82 research outputs found
Classical correlations of defects in lattices with geometrical frustration in the motion of a particle
We map certain highly correlated electron systems on lattices with
geometrical frustration in the motion of added particles or holes to the
spatial defect-defect correlations of dimer models in different geometries.
These models are studied analytically and numerically. We consider different
coverings for four different lattices: square, honeycomb, triangular, and
diamond. In the case of hard-core dimer covering, we verify the existed results
for the square and triangular lattice and obtain new ones for the honeycomb and
the diamond lattices while in the case of loop covering we obtain new numerical
results for all the lattices and use the existing analytical Liouville field
theory for the case of square lattice.The results show power-law correlations
for the square and honeycomb lattice, while exponential decay with distance is
found for the triangular and exponential decay with the inverse distance on the
diamond lattice. We relate this fact with the lack of bipartiteness of the
triangular lattice and in the latter case with the three-dimensionality of the
diamond. The connection of our findings to the problem of fractionalized charge
in such lattices is pointed out.Comment: 6 pages, 6 figures, 1 tabl
Ginzburg-Landau Theory of Josephson Field Effect Transistors
A theoretical model of high-T_c Josephson Field Effect Transistors (JoFETs)
based on a Ginzburg-Landau free energy expression whose parameters are field-
and spatially- dependent is developed. This model is used to explain
experimental data on JoFETs made by the hole-overdoped Ca-SBCO bicrystal
junctions (three terminal devices). The measurements showed a large modulation
of the critical current as a function of the applied voltage due to charge
modulation in the bicrystal junction. The experimental data agree with the
solutions of the theoretical model. This provides an explanation of the large
field effect, based on the strong suppresion of the carrier density near the
grain boundary junction in the absence of applied field and the subsequent
modulation of the density by the field.Comment: REVTEX, 4 figures upon request, submitted to Appl. Phys. Let
On the dephasing time of the chiral metal
In the low-dimensional disordered systems the dephasing time and the
inelastic scattering (out-scattering) time are in general different. We show
that in the case of the two-dimensional chiral metal which is formed at the
surface of a layered three dimensional system, which is exhibiting the integer
quantum Hall effect these two quantities are essentially the same and their
temperature-dependence is T^(-3/2). In particular we show that the results
obtained using the diagramatic technique and the phase uncertainty approach
introduced by A. Stern et al. (Phys. Rev. A 41, 3436 (1990)) for the
out-scattering and the dephasing time respectively, coincide. We furthermore
consider these quantities in the case of the three-dimensional chiral metal,
where similar conclusions are reached.Comment: 6 pages, 1 figure, europhys.st
Entanglement scaling and spatial correlations of the transverse field Ising model with perturbations
We study numerically the entanglement entropy and spatial correlations of the
one dimensional transverse field Ising model with three different
perturbations. First, we focus on the out of equilibrium, steady state with an
energy current passing through the system. By employing a variety of
matrix-product state based methods, we confirm the phase diagram and compute
the entanglement entropy. Second, we consider a small perturbation that takes
the system away from integrability and calculate the correlations, the central
charge and the entanglement entropy. Third, we consider periodically weakened
bonds, exploring the phase diagram and entanglement properties first in the
situation when the weak and strong bonds alternate (period two-bonds) and then
the general situation of a period of n bonds. In the latter case we find a
critical weak bond that scales with the transverse field as =
, where is the strength of the strong bond, of the weak bond
and the transverse field. We explicitly show that the energy current is not
a conserved quantity in this case.Comment: 9 pages, 12 figures, version accepted in PR
Ginzburg-Landau theory and effects of pressure on a two-band superconductor : application to MgB2
We present a model of pressure effects of a two-band superconductor based on
a Ginzburg-Landau free energy with two order parameters. The parameters of the
theory are pressure as well as temperature dependent. New pressure effects
emerge as a result of the competition between the two bands. The theory then is
applied to MgB2. We identify two possible scenaria regarding the fate of the
two subbands under pressure, depending on whether or not both subbands
are above the Fermi energy at ambient pressure. The splitting of the two
subbands is probably caused by the E2g distortion. If only one subband is above
the Fermi energy at ambient pressure (scenario I), application of pressure
diminishes the splitting and it is possible that the lower subband participates
in the superconductivity. The corresponding crossover pressure and Gruneisen
parameter are estimated. In the second scenario both bands start above the
Fermi energy and they move below it, either by pressure or via the substitution
of Mg by Al. In both scenaria, the possibility of electronical topological
transition is emphasized. Experimental signatures of both scenaria are
presented and existing experiments are discussed in the light of the different
physical pictures.Comment: 6 pages; supersedes the first part of cond-mat/0204085 due to new
experiment
Dirac fermion time-Floquet crystal: manipulating Dirac points
We demonstrate how to control the spectra and current flow of Dirac electrons
in both a graphene sheet and a topological insulator by applying either two
linearly polarized laser fields with frequencies and or a
monochromatic (one-frequency) laser field together with a spatially periodic
static potential(graphene/TI superlattice). Using the Floquet theory and the
resonance approximation, we show that a Dirac point in the electron spectrum
can be split into several Dirac points whose relative location in momentum
space can be efficiently manipulated by changing the characteristics of the
laser fields. In addition, the laser-field controlled Dirac fermion band
structure -- Dirac fermion time-Floquet crystal -- allows the manipulation of
the electron currents in graphene and topological insulators. Furthermore, the
generation of dc currents of desirable intensity in a chosen direction occurs
when applying the bi-harmonic laser field which can provide a straightforward
experimental test of the predicted phenomena.Comment: 9 pages, 7 figures, version that will appear in Phys. Rev.
A practical method to detect, analyse and engineer higher order Van Hove singularities in multi-band Hamiltonians
We present a practical method to detect, diagnose and engineer higher order
Van Hove singularities in multiband systems, with no restrictions on the number
of bands and hopping terms. The method allows us to directly compute the Taylor
expansion of the dispersion of any band at arbitrary points in momentum space,
using a generalised extension of the Feynman Hellmann theorem, which we state
and prove. Being fairly general in scope, it also allows us to incorporate and
analyse the effect of tuning parameters on the low energy dispersions, which
can greatly aid the engineering of higher order Van Hove singularities. A
certain class of degenerate bands can be handled within this framework. We
demonstrate the use of the method, by applying it to the Haldane model.Comment: 19 page
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