36 research outputs found
Evidence for 2k_F Electron-Electron Scattering Processes in Coulomb Drag
Measurements and calculations of Coulomb drag between two low density,
closely spaced, two-dimensional electron systems are reported. The
experimentally measured drag exceeds that calculated in the random phase
approximation by a significant, and density dependent, factor. Studies of the
dependence of the measured drag on the difference in density between the two
layers clearly demonstrate that previously ignored q=2k_F scattering processes
can be very important to the drag at low densities and small layer separations.Comment: 5 pages, 5 figure
Microscopic Theory of the Reentrant IQHE in the First and Second Excited LLs
We present a microscopic theory for the recently observed reentrant integral
quantum Hall effect in the n=1 and n=2 Landau levels. Our energy investigations
indicate an alternating sequence of M-electron-bubble and quantum-liquid ground
states in a certain range of the partial filling factor of the n-th level.
Whereas the quantum-liquid states display the fractional quantum Hall effect,
the bubble phases are insulating, and the Hall resistance is thus quantized at
integral values of the total filling factor.Comment: 4 pages, 4 figures; minor corrections include
Breakdown of Particle-Hole Symmetry in the Lowest Landau Level Revealed by Tunneling Spectroscopy
Tunneling measurements on 2D electron gases at high magnetic field reveal a
qualitative difference between the two spin sublevels of the lowest Landau
level. While the tunneling current-voltage characteristic at filling factor
is a single peak shifted from zero bias by a Coulomb pseudogap, the
spectrum at shows a well-resolved double peak structure. This
difference is present regardless of whether and occur at
the same or different magnetic fields. No analogous effect is seen at and 7/2 in the first excited Landau level.Comment: 5 pages, 4 figure
Two-phonon scattering of magnetorotons in fractional quantum Hall liquids
We study the phonon-assisted process of dissociation of a magnetoroton, in a
fractional quantum Hall liquid, into an unbound pair of quasiparticles. Whilst
the dissociation is forbidden to first order in the electron-phonon
interaction, it can occur as a two-phonon process. Depending on the value of
final separation between the quasiparticles, the dissociation is either a
single event involving absorption of one phonon and emission of another phonon
of similar energy, or a two-phonon diffusion of a quasiexciton in momentum
space. The dependence of the magnetoroton dissociation time on the filling
factor of the incompressible liquid is found.Comment: 4 pages, no figure
Quantum fluctuations of classical skyrmions in quantum Hall Ferromagnets
In this article, we discuss the effect of the zero point quantum fluctuations
to improve the results of the minimal field theory which has been applied to
study %SMG the skyrmions in the quantum Hall systems. Our calculation which is
based on the semiclassical treatment of the quantum fluctuations, shows that
the one-loop quantum correction provides more accurate results for the minimal
field theory.Comment: A few errors are corrected. Accepted for publication in Rapid
Communication, Phys. Rev.
Magnetotunneling spectroscopy of mesoscopic correlations in two-dimensional electron systems
An approach to experimentally exploring electronic correlation functions in
mesoscopic regimes is proposed. The idea is to monitor the mesoscopic
fluctuations of a tunneling current flowing between the two layers of a
semiconductor double-quantum-well structure. From the dependence of these
fluctuations on external parameters, such as in-plane or perpendicular magnetic
fields, external bias voltages, etc., the temporal and spatial dependence of
various prominent correlation functions of mesoscopic physics can be
determined. Due to the absence of spatially localized external probes, the
method provides a way to explore the interplay of interaction and localization
effects in two-dimensional systems within a relatively unperturbed environment.
We describe the theoretical background of the approach and quantitatively
discuss the behavior of the current fluctuations in diffusive and ergodic
regimes. The influence of both various interaction mechanisms and localization
effects on the current is discussed. Finally a proposal is made on how, at
least in principle, the method may be used to experimentally determine the
relevant critical exponents of localization-delocalization transitions.Comment: 15 pages, 3 figures include
Double-Layer Systems at Zero Magnetic Field
We investigate theoretically the effects of intralayer and interlayer
exchange in biased double-layer electron and hole systems, in the absence of a
magnetic field. We use a variational Hartree-Fock-like approximation to analyze
the effects of layer separation, layer density, tunneling, and applied gate
voltages on the layer densities and on interlayer phase coherence. In agreement
with earlier work, we find that for very small layer separations and low layer
densities, an interlayer-correlated ground state possessing spontaneous
interlayer coherence (SILC) is obtained, even in the absence of interlayer
tunneling. In contrast to earlier work, we find that as a function of total
density, there exist four, rather than three, distinct noncrystalline phases
for balanced double-layer systems without interlayer tunneling. The newly
identified phase exists for a narrow range of densities and has three
components and slightly unequal layer densities, with one layer being spin
polarized, and the other unpolarized. An additional two-component phase is also
possible in the presence of sufficiently strong bias or tunneling. The
lowest-density SILC phase is the fully spin- and pseudospin-polarized
``one-component'' phase discussed by Zheng {\it et al.} [Phys. Rev. B {\bf 55},
4506 (1997)]. We argue that this phase will produce a finite interlayer Coulomb
drag at zero temperature due to the SILC. We calculate the particle densities
in each layer as a function of the gate voltage and total particle density, and
find that interlayer exchange can reduce or prevent abrupt transfers of charge
between the two layers. We also calculate the effect of interlayer exchange on
the interlayer capacitance.Comment: 35 pages, 19 figures included. To appear in PR
Competition between quantum-liquid and electron-solid phases in intermediate Landau levels
On the basis of energy calculations we investigate the competition between
quantum-liquid and electron-solid phases in the Landau levels n=1,2, and 3 as a
function of their partial filling factor. Whereas the quantum-liquid phases are
stable only in the vicinity of quantized values 1/(2s+1) of the partial filling
factor, an electron solid in the form of a triangular lattice of clusters with
a few number of electrons (bubble phase) is energetically favorable between
these fillings. This alternation of electron-solid phases, which are insulating
because they are pinned by the residual impurities in the sample, and quantum
liquids displaying the fractional quantum Hall effect explains a recently
observed reentrance of the integral quantum Hall effect in the Landau levels
n=1 and 2. Around half-filling of the last Landau level, a uni-directional
charge density wave (stripe phase) has a lower energy than the bubble phase.Comment: 12 pages, 9 figures; calculation of exact exchange potential for
n=1,2,3 included, energies of electron-solid phases now calculated with the
help of the exact potential, and discussion of approximation include
Electromagnetic characteristics of bilayer quantum Hall systems in the presence of interlayer coherence and tunneling
The electromagnetic characteristics of bilayer quantum Hall systems in the
presence of interlayer coherence and tunneling are studied by means of a
pseudospin-texture effective theory and an algebraic framework of the
single-mode approximation, with emphasis on clarifying the nature of the
low-lying neutral collective mode responsible for interlayer tunneling
phenomena. A long-wavelength effective theory, consisting of the collective
mode as well as the cyclotron modes, is constructed. It is seen explicitly from
the electromagnetic response that gauge invariance is kept exact, this
implying, in particular, the absence of the Meissner effect in bilayer systems.
Special emphasis is placed on exploring the advantage of looking into quantum
Hall systems through their response; in particular, subtleties inherent to the
standard Chern-Simons theories are critically examined.Comment: 9 pages, Revtex, to appear in Phys. Rev.
Quantum Ferromagnetism and Phase Transitions in Double-Layer Quantum Hall Systems
Double layer quantum Hall systems have interesting properties associated with
interlayer correlations. At where is an odd integer they exhibit
spontaneous symmetry breaking equivalent to that of spin easy-plane
ferromagnets, with the layer degree of freedom playing the role of spin. We
explore the rich variety of quantum and finite temperature phase transitions in
these systems. In particular, we show that a magnetic field oriented parallel
to the layers induces a highly collective commensurate-incommensurate phase
transition in the magnetic order.Comment: 4 pages, REVTEX 3.0, IUCM93-013, 1 FIGURE, hardcopy available from:
[email protected]