157 research outputs found

### Signatures of neutral quantum Hall modes in transport through low-density constrictions

Constrictions in fractional quantum Hall (FQH) systems not only facilitate
backscattering between counter-propagating edge modes, but also may reduce the
constriction filling fraction $\nu_c$ with respect to the bulk filling fraction
$\nu_b$. If both $\nu_b$ and $\nu_c$ correspond to incompressible FQH states,
at least part of the constriction region is surrounded by composite edges,
whose low energy dynamics is characterized by a charge mode and one or several
neutral modes. In the incoherent regime, decay of neutral modes describes the
equilibration of composite FQH edges, while in the limit of coherent transport,
the presence of neutral modes gives rise to universal conductance fluctuations.
In addition, neutral modes renormalize the strength of scattering across the
constriction, and thus can determine the relative strength of forward and
backwards scattering.Comment: corrected description of the results of Ref. [10], Ref. [17] adde

### Are Microwave Induced Zero Resistance States Necessarily Static?

We study the effect of inhomogeneities in Hall conductivity on the nature of
the Zero Resistance States seen in the microwave irradiated two-dimensional
electron systems in weak perpendicular magnetic fields, and we show that
time-dependent domain patterns may emerge in some situations. For an annular
Corbino geometry, with an equilibrium charge density that varies linearly with
radius, we find a time-periodic non-equilibrium solution, which might be
detected by a charge sensor, such as an SET. For a model on a torus, in
addition to static domain patterns seen at high and low values of the
equilibrium charge inhomogeneity, we find that, in the intermediate regime, a
variety of nonstationary states can also exist. We catalog the possibilities we
have seen in our simulations. Within a particular phenomenological model, we
show that linearizing the nonlinear charge continuity equation about a
particularly simple domain wall configuration and analyzing the eigenmodes
allows us to estimate the periods of the solutions to the full nonlinear
equation.Comment: Submitted to PR

### Interference, Coulomb blockade, and the identification of non-abelian quantum Hall states

We examine the relation between different electronic transport phenomena in a
Fabry-Perot interferometer in the fractional quantum Hall regime. In
particular, we study the way these phenomena reflect the statistics of quantum
Hall quasi-particles. For two series of states we examine, one abelian and one
non-abelian, we show that the information that may be obtained from
measurements of the lowest order interference pattern in an open Fabry-Perot
interferometer is identical to the one that may be obtained from the
temperature dependence of Coulomb blockade peaks in a closed interferometer. We
argue that despite the similarity between the experimental signatures of the
two series of states, interference and Coulomb blockade measurements are likely
to be able to distinguish between abelian and non-abelian states, due to the
sensitivity of the abelian states to local perturbations, to which the
non-abelian states are insensitive.Comment: 10 pages. Published versio

### Driven nonlinear dynamics of two coupled exchange-only qubits

Inspired by creation of a fast exchange-only qubit (Medford et al., Phys.
Rev. Lett., 111, 050501 (2013)), we develop a theory describing the nonlinear
dynamics of two such qubits that are capacitively coupled, when one of them is
driven resonantly at a frequency equal to its level splitting. We include
conditions of strong driving, where the Rabi frequency is a significant
fraction of the level splitting, and we consider situations where the splitting
for the second qubit may be the same or different than the first. We
demonstrate that coupling between qubits can be detected by reading the
response of the second qubit, even when the coupling between them is only of
about $1\%$ of their level splittings, and calculate entanglement between
qubits. Patterns of nonlinear dynamics of coupled qubits and their entanglement
are strongly dependent on the geometry of the system, and the specific
mechanism of inter-qubit coupling deeply influences dynamics of both qubits. In
particular, we describe the development of irregular dynamics in a two-qubit
system, explore approaches for inhibiting it, and demonstrate existence of an
optimal range of coupling strength maintaining stability during the operational
time.Comment: 11 pages, 6 figures; One additional figure with changes to the text
about the results. Additional references include

### Spin Order in Paired Quantum Hall States

We consider quantum Hall states at even-denominator filling fractions,
especially $\nu=5/2$, in the limit of small Zeeman energy. Assuming that a
paired quantum Hall state forms, we study spin ordering and its interplay with
pairing. We give numerical evidence that at $\nu = 5/2$ an incompressible
ground state will exhibit spontaneous ferromagnetism. The Ginzburg-Landau
theory for the spin degrees of freedom of paired Hall states is a perturbed
CP$^2$ model. We compute the coefficients in the Ginzburg-Landau theory by a
BCS-Stoner mean field theory for coexisting order parameters, and show that
even if repulsion is smaller than that required for a Stoner instability,
ferromagnetic fluctuations can induce a partially or fully polarized
superconducting state

### The Physical Significance of Singularities in the Chern--Simons Fermi Liquid Description of a Partially Filled Landau Level

We analyze the linear response of a half filled Landau level to long
wavelength and low frequency driving forces, using Fermi liquid theory for
composite fermions. This response is determined by the composite fermions
quasi--particle effective mass, $m^*$, and quasi--particle Landau interaction
function $f(\theta-\theta')$. Analyzing infra--red divergences of perturbation
theory, we get an exact expression for $m^*$, and conjecture the form of the
$f(\theta-\theta')$. We then conclude that in the limit of infinite cyclotron
frequency, and small ${\bf q},\omega$, the composite fermion excitation
spectrum is continuous for $0<\omega<\gamma \frac{e^2}{\epsilon h}q$, with
$\gamma$ an unknown number. For fractional quantum Hall states near a half
filled Landau level, we derive an exact expression for the energy gap.Comment: 4 pages, RevTeX. This paper, being short and non-technical, could
serve as a useful starting point for reading our manuscript cond-mat/9502032.
The present paper does, however, include results not published in the forme

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