259 research outputs found
Observation of Collective Excitations of the Dilute 2D Electron System
We report inelastic light scattering measurements of dispersive spin and
charge density excitations in dilute 2D electron systems reaching densities
less than 10^{10} cm^{-2}. In the quantum Hall state at nu=2, roton critical
points in the spin inter--Landau level mode show a pronounced softening as r_s
is increased. Instead of a soft mode instability predicted by Hartree--Fock
calculations for r_s ~ 3.3, we find evidence of multiple rotons in the
dispersion of the softening spin excitations. Extrapolation of the data
indicates the possibility of an instability for r_s >~ 11.Comment: Submitted to Physical Review Letter
Resonant Enhancement of Inelastic Light Scattering in the Fractional Quantum Hall Regime at
Strong resonant enhancements of inelastic light scattering from the long
wavelength inter-Landau level magnetoplasmon and the intra-Landau level spin
wave excitations are seen for the fractional quantum Hall state at .
The energies of the sharp peaks (FWHM ) in the profiles of
resonant enhancement of inelastic light scattering intensities coincide with
the energies of photoluminescence bands assigned to negatively charged exciton
recombination. To interpret the observed enhancement profiles, we propose
three-step light scattering mechanisms in which the intermediate resonant
transitions are to states with charged excitonic excitations.Comment: 5 pages, 5 figure
Atypical Fractional Quantum Hall Effect in Graphene at Filling Factor 1/3
We study the recently observed graphene fractional quantum Hall state at a
filling factor using a four-component trial wave function and exact
diagonalization calculations. Although it is adiabatically connected to a 1/3
Laughlin state in the upper spin branch, with SU(2) valley-isospin
ferromagnetic ordering and a completely filled lower spin branch, it reveals
physical properties beyond such a state that is the natural ground state for a
large Zeeman effect. Most saliently, it possesses at experimentally relevant
values of the Zeeman gap low-energy spin-flip excitations that may be unveiled
in inelastic light-scattering experiments.Comment: 4 pages, 3 figures; slightly modified published versio
Light scattering observations of spin reversal excitations in the fractional quantum Hall regime
Resonant inelastic light scattering experiments access the low lying
excitations of electron liquids in the fractional quantum Hall regime in the
range . Modes associated with changes in the charge and
spin degrees of freedom are measured. Spectra of spin reversed excitations at
filling factor and at identify a structure
of lowest spin-split Landau levels of composite fermions that is similar to
that of electrons. Observations of spin wave excitations enable determinations
of energies required to reverse spin. The spin reversal energies obtained from
the spectra illustrate the significant residual interactions of composite
fermions. At energies of spin reversal modes are larger but
relatively close to spin conserving excitations that are linked to activated
transport. Predictions of composite fermion theory are in good quantitative
agreement with experimental results.Comment: Submitted to special issue of Solid State Com
Excitations from Filled Landau Levels in Graphene
We consider graphene in a strong perpendicular magnetic field at zero
temperature with an integral number of filled Landau levels and study the
dispersion of single particle-hole excitations. We first analyze the two-body
problem of a single Dirac electron and hole in a magnetic field interacting via
Coulomb forces. We then turn to the many-body problem, where particle-hole
symmetry and the existence of two valleys lead to a number of effects peculiar
to graphene. We find that the coupling together of a large number of low-lying
excitations leads to strong many-body corrections, which could be observed in
inelastic light scattering or optical absorption. We also discuss in detail how
the appearance of different branches in the exciton dispersion is sensitive to
the number of filled spin and valley sublevels.Comment: 15 pages, 19 figure
Crossover and coexistence of quasiparticle excitations in the fractional quantum Hall regime at nu <= 1/3
New low-lying excitations are observed by inelastic light scattering at filling factors nu=p/(phip+/-1) of the fractional quantum Hall regime with phi=4. Coexisting with these modes throughout the range nuless than or equal to1/3 are phi=2 excitations seen at 1/3. Both phi=2 and phi=4 excitations have distinct behaviors with temperature and filling factor. The abrupt first appearance of the new modes in the low-energy excitation spectrum at nuless than or similar to1/3 suggests a marked change in the quantum ground state on crossing the phi=2-->phi=4 boundary at nu=1/3
Three-point density correlation functions in the fractional quantum Hall regime
In this paper we consider the three-particle density correlation function for
a fractional quantum Hall liquid. The study of this object is motivated by
recent experimental studies of fractional quantum Hall systems using inelastic
light scattering and phonon absorption techniques. Symmetry properties of the
correlation function are noted. An exact sum-rule is derived which this
quantity must obey. This sum-rule is used to assess the convolution
approximation that has been used to estimate the matrix elements for such
experiments. PACS Numbers: 73.40.Hm, 73.20.Mf, 72.10.DiComment: 12 pages + 1 (PS) figur
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