820 research outputs found
Magnetic-Field-Induced Hybridization of Electron Subbands in a Coupled Double Quantum Well
We employ a magnetocapacitance technique to study the spectrum of the soft
two-subband (or double-layer) electron system in a parabolic quantum well with
a narrow tunnel barrier in the centre. In this system unbalanced by gate
depletion, at temperatures T\agt 30 mK we observe two sets of quantum
oscillations: one originates from the upper electron subband in the
closer-to-the-gate part of the well and the other indicates the existence of
common gaps in the spectrum at integer fillings. For the lowest filling factors
and , both the common gap presence down to the point of one- to
two-subband transition and their non-trivial magnetic field dependences point
to magnetic-field-induced hybridization of electron subbands.Comment: Major changes, added one more figure, the latest version to be
published in JETP Let
Quantum Hall effect in single wide quantum wells
We study the quantum Hall states in the lowest Landau level for a single wide
quantum well. Due to a separation of charges to opposite sides of the well, a
single wide well can be modelled as an effective two level system. We provide
numerical evidence of the existence of a phase transition from an
incompressible to a compressible state as the electron density is increased for
specific well width. Our numerical results show a critical electron density
which depends on well width, beyond which a transition incompressible double
layer quantum Hall state to a mono-layer compressible state occurs. We also
calculate the related phase boundary corresponding to destruction of the
collective mode energy gap. We show that the effective tunneling term and the
interlayer separation are both renormalised by the strong magnetic field. We
also exploite the local density functional techniques in the presence of strong
magnetic field at to calculate renormalized . The
numerical results shows good agreement between many-body calculations and local
density functional techniques in the presence of a strong magnetic field at
. we also discuss implications of this work on the
incompressible state observed in SWQW.Comment: 30 pages, 7 figures (figures are not included
Degeneracy of Multi-Component Quantum Hall States Satisfying Periodic Boundary Conditions
In systems subject to periodic boundary conditions, Haldane has shown that
states at arbitrary filling fraction possess a degeneracy with respect to
center of mass translations. An analysis is carried out for multi-component
electron systems and extra degeneracies are shown to exist. Their application
to numerical studies is discussed.Comment: 16 pages, REVTEX v3.0, revised manuscrip
Half-Integral Spin-Singlet Quantum Hall Effect
We provide numerical evidence that the ground state of a short range
interaction model at is incompressible and spin-singlet for a wide
range of repulsive interactions. Furthermore it is accurately described by a
trial wave function studied earlier. For the Coulomb interaction we find that
this wave function provides a good description of the lowest lying spin-singlet
state, and propose that fractional quantum Hall effect would occur at
if this state became the global ground state.Comment: Latex 13 pages, 3 figures upon reques
Collective excitations in double-layer quantum Hall systems
We study the collective excitation spectra of double-layer quantum-Hall
systems using the single mode approximation. The double-layer in-phase density
excitations are similar to those of a single-layer system. For out-of-phase
density excitations, however, both inter-Landau-level and intra-Landau-level
double-layer modes have finite dipole oscillator strengths. The oscillator
strengths at long wavelengths for the latter transitions are shifted upward by
interactions by identical amounts proportional to the interlayer Coulomb
coupling. The intra-Landau-level out-of-phase mode has a gap when the ground
state is incompressible except in the presence of spontaneous inter-layer
coherence. We compare our results with predictions based on the
Chern-Simons-Landau-Ginzburg theory for double-layer quantum Hall systems.Comment: RevTeX, 21 page
Fermionic Chern-Simons theory for the Fractional Quantum Hall Effect in Bilayers
We generalize the fermion Chern-Simons theory for the Fractional Hall Effect
(FQHE) which we developed before, to the case of bilayer systems. We study the
complete dynamic response of these systems and predict the experimentally
accessible optical properties. In general, for the so called
states, we find that the spectrum of collective excitations has a gap, and the
wave function has the Jastrow-Slater form, with the exponents determined by the
coefficients , and . We also find that the states, {\it
i.~e.~}, those states whose filling fraction is , have a gapless mode
which may be related with the spontaneous appearance of the interlayer
coherence. Our results also indicate that the gapless mode makes a contribution
to the wave function of the states analogous to the phonon
contribution to the wave function of superfluid . We calculate the
Hall conductance, and the charge and statistics of the quasiparticles. We also
present an generalization of this theory relevant to spin unpolarized
or partially polarized single layers.Comment: 55 pages, Urbana Prepin
Anomalous Stability of nu=1 Bilayer Quantum Hall State
We have studied the fractional and integer quantum Hall (QH) effects in a
high-mobility double-layer two-dimensional electron system. We have compared
the "stability" of the QH state in balanced and unbalanced double quantum
wells. The behavior of the n=1 QH state is found to be strikingly different
from all others. It is anomalously stable, though all other states decay, as
the electron density is made unbalanced between the two quantum wells. We
interpret the peculiar features of the nu=1 state as the consequences of the
interlayer quantum coherence developed spontaneously on the basis of the
composite-boson picture.Comment: 5 pages, 6 figure
Laughlin liquid - Wigner solid transition at high density in wide quantum wells
Assuming that the phase transition between the Wigner solid and the Laughlin
liquid is first-order, we compare ground-state energies to find features of the
phase diagram at fixed . Rather than use the Coulomb interaction, we
calculate the effective interaction in a square quantum well, and fit the
results to a model interaction with length parameter roughly
proportional to the width of the well. We find a transition to the Wigner solid
phase at high density in very wide wells, driven by the softening of the
interaction at short distances, as well as the more well-known transition to
the Wigner solid at low density, driven by Landau-level mixing.Comment: RevTeX 3.0, 3 Postscript figures appended in uuencoded forma
Phase Transition in \nu=2 Bilayer Quantum Hall State
The Hall-plateau width and the activation energy were measured in the bilayer
quantum Hall state at filling factor \nu=2, 1 and 2/3, by changing the total
electron density and the density ratio in the two quantum wells. Their behavior
are remarkably different from one to another. The \nu=1 state is found stable
over all measured range of the density difference, while the \nu=2/3$ state is
stable only around the balanced point. The \nu=2 state, on the other hand,
shows a phase transition between these two types of the states as the electron
density is changed.Comment: 5 pages including figures, RevTe
Evidence for a Goldstone Mode in a Double Layer Quantum Hall System
The tunneling conductance between two parallel 2D electron systems has been
measured in a regime of strong interlayer Coulomb correlations. At total Landau
level filling the tunnel spectrum changes qualitatively when the
boundary separating the compressible phase from the ferromagnetic quantized
Hall state is crossed. A huge resonant enhancement replaces the strongly
suppressed equilibrium tunneling characteristic of weakly coupled layers. The
possible relationship of this enhancement to the Goldstone mode of the broken
symmetry ground state is discussed.Comment: 4 pages, 3 figures, 2 minor typeos fixe
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