Chemical and Physical Properties of Organic Superconductor ϰ-(BEDT-TTF)₂Cu(NCS)₂

Abstract

An overview of the chemical and physical properties of a new organic superconductor κ-(BEDTTTF)₂Cu(NCS)₂, of which Tc (10.4-11.OK) is the highest among those of organic superconductors so far known, is presented. Distorted-hexagon-shaped crystals of the Cu(NCS)₂ salt were prepared by the electrochemical oxidation of BEDT-TTF in 1,1,2-trichloroethane in the presence of CuSCN, KSCN, and 18-crown-6 ether (or K (18-crown-6 ether)Cu(NCS)₂ complex, or CuSCN and TBA·SCN, as a supporting electrolyte) under a constant current of 1–5 μA. The black shinny crystals were stable against air and moisture and start to decompose above 190 °C.

The crystal structures indicate that both lattice parameters and packing pattern of BEDT-TTF molecules are almost the same to those of κ-(BEDT-TTF) ₂I₃ salt (Tc=3.6K) despite the considerable difference of Tc between them. Two BEDT-TTF molecules are dimerized and the dimers are linked one another by short sulfur..sulfur atomic contacts to construct two-dimensional conducting BEDT-TTF sheets in the bc-plane. The anions, Cu(NCS)₂ , form one-dimensional zigzag polymer along the b-axis and the polymers construct insulating sheets in the bc-plane. Every conducting layer is sandwiched by the insulating layers along the a-axis. Two kinds of layers are linked by hydrogen bonds between terminal ethylene groups of BEDT-TTF and nitrogen and sulfur atoms of the anions. Due to the lack of the inversion center of the crystal, the space group is lowered from P2₁/c of _κ-(BEDT-TTF)₂I₃ to P2₁· Owing to this symmetry the Cu(NCS)₂ salt is optically active (specific rotatory power is about 230° at 632.8nm) and the Fermi surface based on extended Huckel MO is composed of both a cylindrical closed surface (18% of the first Brillouin zone) and a modulated quasi-one-dimensional open surface.

Tc of the BEDT-TTF-h₈ salt is 10.2–10.4K by four-probe d.c. resistivity measurements and Tc decreases with increasing pressure. An inverse isotope effect was observed in our samples so far measured (more than seven each samples). The deuterated samples showed higher Tc by 0.5K measured by the RF penetration depth measurements. Magnetic susceptibility measurements indicate that the salt is non-ideal class II superconductor and almost 100% of the perfect diamagnetism was observed below 7K. Upper critical field measurements showed a dimensional cross-over-like behavior (3D to 2D) when the external field is parallel to the 2D plane. Hc₂ values at 6K were about 13T and O.5T in the 2D plane and normal to it, respectively. The estimated anisotropy in the GL coherence length from the critical field near Tc is ξbc(0):ξa*(0)=182Å:9.6Å=19:1. The Shubnikov-de Haas signal was observed below 1K and above 8T. The period of 0.0015T^-1 corresponds to the area of extremal orbit of 18% of the first Brillouin zone. The anisotropic nature of the thermoelectric power of the crystal in the 2D plane can be explained on the basis of the complicated Fermi surface. No EPR signal ascribed to Cu²⁺ was observed. The linewidth of the EPR signal of BEDT-TTF increased monotonically with decreasing temperature in contrast to the predictions of the Elliot formula for the spin relaxation in metals. A Korringa relation was observed in ¹H NMR measurements between 77K and 10K. Below 10K a big enhancement of the relaxation rate was observed with a peak at considerably lower temperature than Tc. Anisotropic superconducting gaps were detected by tunneling spectroscopic work. Tc of several BEDT-TTF superconductors were discussed on the basis of the effective volume of one electron for the molecular design of new organic superconductors.