Takayuki Isono

Superconductivity is one of the most intriguing topics in solid state physics. Generally, the superconducting Cooper pairs are broken by the Zeeman effect, which gives the so-called Pauli paramagnetic limit H-Pauli. However, when the superconductivity is in the clean limit and the orbital effect is strongly quenched, the Cooper pairs can survive even above H-Pauli, which is the so-called Fulde and Ferrell, and Larkin and Ovchinnikov (FFLO) phase. Extensive efforts have been devoted to the discovery of the FFLO phase. However, vortex phase transitions have given rise to considerable ambiguity in the interpretation of the experimental data. Here, we report comprehensive magnetocaloric and torque studies of the FFLO phase transition in a highly two-dimensional organic superconductor. We observe the FFLO phase transition clearly distinct from vortex melting transitions. The phase diagram provides crucial information on the stability of the FFLO phase in magnetic fields.

A quantum spin liquid (QSL) is an exotic state of matter in condensed-matter systems, where the electron spins are strongly correlated, but conventional magnetic orders are suppressed down to zero temperature because of strong quantum fluctuations. One of the most prominent features of a QSL is the presence of fractionalized spin excitations, called spinons. Despite extensive studies, the nature of the spinons is still highly controversial. Here we report magnetocaloric-effect measurements on an organic spin-1/2 triangular-lattice antiferromagnet, showing that electron spins are decoupled from a lattice in a QSL state. The decoupling phenomena support the gapless nature of spin excitations. We further find that as a magnetic field is applied away from a quantum critical point, the number of spin states that interact with lattice vibrations is strongly reduced, leading to weak spin-lattice coupling. The results are compared with a model of a strongly correlated QSL near a quantum critical point.

Resistance and magnetic torque measurements are reported in a layered organic superconductor, β"-(BEDT-TTF)4[(H3O)Ga(C2O4)3]C6H5NO2 with Tc=4.8 K, where BEDT-TTF stands for bis(ethylenedithio)tetrathiafulvalene. Because of the large anion between the BEDT-TTF conducting layers, the superconductivity of this salt is highly anisotropic. In magnetic fields parallel to the conducting layers for T=0.4 K, the magnetic torque shows a large diamagnetic signal associated with hysteresis up to ∼21 T, suggesting the upper critical field Hc2 21 T at 0.4 K. The large reduction of the diamagnetic signal is observed above 16 T, which shows a Fulde and Ferrell and Larkin and Ovchinnikov (FFLO) phase transition. For T=0.5 K, the interlayer resistance has nonzero value in a wide field region up to Hc2, arising from the Josephson vortex dynamics. Successive dips in the second derivative curves of the resistance are observed between 16 T and Hc2, which are ascribed to the commensurability effect between the Josephson vortex lattice and the order parameter oscillation of the FFLO phase. The commensurability effect is observed only in nearly parallel fields, showing that the FFLO phase is stable in a very limited field angle region. The temperature-field phase diagram is determined.

Resistance and magnetic torque measurements have been performed to investigate vortex phases for a layered organic superconductor κ-(BEDT-TTF)2Cu(NCS)2 [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene], which is modeled as stacks of Josephson junctions. At 25 mK, the out-of-plane resistivity increases at 0.6 T, has a step feature up to 4 T, and then increases again, whereas the in-plane resistivity monotonically increases above 4 T. The results show that both pancake vortices (PVs) and Josephson vortices (JVs) are in solid phases for μ0H&lt 0.6 T, but only JVs are in a liquid phase for 0.6&lt μ0H&lt 4 T. For μ0H&gt 4 T, both PVs and JVs are in liquid phases. These melting transitions are predominantly induced by quantum fluctuations (not by thermal fluctuations). In the magnetic torque curves, the irreversibility transition is clearly observed, roughly corresponding to the melting transition of the PVs but no anomaly is found at the JV melting transition. The detailed vortex phase diagram is determined in a wide temperature region.

Magnetocaloric effect (MCE) and magnetic torque measurements have been carried out in the π–d system λ-(BETS)2FeCl4 [BETS = bis(ethylenedithio)tetraselenafulvalene], which shows an antiferromagnetic insulating (AFI) phase below ∼8.5 K. In the magnetic torque curve, a sharp structure at ∼1.2 T and a step at ∼10 T are observed at low temperatures, which are caused by the spin-flop (SF) transition and the transition from the AFI to paramagnetic metallic (PM) phase, respectively. The MCE, directly related to the magnetic entropy, shows a small sharp peak at the SF transition and a sharp dip at the AFI–PM transition. The overall feature above 3 K is qualitatively interpreted by a simple picture: antiferromagnetic (AF) π spins and paramagnetic 3d spins at the Fe sites. However, a broad dip in the MCE is additionally found at ∼5 T below ∼3 K, which is not explained by the above picture. The results are compared with those of κ-(BETS)2FeBr4, which shows an AF order of the 3d spins at the Fe sites.

Systematic measurements of the magnetic torque tau of the organic conductor lambda-(BETS)(2)FeCl4 have been performed to investigate the magnetic properties. In the magnetic field dependence of t, a very sharp structure is observed at similar to 1.2 T, resulting from the spin-flop transition. A step-like behavior associated with small hysteresis appears at similar to 10 T, which is caused by the antiferromagnetic insulator-paramagnetic metal (AFI-PM) transition. In the angular dependence of tau for magnetic fields in the b*-c plane, it is found that the zero-crossing angles significantly change with field and temperature. The changes provide reasonable evidence of the antiferromagnetic order of the pi spins (not the Fe 3d spins) in the AFI phase. The AFI-PM transition field has a minimum when the magnetization of the 3d spins has a maximum as a function of field angle.

A quantum spin-liquid state, an exotic state of matter, appears when strong quantum fluctuations enhanced by competing exchange interactions suppress a magnetically ordered state. Generally, when an ordered state is continuously suppressed to 0 K by an external parameter, a quantum phase transition occurs. It exhibits critical scaling behaviour, characterized only by a few basic properties such as dimensions and symmetry. Here we report the low-temperature magnetic torque measurements in an organic triangular-lattice antiferromagnet, kappa-(BEDT-TTF)(2)Cu-2(CN)(3), where BEDT-TTF stands for bis(ethylenedithio)-tetrathiafulvalene. It is found that the magnetic susceptibilities derived from the torque data exhibit a universal critical scaling, indicating the quantum critical point at zero magnetic field, and the critical exponents, gamma = 0.83(6) and nu z = 1.0(1). These exponents greatly constrain the theoretical models for the quantum spin liquid, and at present, there is no theory to explain the values, to the best of our knowledge.

Resistance and dielectric constants have been measured in the antiferromagnetic insulating phase of the quasi-two-dimensional organic conductor lambda-(BETS)(2)FeCl4 to understand charge transport. Nonlinear current-voltage characteristics are observed at low temperatures, which are explained by a charge transport model based on the electric-field dependent Coulomb potential between the thermally excited electron and hole. A small dip in the magnetic field dependence of the resistance is found at 1.2 T, which is ascribed to a spin-flop transition. The large difference between the in-plane and out-of-plane dielectric constants shows the two-dimensionality of the Coulomb potential, which is consistent with the charge transport model. The angular dependence of the metal-insulator transition field is determined, which suggests that the Zeeman effect of the 3d spins of the Fe ions plays an essential role.

We report the in-plane anisotropy of the upper critical field (H-c2) and the flux-flow resistivity (FFR) for a layered organic superconductor beta ''-(ET)(2)SF5CH2CF2SO3 with the incoherent nature of the interlayer transport. The in-plane angular dependence of H-c2 showed a fourfold oscillation with maxima in the directions around H parallel to b and a axes. This result is compatible with a d(x2-y2) order parameter. The determined nodal structure is consistent with theoretical predictions of superconductivity mediated by charge fluctuations. Moreover, the dependence of the FFR on in-plane field-orientation showed a fourfold symmetry with cusp-like minima in the directions around H parallel to b and a axes, which is very similar to the FFR for kappa-(ET)(2)Cu(NCS)(2) with d-wave pairing symmetry. From these results, we claim that the vortex dynamics is strongly affected by the superconducting gap structure for highly two-dimensional superconductors with d-wave pairing symmetry.

The anisotropic band structure and electrical resistivity under uniaxial strain were investigated for the moderately dimerized molecular conductor beta-(meso-DMBEDT-TTF)(2)PF6 [DMBEDT-TTF = dimethylbis(ethylenedithio)tetrathiafulvalene], which has an intermediate electronic state between 3/4-filled and effective 1/2-filled band structures at ambient pressure. The calculated energy overlap of the upper and lower bands increased more rapidly by applying uniaxial strain along the c axis than along the (a-b) axis towards the 3/4-filled band structure, which is related to intersite electronic correlation, and the energy gap between these bands is estimated to be larger with application of uniaxial strain along the (a+b) axis towards an effective 1/2-filled band structure, which is related to intrasite electronic correlation. The electrical resistivity under uniaxial strain along the c axis indicates the semiconducting behavior from 300 K and the superconducting (SC) transitions at 4.6-2.8 K under 0.10-0.41 GPa. On the other hand, the metal-insulator transition is not completely suppressed up to 0.8 GPa and SC transitions appear at 4.5-2.6 K under 0.16-0.45 GPa with uniaxial strain along the (a-b) axis. If this uniaxial strain results in semiconducting behavior and positive magnetoresistance related to intersite correlation, the SC states are observed under hydrostatic pressure, which indicates that the SC state is deeply related to the charge-ordered state.

A hydrogen bond (H-bond) is one of the most fundamental and important noncovalent interactions in chemistry, biology, physics, and all other molecular sciences. Especially, the dynamics of a proton or a hydrogen atom in the H-bond has attracted increasing attention, because it plays a crucial role in (bio)chemical reactions and some physical properties, such as dielectricity and proton conductivity. Here we report unprecedented H-bond-dynamics-based switching of electrical conductivity and magnetism in a H-bonded purely organic conductor crystal, kappa-D-3(Cat-EDT-TTF)(2) (abbreviated as kappa-D). This novel crystal kappa-D, a deuterated analogue of kappa-H-3(Cat-EDT-TTF)(2) (abbreviated as kappa-H), is composed only of a H-bonded molecular unit, in which two crystallographically equivalent catechol-fused ethylenedithiotetrathiafulvalene (Cat-EDT-TTF) skeletons with a +0.5 charge are linked by a symmetric anionic [O center dot center dot center dot D center dot center dot center dot O](-1)-type strong H-bond. Although the deuterated and parent hydrogen systems, kappa-D and kappa-H, are isostructural paramagnetic semiconductors with a dimer-Mott-type electronic structure at room temperature (space group: C2/c), only kappa-D undergoes a phase transition at 185 K, to change to a nonmagnetic insulator with a charge-ordered electronic structure (space group: P (1) over bar). The X-ray crystal structure analysis demonstrates that this dramatic switching of the electronic structure and physical properties originates from deuterium transfer or displacement within the H-bond accompanied by electron transfer between the Cat-EDT-TTF pi-systems, proving that the H-bonded deuterium dynamics and the conducting TTF pi-electron are cooperatively coupled. Furthermore, the reason why this unique phase transition occurs only in kappa-D is qualitatively discussed in terms of the H/D isotope effect on the H-bond geometry and potential energy curve.

We report the results of SQUID and torque magnetometry of an organic spin-1/2 triangular-lattice kappa - H(3)dCat-EDT-TTF)(2). Despite antiferromagnetic exchange coupling at 80-100 K, we observed no sign of antiferromagnetic order down to 50 mK owing to spin frustration on the triangular lattice. In addition, we found nearly temperature-independent susceptibility below 3 K associated with Pauli paramagnetism. These observations suggest the development of gapless quantum spin liquid as the ground state. On the basis of a comparative discussion, we point out that the gapless quantum spin liquid states in organic systems share a possible mechanism, namely the formation of a band with a Fermi surface possibly attributed to spinons.

We develop a physical understanding for how anhamonicity of lattice vibration affects transition temperature, T-c, of superconductivity mediated by phonons, via specific heat measurements on beta-pyrochlore oxide superconductors under high pressure and in magnetic fields. We find that T-c demonstrates unimodal dependence on pressure, where T-c increases more than three times by pressure. On the other hand, an electron-phonon coupling constant, lambda, is monotonically enhanced as pressure is applied, leading to the enhancement of effective mass of conduction electrons. From the analysis based on the McMillan equation, moreover, an average phonon frequency, omega(log), is found to reflect a downward trend on all the pressure regions, which is consistent with the results of the previous inelastic neutron scattering and specific heat measurements. The downward trend of omega(log) means that the application of pressure enhances a magnitude of anhamonicity, contrary to a common sense of pressure effects. On the basis of all the results, we indicate that the unimodal pressure variation in T-c is attributed to two competing effects, that is, the enhancement of lambda and the simultaneous suppression of omega(log) caused by the pressure evolution of anhamonicity.

Purely organic materials are generally insulating. Some charge-carrier generation, however, can provide them with electrical conductivity. In multi-component organic systems, carrier generation by intermolecular charge transfer has given many molecular metals. By contrast, in purely organic single-component systems, metallic states have rarely been realized although some neutral-radical semiconductors have been reported. Here we uncover a new type of purely organic single-component molecular conductor by utilizing strong hydrogen-bonding interactions between tetrathiafulvalene-based electron-donor molecules. These conductors are composed of highly symmetric molecular units constructed by the strong intra-unit hydrogen bond. Moreover, we demonstrate that, in this system, charge carriers are produced by the partial oxidation of the donor molecules and delocalized through the formation of the symmetric intra-unit hydrogen bonds. As a result, our conductors show the highest room-temperature electrical conductivity and the metallic state under the lowest physical pressure among the purely organic single-component systems, to our knowledge. © 2013 Macmillan Publishers Limited. All rights reserved.

The metallic state of the molecular conductor beta-(meso-DMBEDT-TTF)(2)X (DMBEDT-TTF = 2-(5,6-dihydro-1,3-dithiolo[4,5-b][1,4] dithiin-2-ylidene)-5,6-dihydro5,6-dimethyl-1,3-dithiolo[4,5-b][1,4]dithiin, X = PF6, AsF6) is transformed into the checkerboard-type charge-ordered state at around 75-80 K with accompanying metal-insulator (MI) transition on the anisotropic triangular lattice. With lowering temperatures, the magnetic susceptibility decreases gradually and reveals a sudden drop at the MI transition. By applying pressure, the charge-ordered state is suppressed and superconductivity appears in beta-(meso-DMBEDT-TTF)(2)AsF6 as well as in the reported beta-(meso-DMBEDT-TTF)(2)PF6. The charge-ordered spin-gapped state and the pressure-induced superconducting state are discussed through the paired-electron crystal (PEC) model, where the spin-bonded electron pairs stay and become mobile in the crystal, namely the valence-bond solid (VBS) and the resonant valence bonded (RVB) state in the quarter-filled band structure.

Catechol-fused tetrathiafulvalene (TIT) derivatives have been designed and synthesized as a new type of pi-electron donor molecules having two phenolic hydroxyl groups. Cyclic voltammetry measurements and quantum chemical calculations demonstrated the electronic effect of the direct fusion of the catechol unit to the TTF pi-skeleton. In the charge-transfer (CT) salts with bromide or chloride anions, a one-dimensional hydrogen-bonded chain was formed by the intermolecular OH center dot center dot center dot X network between the catechol moieties and the halide anions. The slight dissimilarity of the hydrogen-bond distances for the two CT salts gave rise to the significant differences in their overall molecular arrangements and intermolecular interactions as well as the electrical resistivities. (c) 2012 Elsevier Ltd. All rights reserved.

Dependence of the superconducting transition temperature (T-c) and critial superconducting pressure (Pc) of the pressure-induced superconductor beta-(BDA-TTP)(2)I-3 [BDA-TTP = 2,5-bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene] on the orientation of uniaidal strain has been investigated. On the basis of the overlap between the upper and lower bands in the energy dispersion curve, the pressure orientation is thought to change the half-filled band to the quarter-filled one. The observed variations in Tc and Pc are explained by considering the degree of application of the pressure and the degree of contribution of the effective electronic correlation at uniaxial strains with different orientations parallel to the conducting donor layer.

We report the results of the low-temperature specific heat measurements on the β-pyrochlore oxide superconductor RbOs2O6 (T c = 6.3 K) in magnetic field under high pressure. We investigated the upper critical field Hc2 at several pressures and found that the slope of Hc2 at Tc monotonically increases with increasing pressure in low-pressure superconducting phase SC1, indicating that the effective mass m* is enhanced by the pressure. Moreover, the jump of the specific heat at Tc is also enhanced by the application of the pressure in SC1, suggesting that the pressure gives rise to the enhancement of the electron-phonon coupling and consequently enhances T c. © 2011 The Physical Society of Japan.

We report the result of the specific heat and the powder X-ray diffraction measurements on the single crystalline p-pyrochlore oxide superconductors AOs(2)O(6) under high pressure. We found from the specific heat measurements that the superconducting phase in the low pressure region (SC1) suddenly vanishes at the critical pressure P* and an alternative superconducting phase (SC2) with lower T-c compared to that of SC1 appears above P* In addition, we found from the X-ray diffraction measurements that a first-order phase transition of the crystal structure takes place at similar to P*, accompanied with reduction of the crystal symmetry. This result suggests that the sudden suppression of T-c arises from the structural phase transition.

Thermal conductivity tensor has been measured using single crystalline Co-doped BaFe2As2 down to 0.1 K and under magnetic fields up to 7T. We observe peak anomalies both in the thermal conductivity and the thermal Hall conductivity in the superconducting state as an indication of enhancement of the quasiparticle mean-free path. Furthermore, we find a residual T-linear term in the thermal conductivity possibly due to a finite quasiparticle density of states in the superconducting gap induced by impurity pair-breaking. (C) 2010 Elsevier B.V. All rights reserved.

We report the results of the low-temperature specific heat measurements of the single crystalline β-pyrochlore oxide superconductors AOs 2O6 (A=K, Rb, and Cs) under high pressure up to 13 GPa. We find that superconducting transition temperature (Tc) monotonically increases for CsOs2O6 and RbOs2O6, while the one for KOs2O6 decreases by applying the pressure. With further increasing the pressure, Tc is suddenly suppressed at the same lattice volume for all compounds, concomitant with the first-order structural phase transition. © 2010 ElsevierB.V. All rights reserved.

We report the results of the specific heat measurements for β-pyrochlore oxide superconductor RbOs2O6 with superconducting transition temperature Tc ∼ 6.3 K in magnetic field and under pressure. We find that Tc rises up to 9.3 K by applying the pressure concomitant with an increase of an initial slope of the upper critical field Hc2 and a jump of the specific heat at T c. From the analysis of our data, the effective mass of conduction electrons and the electron-phonon coupling appear to increase under the pressure. © Published under licence by IOP Publishing Ltd.

We report the thermal transport coefficients of the single crystalline Co-doped BaFe2As2. Signatures of enhancement of the quasiparticle mean-free path are found both in the thermal conductivity and the thermal Hall conductivity in the superconducting state. Moreover, we find a residual T-linear term in the thermal conductivity possibly due to a finite in-gap state induced by impurity pair-breaking, compatible with a sign-reversing s-wave state. (C) 2010 Elsevier B.V. All rights reserved.

We report the results of the low temperature specific heat of the β-pyrochlore oxide superconductors RbOs2O6 and KOs2O6 under high pressure. The superconducting transition temperature Tc of RbOs2O6 monotonically increases with increasing pressure and then abruptly vanishes at 6.0 GPa. Similar abrupt vanishment is also observed in KOs2O6 at 5.2 GPa. Under high pressure above 5.8 GPa, other two anomalies are found instead of that of the superconductivity for RbOs2O6. It is suggested that the abrupt destruction of the superconductivity and the appearance of anomalies under high pressure are important to understand the origin of the superconductivity in this system. © 2009 Elsevier B.V. All rights reserved.

Thermal transport measurements have been performed on single-crystalline Co-doped BaFe2As2 down to 0.1 K and under magnetic fields up to 7 T. Significant peak anomalies are observed in both thermal conductivity and thermal Hall conductivity below T-c as an indication of the enhancement of the quasiparticle mean-free path. Moreover, we find a sizable residual T-linear term in thermal conductivity, possibly due to a finite quasiparticle density of states in the superconducting gap induced by impurity pair breaking. Our findings support a pairing symmetry compatible with the theoretically predicted sign-reversing s-wave state.