74 research outputs found
Energy relaxation of an excited electron gas in quantum wires: many-body electron LO-phonon coupling
We theoretically study energy relaxation via LO-phonon emission in an excited
one-dimensional electron gas confined in a GaAs quantum wire structure. We find
that the inclusion of phonon renormalization effects in the theory extends the
LO-phonon dominated loss regime down to substantially lower temperatures. We
show that a simple plasmon-pole approximation works well for this problem, and
discuss implications of our results for low temperature electron heating
experiments in quantum wires.Comment: 10 pages, RevTex, 4 figures included. Also available at
http://www-cmg.physics.umd.edu/~lzheng
Fractional quantum Hall effect in higher Landau levels
We investigate, using finite size numerical calculations, the spin-polarized
fractional quantum Hall effect (FQHE) in the first excited Landau level (LL).
We find evidence for the existence of an incompressible state at , but not at . Surprisingly,
the 7/3 state is found to be strongest at a finite thickness. The structure of
the low- lying excited states is found to be markedly different from that in
the lowest LL. This study also rules out FQHE at a large number of
odd-denominator fractions in the lowest LL.Comment: 7 pages RevTex, 4 figure
An exchange-correlation energy for a two-dimensional electron gas in a magnetic field
We present the results of a variational Monte Carlo calculation of the
exchange-correlation energy for a spin-polarized two-dimensional electron gas
in a perpendicular magnetic field. These energies are a necessary input to the
recently developed current-density functional theory. Landau-level mixing is
included in a variational manner, which gives the energy at finite density at
finite field, in contrast to previous approaches. Results are presented for the
exchange-correlation energy and excited-state gap at 1/7, 1/5, 1/3, 1,
and 2. We parameterize the results as a function of and in a form
convenient for current-density functional calculations.Comment: 36 pages, including 6 postscript figure
Interlayer Exchange Interactions, SU(4) Soft Waves and Skyrmions in Bilayer Quantum Hall Ferromagnets
The Coulomb exchange interaction is the driving force for quantum coherence
in quantum Hall systems. We construct a microscopic Landau-site Hamiltonian for
the exchange interaction in bilayer quantum Hall ferromagnets, which is
characterized by the SU(4) isospin structure. By taking a continuous limit, the
Hamiltonian gives rise to the SU(4) nonlinear sigma model in the
von-Neumann-lattice formulation. The ground-state energy is evaluated at
filling factors . It is shown at that there are 3
independent soft waves, where only one soft wave is responsible for the
coherent tunneling of electrons between the two layers. It is also shown at
that there are 3 independent skyrmion states apart from the
translational degree of freedom. They are CP skyrmions enjoying the
spin-charge entanglement confined within the \LLL.Comment: 12 pages, 2 figure
Growth Models and Models of Turbulence : A Stochastic Quantization Perspective
We consider a class of growth models and models of turbulence based on the
randomly stirred fluid. The similarity between the predictions of these models,
noted a decade earlier, is understood on the basis of a stochastic quantization
scheme.Comment: 3 page
Modulation Instability of Ultrashort Pulses in Quadratic Nonlinear Media beyond the Slowly Varying Envelope Approximation
We report a modulational instability (MI) analysis of a mathematical model
appropriate for ultrashort pulses in cascaded quadratic-cubic nonlinear media
beyond the so-called slowly varying envelope approximation. Theoretically
predicted MI properties are found to be in good agreement with numerical
simulation. The study shows the possibility of controlling the generation of MI
and formation of solitons in a cascaded quadratic-cubic media in the few cycle
regimes. We also find that stable propagation of soliton-like few-cycle pulses
in the medium is subject to the fulfilment of the modulation instability
criteria
Electron Localization in a 2D System with Random Magnetic Flux
Using a finite-size scaling method, we calculate the localization properties
of a disordered two-dimensional electron system in the presence of a random
magnetic field. Below a critical energy all states are localized and the
localization length diverges when the Fermi energy approaches the
critical energy, {\it i.e.} . We find that
shifts with the strength of the disorder and the amplitude of the random
magnetic field while the critical exponent () remains unchanged
indicating universality in this system. Implications on the experiment in
half-filling fractional quantum Hall system are also discussed.Comment: 4 pages, RevTex 3.0, 5 figures(PS files available upon request),
#phd1
Persistent spin splitting of a two-dimensional electron gas in tilted magnetic fields
By varying the orientation of the applied magnetic field with respect to the
normal of a two-dimensional electron gas, the chemical potential and the
specific heat reveal persistent spin splitting in all field ranges. The
corresponding shape of the thermodynamic quantities distinguishes whether the
Rashba spin-orbit interaction RSOI, the Zeeman term or both dominate the
splitting. The interplay of the tilting of the magnetic field and RSOI resulted
to an amplified splitting in weak fields. The effects of changing the RSOI
strength and the Landau level broadening are also investigated.Comment: 10 pages, 5 figure
Spin instabilities and quantum phase transitions in integral and fractional quantum Hall states
The inter-Landau-level spin excitations of quantum Hall states at filling
factors nu=2 and 4/3 are investigated by exact numerical diagonalization for
the situation in which the cyclotron (hbar*omega_c) and Zeeman (E_Z) splittings
are comparable. The relevant quasiparticles and their interactions are studied,
including stable spin wave and skyrmion bound states. For nu=2, a spin
instability at a finite value of epsilon=hbar*omega_c-E_Z leads to an abrupt
paramagnetic to ferromagnetic transition, in agreement with the mean-field
approximation. However, for nu=4/3 a new and unexpected quantum phase
transition is found which involves a gradual change from paramagnetic to
ferromagnetic occupancy of the partially filled Landau level as epsilon is
decreased.Comment: 4 pages, 5 figures, submitted to Phys.Rev.Let
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