Thermoelectric signature of quantum critical phase in a doped spin-liquid candidate.

Quantum spin liquid is a nontrivial magnetic state of longstanding interest, in which spins are strongly correlated and entangled but do not order; further intriguing is its doped version, which possibly hosts strange metal and unconventional superconductivity. A promising candidate of the doped spin liquid is a triangular-lattice organic conductor, κ-(BEDT-TTF)4Hg2.89Br8, recently found to hold metallicity, spin-liquid-like magnetism, and BEC-like superconductivity. The nature of the metallic state with the spin-liquid behaviour is awaiting to be further clarified. Here, we report the thermoelectric signature that mobile holes in the spin liquid background are in a quantum critical state and it pertains to the BEC-like superconductivity. The Seebeck coefficient divided by temperature, S/T, is enhanced on cooling with logarithmic divergence indicative of quantum criticality. Furthermore, the logarithmic enhancement is correlated with the superconducting transition temperature under pressure variation, and the temperature and magnetic field profile of S/T upon the superconducting transition change with pressure in a consistent way with the previously suggested BEC-BCS crossover. The present results reveal that the quantum criticality in a doped spin liquid emerges in a phase, not at a point, and is involved in the unconventional BEC-like nature.

Phase Diagram for Light-Induced Superconductivity in κ-(ET)_{2}-X.

Resonant optical excitation of certain molecular vibrations in κ-(BEDT-TTF)_{2}Cu[N(CN)_{2}]Br has been shown to induce transient superconductinglike optical properties at temperatures far above equilibrium T_{c}. Here, we report experiments across the bandwidth-tuned phase diagram of this class of materials, and study the Mott insulator κ-(BEDT-TTF)_{2}Cu[N(CN)_{2}]Cl and the metallic compound κ-(BEDT-TTF)_{2}Cu(NCS)_{2}. We find nonequilibrium photoinduced superconductivity only in κ-(BEDT-TTF)_{2}Cu[N(CN)_{2}]Br, indicating that the proximity to the Mott insulating phase and possibly the presence of preexisting superconducting fluctuations are prerequisites for this effect.

Terahertz-field-induced polar charge order in electronic-type dielectrics.

Ultrafast electronic-phase change in solids by light, called photoinduced phase transition, is a central issue in the field of non-equilibrium quantum physics, which has been developed very recently. In most of those phenomena, charge or spin orders in an original phase are melted by photocarrier generations, while an ordered state is usually difficult to be created from a non-ordered state by a photoexcitation. Here, we demonstrate that a strong terahertz electric-field pulse changes a Mott insulator of an organic molecular compound in κ-(ET)2Cu[N(CN)2]Cl (ET = bis(ethylenedithio)tetrathiafulvalene), to a macroscopically polarized charge-order state; herein, electronic ferroelectricity is induced by the collective intermolecular charge transfers in each dimer. In contrast, in an isostructural compound, κ-(ET)2Cu2(CN)3, which shows the spin-liquid state at low temperatures, a similar polar charge order is not stabilized by the same terahertz pulse. From the comparative studies of terahertz-field-induced second-harmonic-generation and reflectivity changes in the two compounds, we suggest the possibility that a coupling of charge and spin degrees of freedom would play important roles in the stabilization of polar charge order.

Topological charge transport by mobile dielectric-ferroelectric domain walls.

The concept of topology has been widely applied in condensed matter physics, leading to the identification of peculiar electronic states on three-dimensional (3D) surfaces or 2D lines separating topologically distinctive regions. In the systems explored so far, the topological boundaries are built-in walls; thus, their motional degrees of freedom, which potentially bring about new paradigms, have been experimentally inaccessible. Here, working with a quasi-1D organic material with a charge-transfer instability, we show that mobile neutral-ionic (dielectric-ferroelectric) domain boundaries with topological charges carry strongly 1D-confined and anomalously large electrical conduction with an energy gap much smaller than the one-particle excitation gap. This consequence is further supported by nuclear magnetic resonance detection of spin solitons, which are required for steady current of topological charges. The present observation of topological charge transport may open a new channel for broad charge transport-related phenomena such as thermoelectric effects.

Strange metal from a frustration-driven charge order instability.

Interparticle interactions are self-conflicting rather than cooperative on particular lattices. When such geometrical frustration occurs, charge ordering (CO) can be destabilized into non-trivial charge states such as the recently observed charge glass (CG). A more extreme case is the frustration-induced quantum melting of the CO that has been theoretically proposed. Here, we report d.c. charge transport and noise spectroscopy measurements for a triangular-lattice organic conductor situated close to the CO or CG. Our experiments demonstrate that these materials can host a strange metal with unusual charge dynamics, which we attribute to frustration-induced fluctuations of the CO or CG. Our results also show that the anomalous charge fluctuations can freeze into an insulating state when uniaxial stress is applied, which reduces the geometrical frustration. The present observations suggest the existence of the frustration-induced quantum melting of charges analogous to spin liquids.

Evidence for solitonic spin excitations from a charge-lattice-coupled ferroelectric order.

Topological defects have been explored in different fields ranging from condensed matter physics and particle physics to cosmology. In condensed matter, strong coupling between charge, spin, and lattice degrees of freedom brings about emergent excitations with topological characteristics at low energies. One-dimensional (1D) systems with degenerate dimerization patterns are typical stages for the generation of topological defects, dubbed "solitons"; for instance, charged solitons are responsible for high electrical conductivity in doped trans-polyacetylene. Here, we provide evidence based on a nuclear magnetic resonance (NMR) study for mobile spin solitons deconfined from a strongly charge-lattice-coupled spin-singlet ferroelectric order in a quasi-1D organic charge-transfer complex. The NMR spectral shift and relaxation rate associated with static and dynamic spin susceptibilities indicate that the ferroelectric order is violated by dilute solitonic spin excitations, which were further demonstrated to move diffusively by the frequency dependence of the relaxation rate. The traveling solitons revealed here may promise the emergence of anomalous electrical and thermal transport.

Electronic crystal growth.

Interacting atoms or molecules condense into liquid, and, when cooled further, they form a crystal. The time evolution of the atomic or molecular ordering has been widely studied as a nonequilibrium emergence of order from a supercooled liquid or a glass. Interacting electrons in a variety of correlated electron systems also form crystals, but observing the time evolution of electronic crystallization has been experimentally challenging. Here, working with an organic conductor exhibiting a supercooled charge liquid or charge glass as a metastable state, we observed electronic crystal growth through resistivity and nuclear magnetic resonance measurements. The temperature profile of the crystal growth is similar to those observed in classical systems and reveals two distinct regimes for the mechanism of electronic crystallization.

Mott transition by an impulsive dielectric breakdown.

The transition of a Mott insulator to metal, the Mott transition, can occur via carrier doping by elemental substitution, and by photoirradiation, as observed in transition-metal compounds and in organic materials. Here, we show that the application of a strong electric field can induce a Mott transition by a new pathway, namely through impulsive dielectric breakdown. Irradiation of a terahertz electric-field pulse on an ET-based compound, κ-(ET) 2Cu[N(CN) 2]Br (ET:bis(ethylenedithio)tetrathiafulvalene), collapses the original Mott gap of ∼30 meV with a ∼0.1 ps time constant after doublon-holon pair productions by quantum tunnelling processes, as indicated by the nonlinear increase of Drude-like low-energy spectral weights. Additionally, we demonstrate metallization using this method is faster than that by a femtosecond laser-pulse irradiation and that the transition dynamics are more electronic and coherent. Thus, strong terahertz-pulse irradiation is an effective approach to achieve a purely electronic Mott transition, enhancing the understanding of its quantum nature.

Quantum Spin Liquid Emerging from Antiferromagnetic Order by Introducing Disorder.

Quantum spin liquids, which are spin versions of quantum matter, have been sought after in systems with geometrical frustration. We show that disorder drives a classical magnet into a quantum spin liquid through conducting NMR experiments on an organic Mott insulator, κ-(ET)_{2}Cu[N(CN)_{2}]Cl. Antiferromagnetic ordering in the pristine crystal, when irradiated by x rays, disappears. Spin freezing, spin gap, and critical slowing down are not observed, but gapless spin excitations emerge, suggesting a novel role of disorder that brings forth a quantum spin liquid from a classical ordered state.

Pressure-induced Mott transition in an organic superconductor with a finite doping level.

We report the pressure study of a doped organic superconductor with a Hall coefficient and conductivity measurements. We find that maximally enhanced superconductivity and a marginal-Fermi liquid appear around a certain pressure where mobile carriers increase critically, suggesting a possible quantum phase transition between strongly and weakly correlated regimes. This observation points to the presence of a criticality in Mottness for a doped Mott insulator with tunable correlation.

High-frequency nuclear quadrupole resonance apparatus for use in pressure cell.

A high-frequency NMR apparatus for use in pressure cell is described. All components of the resonance circuit are set in the pressure cell. This method makes the resonance frequency much less influenced by large stray capacitance residing at the electrical feedthrough of the pressure cell. With the use of this apparatus, a pressure-induced neutral-ionic phase transition in DMTTF-QBr(4) was successfully observed by (79)Br nuclear quadrupole resonance, whose resonance frequency is ∼300 MHz.

Magnetic and non-magnetic phases of a quantum spin liquid.

A quantum spin-liquid phase is an intriguing possibility for a system of strongly interacting magnetic units in which the usual magnetically ordered ground state is avoided owing to strong quantum fluctuations. It was first predicted theoretically for a triangular-lattice model with antiferromagnetically coupled S = 1/2 spins. Recently, materials have become available showing persuasive experimental evidence for such a state. Although many studies show that the ideal triangular lattice of S = 1/2 Heisenberg spins actually orders magnetically into a three-sublattice, non-collinear 120° arrangement, quantum fluctuations significantly reduce the size of the ordered moment. This residual ordering can be completely suppressed when higher-order ring-exchange magnetic interactions are significant, as found in nearly metallic Mott insulators. The layered molecular system κ-(BEDT-TTF)(2)Cu(2)(CN)(3) is a Mott insulator with an almost isotropic, triangular magnetic lattice of spin-1/2 BEDT-TTF dimers that provides a prime example of a spin liquid formed in this way. Despite a high-temperature exchange coupling, J, of 250 K (ref. 6), no obvious signature of conventional magnetic ordering is seen down to 20 mK (refs 7, 8). Here we show, using muon spin rotation, that applying a small magnetic field to this system produces a quantum phase transition between the spin-liquid phase and an antiferromagnetic phase with a strongly suppressed moment. This can be described as Bose-Einstein condensation of spin excitations with an extremely small spin gap. At higher fields, a second transition is found that suggests a threshold for deconfinement of the spin excitations. Our studies reveal the low-temperature magnetic phase diagram and enable us to measure characteristic critical properties. We compare our results closely with current theoretical models, and this gives some further insight into the nature of the spin-liquid phase.

Spin ordering and enhancement of electronic heat capacity in an organic system of (DI-DCNQI)(2)(Ag(1-x)Cu(x)).

Thermodynamic measurements on the organic system of (DI-DCNQI)(2)(Ag(1-x)Cu(x)) (x = 0,0.05, 0.71, 0.90) were performed to study the change from the charge-ordered (CO) insulating state to the π-d hybridized metallic state. A thermal anomaly associated with the antiferromagnetic transition that occurred in the charge-ordered lattice was observed at 6.2 K from the temperature dependence of the heat capacity of (DI-DCNQI)(2)Ag. We have found that the magnetic entropy around the peak is only 1.5% of Rln2, corresponding to the full entropy expected for the formula unit of (DI-DCNQI)(2)Ag. This anomaly is suppressed down to about 3 K in the x = 0.05 sample owing to the disorders induced in the CO lattice. In the metallic concentration of x = 0.90, the low-temperature electronic heat capacity coefficient, γ was found to be enhanced by up to about 63.6 mJ K(-2) mol(-1) probably owing to the cooperative effect of π-d hybridization and intersite Coulomb interaction (V).

Wigner crystallization in (DI-DCNQI)2Ag detected by synchrotron radiation X-Ray diffraction.

The low-temperature electronic structure of the quarter-filled, quasi-one-dimensional (Q1D) system (DI-DCNQI)2Ag is revealed using synchrotron radiation x-ray diffraction. In spite of the interchain frustration in the twofold superstructure along the 1D chain, the body-centered tetragonal "charge ordering" structure, which consists of 4k_{F} charge ordering columns and 4k_{F} bond order wave columns, is realized. This is the first example of the Q1D system having plural kinds of columns as its ground state. This charge ordered structure is regarded as a Wigner crystal caused by intercolumn Coulomb repulsion.

Mott transition from a spin liquid to a Fermi liquid in the spin-frustrated organic conductor kappa-(ET)2Cu2(CN)3.

The pressure-temperature phase diagram of the organic Mott insulator kappa-(ET)2Cu2(CN)3, a model system of the spin liquid on triangular lattice, has been investigated by 1H NMR and resistivity measurements. The spin-liquid phase is persistent before the Mott transition to the metal or superconducting phase under pressure. At the Mott transition, the spin fluctuations are rapidly suppressed and the Fermi-liquid features are observed in the temperature dependence of the spin-lattice relaxation rate and resistivity. The characteristic curvature of the Mott boundary in the phase diagram highlights a crucial effect of the spin frustration on the Mott transition.

Unconventional critical behaviour in a quasi-two-dimensional organic conductor.

Changing the interactions between particles in an ensemble--by varying the temperature or pressure, for example--can lead to phase transitions whose critical behaviour depends on the collective nature of the many-body system. Despite the diversity of ingredients, which include atoms, molecules, electrons and their spins, the collective behaviour can be grouped into several families (called 'universality classes') represented by canonical spin models. One kind of transition, the Mott transition, occurs when the repulsive Coulomb interaction between electrons is increased, causing wave-like electrons to behave as particles. In two dimensions, the attractive behaviour responsible for the superconductivity in high-transition temperature copper oxide and organic compounds appears near the Mott transition, but the universality class to which two-dimensional, repulsive electronic systems belongs remains unknown. Here we present an observation of the critical phenomena at the pressure-induced Mott transition in a quasi-two-dimensional organic conductor using conductance measurements as a probe. We find that the Mott transition in two dimensions is not consistent with known universality classes, as the observed collective behaviour has previously not been seen. This peculiarity must be involved in any emergent behaviour near the Mott transition in two dimensions.

Collapse of charge order in a quasi-one-dimensional organic conductor with a quarter-filled band.

A charge-ordered insulator, (DI-DCNQI)2Ag, with a quasi-one-dimensional quarter-filled band is metallized by pressure. It was found that the charge order melts into a curious metallic state with cubic-temperature dependence of the resistivity, which implies the unprecedented mechanism of the electron-electron scattering. We constructed the pressure-temperature phase diagram, where the melting line has a tricritical point dividing the second-order line at low pressures and the first-order line at high pressures just before it vanishes.

Magnetic-field-induced Mott transition in a quasi-two-dimensional organic conductor.

We investigated the effect of magnetic field on the highly correlated metal near the Mott transition in the quasi-two-dimensional layered organic conductor, kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Cl, by the resistance measurements under control of temperature, pressure, and magnetic field. It was demonstrated that the marginal metallic phase near the Mott transition is susceptible to the field-induced localization transition of the first order, as was predicted theoretically. The thermodynamic consideration of the present results gives a conceptual pressure-field phase diagram of the Mott transition at low temperatures.

Spin liquid state in an organic Mott insulator with a triangular lattice.

1H NMR and static susceptibility measurements have been performed in an organic Mott insulator with a nearly isotropic triangular lattice, kappa-(BEDT-TTF)2Cu2(CN)(3), which is a model system of frustrated quantum spins. The static susceptibility is described by the spin S=1/2 antiferromagnetic triangular-lattice Heisenberg model with the exchange constant J approximately 250 K. Regardless of the large magnetic interactions, the 1H NMR spectra show no indication of long-range magnetic ordering down to 32 mK, which is 4 orders of magnitude smaller than J. These results suggest that a quantum spin liquid state is realized in the close proximity of the superconducting state appearing under pressure.

d Orbital doping into pi charge-ordered molecular insulator.

A quasi-one-dimensional pi-electron charge-ordered insulator, (DI-DCNQI)2Ag, is metallized by Cu doping into the Ag sites. It is found that with doping the charge gap is diminished and then disorder-induced insulating nature comes up, eventually followed by transition or crossover to a pi-d networked metal. The profile of the charge-gap collapse is consistent with the prediction of the theory highlighting the interplay between electron correlation and disorder. The present doping method is regarded as doping of d orbital, which is different from the conventional charge and/or spin doping developed in cuprates and manganites.