T. Ishiguro

Electrical and magnetic properties of organic semiconductors, (BEDT-TTF)2X (X = IBr2, IBrCl, and ICl2)

Abstract The electrical conductivity and the static magnetic susceptibility of various semiconducting phases of (BEDT-TTF)2X, where X = IBr2( α′), IBrCl (α′) and ICl2 ( β′), are investigated. We show the presence of anomaly in the temperature dependence of conductivity, as well as phase transitions accompanying a distinct change in the magnetic susceptibility. The nature of these phase transitions is discussed.

Magnetic field–temperature phase diagram of the quasi-two-dimensional organic superconductor λ − ( BETS ) 2 GaCl 4 studied via thermal conductivity

The thermal conductivity kappa of the quasi-two-dimensional (Q2D) organic superconductor lambda-(BETS)_2 GaCl_4 was studied in the magnetic field H applied parallel to the Q2D plane. The phase diagram determined from this bulk measurement shows notable dependence on the sample quality. In dirty samples the upper critical field H_{c2} is consistent with the Pauli paramagnetic limiting, and a sharp change is observed in kappa(H) at H_{c2 parallel}. In contrast in clean samples H_{c2}(T) shows no saturation towards low temperatures and the feature in kappa(H) is replaced by two slope changes reminiscent of second-order transitions. The peculiarity was observed below ~ 0.33T_c and disappeared on field inclination to the plane when the orbital suppression of superconductivity became dominant. This behavior is consistent with the formation of a superconducting state with spatially modulated order parameter in clean samples.

Anisotropy of Magnetothermal Conductivity in Sr2RuO4

The dependence of in-plane and interplane thermal conductivities of Sr2RuO4 on temperature, as well as magnetic field strength and orientation, is reported. We found no notable anisotropy in the thermal conductivity for the magnetic field rotation parallel to the conducting plane in the whole range of experimental temperatures and fields, except in the vicinity of the upper critical field H(c2), where the anisotropy of the H(c2) itself plays a dominant role. This finding imposes strong constraints on the possible models of superconductivity in Sr2RuO4 and supports the existence of a superconducting gap with a line of nodes running orthogonal to the Fermi surface cylinder.

Quenching effect of the anion ordering in the organic superconductor (TMTSF)2ClO4: An X-ray study

Abstract In the title compound it is found directly by an X-ray diffuse scattering study that the orientational ordering of ClO4 anions is suppressed when a sample is quenched from higher temperature to 15K. Also the lattice parameter is found to be frozen on quenching. Possible interrelation between these phenomena and the onset of superconductivity is discussed briefly.

Reentrant superconductivity in κ-(BEDT-TTF)2Cu[N(CN)2]Cl and its pressure phase diagram

Abstract Electrical resistivity and magnetization measurements reveal that κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl exhibits reentrant superconductivity (reported here for the first time for an organic solid) in a wide pressure region. The behavior based on the subtle balance between superconductivity and resistive (magnetic) phase is affected dramatically by thermal history. The low temperature state below 300 bar is of inhomogeneous mixture of superconductive and non-metallic phases, while the phase in higher pressure region can be single: rich variety of phase are found in pressure phase diagram.

Overview of organic superconductor κ-(BEDT-TTF)2[Cu(NCS)2] and its related materials

Abstract Crystal structures and physical properties of organic superconductors κ-(BEDT-TTF) 2 Cu(NCS) 2 , κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br and α-(BEDT-TTF) 2 [NH 4 Hg(SCN) 4 ] are discussed. The Cu(NCS) 2 salt shows an inverse isotope effect. The Cu[N(CN) 2 ]Br salt shows Tc of 11.7–11.8K. Conductivities, Hc 2 values and lattice parameters of these κ-type salts are compared.

Universal heat transport in Sr2RuO4.

We present the temperature dependence of the thermal conductivity kappa(T) of the unconventional superconductor Sr2RuO4 down to low temperatures ( approximately 100 mK). In the T-->0 K limit we found a finite residual term in kappa/T, providing clear evidence for the superconducting state with an unconventional pairing. The residual term remains unchanged for samples with different T(c), demonstrating the universal character of heat transport in this spin-triplet superconductor. The low-temperature behavior of kappa suggests the strong impurity scattering with a phase shift close to pi/2. A criterion for the observation of universality is experimentally deduced.

Magnetoresistance and thermoelectric power studies of metal-nonmetal transition in iodine-doped polyacetylene

Abstract Highly conducting polyacetylene iodine-doped to saturation level changed from metallic to nonmetallic states by aging, judging from the temperature dependence of the resistivity. During this change, the thermoelectric power did not vary drastically, suggesting that the transition cannot be ascribed to the change in carrier concentration. By contrast the magnetoresistance changed remarkably on crossing the transition. The transition is ascribed to the effect of disorder.

Anisotropy of the Upper Critical Field of the Organic Superconductor λ-(BETS)2GaCl4

The upper critical field Bc2 of the organic superconductor λ-(BETS)2GaCl4 has been determined from resistance measurements for orientation of the magnetic field along three perpendicular crystallographic directions. The Ginzburg–Landau coherence length was estimated from the slope of Bc2(T) curve near Tc as 12.5 nm, 1.6 nm, and 12.5 nm for the a*-, b*-, and c-directions. Angular dependence of the critical field within highly conducting ac plane at 1.5 K was shown to possess two-fold symmetry, which can be related to the band structure anisotropy.

Magnetic field studies of the peculiar electronic state in the α-(BEDT-TTF)2MHg(SCN)4 family

Abstract Magnetoresistance anomalies exhibited by the members of the α-(BEDT-TTF) 2 MHg(SCN) 4 family with M=Tl, K and Rb are reviewed in connection with possible modification of their electronic structure at low temperatures. Special attention is paid to angular magnetoresistance oscillations (AMRO). The analysis of the AMRO behavior at various temperatures and magnetic fields, both below and above the phase transition temperature, enables us to get an important information about changes in the electronic band structure at the phase transition. Other peculiarities of both semi-classical magnetoresistance and quantum oscillations are discussed within the proposed model of the low-temperature state of these compounds.