A. Ugawa

Evolution of superconductivity of KxC60 upon K-doping : microwave low-field signal and ESR study

Abstract By means of low magnetic-field micowave absorption (Low-Field Signal) and conventional ESR, we have studied the evolution of superconductivity in bulk KxC60 dependent on the variation of K- doping temperature and time. The first LFS as an occurrence of superconductivity is observed in a doping time as short as td = 0.5 h with a discrete Tc ∼ 19 K, and the volume of the superconducting phase increases on further dopings, while Tc changes slightly. The sample in a superconducting state exhibits strong ESR absorptions, which indicates the presence of a non-superconducting metallic phase coexisting with a superconducting phase in inhomogeneously doped KxC60. These phases are closely correlated and disappear at the same time by overdoping.

Structure and superconductivity of single crystalline C60

Abstract We have succeeded in synthesis of C 60 single crsytals with orthorhombic lattice structure contrary to the known cubic form. The structure of the orthorhombic form was confirmed by a four-circle X-ray diffractometer. The intense microwave absorption signal in low magnetic field was detected on the K-doped C 60 single crystals with the onset of superconductivity at Tc = 17.7 K.

Polarized Reflectance Spectra of DCNQI Salts

Abstract Temperature dependence of the polarized reflectance spectra have been measured on the single crystals of (DMe-DCNQI)2Ag, (DMe-DCNQI)2Na, (MeBr-DCNQI)2Cu, and (DMe-DCNQI)2−Cu (DCNQI stands for dicyanoquinonediimine). The optical spectra evidence one-dimensional band structures of Ag- and Na-salts and anisotropic three-dimensional ones of Cu-salts. The dimensionality crossover is found in (MeBr-DCNQI)2Cu around the metal-insulator phase transition.

Reflectance spectra of β -, θ -, and κ-(BEDT-TTF)2I3, and β″- and β′-(BEDT-TTF)2ICl2: Relation between the inter-band transition and the dimeric structure

Abstract The polarized reflectance spectra of the single crystals of the title compounds were measured at room and low temperatures. The correlation was found between the appearance of the extra electronic transitions overlapped on the Drude-tail and the dimeric arrangement of BEDT-TTF. From this relation, this extra transition was assigned to the inter-band transition between the split bands due to the strong dimeric interaction. The Drude parameters were determined by analyzing the low-temperature spectra taking account of the inter-band transition.

Microwave low-field absorption and ESR in Kx(C60)1-y(C70)y and KxC70

Abstract Superconductivity (SC) in K-doped molecular alloys of C 60 and C 70 are studied by means of the nonresonant microwave low-field signal (LFS) and conventional ESR, and a LFS-search for SC in K x C 70 is performed. The critical temperature of K x (C 60 /C 70 ) determined at the onset of hysteretic LFS, decreases to T c =16K (at about 10% C 70 ), and T c =10K (at about 25% C 70 ), compared with T c =19K in pure K x C 60 suggesting the decrease of electron-phonon coupling. A strong multicomponent g=2 ESR signal appears and coexists with LFS at low T for SC composition of K x (C 60 /C 70 ) indicating presence of a non-SC metallic phase. At overdoping LFS disappears and a single ESR line is enormously broadened to Δ H pp (300K) = 80G. In K x C 70 doped under the same conditions the LFS is not detected, indicating absence of SC above 5K, while a narrow ESR single line appears.

Low energy electronic excitations and fano resonance in K doped C60 from Raman scattering excited at 1.16 eV

Abstract We present a Raman scattering study of pristine and K doped C 60 at various doping levels by exciting in the near-IR at 1.16 eV. The normal metallic state of K 3 C 60 is characterized by a broad scattering background and by the resonance of low energy phonons in the range of 250–500 cm −1 . We assign the broad background to an electronic Raman scattering due to low energy electronic excitations. This spectral feature is indicative of an anomalous normal state behaviour and is similar to the case of high temperature ceramic superconductors. In the overdoped K 6 C 60 the squashing mode at 278 cm −1 shows a Fano resonance with the electronic scattering associated with localized electronic excitations which are characteristic of isolated regions of K 3 C 60 into the matrix of K 6 C 60 as a result of inhomogeneous doping. The Fano resonance indicates a specific electron-phonon coupling of this Jahn-Teller mode with low energy excitations and suggests that the symmetry of this electronic excitation is h g (i.e. the same of the coupled phonon mode). We discuss the nature of the anomalous electronic Raman scattering in terms of scattering from low energy excitations involving a low lying singlet band resulting from electron correlation and/or dynamical J-T distortion caused by the squashing mode.

d-π interaction in conducting phthalocyaninatocobalt hexafluoroarsenate, CoPc(AsF6)0.5

Abstract The spectroscopic study of semiconductive CoPc(AsF6)0.5 evidences the ligand-centered oxidation. The possible origin of the semiconductive band gap is related to the magnetic interaction between the itinerant π-electrons on the ligand channel and the unpaired d-electrons interacting antiferromagnetically within the Co2+ channel.

Crystal structure and polarized reflectance spectra of α′-(bis(ethylenedithio)-tetrathiafulvalenium)2-bromoiodoaurate, α′-(BEDT-TTF)2IAuBr

Abstract A single crystal of α′-(BEDT-TTF) 2 IAuBr was synthesized and the crystal structure was determined by X-ray analysis. The structure was found to be isostructural to α′-(BEDT-TTF) 2 AuBr 2 , having the characteristics of α′-type salts in that the donor molecules are stacked to form a dimerized chain. The electronic structure of α′-type salts was proved to be one-dimensional along the stacking direction from the analysis of the polarized reflectance spectra obtained by use of the microspectroscopic technique.

Relation Between the Dimen-sionality of Electronic Structure and the Correlation Effect in (BEDT-TTF)2 X System

Abstract The optical properties of (BEDT-TTF)2X (α ′ −, β-, β ′ −, β ′′ −, δ -, and κ -types) have been studied to elucidate the relationship between the crystal structure and the electronic structure in these charge-transfer salts. Although there is a structurally two-dimensional network of mutually interacting BEDT-TTF molecules in the series compounds, the two-dimensional character of the electronic structure markedly varies depending on the lattice type. The reflectance spectra are analyzed by the Drude-Lorentz model to describe the conduction band in terms of the effective mass approximation. The results indicate that the correlation effect strongly appears when the system is of one- dimensional character, and that the screening of electron correlation becomes more efficient on increasing the dimensionality of electronic structure.

Physical properties of ß″-(BEDT-TTF)2ICl2

Abstract A single crystal of s″-(BEDT-TTF) 2 ICl 2 was synthesized electrochemically, and the crystal structure was determined by X-ray analysis. This s″-type crystal was characterized by means of the electrical resistivity, electron spin resonance, X-ray diffraction, and polarized reflectance spectrum. The temperature dependence of the electrical resistivity which was proportional to T down to 10 K shows a typical metallic behavior. The ESR spectrum exhibited a Dysonian line-shape. The intensity of the spin susceptibility abruptly decreased at 17 K, although the electrical resistivity and the Bragg diffraction intensity showed no discontinuous change: this indicates the occurrence of a magnetic phase transition. The reflectance spectrum for the //c polarization showed a Drude-like dispersion already at room temperature. The anisotropic two-dimensional electronic structure was examined by the reflectance spectrum analysis.