88 research outputs found
Photovoltaic effect for narrow-gap Mott insulators
We discuss the photovoltaic effect at a p-n heterojunction, in which the
illuminated side is a doped Mott insulator, using the simplest description of a
Mott insulator within the Hubbard model. We find that the internal quantum
efficiency of such a device, if we choose an appropriate narrow-gap Mott
insulator, can be significantly enhanced due to impact ionization caused by the
photoexcited ``hot'' electron/hole pairs. Namely, the photoexcited electron
and/or hole can convert its excess energy beyond the Mott-Hubbard gap to
additional electrical energy by creating multiple electron/hole pairs in a time
scale which can be shorter than the time characterizing other relaxation
processes.Comment: 8 latex two-column pages, 5 eps figures (Accepted for publication in
PRB
Three-dimensional model with the Chern-Simons term
We investigate the influence of the Chern-Simons term coupled to the
three-dimensional model. This term endows vortices with an internal
angular momentum and thus gives them arbitrary statistics. The Chern-Simons
term for the model takes an integer value which can be written as a sum
over all vortex lines of the product of the vortex charge and the winding
number of the internal phase angle along that vortex line. We have used the
Monte-Carlo method to study the three-dimensional model with the
Chern-Simons term. Our findings suggest that this model belongs to the
universality class with the critical temperature growing with increasing
internal angular momentum.Comment: 15 pages, uuencoded postscript fil
Boundary effects in superfluid films
We have studied the superfluid density and the specific heat of the XY model
on lattices L x L x H with L >> H (i.e. on lattices representing a film
geometry) using the Cluster Monte Carlo method. In the H-direction we applied
staggered boundary conditions so that the order parameter on the top and bottom
layers is zero, whereas periodic boundary conditions were applied in the
L-directions. We find that the system exhibits a Kosterlitz-Thouless phase
transition at the H-dependent temperature T_{c}^{2D} below the critical
temperature T_{\lambda} of the bulk system. However, right at the critical
temperature the ratio of the areal superfluid density to the critical
temperature is H-dependent in the range of film thicknesses considered here. We
do not find satisfactory finite-size scaling of the superfluid density with
respect to H for the sizes of H studied. However, our numerical results can be
collapsed onto a single curve by introducing an effective thickness H_{eff} = H
+ D (where D is a constant) into the corresponding scaling relations. We argue
that the effective thickness depends on the type of boundary conditions.
Scaling of the specific heat does not require an effective thickness (within
error bars) and we find good agreement between the scaling function f_{1}
calculated from our Monte Carlo results, f_{1} calculated by renormalization
group methods, and the experimentally determined function f_1.Comment: 37 pages,15 postscript figure
Hole spectral functions in lightly doped quantum antiferromagnets
We study the hole and magnon spectral functions as a function of hole doping
in the two-dimensional (2D) t-J and t-t'-t"-J models working within the limits
of the spin-wave theory, by linearizing the hole-spin-deviation interaction and
by adapting the non-crossing approximation (NCA). We find that the staggered
magnetization decreases rather rapidly with doping and it goes to zero at a few
percent of hole concentration in both t-J and the t-t'-t"-J model. We find that
with doping, the residue of the quasiparticle peak at G=(pi/2,pi/2) decreases
rapidly with doping and the spectral function is in agreement with high
resolution angle-resolved photo-emission spectroscopy (ARPES) studies of the
copper-oxide superconductors. The observed large shift of the chemical
potential inside the Mott gap is found to be a result of broadening of the
quasiparticle peak. We find pockets centered at G, similar to those observed by
quantum oscillation measurements, (i) with an elliptical shape with large
eccentricity along the anti-nodal direction in the case of the t-J model, and
(ii) with an almost circular shape in the case of the t-t'-t"-J model. We also
find that the spectral intensity distribution in the doped antiferromagnet has
a waterfall-like patten along the nodal direction of the Brillouin zone, a
feature that is also seen in ARPES measurements.Comment: 13 two-column pages and 21 figure
Scaling of the specific heat of superfluids confined in pores
We investigate the scaling properties of the specific heat of the XY model on
lattices H x H x L with L >> H (i.e. in a bar-like geometry) with respect to
the thickness H of the bar, using the Cluster Monte Carlo method. We study the
effect of the geometry and boundary conditions on the shape of the universal
scaling function of the specific heat by comparing the scaling functions
obtained for cubic, film, and bar-like geometry. In the presence of physical
boundary conditions applied along the sides of the bars we find good agreement
between our Monte Carlo results and the most recent experimental data for
superfluid helium confined in pores.Comment: 10 pages, 4 figures, Revte
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