Nobuo Yamasaki

Superconductivity in Conducting Polymer/Fullerence Composite Doped by Alkali Metal

Superconductivity in poly(3-alkylthiophene)- C60 composite (PAT- C60) is found upon doping by potassium from vapor phase. The superconducting (SC) transition at T C=17 K is detected by superconducting quantum interference device (SQUID), which showed 0.8% of SC fraction for C60 content of 5 mol%. An exceptionally strong low-field microwave absorption (LFMA) implies the existence of granular SC phase. A rather small hysteresis of LFMA indicates the important role of the Josephson weak link network, probably formed between SC Kx C60 clusters separated by conductive Kx PAT barriers. ESR of PAT(C60)y Kx composite shows two types of lines: one from negative polarons P- in PAT chains, and another assigned to Kx C60 clusters and C1-60 radical.

Annealing effect of the superconducting phase of sodium-nitrogen-C60 fulleride, prepared from Na-azide

Abstract By means of low field signal(LFS) of microwave absorption combined with conventional ESR, the evolution of superconducting phase of sodium doped C 60 prepared from sodium azide is studied upon heat treatment. The superconductivity is found to be improved both regarding the superconducting(SC) volume fraction and the critical temperature(Tc) upon low temperature annealing at 200°C. However, the LFS intensity decreased at a rapid rate upon high temperature(350∼400°C) annealing indicating suppression of superconductivity of Na 3 (Na α N β ) y C 60 due to thermally activated decomposition of the spacer cluster Na α N β , which has been tentatively assigned to undecomposed NaN 3 ,(although exact values of α and β are not clear yet). The rate of LFS decrease obeyed Arrhenius relation with the activation energy(Ea)=1.9±0.2eV. The critical temperature Tc is found to decrease smoothly from about 16 K to zero with the rate depending on annealing temperature(T a ), suggesting continuous change of SC composition y, and thus a monotonin decrease of the average lattice constant. Clear correlation between improvement or degradation of the SC phase and the peak-to-peak ESR linewidth at room temperature was observed.

Evolution of superconductivity in Ca doped C60 prepared from azide. Low-field microwave absorption and ESR

Abstract Superconducting (SC) behavior of Ca doped C 60 prepared by calcium azide thermal decomposition is studied by low-field signal (LFS) microwave absorption and conventional ESR. SC phase is obtained for the sample prepared with nominal composition of Ca 6 C 60 and its volume fraction is significantly enlarged by annealing at 350–400°C for 2 h. It is suggested that the SC compound may have two SC phases with slightly different transition temperatures, T C1 (8.2–8.4 K) and T C2 (8.7–8.8 K). The SC 2 phase with higher T C2 is degraded by annealing at 450–500°C and converts to SC 1 . Degradation of SC 2 at high temperature is interpreted to be caused either by (1) thermal reaction of some intercalated Ca with quartz glass and/or (2) dissociation of (CaN) y or N y clusters in the octahedral sites of the Ca 5 (CaN) y C 60 or Ca 5 N y C 60 lattice.

Optical and magnetic properties of polyaniline derivatives having ionic groups

Copolymers of aniline and sulfonated anilines prepared by electrochemical polymerization show pH-dependent absorption spectra such as for the pure polyaniline, except for the difference in critical pH value of the chromic phenomenon. Dependences of electron spin resonance (ESR) spectra of the copolymers corresponding to the polaronic state on solution pH also suggests that protonation and deprotonation of the nitrogen atom occurs in a pH range different from that of polyaniline, and is attributed to effects of the self-doping of sulfonic acid group in the protonated form. ESR spectra of the copolymers show anomalous temperature dependence which is similar to that of antiferromagnetic material. Copolymers of aniline with sulfonated aminonaphthalenes also exhibit antiferromagnetic behavior.

A new metastable superconductor Li2Na1NyC60 prepared from azides

Abstract A new metastable superconductor with nominal composition of Li 2 Na 1 N y C 60 ( y unknown) is prepared by thermal decomposition of lithium azide (LiN 3 ) and sodium azide (NaN 3 ). It is suggested that the superconducting phase in this compound is created by intercalation of the neutral spacer such as NaN 3 . The supeconductivity is examined by a low-field signal of microwave absorption (LFS) and by SQUID. The critical temperature is estimated to be 12 K from SQUID and 14 K from LFS measurement, and the shielding fraction is over 9% at 5 K. Superconducting parameters, such as critical temperature, upper and lower critical fields, coherence length and penetration depth are evaluated and found to be distinct from those in other fulleride superconductors.

Unique pH dependent optical and magnetic properties of self doping type copolymers of aniline and aminonaphthalene derivatives with sulfonic groups

Summary form only given. Electrochemical and chemical copolymerization of aniline and aminonaphthalene derivatives having one or two sulfonic acid groups resulted in novel self doping type copolymers. Absorption spectrum of the resulting copolymers showed unique pH dependence as in the case of polyanifine but at different pH region. Deprotonation and protonation reactions of the nitrogen atom in polyaniline occur at pH about 3-4 but in the case of the self-doping copolymers they occur at pH 8-10. ESR measurements which indicated formation of polarons upon protonation also showed similar pH dependence. Susceptibility of some copolymers at low temperature showed marked decrease upon cooling down after indicating maximum value. This antiferromagnetic behavior was observed only after treatment with strong acid but not after treatments with neutral and basic solutions. Effects of acids, temperature, and polymerization conditions on the optical and magnetic properties have also been studied.

Electron spin resonance spectra of KxC60 films prepared by electrochemical doping

We have found that K-doped C60 films can be prepared by the electrochemical doping technique using a solid electrolyte. From these films, we obtained four electron spin resonance (ESR) peaks at a temperature below 50 K. The intensity of one peak in the high field region was extremely sensitive to slight changes in temperature below 10 K and in microwave power. Spin-lattice relaxation time gives a major effect on the power dependence of the increasing rate of the intensity. The ESR center giving this anomalous peak is tentatively interpreted to be related to the superconducting state.

Superconductivity of CaxC60 prepared by thermal decomposition of azide: ESR and LFS microwave absorption study

Summary form only given. We have prepared C/sub ax/C/sub 60/ by thermal decomposition of CaN6 and studied the X-evolution of its superconducting properties by low field (LFS) microwave absorption and conventional ESR. Superconducting phase is found at x=5 and also x=6 (with lower fraction) with Tc = 8.4-8.8K. The results of the annealing at different temperature (300-450 /spl deg/C) and of aging effect on the intensity of LFS (and hence the amount of superconducting fraction) and ESR g-value and linewidths are presented. Preliminary results on the C/sub 13/ isotope effect on superconducting T/sub c/ in C/sub a5/C/sub 60/ are also discussed.

Thermal stability of sodium-nitrogen-C/sub 60/ superconductor prepared from Na-AZIDE

Abstract Thermal stability of superconducting sodium-nitrogen-C 60 compound prepared from sodium azide has been studied after high temperature treatment by means of low-field signal of microwave absorption(LFS). The LFS amplitude decreased in a short time by high temperature (350∼400°C) annealing. The critical temperature Tc decreased smoothly from about 16K down to zero, indicating continuous change of superconducting composition and the average lattice constant.

Electrochemical and chemical preparations of polyaniline derivatives having ionic groups and their anomalous optical and magnetic properties

Summary form only given. Copolymerizations of aniline, its derivatives and aminonaphthalene derivatives have been achieved by electrochemical and chemical methods and optical and magnetic properties of the copolymers have been studied. Electrochemical copolymerization of aniline with m- and p-sulfonated anilines in aqueous acidic electrolytes resulted in insoluble polymer films on the indium-tin-oxide anode. The copolymers showed unique change in the absorption spectra with the pH value of immersed aqueous solution as in the case of pure polyaniline. The ESR signal also changed with pH value and these changes of optical and magnetic properties can be attributed to protonation and deprotonation of nitrogen atoms in the main chain as in the case of polyaniline. Antiferromagnetic behaviour of these copolymers was observed in ESR measurements at low temperature region, depending on pH value of aqueous solution of pre-treated. Similar antiferromagnetic behaviour was also observed in some copolymers of aniline with sulfonated aminonaphthalene but not in pure polyaniline.