125 research outputs found
Bilayer Quantum Hall Ferromagnet in a Periodic Potential
The bilayer quantum Hall system at a total filling of has long
resisted explanation in terms of a true counterflow superfluid, though many
experimental features can be seen to be "almost" that of a superfluid. It is
widely believed that quenched disorder is the root cause of this puzzle. Here
we model the nonperturbative effects of disorder by investigating the
bilayer in a strong periodic potential. Our model assumes that fermions are
gapped and real spins are fully polarized, and concentrates on the pseudospin
variable (the layer index), with the external potential coupling to the
topological (Pontryagin) density of the pseudospin. We find that as the
potential strength increases, there are ground state transitions in which the
topological content of the pseudospin configuration changes. These transitions
are generically weakly first-order, with a new quadratically dispersing mode
(in addition to the linearly dispersing Goldstone mode) sometimes becoming
nearly gapless near the transition. We show that this leads to strong
suppressions of both the Kosterlitz-Thouless transition temperature and the
interlayer tunneling strength, which we treat perturbatively. We discuss how
these results might extend to the case of true disorder
Magnetic field induced Coulomb blockade in small disordered delta-doped heterostructures
At low densities, electrons confined to two dimensions in a delta-doped
heterostructure can arrange themselves into self-consistent droplets due to
disorder and screening effects. We use this observation to show that at low
temperatures, there should be resistance oscillations in low density two
dimensional electron gases as a function of the gate voltage, that are greatly
enhanced in a magnetic field. These oscillations are intrinsic to small samples
and give way to variable range hopping resistivity at low temperatures in
larger samples. We place our analysis in the context of recent experiments
where similar physical effects have been discussed from the point of view of a
Wigner crystal or charge density wave picture.Comment: 6 pages RevTeX, 2 figures, published versio
Magnetotransport in disordered delta-doped heterostructures
We discuss theoretically how electrons confined to two dimensions in a
delta-doped heterostructure can arrange themselves in a droplet-like spatial
distribution due to disorder and screening effects when their density is low.
We apply this droplet picture to magnetotransport and derive the expected
dependence on electron density of several quantities relevant to this
transport, in the regimes of weak and moderate magnetic fields. We find good
qualitative and quantitative agreement between our calculations and recent
experiments on delta-doped heterostructures.Comment: 10 pages RevTeX, 2 figures, uses psfrag; published versio
Compensation driven superconductor-insulator transition
The superconductor-insulator transition in the presence of strong
compensation of dopants was recently realized in La doped YBCO. The
compensation of acceptors by donors makes it possible to change independently
the concentration of holes n and the total concentration of charged impurities
N. We propose a theory of the superconductor-insulator phase diagram in the
(N,n) plane. It exhibits interesting new features in the case of strong
coupling superconductivity, where Cooper pairs are compact, non-overlapping
bosons. For compact Cooper pairs the transition occurs at a significantly
higher density than in the case of spatially overlapping pairs. We establish
the superconductor-insulator phase diagram by studying how the potential of
randomly positioned charged impurities is screened by holes or by strongly
bound Cooper pairs, both in isotropic and layered superconductors. In the
resulting self-consistent potential the carriers are either delocalized or
localized, which corresponds to the superconducting or insulating phase,
respectively
Structural and transport properties of GaAs/delta<Mn>/GaAs/InxGa1-xAs/GaAs quantum wells
We report results of investigations of structural and transport properties of
GaAs/Ga(1-x)In(x)As/GaAs quantum wells (QWs) having a 0.5-1.8 ML thick Mn
layer, separated from the QW by a 3 nm thick spacer. The structure has hole
mobility of about 2000 cm2/(V*s) being by several orders of magnitude higher
than in known ferromagnetic two-dimensional structures. The analysis of the
electro-physical properties of these systems is based on detailed study of
their structure by means of high-resolution X-ray diffractometry and
glancing-incidence reflection, which allow us to restore the depth profiles of
structural characteristics of the QWs and thin Mn containing layers. These
investigations show absence of Mn atoms inside the QWs. The quality of the
structures was also characterized by photoluminescence spectra from the QWs.
Transport properties reveal features inherent to ferromagnetic systems: a
specific maximum in the temperature dependence of the resistance and the
anomalous Hall effect (AHE) observed in samples with both "metallic" and
activated types of conductivity up to ~100 K. AHE is most pronounced in the
temperature range where the resistance maximum is observed, and decreases with
decreasing temperature. The results are discussed in terms of interaction of
2D-holes and magnetic Mn ions in presence of large-scale potential fluctuations
related to random distribution of Mn atoms. The AHE values are compared with
calculations taking into account its "intrinsic" mechanism in ferromagnetic
systems.Comment: 15 pages, 9 figure
Charge inhomogeneities and transport in semiconductor heterostructures with a manganese -layer
We study experimentally and theoretically the effects of disorder, nonlinear
screening, and magnetism in semiconductor heterostructures containing a
-layer of Mn, where the charge carriers are confined within a quantum
well and hence both ferromagnetism and transport are two-dimensional (2D) and
differ qualitatively from their bulk counterparts. Anomalies in the electrical
resistance observed in both metallic and insulating structures can be
interpreted as a signature of significant ferromagnetic correlations. The
insulating samples turn out to be the most interesting as they can give us
valuable insights into the mechanisms of ferromagnetism in these
heterostructures. At low charge carrier densities, we show how the interplay of
disorder and nonlinear screening can result in the organization of the carriers
in the 2D transport channel into charge droplets separated by insulating
barriers. Based on such a droplet picture and including the effect of magnetic
correlations, we analyze the transport properties of this set of droplets,
compare it with experimental data, and find a good agreement between the model
calculations and experiment. Our analysis shows that the peak or shoulder-like
features observed in temperature dependence of resistance of 2D
heterostructures -doped by Mn lie significantly below the Curie
temperature unlike the three-dimensional case, where it lies above and
close to . We also discuss the consequences of our description for
understanding the mechanisms of ferromagnetism in the heterostructures under
study.Comment: 13 pages, 12 figures, RevTe
A new method for detection of exciton Bose condensation using stimulated two-photon emission
Stimulated two-photon emission by Bose-condensed excitons accompanied by a
coherent two-exciton recombination, i.e., by simultaneous recombination of two
excitons with opposite momenta leaving unchanged the occupation numbers of
excitonic states with nonzero momenta, is investigated. Raman light scattering
accompanied by a similar two-exciton recombination (or generation of two
excitons) is also analyzed. The processes under consideration can occur only if
a system contains Bose condensate, therefore, their detection can be used as a
new method to reveal Bose condensation of excitons. The recoil momentum, which
corresponds to a change in the momentum of the electromagnetic field in the
processes, is transferred to phonons or impurities. If the recoil momentum is
transmitted to optical phonons with frequency , the stimulated
two-photon emission with the coherent two-exciton recombination leads to the
appearance of a line at , where
and is the light frequency corresponding to the recombination of an
exciton with zero momentum. Formulas for the cross sections at finite
temperatures are obtained for the processes under consideration. Our estimates
indicate that a spectral line, corresponding to the stimulated two-photon
emission accompanied by the coherent optical phonon-assisted two-exciton
recombination can be experimentally detected in CuO.Comment: 28 pages, 3 Postscript figure
Manifestation of exciton Bose condensation in induced two-phonon emission and Raman scattering
The unusual two-photon emission by Bose-condensed excitons caused by
simultaneous recombination of two excitons with opposite momenta leaving the
occupation numbers of excitonic states with momenta unchanged
(below coherent two-exciton recombination) is investigated. Raman scattering
accompanied by the analogous two-exciton recombination (or creation) is also
analyzed. The excess momentum equal to the change of the electromagnetic field
momentum in these processes can be transferred to phonons or impurities. The
processes under consideration take place if there is Bose condensation in the
interacting exciton system, and, therefore, can be used as a new method to
reveal exciton Bose condensation. If the frequency of the incident light
( is the frequency corresponding to the recombination
of an exciton with p=0), the coherent two-exciton recombination with the excess
momentum elastically transferred to impurities leads to the appearance of the
spectral line corresponding to the induced two-photon
emission. In this case the anti-Stokes line on frequency also
appears in the Raman spectrum. If , there are both Stokes and
anti-Stokes lines on frequencies in the Raman spectrum. The
induced two-photon emission is impossible in this case. The spectral lines
mentioned above have phonon replicas on frequencies corresponding to the transmission of the excess
momentum (partially or as a whole) to optical phonons of frequency
( is an integer number).Comment: 21 pages, 2 Postscript figures. Submitted to Phys. Rev. B (1998
Collapse of Spin-Splitting in the Quantum Hall Effect
It is known experimentally that at not very large filling factors the
quantum Hall conductivity peaks corresponding to the same Landau level number
and two different spin orientations are well separated. These peaks occur
at half-integer filling factors and so that
the distance between them is unity. As increases
shrinks. Near certain two peaks abruptly merge into a single peak at
. We argue that this collapse of the spin-splitting at low
magnetic fields is attributed to the disorder-induced destruction of the
exchange enhancement of the electron -factor. We use the mean-field approach
to show that in the limit of zero Zeeman energy experiences a
second-order phase transition as a function of the magnetic field. We give
explicit expressions for in terms of a sample's parameters. For example,
we predict that for high-mobility heterostructures where is the spacer width, is the density of the
two-dimensional electron gas, and is the two-dimensional density of
randomly situated remote donors.Comment: 14 pages, compressed Postscript fil
The Effects of Disorder on the Quantum Hall State
A disorder-averaged Hartree-Fock treatment is used to compute the density of
single particle states for quantum Hall systems at filling factor . It
is found that transport and spin polarization experiments can be simultaneously
explained by a model of mostly short-range effective disorder. The slope of the
transport gap (due to quasiparticles) in parallel field emerges as a result of
the interplay between disorder-induced broadening and exchange, and has
implications for skyrmion localization.Comment: 4 pages, 3 eps figure
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