K. Oshima

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).

Electrical properties and crystal structures of mercury (II) thiocyanate salts based upon BEDTTTF with Li+, K+, NH+4, Rb+, and Cs+

Abstract A new series of BEDTTTF salts with a cation (Li + , K + , NH + 4 , Rb + , or Cs + ) as well as an anion [Hg(SCN) 4 ] 2- , was prepared. The crystal structures and the electrical properties were investigated with changing the cation from Li + (0.73 A) to Cs + (1.70 A). The superconductivity of (BEDTTTF) 2 [NH 4 Hg(SCN) 4 ] was confirmed at 0.8 K by the electrical transport measurements.

Novel Pressure-Induced 2k F CDW State in Organic Low-Dimensional Compound θ-(BEDT-TTF) 2CsCo(SCN) 4

A band structure calculation based on the X-ray structure analysis reveals that pressure raises quasi one-dimensionality of the Fermi surface of the title compound. A 2 k F lattice modulation is ob...

g-Shift in Low Dimensional Antiferromagnets at Low Temperatures

Anomalous g -shifts in low dimensional antiferromagnets, copper benzoate Cu(C 6 H 5 COO) 2 3H 2 O and copper tetra-amine sulfate Cu(NH 3 ) 4 SO 4 H 2 O (CTS) were observed mainly below 1 K. These compounds show positive g -shifts as temperature decreases. The results in copper benzoate are qualitatively explained by the theory recently developed by Nagata who introduced the one-dimensional short range order effect between inequivalent spins. The shifts in CTS are discussed from the two-dimensional point of view.

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.

Shubnikov-de Haas Effect in (TMTSF)2ClO4

Angular dependence of the transverse magnetoresistance in (TMTSF) 2 ClO 4 was measured in a -plane down to 0.47 K and H up to 13 T. Two types of Shubnikov-de Haas (SdH) oscillations were observed for the angle |θ| between H and c * -axis less than 60 degrees: One with period 0.015 cos θ T -1 , comparable to that found in (TMTSF) 2 PF 6 , and newly observed one with (0.0036 cos θ+0.0002) T -1 . If these correspond to electron- and hole Fermi surfaces respectively, they support the compensated as well as the quasi-two dimensional feature of this organic metal.

Structural modulation in θ-(BEDT-TTF)2CsM′(SCN)4[M′=Co, Zn]

Abstract X-ray diffuse scattering study reveals presence of diffuse sheets directed along both the 2a*+ c* and 2a* − c* directions in θ-(BEDT-TTF)2CsM′(SCN)4 [M′=Co, Zn] at room temperature. These diffuse streaks condensed into two diffuse rods at the q1=(2/3, k, 1/3) and q2=(0, k, 1/2) reduced reciprocal position below 120K. Around 50K when the resistivity begins to increase, an enhancement of the q2 intensity is observed.

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.

X-ray study on a phase transition of U4O9

Abstract Some features of the transition of U 4 O 9 were investigated by means of X-ray diffractometry. Transition temperature of the U 4 O 9 phase varies from 65 to 96°C with O/U ratios and, at the transition, a notable increase in the intensity of superlattice reflection of U 4 O 9−y with temperature was observed.

Antiferromagnetic Resonance in Copper Benzoate below 1 K

A linear chain antiferromagnet copper benzoate Cu(C 6 H 5 COO) 2 ·3H 2 O was investigated by antiferromagnetic resonance. The Neel temperature was determined as 0.76 K, A clear coexistence of para- and antiferromagnetic resonances was found. The antiferromagnetic resonance was analyzed by introducing the isotropic exchange, symmetric and antisymmetric anisotropic exchange, dipolar and dissimilar spin Zeeman energies. A large spin contraction due to one-dimensional spin fluctuation was found.