507 research outputs found
Stability and effective masses of composite-fermions in the first and second Landau Level
We propose a measure of the stability of composite fermions (CF's) at
even-denominator Landau-level filling fractions. Assuming Landau-level mixing
effects are not strong, we show that the CF liquid at in the
Landau level cannot exist and relate this to the absence of a hierarchy of
incompressible states for filling fractions . We find that
a polarized CF liquid should exist at . We also show that, for CF
states, the variation with system size of the ground state energy of
interacting electrons follows that for non-interacting particles in zero
magnetic field. We use this to estimate the CF effective masses.Comment: 9 pages, Revtex, PSIZ-TP-940
Microscopic non-equilibrium theory of quantum well solar cells
We present a microscopic theory of bipolar quantum well structures in the
photovoltaic regime, based on the non-equilibrium Green's function formalism
for a multi band tight binding Hamiltonian. The quantum kinetic equations for
the single particle Green's functions of electrons and holes are
self-consistently coupled to Poisson's equation, including inter-carrier
scattering on the Hartree level. Relaxation and broadening mechanisms are
considered by the inclusion of acoustic and optical electron-phonon interaction
in a self consistent Born approximation of the scattering self energies.
Photogeneration of carriers is described on the same level in terms of a self
energy derived from the standard dipole approximation of the electron-photon
interaction. Results from a simple two band model are shown for the local
density of states, spectral response, current spectrum, and current-voltage
characteristics for generic single quantum well systems.Comment: 10 pages, 6 figures; corrected typos, changed caption Fig. 1,
replaced Fig.
Excitation gaps in fractional quantum Hall states: An exact diagonalization study
We compute energy gaps for spin-polarized fractional quantum Hall states in
the lowest Landau level at filling fractions nu=1/3, 2/5,3/7 and 4/9 using
exact diagonalization of systems with up to 16 particles and extrapolation to
the infinite system-size limit. The gaps calculated for a pure Coulomb
interaction and ignoring finite width effects, disorder and LL mixing agree
with predictions of composite fermion theory provided the logarithmic
corrections to the effective mass are included. This is in contrast with
previous estimates, which, as we show, overestimated the gaps at nu=2/5 and 3/7
by around 15%. We also study the reduction of the gaps as a result of the
non-zero width of the 2D layer. We show that these effects are accurately
accounted for using either Gaussian or z*Gaussian' (zG) trial wavefunctions,
which we show are significantly better variational wavefunctions than the
Fang-Howard wavefunction. For quantum well parameters typical of
heterostructure samples, we find gap reductions of around 20%. The experimental
gaps, after accounting heuristically for disorder,are still around 40% smaller
than the computed gaps. However, for the case of tetracene layers
inmetal-insulator-semiconductor (MIS) devices we find that the measured
activation gaps are close to those we compute. We discuss possible reasons why
the difference between computed and measured activation gaps is larger in GaAs
heterostructures than in MIS devices. Finally, we present new calculations
using systems with up to 18 electrons of the gap at nu=5/2 including width
corrections.Comment: 18 pages, 17 figure
Second Generation of Composite Fermions in the Hamiltonian Theory
In the framework of a recently developed model of interacting composite
fermions restricted to a single level, we calculate the activation gaps of a
second generation of spin-polarized composite fermions. These composite
particles consist each of a composite fermion of the first generation and a
vortex-like excitation and may be responsible for the recently observed
fractional quantum Hall states at unusual filling factors such as
nu=4/11,5/13,5/17, and 6/17. Because the gaps of composite fermions of the
second generation are found to be more than one order of magnitude smaller than
those of the first generation, these states are less visible than the usual
states observed at filling factors nu=p/(2ps+1). Their stability is discussed
in the context of a pseudopotential expansion of the composite-fermion
interaction potential.Comment: 5 pages, 3 figures; after publication in PRB, we have realized that a
factor was missing in one of the expressions; the erroneous results are now
corrected; an erratum has been sent to PR
Dynamical Correlations in a Half-Filled Landau Level
We formulate a self-consistent field theory for the Chern-Simons fermions to
study the dynamical response function of the quantum Hall system at .
Our scheme includes the effect of correlations beyond the random-phase
approximation (RPA) employed to this date for this system. The resulting
zero-frequency density response function vanishes as the square of the wave
vector in the long-wavelength limit. The longitudinal conductivity calculated
in this scheme shows linear dependence on the wave vector, like the
experimentals results and the RPA, but the absolute values are higher than the
experimental results.Comment: 4 pages, revtex, 3 figures included. Corrected typo
Skyrme Crystal In A Two-Dimensional Electron Gas
The ground state of a two-dimensional electron gas at Landau level filling
factors near is a Skyrme crystal with long range order in the
positions and orientations of the topologically and electrically charged
elementary excitations of the ferromagnetic ground state. The lowest
energy Skyrme crystal is a square lattice with opposing postures for
topological excitations on opposite sublattices. The filling factor dependence
of the electron spin-polarization, calculated for the square lattice Skyrme
crystal, is in excellent agreement with recent experiments.Comment: 3 pages, latex, 3 figures available upon request from
[email protected]
Anisotropic States of Two-Dimensional Electron Systems in High Landau Levels: Effect of an In-Plane Magnetic Field
We report the observation of an acute sensitivity of the anisotropic
longitudinal resistivity of two-dimensional electron systems in half-filled
high Landau levels to the magnitude and orientation of an in-plane magnetic
field. In the third and higher Landau levels, at filling fractions nu=9/2,
11/2, etc., the in-plane field can lead to a striking interchange of the "hard"
and "easy" transport directions. In the second Landau level the normally
isotropic resistivity and the weak nu=5/2 quantized Hall state are destroyed by
a large in-plane field and the transport becomes highly anisotropic.Comment: 5 pages, 4 figures, minor errors correcte
Quasi-Particle Tunneling in Anti-Pfaffian Quantum Hall State
We study tunneling phenomena at the edge of the anti-Pfaffian quantum Hall
state at the filling factor . The edge current in a single
point-contact is considered. We focus on nonlinear behavior of two-terminal
conductance with the increase in negative split-gate voltage. Expecting the
appearance of the intermediate conductance plateau we calculate the value of
its conductance by using the renormalization group (RG) analysis. Further, we
show that non-perturbative quasi-particle tunneling is effectively described as
perturbative electron tunneling by the instanton method. The two-terminals
conductance is written as a function of the gate voltage. The obtained results
enable us to distinguish the anti-Pfaffian state from the Pfaffian state
experimentally.Comment: 5 pages, 4 figure
Possible composite-fermion liquid as a crossover from Wigner crystal to bubble phase in higher Landau level
The ground state cohesive energies per electron of the composite fermion (CF)
Fermi sea, the Laughlin state and the charge density wave (CDW) at higher
Landau levels (LLs) are computed. It is shown that whereas for LL,
the CDW state is generally more energetically preferable than those of the CF
liquid and the Laughlin liquid, the CF liquid state unexpectedly
has lower ground state energy than that of the CDW state. We suggest this CF
liquid between the Wigner crystal and the bubble phase may lead to the
crossover from the normal integer quantum Hall liquid to the novel re-entrant
integer quantum Hall state observed in the recent magneto-transport
experiments
Universal Equilibrium Currents in the Quantum Hall Fluid
The equilibrium current distribution in a quantum Hall fluid that is
subjected to a slowly varying confining potential is shown to generally consist
of strips or channels of current, which alternate in direction, and which have
universal integrated strengths. A measurement of these currents would yield
direct independent measurements of the proper quasiparticle and quasihole
energies in the fractional quantum Hall states.Comment: 4 pages, Revte
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