219 research outputs found
Thermopower of Interacting GaAs Bilayer Hole Systems in the Reentrant Insulating Phase near
We report thermopower measurements of interacting GaAs bilayer hole systems.
When the carrier densities in the two layers are equal, these systems exhibit a
reentrant insulating phase near the quantum Hall state at total filling factor
. Our data show that as the temperature is decreased, the thermopower
diverges in the insulating phase. This behavior indicates the opening of an
energy gap at low temperature, consistent with the formation of a pinned Wigner
solid. We extract an energy gap and a Wigner solid melting phase diagram.Comment: to be published in Phys. Rev. Let
Heat Capacity Evidence for the Suppression of Skyrmions at Large Zeeman Energy
Measurements on a multilayer two-dimensional electron system (2DES) near
Landau level filling =1 reveal the disappearance of the nuclear spin
contribution to the heat capacity as the ratio between the Zeeman
and Coulomb energies exceeds a critical value 0.04. This
disappearance suggests the vanishing of the Skyrmion-mediated coupling between
the lattice and the nuclear spins as the spin excitations of the 2DES make a
transition from Skyrmions to single spin-flips above . Our
experimental is smaller than the calculated =0.054
for an ideal 2DES; we discuss possible origins of this discrepancy.Comment: Experimental paper, 6 figure
Imaging Electron Wave Functions Inside Open Quantum Rings
Combining Scanning Gate Microscopy (SGM) experiments and simulations, we
demonstrate low temperature imaging of electron probability density
in embedded mesoscopic quantum rings (QRs). The tip-induced
conductance modulations share the same temperature dependence as the
Aharonov-Bohm effect, indicating that they originate from electron wavefunction
interferences. Simulations of both and SGM conductance maps
reproduce the main experimental observations and link fringes in SGM images to
.Comment: new titl
Critical Behavior of Nuclear-Spin Diffusion in GaAs/AlGaAs Heterostructures near Landau Level Filling \nu=1
Thermal measurements on a GaAs/AlGaAs heterostructure reveal that the state
of the confined two-dimensional electrons dramatically affects the nuclear-spin
diffusion near Landau level filling factor \nu=1. The experiments provide
quantitative evidence that the sharp peak in the temperature dependence of heat
capacity near \nu=1 is due to an enhanced nuclear-spin diffusion from the GaAs
quantum wells into the AlGaAs barriers. We discuss the physical origin of this
enhancement in terms the possible Skyrme solid-liquid phase transition.Comment: 1 LateX file, 3 figures, submitte
Thermodynamic Phase Diagram of the Quantum Hall Skyrmion System
We numerically study the interacting quantum Hall skyrmion system based on
the Chern-Simons action. By noticing that the action is invariant under global
spin rotations in the spin space with respect to the magnetic field direction,
we obtain the low-energy effective action for a many skyrmion system.
Performing extensive molecular dynamics simulations, we establish the
thermodynamic phase diagram for a many skyrmion system.Comment: 4 pages, RevTex, 2 postscript figure
Wigner and Kondo physics in quantum point contacts revealed by scanning gate microscopy
Quantum point contacts exhibit mysterious conductance anomalies in addition
to well known conductance plateaus at multiples of 2e^2/h. These 0.7 and
zero-bias anomalies have been intensively studied, but their microscopic origin
in terms of many-body effects is still highly debated. Here we use the charged
tip of a scanning gate microscope to tune in situ the electrostatic potential
of the point contact. While sweeping the tip distance, we observe repetitive
splittings of the zero-bias anomaly, correlated with simultaneous appearances
of the 0.7 anomaly. We interpret this behaviour in terms of alternating
equilibrium and non-equilibrium Kondo screenings of different spin states
localized in the channel. These alternating Kondo effects point towards the
presence of a Wigner crystal containing several charges with different
parities. Indeed, simulations show that the electron density in the channel is
low enough to reach one-dimensional Wigner crystallization over a size
controlled by the tip position
Coherent-State Approach to Two-dimensional Electron Magnetism
We study in this paper the possible occurrence of orbital magnetim for
two-dimensional electrons confined by a harmonic potential in various regimes
of temperature and magnetic field. Standard coherent state families are used
for calculating symbols of various involved observables like thermodynamical
potential, magnetic moment, or spatialdistribution of current. Their
expressions are given in a closed form and the resulting Berezin-Lieb
inequalities provide a straightforward way to study magnetism in various limit
regimes. In particular, we predict a paramagnetic behaviour in the
thermodynamical limit as well as in the quasiclassical limit under a weak
field. Eventually, we obtain an exact expression for the magnetic moment which
yields a full description of the phase diagram of the magnetization.Comment: 21 pages, 6 figures, submitted to PR
NMR Determination of 2D Electron Spin Polarization at
Using a `standard' NMR spin-echo technique we determined the spin
polarization of two-dimensional electrons, confined to GaAs quantum wells, from
the hyperfine shift of Ga nuclei in the wells. Concentrating on the temperature
and magnetic field dependencies of spin polarization at Landau level filling
factor , we find that the results are described well by a simple
model of non-interacting composite fermions, although some inconsistencies
remain when the two-dimensional electron system is tilted in the magnetic
field.Comment: 4 pages (REVTEX) AND 4 figures (PS
Long dephasing time and high temperature ballistic transport in an InGaAs open quantum dot
We report on measurements of the magnetoconductance of an open circular
InGaAs quantum dot between 1.3K and 204K. We observe two types of
magnetoconductance fluctuations: universal conductance fluctuations (UCFs), and
'focusing' fluctuations related to ballistic trajectories between openings. The
electron phase coherence time extracted from UCFs amplitude is larger than in
GaAs/AlGaAs quantum dots and follows a similar temperature dependence (between
T^-1 and T^-2). Below 150K, the characteristic length associated with
'focusing' fluctuations shows a slightly different temperature dependence from
that of the conductivity.Comment: 6 pages, 4 figures, proceedings of ICSNN2002, to appear in Physica
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