the coefficient of thermal expansion on the spin-liquid candidate $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ have revealed distinct and strongly anisotropic lattice effects around 6 K - a possible spin-liquid instability. In order to study the effects of a magnetic field on the low-temperature spin-liquid state, dilatometric measurements have been conducted both as a function of temperature at \emph{B} = const. and as a function of field at \emph{T} = const. While the 6 K anomaly is found to be insensitive to magnetic fields \emph{B} $\leq$ 10 T, the maximum field applied, surprisingly strong \emph{B}-induced effects are observed for magnetic fields applied along the in-plane \emph{b}-axis. Above a threshold field of 0.5 T < \emph{B}$_c$ $\leq$ 1 T, a jump-like anomaly is observed in the \emph{b}-axis lattice parameter. This anomaly, which is located at 8.7 K at \emph{B} = 1 T, grows in size and shifts to lower temperatures with increasing the magnetic field. Although the anomaly bears resemblance to a first-order phase transition, the lack of hysteresis suggests otherwise.
We report the effect of Dirac cone tilting on interlayer magnetoresistance in α-(BEDT-TTF)$_2$I$_3$, which is a Dirac semimetal under pressure. Fitting of the experimental data by the theoretical formula suggests that the system is close to a type-II Dirac semimetal.
We investigated the interlayer magnetoresistance in an organic massless Dirac electron system $α$-(BEDT-TTF)$_2$I$_3$ under pressure. We experimentally demonstrate that the width of the zero mode owing to carrier scattering is much narrower than that of the other Landau levels.
Importance of Spin-Orbit Coupling in Organic BEDT-TTF and BEDT-TSF Salts
研究了$α$和$κ$相BEDT-TTF和BEDT-TSF有机盐的自旋轨道耦合效应。相反的假设，SOC在有机物中是可以忽略不计的，由于轻的C，S，H原子，我们在几个代表性的情况下，这样的相互作用的相关性。在弱关联区，SOC主要表现在简并带接触点处能隙的打开。这种效应对于诸如$α$ -（ET）$_2$ I $_3$的狄拉克半金属变得特别重要。此外，在磁绝缘相中，SOC导致额外的各向异性交换相互作用，这为量子自旋液体候选物$κ$ -（ET）$_2$ Cu $_2$（CN）$_3$的有争议的场诱导行为提供了令人信服的解释。最后，我们讨论的重要性SOC的低能量的有机物的属性的描述。
Effect of electronic correlations on the metal-insulator transition of $α$-(BEDT-TTF)$_2$I$_3$: theoretical and experimental investigations of its optical properties
有机盐$α$ -（BEDT-TTF）$_2$ I $_3$被认为是在$T_{\rm CO}=135$ ~K处由于电荷有序引起的金属-绝缘体转变的模型体系。通过基于第一原理密度泛函理论（DFT）的计算，可以很好地描述从$T_{\rm CO}$之上和之下的偏振反射测量获得的光学性质。讨论了电子关联对金属-绝缘体相变的影响。
Since the first observation of weak ferromagnetism in the charge-transfer salt kappa-(BEDT-TTF)2-Cu[N(CN)2]Cl [U. Welp et al., Phys. Rev. Lett. 69, 840 (1992)], no further evidence of ferromagnetism in this class of organic materials has been reported. Here we present static and dynamic spin susceptibility measurements on kappa-(BEDT-TTF)2Hg(SCN)2Br revealing weak ferromagnetism below about TWF = 20 K. We suggest that frustrated spins in the molecular dimers suppress long-range order, forming a spin-glass ground state in the insulating phase.
The low-temperature state of the quantum spin liquid candidate $κ$-(BEDT-TTF)$_{2}$Cu$_{2}$(CN)$_{3}$ emerges via an anomaly at $T^{*}\sim6$ K. Although signatures of this anomaly have been revealed in various quantities, its origin has remained unclear. Here we report inelastic neutron scattering measurements on single crystals of $κ$-(BEDT-TTF)$_{2}$Cu$_{2}$(CN)$_{3}$, aiming at studying phonon renormalization effects at $T^{*}$. A drastic change was observed in the phonon damping across $T^{*}$ for a breathing mode of BEDT-TTF dimers at $E=4.7$ meV. The abrupt change in the phonon damping i…
K(BEDT-TTF)2Cu(NCS)2 has been investigated by Raman scattering in both bulk and nanoparticle compounds. Phonon modes from 20 to 1600 cm-1 have been assigned. Focusing on the unexplored low frequency phonons, a plateau in frequencies is observed in the bulk phonons between 50 and 100 K and assigned to the signature of the bad metal phase. Nanoparticles of K(BEDT-TTF)2Cu(NCS)2 exhibit anomalies at 50 K associated to the crossover from a bad metal to a Fermi liquid whose origins are discussed.
In most Dirac semimetals, time-reversal and inversion symmetries are believed to play a crucial role in their stability. We demonstrate that these symmetries are broken in Dirac fermions in the organic conductor $α$-(BEDT-TTF)$_2$I$_3$ due to the strong electronic correlation. The system is a three-dimensional type-II Dirac semimetal in the coherent inter-layer tunneling regime. A chiral anomaly is predicted to be observed in the magnetoresistance when the magnetic field is tuned to the inter-layer tunneling direction. Our result suggests that $α$-(BEDT-TTF)$_2$I$_3$ is a useful platform to ex…
The electronic properties of molecular conductors can be readily varied via physical or chemical pressure as it increases the bandwidth W; this enables crossing the Mott insulator-to-metal phase transition by reducing electronic correlations U/W. Here we introduce an alternative path by increasing the molecular orbitals when partially replacing sulfur by selenium in the constituting bis-(ethylenedithio)-tetrathiafulvalene (BEDT-TTF) molecules of the title compound. We characterize the tuning of the insulating quantum spin liquid state via a Mott transition to the metallic Fermi-liquid state by…
The electronic structure of weak charge-ordering (CO) state just below the critical pressure in an organic conductor α-(BEDT-TTF)_2I_3 was experimentally investigated using peak structure in the temperature dependence of interlayer magnetoresistance (MR). Based on a minimal model considering multiple Landau levels (LLs), we discuss herein the MR peak as characteristic to multilayer massless/massive Dirac fermion (DF) systems. MR measured in the weak CO state exhibited a clear MR peak, and its magnetic-field dependence was consistent with the LL behavior of a massive DF with a small gap. Result…
Theoretical and experimental studies have supported that the electronic structure of $α$-(BEDT-TTF)$_2$I$_3$ under pressure is described by two-dimensional Dirac fermions. When the interlayer tunneling is coherent, the electronic structure of the system becomes three-dimensional, and we expect the peak structure to appear in the interlayer resistivity under magnetic fields. We theoretically and experimentally show that the peak appears in the interlayer resistivity at low temperatures and high magnetic fields. From the experiment, we estimate that the magnitude of the interlayer tunneling is $…
Site-specific $^{13}$C NMR study on locally distorted triangular lattice of organic conductor $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$
为了验证几何挫折的影响，我们人为扭曲的三角形晶格准二维有机导体$κ$ -（BEDT-TTF）$_2$铜$_2$（CN）$_3$ [BEDT-TTF：双（亚乙基二硫代）四硫富瓦烯]通过类似的分子取代和应用$^{13}$ C NMR的散装和取代的网站，电导率，静态磁化率测量。结果表明，取代样品的磁特性与纯样品的磁特性在数量上相似。此外，在替代位置处的磁特性也与体中相同。这些结果表明，所观察到的磁性可能不是由于几何挫折，但无序的重要性。
ESR studies on the spin-liquid candidate $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_{3}$: anomalous response below $T=8$ K
有机导体$κ$ -（BEDT-TTF）$_2$ Cu $_2$（CN）$_{3}$似乎形成量子自旋液体，尽管在低温下在电荷、自旋和晶格自由度中观察到不寻常的性质。本文报道了$κ$ -（BEDT-TTF）$_2$ Cu $_2$（CN）$_{3}$单晶X波段ESR随温度和角度变化的研究结果。我们发现两个各向异性弛豫机制在低温下的迹象，并将它们与在其他强相关系统中观察到的自旋液体行为进行比较。此外，我们可以认识到在阴离子层的铜离子的电荷不均匀性。这种紊乱可能与在该化合物中测量的介电响应有关。
In order to understand the physical properties of the series of organic conductors $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]$X$ with $X$ = Cl, Br, and I, not only electronic correlations but also the effect of disorder has to be taken into account. While for Cl- and Br-containing salts the influence of both parameters were investigated and a universal phase diagram was proposed, the position of $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]I is still not settled. Here we have conducted transport, infrared, and dielectric measurements on single crystals of the title compound to clarify its electronic state at low tempe…
We report the band structure calculations and the experimental results of resistivity and magnetic susceptibility in a spin-1/2 (BEDT-TTF)$^{\bullet +}$ monomer Mott insulator (BEDT-TTF)Cu[N(CN)$_2$]$_2$. The band calculations indicate a Dirac semimetal state with nodal lines at the Fermi level. The resistivity and the magnetic susceptibility as functions of temperature are well interpreted in terms of the monomer Mott insulating state instead of the expected semimetal state probably owing to strong electron correlation. In addition, we find that an Arrhenius-type steep reduction of the parama…
The charge-ordered insulator $α$-(BEDT-TTF)$_2$I$_3$ gradually evolves to a metal when pressure is applied, and at low temperatures the electronic bands form tilted Dirac-like cones. A metallic state with a frequency-independent optical conductivity indicates the coexistence of the trivial and massless Dirac electrons in the system. Our infrared investigations also reveal that at the boundary between insulating and metallic states an energy gap opens due to correlated massive Dirac fermions, which is gradually suppressed when pressure increases.
Anomalous dielectric response in the dimer Mott insulator $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$
我们测量和分析了二聚体莫特绝缘体$κ-$（BEDT-TTF）$_2$ Cu $_2$（CN）$_3$的介电常数，它被称为自旋液体状态的操场。最出乎意料的是，这种特定的有机盐具有非平凡的电荷自由度，其特征在于低于约60 K的偶极玻璃样介电弛豫。这归因于二聚体内的电荷分离，由于位间库仑排斥。在这方面，这里观察到的偶极子是纯粹的量子力学，从而表现出意想不到的响应。
The ground state of $λ$-(BEDT-TTF)$_2$GaCl$_4$, which has the same structure as the organic superconductor $λ$-(BETS)$_2$GaCl$_4$, was investigated by magnetic susceptibility and $^{13}$C NMR measurements. The temperature dependence of the magnetic susceptibility revealed an antiferromagnetic (AF) correlation with $J/k_{\rm B} \simeq$ 98 K. NMR spectrum splitting and the divergence of $1/T_1$ were observed at approximately 13 K, which is associated with the AF transition. We found that the AF structure is commensurate according to discrete NMR peak splitting, suggesting that the ground state o…
By applying measurements of the dielectric constants and relative length changes to the dimerized molecular conductor $κ$-(BEDT-TTF)$_2$Hg(SCN)$_2$Cl, we provide evidence for order-disorder type electronic ferroelectricity which is driven by charge order within the (BEDT-TTF)$_2$ dimers and stabilized by a coupling to the anions. According to our density functional theory calculations, this material is characterized by a moderate strength of dimerization. This system thus bridges the gap between strongly dimerized materials, often approximated as dimer-Mott systems at 1/2 filling, and non- or …
The emergence of photo-induced topological phases and their phase transitions are theoretically predicted in organic salt $α$-(BEDT-TTF)$_2$I$_3$, which possesses inclined Dirac cones in its band structure. By analyzing a photo-driven tight-binding model describing conduction electrons in the BEDT-TTF layer using the Floquet theorem, we demonstrate that irradiation with circularly polarized light opens a gap at the Dirac points, and the system eventually becomes a Chern insulator characterized by a quantized topological invariant. A rich phase diagram is obtained in plane of amplitude and freq…
Lattice vibrations of the charge-transfer salt $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_{3}$: novel interpretation of the electrodynamic response in a spin-liquid compound
二聚体Mott绝缘体$κ$ -（BEDT-TTF）$_2$ Cu $_2$（CN）$_3$具有不寻常的电动力学性质。电子基态和分子和晶格振动的数值研究揭示了耦合到BEDT-TTF分子的Cu $_2$（CN）$_3^-$阴离子网络的重要性：铜的三重氰化物配位和阴离子结构中的键异构性导致对称性、挫折、无序和域形成的损失。我们的研究结果一致地解释了温度和偏振相关的THz和红外测量，加强了对介电性质的理解，并对量子自旋液体状态具有重要意义，这应该超越二维纯电子模型。
The pressure dependence of many-body interactions in the organic superconductor $κ$-(BEDT-TTF)$_{2}$Cu(SCN)$_{2}$: A comparison of high pressure infrared reflectivity and Raman scattering experiments
理论上研究了theta-（BEDT-TTF）_2X材料各向异性三角晶格上的电荷有序态。使用精确的对角化研究，我们建立的电荷顺序（CO）模式对应于一个“水平”的条纹结构，与…1100... CO沿两个方向具有较大的电子跳跃（p方向），和…1010... C 0沿着第三方向（c方向）。CO是伴随着合作键二聚沿所有三个方向在最高自旋状态。在最低自旋状态下，沿着p方向的键被四聚化。我们的理论解释了电荷诱导的高温跃迁和低温自旋能隙跃迁的发生。
