Božidar Mitrović

On the rate of Tc suppression by interband impurity scattering in MgB2

We calculate the change in the superconducting transition temperature Tc of MgB2 caused by interband nonmagnetic impurity scattering using the Eliashberg theory for the two-band model of this compound. A much slower rate of Tc suppression is obtained compared to the prediction based on the BCS treatment of the two-band model which ignores renormalization and damping associated with the electron–phonon interaction. Hence, the interband impurity scattering rates deduced from experiments on MgB2 using the formula which results from the BCS approach to the two-band model are underestimated. We generalize the BCS treatment of the two-band model to include renormalization effects of the electron–phonon interaction and find an excellent agreement with the full strong coupling calculation.

Effect of disorder on superconductive tunneling into A15 compounds

Abstract One of the current explanations for the degradation of superconductivity in A15 compounds with disorder is in terms of the broadening of a peak in the electronic density of states N (e) near the Fermi level. Here, the Eliashberg gap equations which incorporate both the energy dependence in N (e) and the normal impurity scattering are solved on the real frequency axis at T =0 K for several impurity concentrations. The resulting tunneling characteristics are then inverted to obtain the effective electron-phonon spectral functions α 2 (ohms)F(Ω) assuming that the usual Eliashberg theory ( N ( e )= const .) is applicable. The scale of the effective α 2 F -spectra diminishes with increasing disorder. At the same time there is a transfer of weight under the (α 2 F ) eff to higher frequencies. This trend has been obeserved in tunneling experiments on A15 superconductors with compositional disorder.

Effect of sharp structure in the electron density of states on the electron-phonon spectral function obtained by inversion of normal metal tunneling data

Abstract Recently, Svistunov et al. inverted normal metal tunneling measurements on an AlIPb junction to obtain the frquency (ohms) dependence of the electron-phonon spectral function α 2 F(ohms) for Pb. The agreement with the well-known superconducting tunneling result is remarkably good especially when thermal smearing is taken into account. We examine the possibility of using normal metal tunneling to complement superconducting tunneling in probing the electron-phonon interaction in the notoriously difficult A15 compounds.

Monte Carlo study of the XY-model on Sierpinski gasket

We have performed a Monte Carlo study of the classical XY-model on two-dimensional Sierpinski gaskets (SGs) of several cluster sizes. From the dependence of the helicity modulus on the cluster size we conclude that there is no phase transition in this system at a finite temperature. This is in agreement with previous findings for the harmonic approximation to the XY-model on SG and is analogous to the absence of finite-temperature phase transition for the Ising model on fractals with a finite order of ramification.

Spin-glass behavior of frustrated two-dimensional Penrose lattice in the classical planar model.

Via extensive Monte Carlo studies we show that the frustrated

Absence of phase transition in the XY-model on Menger sponge

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Anisotropy in the helicity modulus of a quantum three-dimensional XY model: application to YBCO

Hamiltonian on a two-dimensional Penrose lattice admits of a spin-glass phase at low temperature. Studies of the Edwards-Anderson order parameter, spin-glass susceptibility, and the local (linear) susceptibility point unequivocally to a paramagnetic-to-spin-glass transition as the temperature is lowered. The specific heat shows a rounded peak at a temperature above the spin-glass transition temperature, as is commonly observed in spin glasses. Our results strongly suggest that the critical-point exponents are the same as obtained by Bhatt and Young in the

Classical planar model on frustrated two-dimensional quasiperiodic lattices: possibility of spin/gauge glass phase

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Eliashberg theory for disordered superconductors. II: Tunneling into disordered two-dimensional Pb films

Ising model on a square lattice. However, unlike in the latter case, the critical temperature is clearly finite (nonzero). The results imply that a quasiperiodic two-dimensional array of superconducting grains in a suitably chosen transverse magnetic field should behave as a superconducting glass at low temperature.

Eliashberg theory for disordered superconductors. I. Tunneling into three-dimensional systems

We have performed a Monte Carlo study of the classical XY-model on a Menger sponge with the Wolff cluster algorithm (U. Wolff, 1989). The Menger sponge is a fractal object with infinite order of ramification and fractal dimension D=log(20)/log(3)=2.7268. From the dependence of the helicity modulus on system size and on boundary conditions, we conclude that there is no phase transition in the system at any finite temperature.