Guy Gusman

Electrical properties of poly(2-vinylpyridine)-iodine films

Abstract The d.c. electrical conductivity of thick films of poly(2-vinylpyridine)-iodine is measured as a function of iodine concentration and temperature. The results are interpreted in terms of electronic hopping between ionic centers dispersed at random in the polymer.

13C NMR spectroscopy of chloroform-solvated-C60

Abstract By means of pulsed Nuclear Magnetic Resonance and Magic Angle Spinning we have studied crystalline C60 × 0.2 C70 × 1.5(CHCl3). We present data of the spin lattice relaxation (T1) of 13C nuclei as a function of temperature between 140 and 310 K. Their analysis has been made in terms of the chemical shift anisotropy model. Our results show that the high temperature free rotation of C60 molecules is not suppressed and that there is no structural phase transition near 260 K as observed in pure crystals of C60.

Spin-lattice relaxation by solitons in 13C enriched iodine doped trans-poly acetylene

Abstract An experimental study of NMR spin-lattice relaxation of 13 C in enriched, iodine doped, polyacetylene is presented together with a theoretical analysis of the results based on the existence of solitons in this system. The good agreement with this model and the numerical values obtained here and those measured by different experimental techniques support firmly the applicability of the soliton picture to I 2 doped polyacetylene.

ROLE OF THE ION CORE IN THE MECHANISM OF SUPERCONDUCTIVITY

Abstract It is shown that if the primary contribution to the bulk modulus of a metal is due to interactions between the ion cores rather than the bulk modulus of the conduction electrons then the effective electron-electron interaction at low frequencies is repulsive. This is in contrast to the usual attractive interaction due to phonon mediated electron-electron interactions. The result is that superconductivity is suppressed for sufficiently strong core interactions. We find that, if at least two thirds of the bulk modulus is due to the core, superconductivity is suppressed. This condition is easily satisfied by the noble metals, so that our mechanism could very possibly explain the absence of superconductivity in these substances.

Benzene-solvated C60:1H nuclear spin-lattice magnetic relaxation

Abstract Crystalline C 60 .4C 6 H 6 was studied by NMR. Using a saturation-recovery method, the spin-lattice relaxation time, T 1 , of the protons of benzene was measured as a function of temperature between 4 K and 300 K. The 1 H data were fitted by a model where four correlation times are present. They correspond to thermally activated processes which follow an Arrhenius law: the activation energies are, respectively, 60 K, 279 K, 1365 K and 2945 K. All constituents of our system are extremely mobile and no structural phase transitions are observed near 260 K and 90 K as in the case of pure C 60 crystals. We have also studied the temperature dependence of the 1 H NMR linewidth which narrows with increasing temperature.

Non-exponential nuclear magnetic relaxation by solitary-waves in K2Pt(CN)4Cl0.303.2H2O

Abstract Pulsed NMR technique is used to study the 195Pt nuclear spin relaxation in KCP (Cl) in the temperature range 44 ⩽ T ⩽ 112 K where solitary waves are expected to be the main excitations of this quasi one-dimensional system. A theoretical explanation of the strongly non-exponential magnetization recovery is presented based on hopping of solitons in connection with NMR relaxation.

NMR spin-lattice relaxation of 1H in iodine doped trans-polyacetylene

Abstract A study of NMR spin-lattice relaxation of 1 H in iodine doped polyacetylene vs temperature is presented. It is shown the doped polymer differs from the undoped one essentially by the presence of a new mechanism of relaxation proportional to kT . The theoretical description of our experimental data vs temperature is in complete agreement with the RF field frequency study made by Nechtshein et al . at room temperature.

13C and 1H nuclear spin lattice magnetic relaxation in undoped polyacetylene

Abstract Pulsed NMR spin lattice relaxation measurements on 13 C and 1 H nuclei in undoped trans -polyacetylene have been carried out between 6 and 295 K. The results indicate that the spin lattice relaxation is due to equilibrium fluctuations of the orientational order parameter for the protons while the carbon relaxation can be attributed to their coupling to paramagnetic impurities. In this temperature range no contribution of solitons has been detected in the relaxation mechanisms.

Mass waves and the metal-semiconductor transition in one-dimensional systems of the type K2Pt(CN)4Br0.3 2.3H2O

Abstract A coupling mechanism of the chain electrons in partially oxydized Tetracyanoplatinates (TCNP) induced by the tunneling of ions in the intermediate medium can explain the existence of a Peierls instability in these compounds. Excitations of mass waves play the role of soft phonons inducing the electronic transition.

Dirty superconductors and isotope effect

Abstract The isotope effect in transition metal superconductors is reexamined. The approximate calculations of Garland are shown to be qualitatively justifiable in the limit of sufficient dirtiness, i.e. ωD−1 ∼ τ ⪢ ϵF−1. Here ωD is the Debye frequency, ϵF the Fermi energy of electrons and τ the relaxation time of electrons due to impurity scattering. Furthermore a necessary condition for Garlands calculation to be valid is that the effective mass ratio m d ∗ m s ∗ be not greatly in excess of unity. If this condition is violated, which appears to be the case in such alloys as Nb3Sn, then a modification ensues wherein the cut-off of Garland corresponding to the band width is replaced by the d band plasma frequency.