N. Kinoshita

Transport properties of organic conductor (BEDT-TTF)2KHg(SCN)4: I. Resistance and magnetoresistance anomaly

Abstract The electrical resistance of the single crystalline organic salt (BEDT-TTF) 2 KHg(SCN) 4 has been measured at temperatures down to 0.5 K and under magnetic fields up to 13 T. The zero-field resistance decreases almost monotonously with decreasing temperature down to 9–10 K and then exhibits a step-like anomaly accompanying the 30–40% resistance drop around below T a =8 K. Under fields perpendicular to the conducting a-c plane, the resistance below T a is so enhanced that an upturn in the temperature-dependent resistance appears and the resistance, for example, at 13 T and 0.5 K, becomes higher by a factor of 12 than the resistance at T a and H =13 T. This anomalous magnetoresistance is never observed under fields precisely aligned parallel to the a-c plane. The dc magnetic susceptibility shows a broad maximum at T a , which seems to be an evidence that weak anti-ferromagnetic correlations like spin-density-wave ordering takes place among p (п) electrons itinerating in the metallic states.

Electron Spin Resonance in Phosphorus Doped Silicon at Low Temperatures

Effects of exchange and motion of electrons on electron spin resonance spectrum of phosphorus doped Si with concentration of 3×10 16 cm -3 ∼3×10 19 cm -3 were investigated at liquid helium temperatures and at about 9300 Mc/sec. At lower concentrations, the intensity of the exchange coupled resonance line was measured as functions of temperature and donor concentration. Electrons are antiferromagnetically coupled, and the exchange energy increases with increasing donor concentration. At higher concentrations, a sharp resonance line was observed. The temperature dependences of the line width and intensity were measured as a function of impurity concentration. The sharp line originates from localized electrons rather than from non-localized electrons contributing to the electronic conduction. At the region of the highest concentrations, where an impurity band is formed, the line width is independent of temperature and becomes larger with increasing concentration. Discussions are given in relation to the impu...

Transport properties of organic conductor (BEDT-TTF)2KHg(SCN)4: II. Shubnikov-de Haas oscillations and spin-splitting effect

Abstract The magnetoresistance, the Shubnikov-de Haas (SdH) effect and their orientation dependence of an organic conductor (BEDT-TTF)2KHg(SCN)4 have been measured at temperatures down to 0.5 K and under magnetic fields up to 13 T. The SdH frequency F=670T, the cyclotron mass mc=1.4m0 and the Dingle temperature TD=1.6K are obtained when the fields are applied perpendicular to the conducting a-c plane. The dependence of F on the angle θ between the field and the b∗-axis (⊥ a-c plane), which deviates slightly from cosθ, indicates the presence of the weakly warped cylindrical Fermi surface with an interlayer transfer integral estimated to be about 1 meV. Below 1 K and above 10 T, the spin-splitting effect is clearly observed and the direct analysis of the splitting oscillations results in the series of the g-factor; g=1.4n±0.6 (n is an odd number). The rigorous g-factor of 2.0 (n=1, + sign), being consistent with ESR measurements, is obtained.

Temperature dependence of hall effect in κ-(BEDT-TTF)2Cu(NCS)2

Abstract The temperature dependence of the Hall coefficient( R H ) of the organic superconductor κ-(BEDT-TTF) 2 Cu(NCS) 2 is studied. The value of R H is positive and slowly increases with decreasing temperature down to 60 K. No remarkable anomaly is seen in R H associated with the anomalous peak in resistivity around 110 K. Below 60 K, R H starts to increase abruptly. This temperature coincides with the inflection point in the temperature variation of resistivity and with various phenomena previously reported. With increasing pressure, the temperature of the inflection point increases. The relation between superconductivity and the new anomalous phase is discussed.

Non-linear conductivity in the spin density wave phase of (TMTSF)2ClO4

Abstract Electrical conductivity was measured in the spin-density wave phase of (TMTSF)2ClO4, which is induced in zero magnetic field when ordering of ClO4 ions is suppressed by rapid quenching. The conductivity along the one-dimensional axis increases above a threshold field, in the range of 10–40mV cm-1. The threshold field increases slightly with increasing temperature. The conductivity increase is presumably assigned to the sliding motion of the spin-density wave, as suggested by Tomic et al. for (TMTSF)2NO3. Significant sample dependence is observed in threshold field-temperature and conductance-field relations; important effects of the interaction between the deformable spin-density wave and pinning centers are inferred.

Cyclotron Mass and Dingle Temperature of Conduction Electrons Moving in Layered Planes of Organic Superconductors: β-(BEDT-TTF)2IBr2, β-(BEDT-TTF)2I3 and κ-(BEDT-TTF)2Cu(NCS)2

The cyclotron mass m c and the Dingle temperature T D in the layered planes of the organic superconductors are determined by the Shubnikov-de Haas effect at ambient pressure; m c =0.4 m 0 and T D =7–8 K, for β-(BEDT-TTF) 2 IBr 2 ( T c =2 K), and (0.4–0.5) m 0 and 11 K for β-(BEDT-TTF) 2 I 3 (=1 K), and 3.5 m 0 and (0.4±0.1) K for κ-(BEDT-TTF) 2 Cu(NCS) 2 ( T c =11 K). A systematic difference is clarified in m c and T D between these salts, suggestive of the origin of different T c s among these salts.

Narrow band noise in SDW sliding

Narrow band noise was observed in the spin-density-wave (SDW) phase of quenched (TMTSF)2ClO4, when electric current larger than the threshold for non-ohmicity was passed parallel to the one-dimensional axis. The noise frequency F increases linearly with the excess current density JSDW with a slope FJSDW = 240 kHz (Acm-2)-1. It is confirmed that the non-linear excess current is carried by the SDW condensate and that the narrow band noise is generated by the sliding motion of the SDW.

Critical Field Anisotropy in an Organic Superconductor β-(BEDT-TTF)2IBr2

A measurement of the anisotropy of the upper critical field H c2 in an organic surerconductor β-(BEDT-TTF) 2 I 3 is reported. A large anisotropy is observed with high critical fields when the magnetic field is applied in the crystallographic a - b plane and a low critical field in the perpendicular direction. ( H c 2 a =9.7 kOe, H c 2 b ′ =9.3 kOe, H c 2 c * =460 Oe at 0.5 K) The result is expressed in terms of the anisotropic coherence lengths and transfer integrals, winch corroborate the two dimensional character of the electronic structure in this organic superconducting material.

Direct Observation of Spin-Splitting of the Shubnikov- de Haas Oscillations in a Quasi-Two-Dimensional Organic Conductor (BEDT-TTF)2KHg(SCN)4

A clear splitting in the Shubnikov-de Haas oscillation has been directly observed for the first time in a quasi-two-dimensional (2D) organic conductor (BEDT-TTF) 2 KHg(SCN) 4 . Analysis of the oscillations yields a 2D nature of the cylindrical Fermi surface, corresponding to 16% of the first Brillouin zone, with a small warping along the direction normal to the 2D layer. The condition for observation of spin-splitting and the evolution of the splitting pattern with a tilted magnetic field are discussed.

Electron Spin Resonance and Electric Resistance Anomalyof (BEDT-TTF)2RbHg(SCN)4

An anomalous behavior was found in the temperature dependence of the ESR spectrum and the electric resistance of (BEDT-TTF) 2 RbHg(SCN) 4 below 20 K. The anomaly was compared with that of the isostructural salt [(BEDT-TTF) 2 KHg(SCN) 4 ] and based on the similarity of the anomaly between the Rb and the K salts, the possibility of the coexistence of a metallic state with a magnetic ordered state at low temperatures was pointed out.