Hatsumi Urayama

Peculiar Critical Field Behaviour in the Recectly Discovered Ambient Pressure Organic Superconductor (BEDT-TTF)2Cu (NCS)2(Tc=10.4K)

An upper critical field measurement in the recently discovered ambient pressure organic superconductor (BEDT-TTF) 2 Cu(NCS) 2 ( T c =10.4K) is reported. Three- to two-dimensional crossover behaviour is observed in the temperature dependence of the parallel critical field. The anisotropy of the coherence length near T c , ξ c /ξ a * , is about 19 and ξ a * (0) is smaller than the lattice spacing along the a * -axis. An unusual temperature dependence of the perpendicular critical field is observed far below T c . It is confirmed that deuterated salt is also an ambient pressure superconductor with a slightly higher T c (11 K).

A new ambient pressure organic superconductor (BEDT-TTF)2Cu(NCS)2 with Tc above 10 K

An ambient pressure superconductivity in (BEDT-TTF)2Cu(NCS)2 is reported. The superconducting critical temperature is the highest among the organic superconductors so far obtained (Tc=10.4 K). The salt prepared using deuterated BEDT-TTF is also an ambient pressure superconductor with a slightly higher Tc (11.0K). The crystal structure analysis and resistivity measurement revealed the highly two-dimensional nature of this salt. The temperature dependence of normal resistivity, superconducting critical field, quantum oscillation of resistivity and so on are reported down to 0.5 K and up to 13.5 T. The superconducting upper critical field shows a peculiar temperature dependence, and the parallel critical field behaviour is ascribed to the dimensional crossover effect. The quantum oscillation is understood as the Shubnikov-de Haas effect, and the possible Fermi surface is presented. The possible superconducting mechanisms are also discussed.

Chemical and physical properties of a new ambient pressure organic superconductor with Tc higher than 10K

Abstract An overview of the chemical and physical properties of a new organic superconductor (BEDT-TTF)2Cu(NCS)2, of which Tc (10.4–11.0K) 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 detected by DSC, TG and DTG. 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 0.5T in the 2D plane and normal to it, respectively. The estimated anisotropy in the GL coherence length from the critical field near Tc is ξ bc (0):ξ a ∗(0)=182 A :9.6 A =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.

NMR relaxation in the organic superconductor (BEDT-TTF)2Cu(NCS)2

Abstract 1H-NMR relaxation rate 1/T1 in the organic superconductor [bis(ethylenedithiolo)tetrathiafulvalene]2Cu(NCS)2 was measured in the temperature range between 0.5 K and room temperature. It was found that 1/T1 exhibits an enormous enhancement below Tc, forming a sharp peak around 4 K (at 3.28 kOe), and then decreases rapidly at lower temperatures. The peak value of 1/T1 is 30 times as large as that at Tc at the field of 3.28 kOe. At higher magnetic field the enhancement tends to be depressed. Above Tc, the Korringa relation T1T=1100 secK is satisfied up to 80 K, and then the contribution of molecular vibrations becomes dominant. Possible mechanisms responsible for the low temperature anomaly are discussed.

Superconductivity and deuteration effect in (BEDT-TTF)2Cu(NCS)2

We report on the critical temperature, deuteration effect on Tc and related experimental results in (BEDT-TTF)2Cu(NCS)2. Tc defined at the midpoint of the normal to superconducting resistance transition ranges from 10.2 to 10.4 K at ambient pressure and at zero field. On the contrary samples using deuterated BEDT-TTF has a little higher Tc of 11.0 K. Therefore we found an inverse isotope effect (ΔTc/Tc=0.06). We confirmed this result by RF penetration measurement. Possible mechanism to raise Tc by the deuteration is discussed considering the electron-phonon interaction mechanism. We stress the importance of the hydrogen-bond coupling between BEDT-TTF molecule and Cu(NCS)2 anions which constitute linear chains.

Anomalous NMR relaxation in an organic superconductor (BEDT-TTF)2Cu(NCS)2

Abstract We report the existence of drastic enhancement of NMR relaxation rate 1/T 1 in the superconducting state of (BEDT-TTF) 2 Cu(NCS) 2 , a recently synthesized organic superconductor. It was found that 1/T 1 exhibits a sharp peak around 4 K, well below the transition temperature T c = 10.4 K. The peak value of 1/T 1 is 30 times as large as that at T c at the external field of 3.28 kOe. At higher magnetic field the enhancement tends to be depressed. Above T c , T 1 T keeps nearly constant up to 77 K. Possible mechanisms responsible for the T 1 anomaly at low temperatures are discussed.

Fermi surface and pressure effect in (BEDT-TTF)2Cu(NCS)2

Abstract We have recently reported on the Shubnikov-de Haas (SdH) effect and the Fermi surface in the title compound. The SdH signal observed corresponds to the cylindrical closed Fermi surface with the area of the extremal orbit, 18 % of the first Brillouin zone. The corresponding Fermi surface can be obtained using the tight-binding band calculation based on the extended Huckel approximation. In this case the validity of the simple theory is confirmed. We report on the pressure effect in the SdH effect. The SdH period decreases as the pressure increases. The preliminary result indicates the decrease in the effective mass. The decrease of the oscillation period gives the same order of compressibility of the crystal as the s-(BEDT-TTF) 2 I 3 salt. The decrease in the effective mass cannot be understood by the simple band theory. The decrease may correspond to the decrease in the electron-photon interaction resulting in the lower superconducting transition temperature.

Low-Temperature Specific Heat of Organic Superconductor κ-(BEDT-TTF)2Cu(NCS)2

Specific heat C of κ-(BEDT-TTF) 2 Cu(NCS) 2 was measured between T c (10.4 K) and 3 K. An anomaly around T c was observed. The specific heat data show unusual bending below T c in the C / T versus T 2 plot. At the lowest temperatures, C obeyed the T 3 law and no residual T -linear contribution was observed.

Crystal and electronic structures and physical properties of Tc=10.4 K superconductor, (BEDT-TTF)2Cu(NCS)2

Abstract (BEDT-TTF)2Cu(NCS)2 was found to be the first ambient pressure organic superconductor with the Tc higher than 10 K. The salt has a layered structure which is composed of donor and anion sheets. The packing pattern of donors of the Cu(NCS)2 salt is nearly analogous to κ-(BEDT-TTF)2I3. In anion layers CuI is trigonal coordinated to two N atoms and one S atom to construct a one-dimensional planar polymer. The valence state of CuI was observed by ESCA at 298 K and ESR at 298-4 K. One broad Lorentzian ESR signal ascribed to a BEDT-TTF cation radical was obtained. The g-values are independent of temperature, while the linewidth shows broadening with lowering temperature even though the resistivity decreases below 90 K. The ESR magnetic susceptibility is constant down to 90 K, which is suggesting Pauli-like paramagnetism. The thermopower is anisotropic in the two-dimensional bc plane, which is originated from the anisotropy of the band structure.

Tunneling Spectroscopic Study on the Superconducting Gap of (BEDT-TTF)2Cu(NCS)2 Crystals

Abstract The superconducting gaps of newly discovered organic superconductor, (BEDT-TTF)2Cu(NCS)2, have been measured with a tunneling spectroscopic method. One of the observed gap data, 4meV, gives 4.5 for the 2Δ kT c (2Δ=4meV and Tc=10.4K) which is a little larger than that of BCS ratio, 3.52. The other spectra observed in different crystals, however, are exhibiting much smaller gap electronic structure. This fact is suggestive of the anisotropic nature of the superconductivity in this crystal.