19 research outputs found
Quasiparticle spectrum of d-wave superconductors in the mixed state: a large Fermi-velocity anisotropy study
The quasiparticle spectrum of a two-dimensional d-wave superconductor in the
mixed state, H_c1 << H << H_c2, is studied for large values of the ``anisotropy
ratio'' alpha_D = v_F/v_Delta. For a square vortex lattice rotated by 45
degrees from the quasiparticle anisotropy axes (and the usual choice of
Franz--Tesanovic singular gauge transformation) we determine essential features
of the band structure asymptotically for large alpha_D, using an effective
one-dimensional model, and compare them to numerical calculations. We find that
several features of the band structure decay to zero exponentially fast for
large alpha_D. Using a different choice of singular gauge transformation, we
obtain a different band structure, but still find qualitative agreement between
the 1D and full 2D calculations. Finally, we distort the square lattice into a
non-Bravais lattice. Both the one- and two-dimensional numerical calculations
of the energy spectra show a gap around zero-energy, with our gauge choice, and
the two excitation spectra agree reasonably well.Comment: 14 pages, 13 figures, revte
Calculation of excited polaron states in the Holstein model
An exact diagonalization technique is used to investigate the low-lying
excited polaron states in the Holstein model for the infinite one-dimensional
lattice. For moderate values of the adiabatic ratio, a new and comprehensive
picture, involving three excited (coherent) polaron bands below the phonon
threshold, is obtained. The coherent contribution of the excited states to both
the single-electron spectral density and the optical conductivity is evaluated
and, due to the invariance of the Hamiltonian under the space inversion, the
two are shown to contain complementary information about the single-electron
system at zero temperature. The chosen method reveals the connection between
the excited bands and the renormalized local phonon excitations of the
adiabatic theory, as well as the regime of parameters for which the electron
self-energy has notable non-local contributions. Finally, it is shown that the
hybridization of two polaron states allows a simple description of the ground
and first excited state in the crossover regime.Comment: 12 pages, 9 figures, submitted to PR
Phase diagram of the Holstein polaron in one dimension
The behavior of the 1D Holstein polaron is described, with emphasis on
lattice coarsening effects, by distinguishing between adiabatic and
nonadiabatic contributions to the local correlations and dispersion properties.
The original and unifying systematization of the crossovers between the
different polaron behaviors, usually considered in the literature, is obtained
in terms of quantum to classical, weak coupling to strong coupling, adiabatic
to nonadiabatic, itinerant to self-trapped polarons and large to small
polarons. It is argued that the relationship between various aspects of polaron
states can be specified by five regimes: the weak-coupling regime, the regime
of large adiabatic polarons, the regime of small adiabatic polarons, the regime
of small nonadiabatic (Lang-Firsov) polarons, and the transitory regime of
small pinned polarons for which the adiabatic and nonadiabatic contributions
are inextricably mixed in the polaron dispersion properties. The crossovers
between these five regimes are positioned in the parameter space of the
Holstein Hamiltonian.Comment: 19 pages, 9 figure
Vortex molecules in thin films of layered superconductors
The seminal work of A.A. Abrikosov on the vortex lattices in type-II superconductors has triggered the creation and rapid development of the whole new field of condensed matter physics, namely, the physics of vortex matter in a large variety of superfluid and superconducting systems. Among these systems and compounds the layered superconductors are known to form a very important class which is associated with a number of fascinating new discoveries in vortex physics. The phenomenon of the vortex attraction in tilted magnetic fields provides one of the examples of such findings which affected theoretical and experimental research in the field for almost two decades. In our paper we review some recent advances in this direction focusing on the intervortex interaction and equilibrium vortex structures in thin films of layered superconductors in magnetic field tilted with respect to the layers. In such a case the magnetic field penetrates superconductor in the form of tilted vortices or a crossing array of Josephson vortices and pancake stacks. We study the interplay between two different long-range potentials: (i) attraction of tilted vortices or deformed stacks; (ii) the Pearl's repulsion. This interplay is responsible for the formation of the minimum in the total interaction energy and resulting decay of vortex chains in clusters. The number of Abrikosov vortices in these clusters (or vortex molecules) depends on field tilting angle and film thickness