M. Weger

Competition Between Sdw and Superconductivity in (TMTSF)2X Compounds

Abstract The phase diagram of a quasi-one-dimensional metal with retarded and nonretarded forward and backward scattering is derived. It is shown that singly charged acceptor molecules produce a potential along the conducting chains which leads to umklapp scattering. This umklapp scattering depends strongly on the phonon frequency and can explain the crossover from spin density waves to superconductivity as observed in the (TMTSF)2X compounds. The relevance of recent magnetoresistance measurements to the proposed model is discussed. We have shown that the discovery of the (TMTSF)2X Compounds requires some modifications in the “g-ology” picture which accompanied the field of one-dimensional metals since the beginning. In particular, the inclusion of retardation effects and umklapp scattering seems to be essential. When these are included, a simple and natural description of the behaviour of the (TMTSF)2X compounds is obtained.

Crystalline electric field levels of U in UPd3AN inelastic neutron scattering study

Abstract Experimental evidence is presented of an unambiguous observation ny neutron scattering of energy-localized excited crystalline electric-field states of an actinide ion in a metallic compound. Energy transfer lines were found in an inelastic neutron-scattering experiment using a UPd3 powder sample. These lines exhibit the following features: (a) The energies of the lines are Q independent. (b) The Q dependence of the line intensities concurs with the magnetic form factor of U4+. (c) The intensity of the line due to transitions from the ground state decreases as the temperature increases.

Role of the fermi surface in the superconductivit y of A15 compounds

Abstract The Fermi surface of two A15 compounds, Nb 3 Ir and Cr 3 Si, has been determined by positron annihilation using both 1D and 2D angular correlation techniques. These results are compared with those obtained by linear muffin-tin orbitals band structure calculations. We point out some differences in the Fermi surface topology between high- and low- T c A15 materials.

New Nuclear Spin Relaxation Process in Two Dimensional Metals in High Magnetic Fields

Abstract An unusual linear dependence of the nuclear relaxation rate upon the magnetic field is observed in the incommensurate linear chain compound Hg3-δAsF6. It is shown that the unique two-dimensional character of the conduction electrons motion in this metal enhances the relaxation rate in strong magnetic fields. The incommensurate lattice structure gives rise to very high cyclotron frequency in these fields and makes it possible to observe the enhancement.

Uniformly attractive pairing — interaction in an electronic mechanism of superconductivity

Abstract A full calculation is performed for the interaction between states of carrier pairs with a model in which the conduction band is coupled to another lying entirely above (electronic conduction) or below (hole conduction) the Fermi energy. The coupling proves to be attractive, leading to pair-stabilization, for the full range of the conduction band. Thus our calculation justifies the model as a framework for high temperature superconductivity.

Superconductivity in the Mercury-Chain Compound Hg3-δAsF6

Studies for the zero-field diamagnetic susceptibility of the mercury-chain compound Hg3-δAsF6 is reviewed, using the anisotropic Ginzburg-Landau (GL) theory, with a model BCS Hamiltonian with only interchain coupling.

One dimensional BCS Hamiltonian in Hg3−δAsF6

Abstract The structure of the linear chain compound Hg3−δAsF6 gives rise to an unusual electron-phonon interaction which leads to a reduced BCS Hamiltonian. The effective electron-electron interaction exists only when the two electrons of a Cooper-pair jump together from one chain to the next. The calculation on the paraconductivity due to this unusual Hamiltonian is briefly reviewed and the diamagnetic susceptibility is calculated. When the two-dimensional flucturation of Cooper-pairs along the a- and b-chains is assumed, the susceptibility χc for a field H in the c-direction is larger than χa,b (H in the a-b plane).