10,003 research outputs found
Understanding Variations in Circularly Polarized Photoluminescence in Monolayer Transition Metal Dichalcogenides
Monolayer transition metal dichalcogenides are promising materials for
valleytronic operations. They exhibit two inequivalent valleys in the Brillouin
zone, and the valley populations can be directly controlled and determined
using circularly polarized optical excitation and emission. The
photoluminescence polarization reflects the ratio of the two valley
populations. A wide range of values for the degree of circularly polarized
emission, Pcirc, has been reported for monolayer WS2, although the reasons for
the disparity are unclear. Here we optically populate one valley, and measure
Pcirc to explore the valley population dynamics at room temperature in a large
number of monolayer WS2 samples synthesized via chemical vapor deposition.
Under resonant excitation, Pcirc ranges from 2% to 32%, and we observe a
pronounced inverse relationship between photoluminescence (PL) intensity and
Pcirc. High quality samples exhibiting strong PL and long exciton relaxation
time exhibit a low degree of valley polarization, and vice versa. This behavior
is also demonstrated in monolayer WSe2 samples and transferred WS2, indicating
that this correlation may be more generally observed and account for the wide
variations reported for Pcirc. Time resolved PL provides insight into the role
of radiative and non-radiative contributions to the observed polarization.
Short non-radiative lifetimes result in a higher measured polarization by
limiting opportunity for depolarizing scattering events
Where are the Hedgehogs in Nematics?
In experiments which take a liquid crystal rapidly from the isotropic to the
nematic phase, a dense tangle of defects is formed. In nematics, there are in
principle both line and point defects (``hedgehogs''), but no point defects are
observed until the defect network has coarsened appreciably. In this letter the
expected density of point defects is shown to be extremely low, approximately
per initially correlated domain, as result of the topology
(specifically, the homology) of the order parameter space.Comment: 6 pages, latex, 1 figure (self-unpacking PostScript)
Geometric quantum computation using fictitious spin- 1/2 subspaces of strongly dipolar coupled nuclear spins
Geometric phases have been used in NMR, to implement controlled phase shift
gates for quantum information processing, only in weakly coupled systems in
which the individual spins can be identified as qubits. In this work, we
implement controlled phase shift gates in strongly coupled systems, by using
non-adiabatic geometric phases, obtained by evolving the magnetization of
fictitious spin-1/2 subspaces, over a closed loop on the Bloch sphere. The
dynamical phase accumulated during the evolution of the subspaces, is refocused
by a spin echo pulse sequence and by setting the delay of transition selective
pulses such that the evolution under the homonuclear coupling makes a complete
rotation. A detailed theoretical explanation of non-adiabatic geometric
phases in NMR is given, by using single transition operators. Controlled phase
shift gates, two qubit Deutsch-Jozsa algorithm and parity algorithm in a
qubit-qutrit system have been implemented in various strongly dipolar coupled
systems obtained by orienting the molecules in liquid crystal media.Comment: 37 pages, 17 figure
Direct vs. indirect optical recombination in Ge films grown on Si substrates
The optical emission spectra from Ge films on Si are markedly different from
their bulk Ge counterparts. Whereas bulk Ge emission is dominated by the
material's indirect gap, the photoluminescence signal from Ge films is mainly
associated with its direct band gap. Using a new class of Ge-on-Si films grown
by a recently introduced CVD approach, we study the direct and indirect
photoluminescence from intrinsic and doped samples and we conclude that the
origin of the discrepancy is the lack of self-absorption in thin Ge films
combined with a deviation from quasi-equilibrium conditions in the conduction
band. The latter is confirmed by a simple model suggesting that the deviation
from quasi-equilibrium is caused by the much shorter recombination lifetime in
the films relative to bulk Ge
One-dimensional array of ion chains coupled to an optical cavity
We present a novel hybrid system where an optical cavity is integrated with a
microfabricated planar-electrode ion trap. The trap electrodes produce a
tunable periodic potential allowing the trapping of up to 50 separate ion
chains spaced by 160 m along the cavity axis. Each chain can contain up to
20 individually addressable Yb\textsuperscript{+} ions coupled to the cavity
mode. We demonstrate deterministic distribution of ions between the sites of
the electrostatic periodic potential and control of the ion-cavity coupling.
The measured strength of this coupling should allow access to the strong
collective coupling regime with 10 ions. The optical cavity could
serve as a quantum information bus between ions or be used to generate a strong
wavelength-scale periodic optical potential.Comment: 15 pages, 6 figures, submitted to New Journal of Physic
Ground-state configuration space heterogeneity of random finite-connectivity spin glasses and random constraint satisfaction problems
We demonstrate through two case studies, one on the p-spin interaction model
and the other on the random K-satisfiability problem, that a heterogeneity
transition occurs to the ground-state configuration space of a random
finite-connectivity spin glass system at certain critical value of the
constraint density. At the transition point, exponentially many configuration
communities emerge from the ground-state configuration space, making the
entropy density s(q) of configuration-pairs a non-concave function of
configuration-pair overlap q. Each configuration community is a collection of
relatively similar configurations and it forms a stable thermodynamic phase in
the presence of a suitable external field. We calculate s(q) by the
replica-symmetric and the first-step replica-symmetry-broken cavity methods,
and show by simulations that the configuration space heterogeneity leads to
dynamical heterogeneity of particle diffusion processes because of the entropic
trapping effect of configuration communities. This work clarifies the fine
structure of the ground-state configuration space of random spin glass models,
it also sheds light on the glassy behavior of hard-sphere colloidal systems at
relatively high particle volume fraction.Comment: 26 pages, 9 figures, submitted to Journal of Statistical Mechanic
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