245 research outputs found
Freezing of the quantum Hall liquid at 1/7 and 1/9
We compare the free energy computed from the ground state energy and
low-lying excitations of the 2-D Wigner solid and the fractional quantum Hall
liquid, at magnetic filling factors and 1/9. We find that the
Wigner solid melts into the fractional quantum Hall liquid at roughly the same
temperature as that of some recent luminescence experiments, while it remains a
solid at the lower temperatures characteristic of the transport experiments. We
propose this melting as a consistent interpretation of both sets of
experiments.Comment: uses RevTeX 2.0 or 3.
Laughlin liquid - Wigner solid transition at high density in wide quantum wells
Assuming that the phase transition between the Wigner solid and the Laughlin
liquid is first-order, we compare ground-state energies to find features of the
phase diagram at fixed . Rather than use the Coulomb interaction, we
calculate the effective interaction in a square quantum well, and fit the
results to a model interaction with length parameter roughly
proportional to the width of the well. We find a transition to the Wigner solid
phase at high density in very wide wells, driven by the softening of the
interaction at short distances, as well as the more well-known transition to
the Wigner solid at low density, driven by Landau-level mixing.Comment: RevTeX 3.0, 3 Postscript figures appended in uuencoded forma
Connecting the Reentrant Insulating Phase and the Zero Field Metal-Insulator Transition in a 2D Hole System
We present the transport and capacitance measurements of 10nm wide GaAs
quantum wells with hole densities around the critical point of the 2D
metal-insulator transition (critical density down to
0.8/cm, 36). For metallic hole density /cm, a reentrant insulating phase (RIP) is observed
between the =1 quantum Hall state and the zero field metallic state and is
attributed to the formation of pinned Wigner crystal. Through studying the
evolution of the RIP versus 2D hole density by transport and capacitance
experiments, we show that the RIP is incompressible and continuously connected
to the zero field insulator, suggesting a similar origin for these two phases.Comment: pdf with higher resolution figures and other related papers can be
found at http://gaogroup.case.ed
Resonance effects due to the excitation of surface Josephson plasma waves in layered superconductors
We analytically examine the excitation of surface Josephson plasma waves
(SJPWs) in periodically-modulated layered superconductors. We show that the
absorption of the incident electromagnetic wave can be substantially increased,
for certain incident angles, due to the resonance excitation of SJPWs. The
absorption increase is accompanied by the decrease of the specular reflection.
Moreover, we find the physical conditions guaranteeing the total absorption
(and total suppression of the specular reflection). These conditions can be
realized for Bi2212 superconductor films.Comment: 17 pages, 3 figure
Dynamical Decoupling Using Slow Pulses: Efficient Suppression of 1/f Noise
The application of dynamical decoupling pulses to a single qubit interacting
with a linear harmonic oscillator bath with spectral density is studied,
and compared to the Ohmic case. Decoupling pulses that are slower than the
fastest bath time-scale are shown to drastically reduce the decoherence rate in
the case. Contrary to conclusions drawn from previous studies, this shows
that dynamical decoupling pulses do not always have to be ultra-fast. Our
results explain a recent experiment in which dephasing due to charge
noise affecting a charge qubit in a small superconducting electrode was
successfully suppressed using spin-echo-type gate-voltage pulses.Comment: 5 pages, 3 figures. v2: Many changes and update
Magnetoroton scattering by phonons in the fractional quantum Hall regime
Motivated by recent phonon spectroscopy experiments in the fractional quantum
Hall regime we consider processes in which thermally excited magnetoroton
excitations are scattered by low energy phonons. We show that such scattering
processes can never give rise to dissociation of magnetorotons into unbound
charged quasiparticles as had been proposed previously. In addition we show
that scattering of magnetorotons to longer wavelengths by phonon absorption is
possible because of the shape of the magnetoroton dispersion curve and it is
shown that there is a characteristic cross-over temperature above which the
rate of energy transfer to the electron gas changes from an exponential
(activated) to a power law dependence on the effective phonon temperature.Comment: LaTex document, 3 eps figures. submitted to Phys Rev
Transition from an electron solid to the sequence of fractional quantum Hall states at very low Landau level filling factor
At low Landau level filling of a two-dimensional electron system, typically
associated with the formation of an electron crystal, we observe local minima
in Rxx at filling factors nu=2/11, 3/17, 3/19, 2/13, 1/7, 2/15, 2/17, and 1/9.
Each of these developing fractional quantum Hall (FQHE) states appears only
above a filling factor-specific temperature. This can be interpreted as the
melting of an electron crystal and subsequent FQHE liquid formation. The
observed sequence of FQHE states follow the series of composite fermion states
emanating from nu=1/6 and nu=1/8
Coupled phonon-ripplon modes in a single wire of electrons on the liquid-helium surface
The coupled phonon-ripplon modes of the quasi-one-dimensional electron chain
on the liquid helium sutface are studied. It is shown that the electron-ripplon
coupling leads to the splitting of the collective modes of the wire with the
appearance of low-frequency modes and high-frequency optical modes starting
from threshold frequencies. The effective masses of an electron plus the
associated dimple for low frequency modes are estimated and the values of the
threshold frequencies are calculated. The results obtained can be used in
experimental attempts to observe the phase transition of the electron wire into
a quasi-ordered phase.Comment: 5 pages, 1 figure, Physical Review (in press
Interaction of photons with plasmas and liquid metals: photoabsorption and scattering
Formulas to describe the photoabsorption and the photon scattering by a
plasma or a liquid metal are derived in a unified manner with each other. It is
shown how the nuclear motion, the free-electron motion and the core-electron
behaviour in each ion in the system determine the structure of photoabsorption
and scattering in an electron-ion mixture. The absorption cross section in the
dipole approximation consists of three terms which represent the absorption
caused by the nuclear motion, the absorption owing to the free-electron motion
producing optical conductivity or inverse Bremsstrahlung, and the absorption
ascribed to the core-electron behaviour in each ion with the Doppler
correction. Also, the photon scattering formula provides an analysis method for
experiments observing the ion-ion dynamical structure factor (DSF), the
electron-electron DSF giving plasma oscillations, and the core-electron DSF
yielding the X-ray Raman (Compton) scattering with a clear definition of the
background scattering for each experiment, in a unified manner. A formula for
anomalous X-ray scattering is also derived for a liquid metal. At the same
time, Thomson scattering in plasma physics is discussed from this general point
of view.Comment: LaTeX file: 18 pages without figur
Power of Anisotropic Exchange Interactions: Universality and Efficient Codes for Quantum Computing
We study the quantum computational power of a generic class of anisotropic
solid state Hamiltonians. A universal set of encoded logic operations are found
which do away with difficult-to-implement single-qubit gates in a number of
quantum computer proposals, e.g., quantum dots and donor atom spins with
anisotropic exchange coupling, quantum Hall systems, and electrons floating on
helium.We show how to make the corresponding Hamiltonians universal by encoding
one qubit into two physical qubits, and by controlling nearest neighbor
interactions.Comment: 5 pages, no figures. v4: Title and abstract changed. Added paragraph
on state preparation and measurement. Parafermionic notation replaced with
standard qubit notation. For treatment of qubits as parafermions see
http://xxx.lanl.gov/abs/quant-ph/010907
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