M.A. Tanatar

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.

Superconducting double transition and the upper critical field limit of Sr2RuO4 in parallel magnetic fields

We studied the specific heat and thermal conductivity of the spin-triplet superconductor Sr 2 RuO 4 at low temperatures and under oriented magnetic fields H . We resolved a double peak structure of the superconducting transition under magnetic field for the first time, which provides thermodynamic evidence for the existence of multiple superconducting phases. We also found a clear limiting of the upper critical field H c2 for the field direction parallel to the RuO 2 plane only within ±2°. The limiting of H c2 occurs in the same H – T domain of the second superconducting phase; we suggest that the two phenomena have the same physical origin.

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.

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.

Electronic phase diagrams and fermi surfaces of κ-(ET)2X, the high Tc organic superconductors

Following the introduction of κ-(ET) 2 X (X=cu[N(cN) 2 ]Cl, Cu[N(CN) 2 ]Br, CU(CN)[N(CN) 2 ], CU(NCS) 2 ) providing high-T c organic superconductors, the pressure phase diagram of k-(ET) 2 Cu[N(CN) 2 ]Cl giving the highest Tc is described, as an epitome of dramatic interplay between electronic and magnetic phases in π electron system. This demonstrates the significance of two-dimensionality, electron correlation and soft lattice. The topology of the Fermi surfaces of k-(ET) 2 X is discussed in relation to the magnetooscillatory phenomena such as Shubnikov-de Haas effect, angle-dependent magnetoresistance and thermomagnetic effects.

Multiple superconducting phases of Sr2RuO4

Abstract A number of key experiments have provided strong evidence for spin–triplet symmetry in the superconductivity of Sr 2 RuO 4 with T c =1.5 K . However, there still remain important issues to be clarified before its superconducting state is fully understood. We will focus on two important issues, which appear to be intimately related to each other. First, we present evidence for multiple superconducting phases of Sr 2 RuO 4 in magnetic fields precisely parallel to the RuO 2 plane. Second, we show indications that the upper critical field H c2 is strongly suppressed when the magnetic field is precisely parallel to the RuO 2 plane. We discuss the relationship between these two phenomena, which are both characteristic of the parallel field configuration.

Transport properties of thallium series compounds Tl2Ba2Can−1CunO2n+4+x (n=1,2,3) with different oxygen content

Abstract Transport properties studies of Tl 2 Ba 2 Ca n−1 Cu 2 O x compounds with different doping levels have been performed. Resistivity, Hall effect and thermopower temperature dependencies were shown to be strongly dependent on doping level with respect to T c (X) curve maximum.

Thermal conductivity of organic superconductors in oriented magnetic field

We report study of thermal conductivity of organic superconductors in magnetic field of varying orientation with respect to the conducting plane, paying special attention to the parallel case. At low temperatures the field dependence of thermal conductivity in this configuration shows notable difference from usual behavior, both for electronic and phononic contributions. We show that the behavior of the electronic thermal conductivity in organic superconductor λ-(BETS) 2 GaCl 4 is consistent with the formation of an inhomogeneous Fulde-Ferrel-Larkin-Ovchinnikov superconducting state. In a closely related antiferromagnetic superconductor κ-(BETS) 2 FeBr 4 interaction with magnetic sublattice leads to a first order transition at H c2 in parallel magnetic field and reentrant superconductivity at high fields. The phonon thermal conductivity is shown to be a good tool for picking up a dimensional crossover and lock-in transition in the vortex system.

Pressure-temperature phase diagram of the organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]I

The pressure-temperature phase diagram of the organic superconductor K-(BEDT-TTF) 2 Cu[N(CN) 2 ]I determined by resistive measurements is presented. Under a hydrostatic pressure of about I kbar an insulating state is transformed into a metallic state, showing superconductivity with a transition temperature T c up to 8 K. A resistivity maximum develops in the boundary region of insulator-metal transformation and its position gradually shifts to high temperatures with pressure. These properties of the salt are in line with the generic phase diagram for K-phase materials, although the salt is located deep in the insulating domain of the phase diagram, contrary to the expectation based on the ionic radius of the halogen atom. We show the relation of the shape of the phase diagram to the degree of structural order in the material.