32 research outputs found
A low power photoemission source for electrons on liquid helium
Electrons on the surface of liquid helium are a widely studied system that
may also provide a promising method to implement a quantum computer. One
experimental challenge in these studies is to generate electrons on the helium
surface in a reliable manner without heating the cryo-system. An electron
source relying on photoemission from a zinc film has been previously described
using a high power continuous light source that heated the low temperature
system. This work has been reproduced more compactly by using a low power
pulsed lamp that avoids any heating. About 5e3 electrons are collected on 1
cm^2 of helium surface for every pulse of light. A time-resolved experiment
suggests that electrons are either emitted over or tunnel through the 1eV
barrier formed by the thin superfluid helium film on the zinc surface. No
evidence of trapping or bubble formation is seen.Comment: 9 pages, 3 figures, submitted to J. Low Temp. Phy
Quantum Hall Effect induced by electron-electron interaction in disordered GaAs layers with 3D spectrum
It is shown that the observed Quantum Hall Effect in epitaxial layers of
heavily doped n-type GaAs with thickness (50-140 nm) larger the mean free path
of the conduction electrons (15-30 nm) and, therefore, with a three-dimensional
single-particle spectrum is induced by the electron-electron interaction. The
Hall resistance R_xy of the thinnest sample reveals a wide plateau at small
activation energy E_a=0.4 K found in the temperature dependence of the
transverse resistance R_xx. The different minima in the transverse conductance
G_xx of the different samples show a universal temperature dependence
(logarithmic in a large range of rescaled temperatures T/T_0) which is
reminiscent of electron-electron-interaction effects in coherent diffusive
transport.Comment: 6 pages, 3 figures, 1 tabl
Anomalous NMR Spin-Lattice Relaxation in SrB_{6} and Ca_{1-x}La_{x}B_{6}
We report the results of {11}B nuclear magnetic resonance (NMR) measurements
of SrB_{6} and Ca_{0.995}La_{0.05}B_{6} below room temperature. Although the
electrical resistivities of these two materials differ substantially, their
{11}B-NMR responses exhibit some strikingly common features. Both materials
exhibit ferromagnetic order, but their {11}B-NMR spectra reveal very small
hyperfine fields at the Boron sites. The spin lattice relaxation T_{1}^{-1}
varies considerably with external field but changes with temperature only below
a few K. We discuss these unusual results by considering various different
scenarios for the electronic structure of these materials.Comment: Accepted for publication in Phys. Rev. B Rapid communication, 4
pages, 3 figures. This manuscript replaces an earlier version and includes
some minor changes in the text and in Fig.
Disordered Hubbard Model with Attraction: Coupling Energy of Cooper Pairs in Small Clusters
We generalize the Cooper problem to the case of many interacting particles in
the vicinity of the Fermi level in the presence of disorder. On the basis of
this approach we study numerically the variation of the pair coupling energy in
small clusters as a function of disorder. We show that the Cooper pair energy
is strongly enhanced by disorder, which at the same time leads to the
localization of pairs.Comment: revtex, 5 pages, 6 figure
Symmetry in the insulator - quantum Hall - insulator transitions observed in a Ge/SiGe quantum well
We examine the magnetic field driven insulator-quantum Hall-insulator
transitions of the two dimensional hole gas in a Ge/SiGe quantum well. We
observe direct transitions between low and high magnetic field insulators and
the quantum Hall state. With increasing magnetic field, the transitions
from insulating to quantum Hall and quantum Hall to insulating are very similar
with respect to their transport properties. We address the temperature
dependence around the transitions and show that the characteristic energy scale
for the high field transition is larger.Comment: 4 page
Two-subband electron transport in nonideal quantum wells
Electron transport in nonideal quantum wells (QW) with large-scale variations
of energy levels is studied when two subbands are occupied. Although the mean
fluctuations of these two levels are screened by the in-plane redistribution of
electrons, the energies of both levels remain nonuniform over the plane. The
effect of random inhomogeneities on the classical transport is studied within
the framework of a local response approach for weak disorder. Both short-range
and small-angle scattering mechanisms are considered. Magnetotransport
characteristics and the modulation of the effective conductivity by transverse
voltage are evaluated for different kinds of confinement potentials (hard wall
QW, parabolic QW, and stepped QW).Comment: 10 pages, 6 figure
Quantum Correlated Interstitials and the Hall Resistivity of the Magnetically Induced Wigner Crystal
We study a trial wavefunction for an interstitial in a Wigner crystal. We
find that the electron correlations, ignored in a conventional Hartree-Fock
treatment, dramatically lower the interstitial energy, especially at fillings
close to an incompressible liquid state. The correlation between the
interstitial electron and the lattice electrons at is introduced by
constructing a trial wave- function which bears a Jastrow factor of a Laughlin
state at . For fillings close to but just below , we find
that a perfect Wigner crystal becomes unstable against formation of such
interstitials. It is argued that conduction due to correlated interstitials in
the presence of weak disorder leads to the {\it classical} Hall resistivity, as
seen experimentally.Comment: 10 pages, RevTe
On The Mobile Behavior of Solid He at High Temperatures
We report studies of solid helium contained inside a torsional oscillator, at
temperatures between 1.07K and 1.87K. We grew single crystals inside the
oscillator using commercially pure He and He-He mixtures containing
100 ppm He. Crystals were grown at constant temperature and pressure on the
melting curve. At the end of the growth, the crystals were disordered,
following which they partially decoupled from the oscillator. The fraction of
the decoupled He mass was temperature and velocity dependent. Around 1K, the
decoupled mass fraction for crystals grown from the mixture reached a limiting
value of around 35%. In the case of crystals grown using commercially pure
He at temperatures below 1.3K, this fraction was much smaller. This
difference could possibly be associated with the roughening transition at the
solid-liquid interface.Comment: 15 pages, 6 figure
Time dependent mean field theory of the superfluid-insulator phase transition
We develop a time-dependent mean field approach, within the time-dependent
variational principle, to describe the Superfluid-Insulator quantum phase
transition. We construct the zero temperature phase diagram both of the
Bose-Hubbard model (BHM), and of a spin-S Heisenberg model (SHM) with the XXZ
anisotropy. The phase diagram of the BHM indicates a phase transition from a
Mott insulator to a compressibile superfluid phase, and shows the expected
lobe-like structure. The SHM phase diagram displays a quantum phase transition
between a paramagnetic and a canted phases showing as well a lobe-like
structure. We show how the BHM and Quantum Phase model (QPM) can be rigorously
derived from the SHM. Based on such results, the phase boundaries of the SHM
are mapped to the BHM ones, while the phase diagram of the QPM is related to
that of the SHM. The QPM's phase diagram obtained through the application of
our approach to the SHM, describes the known onset of the macroscopic phase
coherence from the Coulomb blockade regime for increasing Josephson coupling
constant. The BHM and the QPM phase diagrams are in good agreement with Quantum
Monte Carlo results, and with the third order strong coupling perturbative
expansion.Comment: 15 pages, 8 figures. To be published in Phys. Rev.
A glassy contribution to the heat capacity of hcp He solids
We model the low-temperature specific heat of solid He in the hexagonal
closed packed structure by invoking two-level tunneling states in addition to
the usual phonon contribution of a Debye crystal for temperatures far below the
Debye temperature, . By introducing a cutoff energy in the
two-level tunneling density of states, we can describe the excess specific heat
observed in solid hcp He, as well as the low-temperature linear term in the
specific heat. Agreement is found with recent measurements of the temperature
behavior of both specific heat and pressure. These results suggest the presence
of a very small fraction, at the parts-per-million (ppm) level, of two-level
tunneling systems in solid He, irrespective of the existence of
supersolidity.Comment: 11 pages, 4 figure