425 research outputs found
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
Girvin-MacDonald-Platzman Collective Mode at General Filling Factors: Magneto-Roton Minimum at Half-Filled Landau Level
The single mode approximation has proved useful for the excitation spectrum
at . We apply it to general fractions and find that it predicts
magneto-roton minima in the dispersion of the Girvin-MacDonald-Platzman
collective mode for the fractional quantum Hall states at , and
one magneto-roton minimum for both the composite Fermi sea and the paired
composite fermion state. Experimental relevance of the results will be
considered.Comment: 5 pages, 6 figure
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
Electric Field Effect Tuning of Electron-Phonon Coupling in Graphene
Gate-modulated low-temperature Raman spectra reveal that the electric field
effect (EFE), pervasive in contemporary electronics, has marked impacts on long
wavelength optical phonons of graphene. The EFE in this two dimensional
honeycomb lattice of carbon atoms creates large density modulations of carriers
with linear dispersion (known as Dirac fermions). Our EFE Raman spectra display
the interactions of lattice vibrations with these unusual carriers. The changes
of phonon frequency and line-width demonstrate optically the particle-hole
symmetry about the charge-neutral Dirac-point. The linear dependence of the
phonon frequency on the EFE-modulated Fermi energy is explained as the
electron-phonon coupling of mass-less Dirac fermions.Comment: 4 pages, 4 figure
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
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
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
Observation of large many-body Coulomb interaction effects in a doped quantum wire
We demonstrate strong one dimensional (1-D) many-body interaction effects in
photoluminescence (PL) in a GaAs single quantum wire of unprecedented optical
quality, where 1-D electron plasma densities are controlled via electrical
gating. We observed PL of 1-D charged excitons with large binding energy of 2.3
meV relative to the neutral excitons, and its evolution to a Fermi-edge
singularity at high electron density. Furthermore, we find a strong band-gap
renormalization in the 1-D wire, or a large red-shift of PL with increased
electron plasma density. Such a large PL red-shift is not observed when we
create a high density neutral electron-hole plasma in the same wire, due
probably to cancellation of the Coulomb interaction energy in the neutral
plasma.Comment: 5 pages, 4 figures, RevTeX, to be published in Solid State
Communication
One-dimensional continuum and exciton states in quantum wires
High-quality T-shaped quantum wires are fabricated by cleaved-edge overgrowth
with the molecular beam epitaxy on the interface improved by a growth-interrupt
high-temperature anneal. Characterization by micro-photoluminescence (PL) and
PL excitation (PLE) spectroscopy at 5 K reveals high uniformity, a sharp
spectral width, and a small Stokes shift of one-dimensional (1-D) excitons. The
PLE spectrum for 1-D states shows a large peak of ground-state excitons and a
small absorption band ascribed to 1-D continuum states with an onset at 11 meV
above the exciton peak.Comment: 4 pages, 4 figures, RevTe
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