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