Superconductivity in the presence of magnetic field

We study the influence of a strong magnetic field on a superconducting state of electron gas in a two-dimensional square lattice. The Harper equation is extended in order to include pairing interactions between electrons. We examine the effects of superconductivity with different pairing symmetries on the Hofstadter energy spectra

Cosmic strings in extra-U (1) model

In this work a cosmic string arising as a result of spontaneous breaking of the SU(2)l x x U ( 1 ) y x U ( 1 ) e symmetry is investigated

Correlations in hexagonal lattice systems : application to carbon nanotubes

We present exact diagonalization studies of two-dimensional electron gas on hexagonal lattice. Using Lanczos method we analyze the influence of the Coulomb correlations on the density of states and spectral functions. Choosing appropriate boundary conditions we simulate the geometry of a single wall carbon nanotube. In particular, integration over the boundary condition in one direction and summation in the other one allows us to perform cluster calculations for a tube-like system with a finite diameter and infinite length

The Falicov-Kimball Model in External Magnetic Field : Orbital Effects

We study thermodynamic properties of the two-dimensional (2D) Falicov–Kimball model in the presence of external magnetic field perpendicular to the lattice. The field is taken into account by the Peierls substitution in the hopping term. We show how the Hofstadter butterfly is affected by electronic correlations. In the non-interacting case the field dependent energy spectrum forms the famous Hofstadter butterfly. Our results indicate that for arbitrary nonzero interaction strength and arbitrary magnetic field there is a gap in the energy spectrum at sufficiently low temperature. The gap vanishes with increase of temperature for weak coupling, however, it persists at high temperatures if the coupling is strong enough. Numerical results have been obtained with the help of Monte Carlo technique based on a modified Metropolis algorithm

Pseudogap and vortices in high-temperature superconductors

The origin of the pseudogap is one of the most puzzling features of the high-temperature superconductors. There are two main scenarios: the first one assumes the presence of a hidden order competing or coexisting with superconductivity; within the framework of the second one the pseudogap is a precursor of the superconducting gap. In this paper we present some aspects of the hidden order pseudogap scenario. In particular, we discuss how the competing order modifies the structure of vortices in high-temperaturę superconductors. We demonstrate that the presence of the hidden order can explain some features of vortices observed in scanning tunneling microscopy experiments

Upper critical field in a stripe-phase

We study the problem of the upper critical field (Hc2 ) for tight-binding electrons in a phase with stripes. Carrying out calculations for finite systems we analyze the influence of the external field in the commensurable and incommensurable case on an equal footing. The upper critical field is discussed for anisotropic intersite pairing as a function of the width of stripe. We show that the upper critical field increases with a decrease of the width of stripe. This effect is of particular importance close to the superconducting transition temperature

The Friedel oscillations in the presence of transport currents

We investigate the Friedel oscillations in a nanowire coupled to two macroscopic electrodes of different potentials. We show that the wave-length of the density oscillations monotonically increases with the bias voltage, whereas the amplitude and the spatial decay exponent of the oscillations remain intact. Using the nonequilibrium Keldysh Green functions, we derive an explicit formula that describes voltage dependence of the wave-length of the Friedel oscillations.Comment: 5 pages, 3 figures, RevTe

Superconductivity in the stripe phase magnetic properties

One of unusual features of high-Tc superconductors, that we discuss in the present report, is related to inhomogeneous distribution of holes. It results in a stripe-phase which consists of antiferromagnetic domains separated by hole-rich domain walls. We study how the upper critical field is affected by this specific distribution of carriers. We consider a twodimensional square lattice immersed in a perpendicular uniform magnetic field. In order to simulate the presence of a stripe-phase we carry out the calculations for a system with modulated hopping integral. Namely, the magnitude of the hopping integral is constant along the stripe, whereas it oscillates in the opposite direction

Next-nearest-neighbor hopping in the Falicov-Kimball model

Results of Monte Carlo simulations for the spinless Falicov-Kimball model with the next-nearest-neighbor hopping are presented. We find the critical value of the next-nearest-neighbor hopping integral, below which the low temperature configuration of the localized particles is the same as in the presence of only the nearest-neighbor hopping. Beyond this critical value the localized particles form horizontal or vertical stripes