4,593 research outputs found

### Real-Space Imaging of Alternate Localization and Extension of Quasi Two-Dimensional Electronic States at Graphite Surfaces in Magnetic Fields

We measured the local density of states (LDOS) of a quasi two-dimensional
(2D) electron system near point defects on a surface of highly oriented
pyrolytic graphite (HOPG) with scanning tunneling microscopy and spectroscopy.
Differential tunnel conductance images taken at very low temperatures and in
high magnetic fields show a clear contrast between localized and extended
spatial distributions of the LDOS at the valley and peak energies of the Landau
level spectrum, respectively. The localized electronic state has a single
circular distribution around the defects with a radius comparable to the
magnetic length. The localized LDOS is in good agreement with a spatial
distribution of a calculated wave function for a single electron in 2D in a
Coulomb potential in magnetic fields.Comment: 4 pages, 4 figure

### Anlaytic mean-field Hall crystal solution at nu=1/3: composite fermion like sub-bands and correlation effects

An analytic solution of the Hartree-Fock problem for a 2DEG at filling 1/3
and half an electron per unit cell is presented. The Coulomb interaction
dynamically breaks the first Landau level in three narrow sub-bands, one of
which is fully occupied and the other empty, as in the composite fermion model.
The localized orbitals associated to the Bloch like single electron
wavefunctions are nearly static, resembling the angular momentum eigenstates
within a Landau level for non-interacting fermions. Strong correlations are
expected owing to the large charge density overlap between neighboring
plaquettes. A numerical evaluation brings the cohesive energy close to that of
the best present day models. It is also found that correlations are long range,
requiring over 50 particles spread over a finite sample to approach
convergence. Since presently allowed exact calculations are far from this
number, the question of how relevant the considered wave-function is for the
description of the ground state of the 2DEG system remains open.Comment: 24 pages, 4 figures, content exposed in the: Symposium Quantum Hall
Effect: "Past, Present and Future", Sttutgart, Germany, 2-5 July 2003 and in
the Third Stig Lundquist Conference on Advancing Frontiers of Condensed
Matter Physics 11-15 August 2003, ICTP, Trieste Ital

### Localized Distributions of Quasi Two-Dimensional Electronic States near Defects Artificially Created at Graphite Surfaces in Magnetic Fields

We measured the local density of states of a quasi two-dimensional electron
system (2DES) near defects, artificially created by Ar-ion sputtering, on
surfaces of highly oriented pyrolytic graphite (HOPG) with scanning tunneling
spectroscopy (STS) in high magnetic fields. At valley energies of the Landau
level spectrum, we found two typical localized distributions of the 2DES
depending on the defects. These are new types of distributions which are not
observed in the previous STS work at the HOPG surface near a point defect [Y.
Niimi \textit{et al}., Phys. Rev. Lett. {\bf 97}, 236804 (2006).]. With
increasing energy, we observed gradual transformation from the localized
distributions to the extended ones as expected for the integer quantum Hall
state. We show that the defect potential depth is responsible for the two
localized distributions from comparison with theoretical calculations.Comment: 4 pages, 3 figure

### Magnetoelasticity theory of incompressible quantum Hall liquids

A simple and physically transparent magnetoelasticity theory is proposed to
describe linear dynamics of incompressible fractional quantum Hall states. The
theory manifestly satisfies the Kohn theorem and the $f$-sum rule, and predicts
a gaped intra-Landau level collective mode with a roton minimum. In the limit
of vanishing bare mass $m$ the correct form of the static structure factor,
$s(q)\sim q^4$, is recovered. We establish a connection of the present approach
to the fermionic Chern-Simons theory, and discuss further extensions and
applications. We also make an interesting analogy of the present theory to the
theory of visco-elastic fluids.Comment: RevTeX 4, 6 pages; expanded version to appear in PRB; more technical
details, and discussions of the physics adde

### Broadening effects due to alloy scattering in Quantum Cascade Lasers

We report on calculations of broadening effects in QCL due to alloy
scattering. The output of numerical calculations of alloy broadened Landau
levels compare favorably with calculations performed at the self-consistent
Born approximation. Results for Landau level width and optical absorption are
presented. A disorder activated forbidden transition becomes significant in the
vicinity of crossings of Landau levels which belong to different subbands. A
study of the time dependent survival probability in the lowest Landau level of
the excited subband is performed. It is shown that at resonance the population
relaxation occurs in a subpicosecond scale.Comment: 7 pages, 8 figure

### Fractional quantum Hall states in two-dimensional electron systems with anisotropic interactions

We study the anisotropic effect of the Coulomb interaction on a 1/3-filling
fractional quantum Hall system by using an exact diagonalization method on
small systems in torus geometry. For weak anisotropy the system remains to be
an incompressible quantum liquid, although anisotropy manifests itself in
density correlation functions and excitation spectra. When the strength of
anisotropy increases, we find the system develops a Hall-smectic-like phase
with a one-dimensional charge density wave order and is unstable towards the
one-dimensional crystal in the strong anisotropy limit. In all three phases of
the Laughlin liquid, Hall-smectic-like, and crystal phases the ground state of
the anisotropic Coulomb system can be well described by a family of model wave
functions generated by an anisotropic projection Hamiltonian. We discuss the
relevance of the results to the geometrical description of fractional quantum
Hall states proposed by Haldane [ Phys. Rev. Lett. 107 116801 (2011)].Comment: 8 pages, 8 figure

### On transport in quantum Hall systems with constrictions

Motivated by recent experimental findings, we study transport in a simple
phenomenological model of a quantum Hall edge system with a gate-voltage
controlled constriction lowering the local filling factor. The current
backscattered from the constriction is seen to arise from the matching of the
properties of the edge-current excitations in the constriction ($\nu_{2}$) and
bulk ($\nu_{1}$) regions. We develop a hydrodynamic theory for bosonic edge
modes inspired by this model, finding that a competition between two tunneling
process, related by a quasiparticle-quasihole symmetry, determines the fate of
the low-bias transmission conductance. In this way, we find satisfactory
explanations for many recent puzzling experimental results.Comment: 4 pages, 4 figure

### Electron interferometry in quantum Hall regime: Aharonov-Bohm effect of interacting electrons

An apparent h/fe Aharonov-Bohm flux period, where f is an integer, has been
reported in coherent quantum Hall devices. Such sub-period is not expected for
non-interacting electrons and thus is thought to result from interelectron
Coulomb interaction. Here we report experiments in a Fabry-Perot interferometer
comprised of two wide constrictions enclosing an electron island. By carefully
tuning the constriction front gates, we find a regime where interference
oscillations with period h/2e persist throughout the transition between the
integer quantum Hall plateaus 2 and 3, including half-filling. In a large
quantum Hall sample, a transition between integer plateaus occurs near
half-filling, where the bulk of the sample becomes delocalized and thus
dissipative bulk current flows between the counterpropagating edges
("backscattering"). In a quantum Hall constriction, where conductance is due to
electron tunneling, a transition between forward- and back-scattering is
expected near the half-filling. In our experiment, neither period nor amplitude
of the oscillations show a discontinuity at half-filling, indicating that only
one interference path exists throughout the transition. We also present
experiments and an analysis of the front-gate dependence of the phase of the
oscillations. The results point to a single physical mechanism of the observed
conductance oscillations: Aharonov-Bohm interference of interacting electrons
in quantum Hall regime.Comment: 10 pages, 4 Fig

- â€¦