191 research outputs found
Massive skyrmions in quantum Hall ferromagnets
We apply the theory of elasticity to study the effects of skyrmion mass on
lattice dynamics in quantum Hall systems. We find that massive Skyrme lattices
behave like a Wigner crystal in the presence of a uniform perpendicular
magnetic field. We make a comparison with the microscopic Hartree-Fock results
to characterize the mass of quantum Hall skyrmions at and investigate
how the low temperature phase of Skyrme lattices may be affected by the
skyrmion mass.Comment: 6 pages and 2 figure
Statistical significance of fine structure in the frequency spectrum of Aharonov-Bohm conductance oscillations
We discuss a statistical analysis of Aharonov-Bohm conductance oscillations
measured in a two-dimensional ring, in the presence of Rashba spin-orbit
interaction. Measurements performed at different values of gate voltage are
used to calculate the ensemble-averaged modulus of the Fourier spectrum and, at
each frequency, the standard deviation associated to the average. This allows
us to prove the statistical significance of a splitting that we observe in the
h/e peak of the averaged spectrum. Our work illustrates in detail the role of
sample specific effects on the frequency spectrum of Aharonov-Bohm conductance
oscillations and it demonstrates how fine structures of a different physical
origin can be discriminated from sample specific features.Comment: accepted for publication in PR
Solitons in polarized double layer quantum Hall systems
A new manifestation of interlayer coherence in strongly polarized double
layer quantum Hall systems with total filling factor
in the presence of a small or zero tunneling is theoretically
predicted. It is shown that moving (for small tunneling) and spatially
localized (for zero tunneling) stable pseudospin solitons develop which could
be interpreted as mobile or static charge-density excitations.
The possibility of their experimental observation is also discussed.Comment: Phys. Rev. B (accepted
Broken-Symmetry States in Quantum Hall Superlattices
We argue that broken-symmetry states with either spatially diagonal or
spatially off-diagonal order are likely in the quantum Hall regime, for clean
multiple quantum well (MQW) systems with small layer separations. We find that
for MQW systems, unlike bilayers, charge order tends to be favored over
spontaneous interlayer coherence. We estimate the size of the interlayer
tunneling amplitude needed to stabilize superlattice Bloch minibands by
comparing the variational energies of interlayer-coherent superlattice miniband
states with those of states with charge order and states with no broken
symmetries. We predict that when coherent miniband ground states are stable,
strong interlayer electronic correlations will strongly enhance the
growth-direction tunneling conductance and promote the possibility of Bloch
oscillations.Comment: 9 pages LaTeX, 4 figures EPS, to be published in PR
Mass spectrum and elastic scattering in the massive SU(2)_f Schwinger model on the lattice
We calculate numerically scattering phases for elastic meson-meson scattering
processes in the strongly coupled massive Schwinger-model with an SU(2) flavour
symmetry. These calculations are based on Luescher's method in which finite
size effects in two-particle energies are exploited. The results from
Monte-Carlo simulations with staggered fermions for the lightest meson ("pion")
are in good agreement with the analytical strong-coupling prediction.
Furthermore, the mass spectrum of low-lying mesonic states is investigated
numerically. We find a surprisingly rich spectrum in the mass region [m_\pi,4
m_\pi].Comment: 43 pages, 15 figures, LaTeX, uses feynmf.st
Crossover between ionic/covalent and pure ionic bonding in magnesium oxyde clusters
An empirical potential with fluctuating charges is proposed for modelling
(MgO)_n clusters in both the molecular (small n) and bulk (n->infty) regimes.
Vectorial polarization forces are explicitely taken into account in the
self-consistent determination of the charges. Our model predicts cuboid cluster
structures, in agreement with previous experimental and theoretical results.
The effective charge transferred between magnesium and oxygen smoothly
increases from 1 to 2, with an estimated crossover size above 300 MgO
molecules
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
Bias-voltage induced phase-transition in bilayer quantum Hall ferromagnets
We consider bilayer quantum Hall systems at total filling factor in
presence of a bias voltage which leads to different filling factors
in each layer. We use auxiliary field functional integral approach to study
mean-field solutions and collective excitations around them. We find that at
large layer separation, the collective excitations soften at a finite wave
vector leading to the collapse of quasiparticle gap. Our calculations predict
that as the bias voltage is increased, bilayer systems undergo a phase
transition from a compressible state to a phase-coherent state {\it
with charge imbalance}. We present simple analytical expressions for
bias-dependent renormalized charge imbalance and pseudospin stiffness which are
sensitive to the softening of collective modes.Comment: 12 pages, 5 figures. Minor changes, one reference adde
Lattice Pseudospin Model for Quantum Hall Bilayers
We present a new theoretical approach to the study of quantum Hall
bilayer that is based on a systematic mapping of the microscopic Hamiltonian to
an anisotropic SU(4) spin model on a lattice. To study the properties of this
model we generalize the Heisenberg model Schwinger boson mean field theory
(SBMFT) of Arovas and Auerbach to spin models with anisotropy. We calculate the
temperature dependence of experimentally observable quantities, including the
spin magnetization, and the differential interlayer capacitance. Our theory
represents a substantial improvement over the conventional Hartree-Fock picture
which neglects quantum and thermal fluctuations, and has advantages over
long-wavelength effective models that fail to capture important microscopic
physics at all realistic layer separations. The formalism we develop can be
generalized to treat quantum Hall bilayers at filling factor .Comment: 26 pages, 10 figures. The final version, to appear in PR
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