73 research outputs found
Magnetic Field Effect for Two Electrons in a Two Dimensional Random Potential
We study the problem of two particles with Coulomb repulsion in a
two-dimensional disordered potential in the presence of a magnetic field. For
the regime, when without interaction all states are well localized, it is shown
that above a critical excitation energy electron pairs become delocalized by
interaction. The transition between the localized and delocalized regimes goes
in the same way as the metal-insulator transition at the mobility edge in the
three dimensional Anderson model with broken time reversal symmetry.Comment: revtex, 7 pages, 6 figure
Metal-insulator transition in one-dimensional lattices with chaotic energy sequences
We study electronic transport through a one-dimensional array of sites by
using a tight binding Hamiltonian, whose site-energies are drawn from a chaotic
sequence. The correlation degree between these energies is controlled by a
parameter regulating the dynamic Lyapunov exponent measuring the degree of
chaos. We observe the effect of chaotic sequences on the localization length,
conductance, conductance distribution and wave function, finding evidence of a
Metal-Insulator Transition (MIT) at a critical degree of chaos. The
one-dimensional metallic phase is characterized by a Gaussian conductance
distribution and exhibits a peculiar non-selfaveraging.Comment: 5 pages, 5 figures (one figure replaced). Includes new results and a
few additional references. Improved style for publication. Accepted in
Physics Letters
Disordered Hubbard Model with Attraction: Coupling Energy of Cooper Pairs in Small Clusters
We generalize the Cooper problem to the case of many interacting particles in
the vicinity of the Fermi level in the presence of disorder. On the basis of
this approach we study numerically the variation of the pair coupling energy in
small clusters as a function of disorder. We show that the Cooper pair energy
is strongly enhanced by disorder, which at the same time leads to the
localization of pairs.Comment: revtex, 5 pages, 6 figure
Critical fluctuation conductivity in layered superconductors in strong electric field
The paraconductivity, originating from critical superconducting
order-parameter fluctuations in the vicinity of the critical temperature in a
layered superconductor is calculated in the frame of the self-consistent
Hartree approximation, for an arbitrarily strong electric field and zero
magnetic field. The paraconductivity diverges less steep towards the critical
temperature in the Hartree approximation than in the Gaussian one and it shows
a distinctly enhanced variation with the electric field. Our results indicate
that high electric fields can be effectively used to suppress order-parameter
fluctuations in high-temperature superconductors.Comment: 11 pages, 2 figures, to be published in Phys. Rev.
Magnetic Field Effects on the Transport Properties of One-sided Rough Wires
We present a detailed numerical analysis of the effect of a magnetic field on
the transport properties of a `small-' one-sided surface disordered wire.
When time reversal symmetry is broken due to a magnetic field , we find a
strong increase with not only of the localization length but also of
the mean free path caused by boundary states. Despite this, the
universal relationship between and does hold. We also analyze the
conductance distribution at the metal-insulator crossover, finding a very good
agreement with Random Matrix Theory with two fluctuating channels within the
Circular Orthogonal(Unitary) Ensemble in absence(presence) of Comment: 5 pages, 4 figures, to appear in Phys. Rev.
Extending the lifetime of 3D black hole computations with a new hyperbolic system of evolution equations
We present a new many-parameter family of hyperbolic representations of
Einstein's equations, which we obtain by a straightforward generalization of
previously known systems. We solve the resulting evolution equations
numerically for a Schwarzschild black hole in three spatial dimensions, and
find that the stability of the simulation is strongly dependent on the form of
the equations (i.e. the choice of parameters of the hyperbolic system),
independent of the numerics. For an appropriate range of parameters we can
evolve a single 3D black hole to -- , and are
apparently limited by constraint-violating solutions of the evolution
equations. We expect that our method should result in comparable times for
evolutions of a binary black hole system.Comment: 11 pages, 2 figures, submitted to PR
The Parallel Magnetoconductance of Interacting Electrons in a Two Dimensional Disordered System
The transport properties of interacting electrons for which the spin degree
of freedom is taken into account are numerically studied for small two
dimensional diffusive clusters. On-site electron-electron interactions tend to
delocalize the electrons, while long-range interactions enhance localization.
On careful examination of the transport properties, we reach the conclusion
that it does not show a two dimensional metal insulator transition driven by
interactions. A parallel magnetic field leads to enhanced resistivity, which
saturates once the electrons become fully spin polarized. The strength of the
magnetic field for which the resistivity saturates decreases as electron
density goes down. Thus, the numerical calculations capture some of the
features seen in recent experimental measurements of parallel
magnetoconductance.Comment: 10 pages, 6 figure
Unconventional particle-hole mixing in the systems with strong superconducting fluctuations
Development of the STM and ARPES spectroscopies enabled to reach the
resolution level sufficient for detecting the particle-hole entanglement in
superconducting materials. On a quantitative level one can characterize such
entanglement in terms of the, so called, Bogoliubov angle which determines to
what extent the particles and holes constitute the spatially or momentum
resolved excitation spectra. In classical superconductors, where the phase
transition is related to formation of the Cooper pairs almost simultaneously
accompanied by onset of their long-range phase coherence, the Bogoliubov angle
is slanted all the way up to the critical temperature Tc. In the high
temperature superconductors and in superfluid ultracold fermion atoms near the
Feshbach resonance the situation is different because of the preformed pairs
which exist above Tc albeit loosing coherence due to the strong quantum
fluctuations. We discuss a generic temperature dependence of the Bogoliubov
angle in such pseudogap state indicating a novel, non-BCS behavior. For
quantitative analysis we use a two-component model describing the pairs
coexisting with single fermions and study their mutual feedback effects by the
selfconsistent procedure originating from the renormalization group approach.Comment: 4 pages, 4 figure
Quantitative comparison of single- and two-particle properties in the cuprates
We explore the strong variations of the electronic properties of
copper-oxygen compounds across the doping phase diagram in a quantitative way.
To this end we calculate the electronic Raman response on the basis of results
from angle-resolved photoemission spectroscopy (ARPES). In the limits of our
approximations we find agreement on the overdoped side and pronounced
discrepancies at lower doping. In contrast to the successful approach for the
transport properties at low energies, the Raman and the ARPES data cannot be
reconciled by adding angle-dependent momentum scattering. We discuss possible
routes towards an explanation of the suppression of spectral weight close to
the points which sets in abruptly close to 21% doping.Comment: 7 pages, 4 figure
Weak Localization Effect in Superconductors by Radiation Damage
Large reductions of the superconducting transition temperature and
the accompanying loss of the thermal electrical resistivity (electron-phonon
interaction) due to radiation damage have been observed for several A15
compounds, Chevrel phase and Ternary superconductors, and in
the high fluence regime. We examine these behaviors based on the recent theory
of weak localization effect in superconductors. We find a good fitting to the
experimental data. In particular, weak localization correction to the
phonon-mediated interaction is derived from the density correlation function.
It is shown that weak localization has a strong influence on both the
phonon-mediated interaction and the electron-phonon interaction, which leads to
the universal correlation of and resistance ratio.Comment: 16 pages plus 3 figures, revtex, 76 references, For more information,
Plesse see http://www.fen.bilkent.edu.tr/~yjki
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