727 research outputs found
Gating of high-mobility InAs metamorphic heterostructures
We investigate the performance of gate-defined devices fabricated on high
mobility InAs metamorphic heterostructures. We find that heterostructures
capped with InGaAs often show signs of parallel conduction
due to proximity of their surface Fermi level to the conduction band minimum.
Here, we introduce a technique that can be used to estimate the density of this
surface charge that involves cool-downs from room temperature under gate bias.
We have been able to remove the parallel conduction under high positive bias,
but achieving full depletion has proven difficult. We find that by using
InAlAs as the barrier without an InGaAs
capping, a drastic reduction in parallel conduction can be achieved. Our
studies show that this does not change the transport properties of the quantum
well significantly. We achieved full depletion in InAlAs capped
heterostructures with non-hysteretic gating response suitable for fabrication
of gate-defined mesoscopic devices
Vanishing quantum discord is necessary and sufficient for completely positive maps
Two long standing open problems in quantum theory are to characterize the
class of initial system-bath states for which quantum dynamics is equivalent to
(1) a map between the initial and final system states, and (2) a completely
positive (CP) map. The CP map problem is especially important, due to the
widespread use of such maps in quantum information processing and open quantum
systems theory. Here we settle both these questions by showing that the answer
to the first is "all", with the resulting map being Hermitian, and that the
answer to the second is that CP maps arise exclusively from the class of
separable states with vanishing quantum discord.Comment: 4 pages, no figures. v2: Accepted for publication in Phys. Rev. Let
Correlated errors can lead to better performance of quantum codes
A formulation for evaluating the performance of quantum error correcting
codes for a general error model is presented. In this formulation, the
correlation between errors is quantified by a Hamiltonian description of the
noise process. We classify correlated errors using the system-bath interaction:
local versus nonlocal and two-body versus many-body interactions. In
particular, we consider Calderbank-Shor-Steane codes and observe a better
performance in the presence of correlated errors depending on the timing of the
error recovery. We also find this timing to be an important factor in the
design of a coding system for achieving higher fidelities.Comment: 5 pages, 3 figures. Replaced by the published version. Title change
Locally Optimal Control of Quantum Systems with Strong Feedback
For quantum systems with high purity, we find all observables that, when
continuously monitored, maximize the instantaneous reduction in the von Neumann
entropy. This allows us to obtain all locally optimal feedback protocols with
strong feedback, and explicit expressions for the best such protocols for
systems of size N <= 4. We also show that for a qutrit the locally optimal
protocol is the optimal protocol for a given range of control times, and derive
an upper bound on all optimal protocols with strong feedback.Comment: 4 pages, Revtex4. v2: published version (some errors corrected
Anisotropic low-temperature piezoresistance in (311)A GaAs two-dimensional holes
We report low-temperature resistance measurements in a modulation-doped,
(311)A GaAs two-dimensional hole system as a function of applied in-plane
strain. The data reveal a strong but anisotropic piezoresistance whose
magnitude depends on the density as well as the direction along which the
resistance is measured. At a density of cm and for a
strain of about applied along [01], e.g., the
resistance measured along this direction changes by nearly a factor of two
while the resistance change in the [33] direction is less than 10% and
has the opposite sign. Our accurate energy band calculations indicate a
pronounced and anisotropic deformation of the heavy-hole dispersion with
strain, qualitatively consistent with the experimental data. The extremely
anisotropic magnitude of the piezoresistance, however, lacks a quantitative
explanation.Comment: 4 pages. Submitted to Applied Physics Letter
The galaxy rotation curves in the modified gravity formalism
Astronomical data have shown that the galaxy rotation curves are mostly flat
in the far distance of the galactic cores, which reveals the insufficiency of
our knowledges about how gravity works in these regimes. In this paper we
introduce a resolution of this issue from the modified gravity
formalism perspective. By investigating two classes of models with separable
(minimal coupling model) and inseparable (non-minimal coupling model) parts of
the Ricci scalar and trace of the energy-momentum tensor , we find that
only in the latter models it is possible to attain flat galaxy rotation curves.Comment: 9 pages, 4 figure
Geometrical effects on energy transfer in disordered open quantum systems
We explore various design principles for efficient excitation energy
transport in complex quantum systems. We investigate energy transfer efficiency
in randomly disordered geometries consisting of up to 20 chromophores to
explore spatial and spectral properties of small natural/artificial
Light-Harvesting Complexes (LHC). We find significant statistical correlations
among highly efficient random structures with respect to ground state
properties, excitonic energy gaps, multichromophoric spatial connectivity, and
path strengths. These correlations can even exist beyond the optimal regime of
environment-assisted quantum transport. For random configurations embedded in
spatial dimensions of 30 A and 50 A, we observe that the transport efficiency
saturates to its maximum value if the systems contain 7 and 14 chromophores
respectively. Remarkably, these optimum values coincide with the number of
chlorophylls in (Fenna-Matthews-Olson) FMO protein complex and LHC II monomers,
respectively, suggesting a potential natural optimization with respect to
chromophoric density.Comment: 11 pages, 10 figures. Expanded from the former appendix to
arXiv:1104.481
Inhibitory effect of citric acid on rancidity of frozen catfish (Silurus glanis)fillets
The effect of aqueous solution of citric acid (CA) on the lipid stability of the European catfish (Silurus glanis) fillets during frozen storage (up to 6 months) was investigated. Rancidity development was measured by several biochemical indices (free fatty acids, peroxides and TBA) and complemented by the measurement of expressible moisture and sensory evaluation (flesh odor, consistency and flesh appearance). The CA treatment led to some lower free fatty acids (6.43% oleic acid in total lipid at month 6), peroxides (7.98meq oxygen/kg lipids, at month 6) and secondary compounds formation (2.16mg malondialdehyde/kg fish sample). Lower peroxide, thiobarbituric acid-reactive substance (TBA), free fatty acids (during 0,3 and 6 months frozen storage), and expressible moisture (month 6) values were obtained from CA-treated fish fillets than the untreated (blank control) ones (P<0.05)
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