835 research outputs found
Scanning Raman spectroscopy of graphene antidot lattices: Evidence for systematic p-type doping
We have investigated antidot lattices, which were prepared on exfoliated
graphene single layers via electron-beam lithography and ion etching, by means
of scanning Raman spectroscopy. The peak positions, peak widths and intensities
of the characteristic phonon modes of the carbon lattice have been studied
systematically in a series of samples. In the patterned samples, we found a
systematic stiffening of the G band mode, accompanied by a line narrowing,
while the 2D mode energies are found to be linearly correlated with the G mode
energies. We interpret this as evidence for p-type doping of the nanostructured
graphene
Time-Resolved Studies of a Rolled-Up Semiconductor Microtube Laser
We report on lasing in rolled-up microtube resonators. Time-resolved studies
on these semiconductor lasers containing GaAs quantum wells as optical gain
material reveal particularly fast turn-on-times and short pulse emissions above
the threshold. We observe a strong red-shift of the laser mode during the pulse
emission which is compared to the time evolution of the charge-carrier density
calculated by rate equations
Low-temperature photocarrier dynamics in monolayer MoS2
The band structure of MoS strongly depends on the number of layers, and a
transition from indirect to direct-gap semiconductor has been observed recently
for a single layer of MoS. Single-layer MoS therefore becomes an
efficient emitter of photoluminescence even at room temperature. Here, we
report on scanning Raman and on temperature-dependent, as well as time-resolved
photoluminescence measurements on single-layer MoS flakes prepared by
exfoliation. We observe the emergence of two distinct photoluminescence peaks
at low temperatures. The photocarrier recombination at low temperatures occurs
on the few-picosecond timescale, but with increasing temperatures, a
biexponential photoluminescence decay with a longer-lived component is
observed.Comment: 3 pages, 4 figure
Spin dynamics in p-doped semiconductor nanostructures subject to a magnetic field tilted from the Voigt geometry
We develop a theoretical description of the spin dynamics of resident holes
in a p-doped semiconductor quantum well (QW) subject to a magnetic field tilted
from the Voigt geometry. We find the expressions for the signals measured in
time-resolved Faraday rotation (TRFR) and resonant spin amplification (RSA)
experiments and study their behavior for a range of system parameters. We find
that an inversion of the RSA peaks can occur for long hole spin dephasing times
and tilted magnetic fields. We verify the validity of our theoretical findings
by performing a series of TRFR and RSA experiments on a p-modulation doped
GaAs/Al_{0.3}Ga_{0.7}As single QW and showing that our model can reproduce
experimentally observed signals.Comment: 9 pages, 3 figures; corrected typo
Spin dephasing and photoinduced spin diffusion in high-mobility 110-grown GaAs-AlGaAs two-dimensional electron systems
We have studied spin dephasing and spin diffusion in a high-mobility
two-dimensional electron system, embedded in a GaAs/AlGaAs quantum well grown
in the [110] direction, by a two-beam Hanle experiment. For very low excitation
density, we observe spin lifetimes of more than 16 ns, which rapidly decrease
as the pump intensity is increased. Two mechanisms contribute to this decrease:
the optical excitation produces holes, which lead to a decay of electron spin
via the Bir-Aranov-Pikus mechanism and recombination with spin-polarized
electrons. By scanning the distance between the pump and probe beams, we
observe the diffusion of spin-polarized electrons over more than 20 microns.
For high pump intensity, the spin polarization in a distance of several microns
from the pump beam is larger than at the pump spot, due to the reduced
influence of photogenerated holes.Comment: 4 pages, 3 figure
Gate control of low-temperature spin dynamics in two-dimensional hole systems
We have investigated spin and carrier dynamics of resident holes in
high-mobility two-dimensional hole systems in GaAs/AlGaAs
single quantum wells at temperatures down to 400 mK. Time-resolved Faraday and
Kerr rotation, as well as time-resolved photoluminescence spectroscopy are
utilized in our study. We observe long-lived hole spin dynamics that are
strongly temperature dependent, indicating that in-plane localization is
crucial for hole spin coherence. By applying a gate voltage, we are able to
tune the observed hole g factor by more than 50 percent. Calculations of the
hole g tensor as a function of the applied bias show excellent agreement with
our experimental findings.Comment: 8 pages, 7 figure
Theory of Resonant Raman Scattering in One Dimensional Electronic systems
A theory of resonant Raman scattering spectroscopy of one dimensional
electronic systems is developed on the assumptions that (i) the excitations of
the one dimensional electronic system are described by the Luttinger Liquid
model, (ii) Raman processes involve virtual excitations from a filled valence
band to an empty state of the one dimensional electronic system and (iii)
excitonic interactions between the valence and conduction bands may be
neglected. Closed form analytic expressions are obtained for the Raman
scattering cross sections, and are evaluated analytically and numerically for
scattering in the polarized channel, revealing a "double-peak" structure with
the lower peak involving multispinon excitations with total spin S=0 and the
higher peak being the conventional plasmon. A key feature of our results is a
nontrivial power law dependence, involving the Luttinger Liquid exponents, of
the dependence of the Raman cross sections on the difference of the laser
frequency from resonance. We find that near resonance the calculated ratio of
intensity in the lower energy feature to the intensity in the higher energy
feature saturates at a value of the order of unity (times a factor of the ratio
of the velocities of the two modes). We explicate the differences between the
'Luttinger liquid' and 'Fermi liquid' calculations of RRS spectra and argue
that excitonic effects, neglected in all treatments so far, are essential for
explaining the intensity ratios observed in quantum wires. We also discuss
other Luttinger liquid features which may be observed in future RRS
experiments
- …