173 research outputs found
Quantized Thermal Transport in the Fractional Quantum Hall Effect
We analyze thermal transport in the fractional quantum Hall effect (FQHE),
employing a Luttinger liquid model of edge states. Impurity mediated
inter-channel scattering events are incorporated in a hydrodynamic description
of heat and charge transport. The thermal Hall conductance, , is shown to
provide a new and universal characterization of the FQHE state, and reveals
non-trivial information about the edge structure. The Lorenz ratio between
thermal and electrical Hall conductances {\it violates} the free-electron
Wiedemann-Franz law, and for some fractional states is predicted to be {\it
negative}. We argue that thermal transport may provide a unique way to detect
the presence of the elusive upstream propagating modes, predicted for fractions
such as and .Comment: 6 pages REVTeX, 2 postscript figures (uuencoded and compressed
Randomness at the Edge: Theory of Quantum Hall transport at filling
Current Luttinger liquid edge state theories for filling predict a
non-universal Hall conductance, in disagreement with experiment. Upon inclusion
of random edge tunnelling we find a phase transition into a new
disordered-dominated edge phase. An exact solution of the random model in this
phase gives a quantized Hall conductance of 2/3 and a neutral mode propagating
upstream. The presence of the neutral mode changes the predicted temperature
dependence for tunnelling through a point contact from to .Comment: 12 pages 1 postscript figure appended, REVTEX 3.
Edge Dynamics in Quantum Hall Bilayers II: Exact Results with Disorder and Parallel Fields
We study edge dynamics in the presence of interlayer tunneling, parallel
magnetic field, and various types of disorder for two infinite sequences of
quantum Hall states in symmetric bilayers. These sequences begin with the 110
and 331 Halperin states and include their fractional descendants at lower
filling factors; the former is easily realized experimentally while the latter
is a candidate for the experimentally observed quantum Hall state at a total
filling factor of 1/2 in bilayers. We discuss the experimentally interesting
observables that involve just one chiral edge of the sample and the correlation
functions needed for computing them. We present several methods for obtaining
exact results in the presence of interactions and disorder which rely on the
chiral character of the system. Of particular interest are our results on the
331 state which suggest that a time-resolved measurement at the edge can be
used to discriminate between the 331 and Pfaffian scenarios for the observed
quantum Hall state at filling factor 1/2 in realistic double-layer systems.Comment: revtex+epsf; two-up postscript at
http://www.sns.ias.edu/~leonid/ntwoup.p
Impurity scattering and transport of fractional Quantum Hall edge state
We study the effects of impurity scattering on the low energy edge state
dynamic s for a broad class of quantum Hall fluids at filling factor , for integer and even integer . When is positive all
of the edge modes are expected to move in the same direction, whereas for
negative one mode moves in a direction opposite to the other modes.
Using a chiral-Luttinger model to describe the edge channels, we show that for
an ideal edge when is negative, a non-quantized and non-universal Hall
conductance is predicted. The non-quantized conductance is associated with an
absence of equilibration between the edge channels. To explain the robust
experimental Hall quantization, it is thus necessary to incorporate impurity
scattering into the model, to allow for edge equilibration. A perturbative
analysis reveals that edge impurity scattering is relevant and will modify the
low energy edge dynamics. We describe a non-perturbative solution for the
random channel edge, which reveals the existence of a new
disorder-dominated phase, characterized by a stable zero temperature
renormalization group fixed point. The phase consists of a single propagating
charge mode, which gives a quantized Hall conductance, and neutral modes.
The neutral modes all propagate at the same speed, and manifest an exact SU(n)
symmetry. At finite temperatures the SU(n) symmetry is broken and the neutral
modes decay with a finite rate which varies as at low temperatures.
Various experimental predictions and implications which follow from the exact
solution are described in detail, focusing on tunneling experiments through
point contacts.Comment: 19 pages (two column), 5 post script figures appended, 3.0 REVTE
Single Spin Measurement using Single Electron Transistors to Probe Two Electron Systems
We present a method for measuring single spins embedded in a solid by probing
two electron systems with a single electron transistor (SET). Restrictions
imposed by the Pauli Principle on allowed two electron states mean that the
spin state of such systems has a profound impact on the orbital states
(positions) of the electrons, a parameter which SET's are extremely well suited
to measure. We focus on a particular system capable of being fabricated with
current technology: a Te double donor in Si adjacent to a Si/SiO2 interface and
lying directly beneath the SET island electrode, and we outline a measurement
strategy capable of resolving single electron and nuclear spins in this system.
We discuss the limitations of the measurement imposed by spin scattering
arising from fluctuations emanating from the SET and from lattice phonons. We
conclude that measurement of single spins, a necessary requirement for several
proposed quantum computer architectures, is feasible in Si using this strategy.Comment: 22 Pages, 8 Figures; revised version contains updated references and
small textual changes. Submitted to Phys. Rev.
An NMR-based nanostructure switch for quantum logic
We propose a nanostructure switch based on nuclear magnetic resonance (NMR)
which offers reliable quantum gate operation, an essential ingredient for
building a quantum computer. The nuclear resonance is controlled by the magic
number transitions of a few-electron quantum dot in an external magnetic field.Comment: 4 pages, 2 separate PostScript figures. Minor changes included. One
reference adde
Voltage-tunable singlet-triplet transition in lateral quantum dots
Results of calculations and high source-drain transport measurements are
presented which demonstrate voltage-tunable entanglement of electron pairs in
lateral quantum dots. At a fixed magnetic field, the application of a
judiciously-chosen gate voltage alters the ground-state of an electron pair
from an entagled spin singlet to a spin triplet.Comment: 8.2 double-column pages, 10 eps figure
Vacuum structure of Toroidal Carbon Nanotubes
Low energy excitations in carbon nanotubes can be described by an effective
field theory of two components spinor. It is pointed out that the chiral
anomaly in 1+1 dimensions should be observed in a metallic toroidal carbon
nanotube on a planar geometry with varying magnetic field. We propose an
experimental setup for studying this quantum effect. We also analyze the vacuum
structure of the metallic toroidal carbon nanotube including the Coulomb
interactions and discuss some effects of external charges on the vacuum.Comment: 10 pages, 11 figure
Double quantum dot turnstile as an electron spin entangler
We study the conditions for a double quantum dot system to work as a reliable
electron spin entangler, and the efficiency of a beam splitter as a detector
for the resulting entangled electron pairs. In particular, we focus on the
relative strengths of the tunneling matrix elements, the applied bias and gate
voltage, the necessity of time-dependent input/output barriers, and the
consequence of considering wavepacket states for the electrons as they leave
the double dot to enter the beam splitter. We show that a double quantum dot
turnstile is, in principle, an efficient electron spin entangler or
entanglement filter because of the exchange coupling between the dots and the
tunable input/output potential barriers, provided certain conditions are
satisfied in the experimental set-up.Comment: published version; minor error correcte
RHESSI Results -- Time For a Rethink?
Hard X-rays and gamma-rays are the most direct signatures of energetic
electrons and ions in the sun's atmosphere which is optically thin at these
energies and their radiation involves no coherent processes. Being collisional
they are complementary to gyro-radiation in probing atmospheric density as
opposed to magnetic field and the electrons are primarily 10--100 keV in
energy, complementing the (>100 keV) electrons likely responsible for microwave
bursts.
The pioneering results of the Ramaty High Energy Solar Spectroscopic Imager
(RHESSI) are raising the first new major questions concerning solar energetic
particles in many years. Some highlights of these results are discussed --
primarily around RHESSI topics on which the authors have had direct research
involvement -- particularly when they are raising the need for re-thinking of
entrenched ideas. Results and issues are broadly divided into discoveries in
the spatial, temporal and spectral domains, with the main emphasis on flare
hard X-rays/fast electrons but touching also on gamma-rays/ions, non-flare
emissions, and the relationship to radio bursts.Comment: Proceedings CESRA Workshop 2004: "The High Energy Solar Corona:
Waves, Eruptions, Particles", Lecture Notes in Physics, 2006 (accepted
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