Exchange interaction and Fano resonances in diatomic molecular systems

We propose a mechanism to use scanning tunneling microscopy (STM) for direct measurements of the two-electron singlet-triplet exchange splitting $J$ in diatomic molecular systems, unsing the coupling between the molecule and the substrate electrons. The different pathways for electrons lead to interference effects and generate kinks in the differential conductance at the energies for the singlet and triplet states. These features are related to Fano resonance due to the branched electron wave functions. The ratio between the tunneling amplitudes through the two atoms can be modulated by spatial movements of the tip along the surface.Comment: 4 pages, 2 figures, submitted - Changes in Fig. 1 (panel c) added), and minor modification in the main text - new version, as publishe

Impurities and Conductivity in a D-wave Superconductor

Impurity scattering in the unitary limit produces low energy quasiparticles with anisotropic spectrum in a two-dimensional $d$-wave superconductor. We describe a new {\em quasi-one-dimensional } limit of the quasiparticle scattering, which might occur in a superconductor with short coherence length and with {\em finite} impurity potential range. The dc conductivity in a $d$-wave superconductor is predicted to be proportional to the normal state scattering rate and is impurity-{\em dependent}. We show that {\em quasi-one-dimensional } regime might occur in high-$T_c$ superconductors with Zn impurities at low temp\ eratures $T\lesssim 10$~KComment: 6 pages , Revtex 3, Los Alamos Preprint LA-UR-94-9

Doping induced inhomogeneity in high-Tc superconductors

Doping and disorder are inseparable in the superconducting cuprates. Assuming the simplest possible disordered doping, we construct a semiphenomenological model and analyze its experimental consequences. Among the affected experimental quantities are the ARPES spectra and thermodynamic properties. From our model we make a prediction for the width of the local superconducting gap distribution with the only experimentally unknown parameter being the superconducting correlation length. Thus, our model provides a direct way of determining the superconducting correlation length from a known experimental gap distribution.Comment: 5 pages, 3 eps figures. Expanded version of talk presented by AVB at the ISS 2000, Oct 14-16, Tokyo, Japa