2,018 research outputs found
Scaling Behavior of the Activated Conductivity in a Quantum Hall Liquid
We propose a scaling model for the universal longitudinal conductivity near
the mobility edge for the integer quantum Hall liquid. We fit our model with
available experimental data on exponentially activated conductance near the
Landau level tails in the integer quantum Hall regime. We obtain quantitative
agreement between our scaling model and the experimental data over a wide
temperature and magnetic field range.Comment: 9 pages, Latex, 2 figures (available upon request), #phd0
A Unified Model for Two Localisation Problems: Electron States in Spin-Degenerate Landau Levels, and in a Random Magnetic Field
A single model is presented which represents both of the two apparently
unrelated localisation problems of the title. The phase diagram of this model
is examined using scaling ideas and numerical simulations. It is argued that
the localisation length in a spin-degenerate Landau level diverges at two
distinct energies, with the same critical behaviour as in a spin-split Landau
level, and that all states of a charged particle moving in two dimensions, in a
random magnetic field with zero average, are localised.Comment: 7 pages (RevTeX 3.0) plus 4 postscript figure
Rheological evaluation of the fabrication parameters of cellulose acetate butyrate membrane on CO2/N2 separation performance
The rise in emission of greenhouse gases (GHGs) mainly carbon dioxide (CO2) in recent years due to rapid development of modern civilisation, has been listed as the primary contributor to global warming. To address this global issue, membrane technology was applied and developed intensively because of its superior performance in terms of efficiency and economic advantages. In this study, the cellulose acetate butyrate (CAB) polymer was selected as the polymer matrix material since it exhibited excellent film-forming properties. In addition, the wet-phase inversion technique was adopted to synthesise the membrane based on different casting conditions. The optimum outcomes of the fabrication conditions were then characterised with the scanning electron micrograph (SEM) to determine the best CAB membrane for CO2/N2 separation. The results showed that CAB-70000 fabricated with 4 wt% of CAB polymer concentration, casting thickness of 250 µm, solvent evaporation time of 5 minutes, and 30 minutes of solvent exchange for isopropyl alcohol and n-hexane, exhibited the best gas separation performance. Further, CAB-70000 showed an average selectivity of 6.12 ± 0.09 and permeance up to 227.95 ± 0.39 GPU for CO2 and 37.28 ± 0.54 GPU for N2, respectively. In summary, this study is expected to show a detailed outline of the future direction and perspective of the novel CAB polymeric membrane that is suitable to be applied in the industry, and serves as an insight for researchers and manufacturers working in the related field of gas separation
Universal flow diagram for the magnetoconductance in disordered GaAs layers
The temperature driven flow lines of the diagonal and Hall magnetoconductance
data (G_{xx},G_{xy}) are studied in heavily Si-doped, disordered GaAs layers
with different thicknesses. The flow lines are quantitatively well described by
a recent universal scaling theory developed for the case of duality symmetry.
The separatrix G_{xy}=1 (in units e^2/h) separates an insulating state from a
spin-degenerate quantum Hall effect (QHE) state. The merging into the insulator
or the QHE state at low temperatures happens along a semicircle separatrix
G_{xx}^2+(G_{xy}-1)^2=1 which is divided by an unstable fixed point at
(G_{xx},G_{xy})=(1,1).Comment: 10 pages, 5 figures, submitted to Phys. Rev. Let
Density perturbations in warm inflation and COBE normalization
Starting from a gauge invariant treatment of perturbations an analytical
expression for the spectrum of long wavelength density perturbations in warm
inflation is derived. The adiabatic and entropy modes are exhibited explicitly.
As an application of the analytical results, we determined the observational
constraint for the dissipation term compatible with COBE observation of the
cosmic microwave radiation anisotropy for some specific models. In view of the
results the feasibility of warm inflation is discussed.Comment: 11 pages, no figure
A Farewell to Liouvillians
We examine the Liouvillian approach to the quantum Hall plateau transition,
as introduced recently by Sinova, Meden, and Girvin [Phys. Rev. B {\bf 62},
2008 (2000)] and developed by Moore, Sinova and Zee [Phys. Rev. Lett. {\bf 87},
046801 (2001)]. We show that, despite appearances to the contrary, the
Liouvillian approach is not specific to the quantum mechanics of particles
moving in a single Landau level: we formulate it for a general disordered
single-particle Hamiltonian. We next examine the relationship between
Liouvillian perturbation theory and conventional calculations of
disorder-averaged products of Green functions and show that each term in
Liouvillian perturbation theory corresponds to a specific contribution to the
two-particle Green function. As a consequence, any Liouvillian approximation
scheme may be re-expressed in the language of Green functions. We illustrate
these ideas by applying Liouvillian methods, including their extension to Liouvillian flavors, to random matrix ensembles, using numerical
calculations for small integer and an analytic analysis for large .
We find that behavior at is different in qualitative ways from that
at . In particular, the limit expressed using Green
functions generates a pathological approximation, in which two-particle
correlation functions fail to factorize correctly at large separations of their
energy, and exhibit spurious singularities inside the band of random matrix
energy levels. We also consider the large treatment of the quantum Hall
plateau transition, showing that the same undesirable features are present
there, too
Evolution of active and polar photospheric magnetic fields during the rise of Cycle 24 compared to previous cycles
The evolution of the photospheric magnetic field during the declining phase
and minimum of Cycle 23 and the recent rise of Cycle 24 are compared with the
behavior during previous cycles. We used longitudinal full-disk magnetograms
from the NSO's three magnetographs at Kitt Peak, the Synoptic Optical Long-term
Investigations of the Sun (SOLIS) Vector Spectro-Magnetograph (VSM), the
Spectromagnetograph and the 512-Channel Magnetograph instruments, and
longitudinal full-disk magnetograms from the Mt. Wilson 150-foot tower. We
analyzed 37 years of observations from these two observatories that have been
observing daily, weather permitting, since 1974, offering an opportunity to
study the evolving relationship between the active region and polar fields in
some detail over several solar cycles. It is found that the annual averages of
a proxy for the active region poloidal magnetic field strength, the magnetic
field strength of the high-latitude poleward streams, and the time derivative
of the polar field strength are all well correlated in each hemisphere. These
results are based on statistically significant cyclical patterns in the active
region fields and are consistent with the Babcock-Leighton phenomenological
model for the solar activity cycle. There was more hemispheric asymmetry in the
activity level, as measured by total and maximum active region flux, during
late Cycle 23 (after around 2004), when the southern hemisphere was more
active, and Cycle 24 up to the present, when the northern hemisphere has been
more active, than at any other time since 1974. The active region net proxy
poloidal fields effectively disappeared in both hemispheres around 2004, and
the polar fields did not become significantly stronger after this time. We see
evidence that the process of Cycle 24 field reversal has begun at both poles.Comment: Accepted for publication in Solar Physic
Liouvillian Approach to the Integer Quantum Hall Effect Transition
We present a novel approach to the localization-delocalization transition in
the integer quantum Hall effect. The Hamiltonian projected onto the lowest
Landau level can be written in terms of the projected density operators alone.
This and the closed set of commutation relations between the projected
densities leads to simple equations for the time evolution of the density
operators. These equations can be used to map the problem of calculating the
disorder averaged and energetically unconstrained density-density correlation
function to the problem of calculating the one-particle density of states of a
dynamical system with a novel action. At the self-consistent mean-field level,
this approach yields normal diffusion and a finite longitudinal conductivity.
While we have not been able to go beyond the saddle point approximation
analytically, we show numerically that the critical localization exponent can
be extracted from the energetically integrated correlation function yielding
in excellent agreement with previous finite-size scaling
studies.Comment: 9 pages, submitted to PR
Lattice calculations for A=3,4,6,12 nuclei using chiral effective field theory
We present lattice calculations for the ground state energies of tritium,
helium-3, helium-4, lithium-6, and carbon-12 nuclei. Our results were
previously summarized in a letter publication. This paper provides full details
of the calculations. We include isospin-breaking, Coulomb effects, and
interactions up to next-to-next-to-leading order in chiral effective field
theory.Comment: 38 pages, 11 figures, final publication versio
Exact Master Equation and Non-Markovian Decoherence for Quantum Dot Quantum Computing
In this article, we report the recent progress on decoherence dynamics of
electrons in quantum dot quantum computing systems using the exact master
equation we derived recently based on the Feynman-Vernon influence functional
approach. The exact master equation is valid for general nanostructure systems
coupled to multi-reservoirs with arbitrary spectral densities, temperatures and
biases. We take the double quantum dot charge qubit system as a specific
example, and discuss in details the decoherence dynamics of the charge qubit
under coherence controls. The decoherence dynamics risen from the entanglement
between the system and the environment is mainly non-Markovian. We further
discuss the decoherence of the double-dot charge qubit induced by quantum point
contact (QPC) measurement where the master equation is re-derived using the
Keldysh non-equilibrium Green function technique due to the non-linear coupling
between the charge qubit and the QPC. The non-Markovian decoherence dynamics in
the measurement processes is extensively discussed as well.Comment: 15 pages, Invited article for the special issue "Quantum Decoherence
and Entanglement" in Quantum Inf. Proces
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