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

Properties of Odd Gap Superconductors

A new class of superconductors with the gap function {\it odd} under time reversal is considered. Some of the physical properties of these superconductors such as the Meissner effect, composite condensate, gapless spectrum and transition from the {\it odd} gap superconductor to the BCS state at lower temperatures are discussed.Comment: 9 pages + 2 fig, LA-UR-93-299

Spin Singlet Quantum Hall Effect and Nonabelian Landau-Ginzburg Theory

We show that the Halperin-Haldane SQHE wave function can be written in the form of a product of a wave function for charged semions in a magnetic field and a wave function for the Chiral Spin Liquid of neutral spin-\12 semions. We introduce field-theoretic model in which the electron operators are factorized in terms of charged spinless semions (holons) and neutral spin-\12 semions (spinons). Broken time reversal symmetry and short ranged spin correlations lead to $SU(2)_{k=1}$ Chern-Simons term in Landau-Ginzburg action for SQHE phase. We construct appropriate coherent states for SQHE phase and show the existence of $SU(2)$ valued gauge potential. This potential appears as a result of ``spin rigidity" of the ground state against any displacements of nodes of wave function from positions of the particles and reflects the nontrivial monodromy in the presence of these displacements. We argue that topological structure of $SU(2)_{k=1}$ Chern-Simons theory unambiguously dictates {\it semion} statistics of spinons

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

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

Clapping modes in unconventional superconductors

We consider a superconducting state with a mixed symmetry order parameter components, e.g. $d+is$ or $d+id'$ with $d'= d_{xy}$. We argue for the existence of the new orbital magnetization mode which corresponds to the oscillations of relative phase $\phi$ between two components around an equilibrium value of $\phi = \frac{\pi}{2}$. It is similar to the so called ``clapping'' mode in superfluid $^3He-A$. We estimate the frequency of this mode $\omega_0(B,T)$ depending on the field and temperature for the specific case of magnetic field induced $d'$ state. We find that this mode is {\em tunable} with an applied magnetic field with \omega_0(B,T) \propto B \0, where \0 is the magnitude of the d-wave order parameter. We argue also that similar filed induced clapping mode should be present in an organic p-wave superconductors.Comment: M2S-HTSC-VI Proceedings, Feb 2000, Houston, TX, USA; 4 pages. Physica C, to be publishe