K. Kanoda

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.

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.

Superconductivity and magnetism of Bi2Sr2CaCu2Ox Effect of quenching and pulverization

Abstract The effect of quenching on Bi 2 Sr 2 CaCu 2 O x (BSCCO) has been investigated. Quenching enhances the critical temperature T c . The hole concentration n h (∼ 5 × 10 21 cm −3 ) decreases as the quenching temperature T q increases. The normal-state static susceptibility χ DC (∼ 5 × 10 −7 emu/g) of the BSCCO pellet is almost independent of temperature but its magnitude decreases with the increase of T q . Thermogravimetry (TG) analysis shows that the change in the oxygen content is very small when the sample is heated. The Cu nuclear quadrupole resonance (NQR) spectra are very broad. In addition, we have observed a large Curie-Weiss term in χ DC and χ ESR of powdered samples. From the Curie constant, the number of localized Cu 2+ spins is estimated to reach 30% of total Cu atoms. This component almost disappears by re-annealing. In all cases, χ ESR has almost the same magnitude as χ DC , in marked contrast with YBa 2 Cu 3 O 7−δ . The origin of the normal-state susceptibility is discussed.

Inner structure of the low-field SDW state of (TMTSF)2PF6

Abstract The observation of relaxation anomalies within the SDW phase of (TMTSF) 2 PF 6 has suggested the existence of successive transitions below the SDW transition. The low-field SDW phase seemed to be divided into three or more sub-phases. The ‘phase boundaries’ were determined by the extension of 1 H nuclear spin-lattice relaxation rate measurements under pressure. In the temperature dependence of resistivity, slight breaks in the activation plot were found at the boundaries. Possible mechanisms to make the inner structure of the SDW phase are discussed.

EPR studies of organic conductors (DMET)2X

Abstract We have performed EPR studies on the family of organic conductors, (DMET)2X, [X= PF6, BF4, Au(CN)2, I3 and AuBr2]. In spite of large difference in conductive behaviors, the four salts, except the AuBr2 salt, show similar magnitude and temperature-dependence of the spin susceptibility as a whole. Below 20–25 K, the PF6, BF4 and Au(CN)2 salts exhibit an abrupt decrease of the susceptibility, suggesting some phase transitions. The AuBr2 salt shows temperature-independent spin susceptibility, greater than those of other salts, and an anomalous temperature dependence of the linewidth. The physical implication of the results are discussed.

Magnetism of DCNQI-Cu salts

Abstract Magnetic properties of (DMe-DCNQI)2Cu, (DMeO-DCNQI)2Cu and (DBr-DCNQI)2Cu were investigated by 1H-NMR, ESR and magnetic susceptibility measurements. 1H-T1−1 was found to exhibit a step-like enhancement at the metal-insulator (M-I) transitions of DBr-salt at ambient pressure and of DMe- and DMeO-salts under pressure. This reflects the appearance of localized Cu2+ spins at the M-I transition. An antiferromagnetic (AF) ordering was observed in DBr- (at ambient pressure) and DMe-salts (under pressure). In both salts, 1H-NMR relaxation rates, T 1 −1 , T 2 ∗−1 and T 2 −1 , indicate the onset of inhomogeneous local field at the transition temperature, TN: The lineshape below TN is well explained by the assumption that the magnetic Cu2+-sites with μB form a regular three-fold lattice along the c-axis. In the DMe-salt, the pressure dependence of TN was determined. It was shown that the enhancement of static susceptibility in the metallic state of DMe-salt was due to the insulating domain induced by some stress within the sample.

Observation of the coherence peak of 1H-NMR relaxation rate in the superconducting state of (MDT-TTF)2Aul2

Abstract We have investigated the superconductivity in the organic conductor, (MDT-TTF)2Aul2 (Tc = 4.1 K) from the temperature dependence of 1H nuclear relaxation rate, 1 T 1 , measured by the field cycling method. The nuclear relaxation above 3 K is well characterized by a single relaxation time for nearly two decades. In the normal state, the standard Korringa relaxation holds. 1 T 1 just below Tc showed a clear enhancement, followed by a falling-off at lower temperatures. This behavior is just as predicted in the BCS theory (coherence peak) and implies that this material is a conventional superconductor with an isotropic gap. The decay of the nuclear magnetization below 3 K appeared non-sinle exponential. This is explained as the effect of trapped vortices.

Organic superconductors, (DMET)2X

Abstract The physical properties of (DMET)2X are summarized. DMET salts are classified into five groups on the basis of the temperature dependence of electrical conductivities. Among them, seven salts show superconductivities. The crystal structures of typical DMET salts within each group are described. These salts have the crystal structures characteristic to each group which are reflected in their physical properties. The phase diagram of the first DMET superconductor, (DMET)2Au(CN)2 is also presented.

EPR and static susceptibility of YBa2Cu3O7−x with various oxygen content

Oxygen-deficiency dependent magnetism of YBa 2 Cu 3 O 7−x has been investigated by means of static susceptibility and EPR measurements. The static susceptibility for x 2 planes.

Anomalous spin dynamics of conduction electrons in the organic conductor, [dimethyl(ethylenedithio)diselenadithiafulvalene]2AuBr2 [(DMET)2AuBr2]

Abstract Conduction electron spin resonance study on the organic conductor (DMET)2AuBr2 was performed in the temperature range between 10 and 300 K. The linewidth is exceptionally large compared with the usual organic conductors and has an anomalous temperature dependence: with decreasing temperature, the linewidth increases gradually, forms a broad peak around 150 K, and then decreases rapidly. The peak value is as large as 1000 G. The temperature of the linewidth maximum corresponds well to that of resistivity maximum. On the other hand, the spin susceptibility is insensitive to temperature. The anomaly in the linewidth and the resistivity is inferred to be dynamical in origin.