29 research outputs found
Wigner Crystals Phases in Bilayer Quantum Hall Systems
(This is a substantially shortened version of the original abstract:)
The Wigner crystal phase diagram of the bilayer systems have been studied
using variational methods. Five crystal phases are obtained. As the layer
spacing increases, the system will undergo a sequence of phase transitions. A
common feature of most bilayer Wigner crystals is that they have mixed
(pseudo-spin) ferromagnetic and antiferromagnetic order.Comment: 19 figures. Figures will be provided upon request. Submitted in PRB
in Nov 94
New insulating phases of two-dimensional electrons in high Landau levels: observation of sharp thresholds to conduction
The intriguing re-entrant integer quantized Hall states recently discovered
in high Landau levels of high-mobility 2D electron systems are found to exhibit
extremely non-linear transport. At small currents these states reflect
insulating behavior of the electrons in the uppermost Landau level. At larger
currents, however, a discontinuous and hysteretic transition to a conducting
state is observed. These phenomena, found only in very narrow magnetic field
ranges, are suggestive of the depinning of a charge density wave state, but
other explanations can also be constructed.Comment: 5 pages, 5 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
Topological Defects, Orientational Order, and Depinning of the Electron Solid in a Random Potential
We report on the results of molecular dynamics simulation (MD) studies of the
classical two-dimensional electron crystal in the presence disorder. Our study
is motivated by recent experiments on this system in modulation doped
semiconductor systems in very strong magnetic fields, where the magnetic length
is much smaller than the average interelectron spacing , as well as by
recent studies of electrons on the surface of helium. We investigate the low
temperature state of this system using a simulated annealing method. We find
that the low temperature state of the system always has isolated dislocations,
even at the weakest disorder levels investigated. We also find evidence for a
transition from a hexatic glass to an isotropic glass as the disorder is
increased. The former is characterized by quasi-long range orientational order,
and the absence of disclination defects in the low temperature state, and the
latter by short range orientational order and the presence of these defects.
The threshold electric field is also studied as a function of the disorder
strength, and is shown to have a characteristic signature of the transition.
Finally, the qualitative behavior of the electron flow in the depinned state is
shown to change continuously from an elastic flow to a channel-like, plastic
flow as the disorder strength is increased.Comment: 31 pages, RevTex 3.0, 15 figures upon request, accepted for
publication in Phys. Rev. B., HAF94MD
Normal-state conductivity in underdoped La_{2-x}Sr_xCuO_4 thin films: Search for nonlinear effects related to collective stripe motion
We report a detailed study of the electric-field dependence of the
normal-state conductivity in La_{2-x}Sr_xCuO_4 thin films for two
concentrations of doped holes, x=0.01 and 0.06, where formation of diagonal and
vertical charged stripes was recently suggested. In order to elucidate whether
high electric fields are capable of depinning the charged stripes and inducing
their collective motion, we have measured current-voltage characteristics for
various orientations of the electric field with respect to the crystallographic
axes. However, even for the highest possible fields (~1000 V/cm for x=0.01 and
\~300 V/cm for x=0.06) we observed no non-linear-conductivity features except
for those related to the conventional Joule heating of the films. Our analysis
indicates that Joule heating, rather than collective electron motion, may also
be responsible for the non-linear conductivity observed in some other 2D
transition-metal oxides as well. We discuss that a possible reason why moderate
electric fields fail to induce a collective stripe motion in layered oxides is
that fairly flexible and compressible charged stripes can adjust themselves to
the crystal lattice and individual impurities, which makes their pinning much
stronger than in the case of conventional rigid charge-density waves.Comment: 10 pages, 10 figures, accepted for publication in Phys. Rev.
The two dimensional electron solid charge density wave
A brief presentation of the physics and physical realisations of classical and quantum 2 dimensional electron solids is followed by a tourist guide to some of the experiments
Mobility of ions in solid helium
We have studied the mobilities of positive and negative ions in solid helium 3 and 4, by measuring directly the time of flight in an electric field. These mobilities vary exponentially with inverse temperature, leading to an activation energy. In addition, we have detected trapping of negative ions. Finally, in hexagonal helium 3, we have observed a discontinuity in the temperature variation of the mobility of positive ions.Nous avons étudié les mobilités des ions positifs et négatifs dans l'hélium 3 et 4 solide, par mesure directe du temps de vol en présence d'un champ électrique. Ces mobilités varient exponentiellement avec l'inverse de la température, ce qui conduit à une énergie d'activation. De plus, nous avons détecté un piégeage pour les ions négatifs. Enfin, nous avons observé, dans l'hélium 3 hexagonal, une discontinuité dans la variation de la mobilité des ions positifs en fonction de la température
Polarization of a paramagnet by a fast high intensity magnetic field pulse : spin and phonon relaxation, phonon spectroscopy
The low temperature transient response of the polarization of a paramagnet to a fast high intensity (1 ms; 250 kG; 1/2 sine-wave) field pulse is shown to give information on spin-lattice coupling, density and relaxation of phonon modes and, sometimes, magnetic ordering temperatures.On démontre que la réponse dynamique de la polarisation paramagnétique à une impulsion rapide et intense de champ magnétique fournit des informations sur le couplage spin-phonon, la densité et la relaxation des modes de phonons et, éventuellement sur la température de transition à l'état ordonné