1,135 research outputs found
Elastic theory of quantum Hall smectics: effects of disorder
We study the effect of disorder on quantum Hall smectics within the framework
of an elastic theory. Based on a renormalization group calculation, we derive
detailed results for the degrees of translational and orientational order of
the stripe pattern at zero temperature and carefully map out the disorder and
length-scale regimes in which the system effectively exhibits smectic, nematic,
or isotropic behavior. We show that disorder always leads to a finite density
of free dislocations and estimate the scale on which they begin to appear.Comment: 4 pages latex with 1 EPS figur
Parametric resonance of a two-dimensional electron gas under bichromatic irradiation
In an ultrahigh mobility 2D electron gas, even a weak nonparabolicity of the
electron dispersion, by violating Kohn's theorem, can have a drastic effect on
dc magnetotransport under ac drive. We study theoretically the manifestation of
this effect in the dc response to the combined action of two driving ac-fields
(bichromatic irradiation). Compared to the case of monochromatic irradiation,
which is currently intensively studied both experimentally and theoretically,
the presence of a second microwave source provides additional insight into the
properties of an ac-driven 2D electron gas. In particular, we find that
nonparabolicity, being the simplest cause for a violation of Kohn's theorem,
gives rise to new qualitative effects specific to bichromatic irradiation.
Namely, when the frequencies and are well away from the
cyclotron frequency, , our simple classical considerations
demonstrate that the system becomes parametrically unstable with respect to
fluctuations with frequency . As an additional effect of
nonparabolicity, this parametric instability can manifest itself in the dc
properties of the system. This happens when , and
are related as 3:1:2, respectively. Even for weak detuning between
and , the effect of the bichromatic irradiation on the dc
response in the presence of nonparabolicity can differ dramatically from the
monochromatic case. In particular, the equations of motion can acquire
multistable solutions. As a result, the diagonal dc-conductivity can assume
several stable negative values at the same magnetic field.Comment: 11 pages, 10 figure
General Localization Lengths for Two Interacting Particles in a Disordered Chain
The propagation of an interacting particle pair in a disordered chain is
characterized by a set of localization lengths which we define. The
localization lengths are computed by a new decimation algorithm and provide a
more comprehensive picture of the two-particle propagation. We find that the
interaction delocalizes predominantly the center-of-mass motion of the pair and
use our approach to propose a consistent interpretation of the discrepancies
between previous numerical results.Comment: 4 pages, 2 epsi figure
Readout of Majorana qubits
Schemes for topological quantum computation with Majorana bound states rely heavily on the ability to projectively measure products of Majorana operators. Here, we employ Markovian quantum measurement theory, including the readout device, to analyze such measurements. Specifically, we focus on the readout of Majorana qubits via continuous charge sensing of a tunnel-coupled quantum dot by a quantum point contact. We show that projective measurements of Majorana products Pi(i) (gamma) over cap (i) can be implemented by continuous charge sensing under quite general circumstances. Essential requirements are that a combined local parity (pi) over cap, involving the quantum dot charge along with the Majorana product of interest, be conserved, and that the two eigenspaces of the combined parity (pi) over cap generate distinguishable measurement signals. We find that qubit readout may have to rely on measuring noise correlations of the quantum-point-contact current. The average current encodes the qubit readout only transiently for fine-tuned parameters or in the presence of relaxation processes. We also discuss the corresponding measurement and decoherence times and consider processes, such as residual Majorana hybridizations, which are detrimental to the measurement protocol. Finally, we emphasize that the underlying mechanism-which we term symmetry-protected readout-is quite general and has further implications for both Majorana and non-Majorana systems
Discrete charging of a quantum dot strongly coupled to external leads
We examine a quantum dot with levels which is strongly coupled
to leads for varying number of channels in the leads. It is shown both
analytically and numerically that for strong couplings between the dot and the
leads, at least bound states (akin to subradiant states in
optics) remain on the dot. These bound states exhibit discrete charging and,
for a significant range of charging energies, strong Coulomb blockade behavior
as function of the chemical potential. The physics changes for large charging
energy where the same (superradiant) state is repeatedly charged.Comment: 5 pages, 3 figures (accepted for publication in EPL
Chaos and Interacting Electrons in Ballistic Quantum Dots
We show that the classical dynamics of independent particles can determine
the quantum properties of interacting electrons in the ballistic regime. This
connection is established using diagrammatic perturbation theory and
semiclassical finite-temperature Green functions. Specifically, the orbital
magnetism is greatly enhanced over the Landau susceptibility by the combined
effects of interactions and finite size. The presence of families of periodic
orbits in regular systems makes their susceptibility parametrically larger than
that of chaotic systems, a difference which emerges from correlation terms.Comment: 4 pages, revtex, includes 3 postscript fig
Semiclassical Approach to Orbital Magnetism of Interacting Diffusive Quantum Systems
We study interaction effects on the orbital magnetism of diffusive mesoscopic
quantum systems. By combining many-body perturbation theory with semiclassical
techniques, we show that the interaction contribution to the ensemble averaged
quantum thermodynamic potential can be reduced to an essentially classical
operator. We compute the magnetic response of disordered rings and dots for
diffusive classical dynamics. Our semiclassical approach reproduces the results
of previous diagrammatic quantum calculations.Comment: 8 pages, revtex, includes 1 postscript fi
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