Mott transition in a valence-bond solid insulator with a triangular lattice

Chasing the spin gap through the phase diagram of a frustrated Mott insulator

The quest for entangled spin excitations has stimulated intense research on frustrated magnetic systems. For almost two decades, the triangular-lattice Mott insulator κ-(BEDT-TTF)₂Cu₂(CN)₃ has been one of the hottest candidates for a gapless quantum spin liquid with itinerant spinons. Very recently, however, this scenario was overturned as electron-spin-resonance (ESR) studies unveiled a spin gap, calling for reevaluation of the magnetic ground state. Here we achieve a precise mapping of this spin-gapped phase through the Mott transition by ultrahigh-resolution strain tuning. Our transport experiments reveal a reentrance of charge localization below T^⋆ = 6 K associated with a gap size of 30-50 K. The negative slope of the insulator-metal boundary, dT^⋆/dp < 0, evidences the low-entropy nature of the spin-singlet ground state. By tuning the enigmatic '6K anomaly' through the phase diagram of κ-(BEDT-TTF)₂Cu₂(CN)₃, we identify it as the transition to a valence-bond-solid phase, in agreement with previous thermal expansion and magnetic resonance studies. This spin-gapped insulating state persists at T → 0 until unconventional superconductivity and metallic transport proliferate.

Contrasting Pressure-Induced Metallization Processes in Layered Perovskites, α-Sr_{2}MO_{4} (M=V, Cr)

Electric resistivity, magnetic susceptibility, and x-ray diffraction measurements under high pressure are performed in both α-Sr_{2}VO_{4} and α-Sr_{2}CrO_{4}, which are carefully prepared with regard to their stoichiometry. These measurements reveal contrasting and peculiar metallization processes of these compounds with increasing pressure. In contrast to a previously reported one in a V compound, we find two kinds of pressure-induced metallic states at low- (T<50  K) and high-temperature (T>100  K) regions. The high-temperature one seems to emerge beyond the pressure-induced Mott transition. The low-temperature one might imply a topological nature of the V compound, which is expected in the spin-orbit coupled 3d^{1} state that arises from their degenerated d_{zx} and d_{yz} orbits.

Slow dynamics of electrons at a metal-Mott insulator boundary in an organic system with disorder

The Mott transition-a metal-insulator transition caused by repulsive Coulomb interactions between electrons-is a central issue in condensed matter physics because it is the mother earth of various attractive phenomena. Outstanding examples are high-Tc (critical temperature) cuprates and manganites exhibiting colossal magnetoresistance. Furthermore, spin liquid states, which are quantum-fluctuation-driven disordered ground states in antiferromagnets, have recently been found in magnetic systems very near the Mott transition. To date, intensive studies on the Mott transition have been conducted and appear to have established a nearly complete framework for understanding the Mott transition. We found an unknown type of Mott transition in an organic spin liquid material with a slightly disordered lattice. Around the Mott transition region of this material under pressure, nuclear magnetic resonance experiments capture the emergence of slow electronic fluctuations of the order of kilohertz or lower, which is not expected in the conventional Mott transition that appears as a clear first-order transition at low temperatures. We suggest that they are due to the unconventional metal-insulator fluctuations emerging around the disordered Mott transition in analogy to the slowly fluctuating spin phase, or Griffiths phase, realized in Ising spin systems with disordered lattices.

Breakdown of Hooke's law of elasticity at the Mott critical endpoint in an organic conductor

The Mott metal-insulator transition, a paradigm of strong electron-electron correlations, has been considered as a source of intriguing phenomena. Despite its importance for a wide range of materials, fundamental aspects of the transition, such as its universal properties, are still under debate. We report detailed measurements of relative length changes ΔL/L as a function of continuously controlled helium-gas pressure P for the organic conductor κ-(BEDT-TTF)₂Cu[N(CN)₂]Cl across the pressure-induced Mott transition. We observe strongly nonlinear variations of ΔL/L with pressure around the Mott critical endpoint, highlighting a breakdown of Hooke's law of elasticity. We assign these nonlinear strain-stress relations to an intimate, nonperturbative coupling of the critical electronic system to the lattice degrees of freedom. Our results are fully consistent with mean-field criticality, predicted for electrons in a compressible lattice with finite shear moduli. We argue that the Mott transition for all systems that are amenable to pressure tuning shows the universal properties of an isostructural solid-solid transition.

Universality class of the mott transition

Pressure dependence of the conductivity and thermoelectric power is measured through the Mott transition in the layer organic conductor EtMe_{3}P[Pd(dmit)_{2}]_{2}. The critical behavior of the thermoelectric effect provides a clear and objective determination of the Mott-Hubbard transition during the isothermal pressure sweep. Above the critical end point, the metal-insulator crossing, determined by the thermoelectric effect minimum value, is not found to coincide with the maximum of the derivative of the conductivity as a function of pressure. We show that the critical exponents of the Mott-Hubbard transition fall within the Ising universality class regardless of the dimensionality of the system.

Absence of superconductivity and valence bond order in the Hubbard-Heisenberg model for organic charge-transfer solids

A frustrated, effective ½-filled band Hubbard-Heisenberg model has been proposed for describing the strongly dimerized charge-transfer solid families κ-(ET)2X and Z[Pd(dmit)2]2. In addition to showing unconventional superconductivity, these materials also exhibit antiferromagnetism, candidate spin-liquid phases, and, in the case of Z=EtMe3P, a spin-gapped phase that has sometimes been referred to as a valence bond solid. We show that neither superconductivity nor the valence bond order phase occurs within the Hubbard-Heisenberg model. We suggest that a description based on ¼-filling, that is reached when the carrier concentration per molecule instead of per dimer is considered, thus may be appropriate.

Emergence of superconductivity, valence bond order, and Mott insulators in Pd[(dmit)2] based organic salts

The EtMe(3)P and EtMe(3)Sb triangular organic salts are distinguished from other Pd[(dmit)(2)] based salts, as they display valence bond and no long-range order, respectively. Under pressure, a superconducting phase is revealed in EtMe(3)P near the boundary of valence bond order. We use slave-rotor theory with an enlarged unit cell to study competition between uniform and broken translational symmetry states, offering a theoretical framework capturing the superconducting, valence bond order, spin liquid, and metallic phases on an isotropic triangular lattice. Our finite temperature phase diagram manifests a remarkable resemblance to the phase diagram of the EtMe(3)P salt, where the reentrant transition of insulator-metal-insulator type can be explained by an entropy difference between the metal and U(1) spin liquid. We predict different temperature dependence of the specific heat between the spin liquid and metal.

The paired-electron crystal in the two-dimensional frustrated quarter-filled band

The competition between antiferromagnetic and spin-singlet ground states within quantum spin models and the ½-filled band Hubbard model has received intense scrutiny. Here we demonstrate a frustration-induced transition from Néel antiferromagnetism to a spin-singlet state in the interacting ¼-filled band on an anisotropic triangular lattice. While the antiferromagnetic state has equal charge densities, 0.5, on all sites, the spin-singlet state is a paired-electron crystal, with pairs of charge-rich sites separated by pairs of charge-poor sites. The paired-electron crystal provides a natural description of the spin-gapped state proximate to superconductivity in many organic charge transfer solids. Pressure-induced superconductivity in these correlated-electron systems is likely a result of a transition from the ¼-filled band valence bond solid to a valence bond liquid.

Spin liquids in frustrated magnets

Frustrated magnets are materials in which localized magnetic moments, or spins, interact through competing exchange interactions that cannot be simultaneously satisfied, giving rise to a large degeneracy of the system ground state. Under certain conditions, this can lead to the formation of fluid-like states of matter, so-called spin liquids, in which the constituent spins are highly correlated but still fluctuate strongly down to a temperature of absolute zero. The fluctuations of the spins in a spin liquid can be classical or quantum and show remarkable collective phenomena such as emergent gauge fields and fractional particle excitations. This exotic behaviour is now being uncovered in the laboratory, providing insight into the properties of spin liquids and challenges to the theoretical description of these materials.

Anomalous metallic state in the vicinity of metal to valence-bond solid insulator transition in LiVS2

We investigate LiVS2 and LiVSe2 with a triangular lattice as itinerant analogues of LiVO2 known for the formation of a valence-bond solid (VBS) state out of an S=1 frustrated magnet. LiVS2, which is located at the border between a metal and a correlated insulator, shows a first order transition from a paramagnetic metal to a VBS insulator at Tc approximately 305 K upon cooling. The presence of a VBS state in the close vicinity of insulator-metal transition may suggest the importance of itinerancy in the formation of a VBS state. We argue that the high temperature metallic phase of LiVS2 has a pseudogap, likely originating from the VBS fluctuation. LiVSe2 was found to be a paramagnetic metal down to 2 K.

Dynamics and transport of the Z2 spin liquid: application to kappa-(ET)2Cu2(CN)3

We describe neutron scattering, NMR relaxation, and thermal transport properties of Z2 spin liquids in two dimensions. Comparison to recent experiments on the spin S=1/2 triangular lattice antiferromagnet in kappa-(ET)2Cu2(CN)3 shows that this compound may realize a Z2 spin liquid. We argue that the topological "vison" excitations dominate thermal transport, and that recent thermal conductivity experiments by M. Yamashita et al. have observed the vison gap.

Variety of valence bond states formed of frustrated spins on triangular lattices based on a two-level system Pd(dmit)2

Recent studies on the physical properties of the triangular system based on the Pd(dmit)2 salts (dmit=1,3-dithiole-2-thione-4,5-dithiolate) are reviewed. Quantum chemical architectures of the Pd(dmit)2 molecule and its dimer are introduced with emphasis on the strong dimerization of a two-level system, which provides unique physical properties of the salts. The magnetic properties are outlined in view of the magneto-structural correlation specific to the frustrated spin systems. Some newly discovered ground states and their origins are discussed, for which the valence bond formation plays a key role. Among them, the two-level structure is crucial for the novel charge-separated state found in two salts. The valence bond ordering, similar to the spin-Peierls transition, has been found in a two-dimensional frustrated spin system. The physical aspects and possible relation to the pressure-induced superconductivity are discussed.

Absence of superconductivity in the half-filled band Hubbard model on the anisotropic triangular lattice

We report exact calculations of magnetic and superconducting pair-pair correlations for the half-filled band Hubbard model on an anisotropic triangular lattice. Our results for the magnetic phases are similar to those obtained with other techniques. The superconducting pair-pair correlations at distances beyond nearest neighbor decrease monotonically with increasing Hubbard interaction U for all anisotropy, indicating the absence of frustration-driven superconductivity within the model.