908 research outputs found
Dependence of spin susceptibility of a two-dimensional electron system on the valley degree of freedom
We report measurements of the spin susceptibility, ,
in an AlAs two-dimensional electron system where, via the application of
in-plane stress, we transfer electrons from one conduction-band valley to
another ( is the valley degeneracy, and and are the electron
effective mass and g-factor). At a given density, when the two valleys are
equally populated (), the measured is smaller than when only
one valley is occupied (). This observation counters the common
assumption that a two-valley two-dimensional system is effectively more dilute
than a single-valley system because of its smaller Fermi energy.Comment: 4 pages, 3 figure
High-contrast imaging at small separation: impact of the optical configuration of two deformable mirrors on dark holes
The direct detection and characterization of exoplanets will be a major
scientific driver over the next decade, involving the development of very large
telescopes and requires high-contrast imaging close to the optical axis. Some
complex techniques have been developed to improve the performance at small
separations (coronagraphy, wavefront shaping, etc). In this paper, we study
some of the fundamental limitations of high contrast at the instrument design
level, for cases that use a combination of a coronagraph and two deformable
mirrors for wavefront shaping. In particular, we focus on small-separation
point-source imaging (around 1 /D). First, we analytically or
semi-analytically analysing the impact of several instrument design parameters:
actuator number, deformable mirror locations and optic aberrations (level and
frequency distribution). Second, we develop in-depth Monte Carlo simulation to
compare the performance of dark hole correction using a generic test-bed model
to test the Fresnel propagation of multiple randomly generated optics static
phase errors. We demonstrate that imaging at small separations requires large
setup and small dark hole size. The performance is sensitive to the optic
aberration amount and spatial frequencies distribution but shows a weak
dependence on actuator number or setup architecture when the dark hole is
sufficiently small (from 1 to 5 /D).Comment: 13 pages, 18 figure
Classical and quantum massive cosmology for the open FRW universe
In an open Friedmann-Robertson-Walker (FRW) space background, we study the
classical and quantum cosmological models in the framework of the recently
proposed nonlinear massive gravity theory. Although the constraints which are
present in this theory prevent it from admitting the flat and closed FRW models
as its cosmological solutions, for the open FRW universe, it is not the case.
We have shown that, either in the absence of matter or in the presence of a
perfect fluid, the classical field equations of such a theory adopt physical
solutions for the open FRW model, in which the mass term shows itself as a
cosmological constant. These classical solutions consist of two distinguishable
branches: One is a contacting universe which tends to a future singularity with
zero size, while another is an expanding universe having a past singularity
from which it begins its evolution. A classically forbidden region separates
these two branches from each other. We then employ the familiar canonical
quantization procedure in the given cosmological setting to find the
cosmological wave functions. We use the resulting wave function to investigate
the possibility of the avoidance of classical singularities due to quantum
effects. It is shown that the quantum expectation values of the scale factor,
although they have either contracting or expanding phases like their classical
counterparts, are not disconnected from each other. Indeed, the classically
forbidden region may be replaced by a bouncing period in which the scale factor
bounces from the contraction to its expansion eras. Using the Bohmian approach
of quantum mechanics, we also compute the Bohmian trajectory and the quantum
potential related to the system, which their analysis shows are the direct
effects of the mass term on the dynamics of the universe.Comment: 18 pages, 7 figures, typos corrected, refs. adde
Dependence of persistent gaps at Landau level crossings on relative spin
We report measurements of the quantum Hall state energy gap at avoided
crossings between Landau levels originating from different conduction band
valleys in AlAs quantum wells. These gaps exhibit an approximately linear
dependence on magnetic field over a wide range of fields and filling factors.
More remarkably, we observe an unexpected dependence of the gap size on the
relative spin orientation of the crossing levels, with parallel-spin crossings
exhibiting larger gaps than antiparallel-spin crossings.Comment: 4 pages, 3 figures, submitted for publicatio
Valley susceptibility of an interacting two-dimensional electron system
We report direct measurements of the valley susceptibility, the change of
valley population in response to applied symmetry-breaking strain, in an AlAs
two-dimensional electron system. As the two-dimensional density is reduced, the
valley susceptibility dramatically increases relative to its band value,
reflecting the system's strong electron-electron interaction. The increase has
a remarkable resemblance to the enhancement of the spin susceptibility and
establishes the analogy between the spin and valley degrees of freedom.Comment: 5 pages, 2 figure
Valley polarization and susceptibility of composite fermions around nu=3/2
We report magnetotransport measurements of fractional quantum Hall states in
an AlAs quantum well around Landau level filling factor nu = 3/2, demonstrating
that the quasiparticles are composite Fermions (CFs) with a valley degree of
freedom. By monitoring the valley level crossings for these states as a
function of applied symmetry-breaking strain, we determine the CF valley
susceptibility and polarization. The data can be explained well by a simple
Landau level fan diagram for CFs, and are in nearly quantitative agreement with
the results reported for CF spin polarization.Comment: to appear in Phys. Rev. Let
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