Electron-hole symmetry and solutions of Richardson pairing model

Richardson approach provides an exact solution of the pairing Hamiltonian. This Hamiltonian is characterized by the electron-hole pairing symmetry, which is however hidden in Richardson equations. By analyzing this symmetry and using an additional conjecture, fulfilled in solvable limits, we suggest a simple expression of the ground state energy for an equally-spaced energy-level model, which is applicable along the whole crossover from the superconducting state to the pairing fluctuation regime. Solving Richardson equations numerically, we demonstrate a good accuracy of our expression.Comment: 9 pages, 1 figure; accepted for publication in Eur. Phys. J.

Little-Parks effect in a superconducting loop with magnetic dot

We have studied the nucleation of superconductivity in a mesoscopic Al loop, enclosing magnetic dot with perpendicular magnetization. The superconducting phase boundary Tc(B), determined from transport measurements, is asymmetric with respect to the polarity of an applied magnetic field. The maximum critical temperature has been found for a finite applied magnetic field, which is antiparallel to the magnetization of the dot. Theoretical phase boundary shows a good agreement with the experimental data.Comment: to be published in Phys. Rev. B - Brief Report

A mechanism for pair formation in strongly correlated systems

We start from a Hamiltonian describing non-interacting fermions and add bosons to the model, with a Jaynes-Cummings-like interaction between the bosons and fermions. Because of the specific form of the interaction the model can be solved exactly. In the ground state, part of the electrons form bound pairs with opposite momentum and spin. The model also shows a gap in the kinetic energy of the fermions, but not in the spectrum of the full Hamiltonian. This gap is not of a mean-field nature, but is due to the Pauli exclusion principle.Comment: 13 pages, corrected some notations and made some clarification

Optical conductivity of metal nanofilms and nanowires: The rectangular-box model

The conductivity tensor is introduced for the low-dimensional electron systems. Within the particle-in-a-box model and the diagonal response approximation, components of the conductivity tensor for a quasi-homogeneous ultrathin metal film and wire are calculated under the assumption $d\cong \lambda_{\rm F}$ (where $d$ is the characteristic small dimension of the system, $\lambda_{\rm F}$ is the Fermi wavelength for bulk metal). We find the transmittance of ultrathin films and compare these results with available experimental data. The analytical estimations for the size dependence of the Fermi level are presented, and the oscillations of the Fermi energy in ultrathin films and wires are computed. Our results demonstrate the strong size and frequency dependences of the real and imaginary parts of the conductivity components in the infrared range. A sharp distinction of the results for Au and Pb is observed and explained by the difference in the relaxation time of these metals.Comment: 13 pages, 8 figure

Cyclic phase in F=2 spinor condensate: Long-range order, kinks, and roughening transition

We study the effect of thermal fluctuations on homogeneous infinite Bose-Einstein condensate with spin F=2 in the cyclic state, when atoms occupy three hyperfine states with $m_{F}=0, \pm 2$. We use both the approach of small-amplitude oscillations and mapping of our model on the sine-Gordon model. We show that thermal fluctuations lead to the existence of the rough phase in one- and two-dimensional systems, when presence of kinks is favorable. The structure and energy of a single kink are found. We also discuss the effect of thermal fluctuations on spin degrees of freedom in F=1 condensate.Comment: 6 pages, 1 figure; final version, accepted for publication in Phys. Rev.