158 research outputs found
Trion Species-Resolved Quantum Beats in MoSe2
Monolayer photonic materials offer a tremendous potential for on-chip
optoelectronic devices. Their realization requires knowledge of optical
coherence properties of excitons and trions that have so far been limited to
nonlinear optical experiments carried out with strongly inhomogenously
broadened material. Here we employ h-BN encapsulated and electrically gated
MoSe2 to reveal coherence properties of trion-species directly in the linear
optical response. Autocorrelation measurements reveal long dephasing times up
to T2=1.16+-0.05 ps for positively charged excitons. Gate dependent
measurements provide evidence that the positively-charged trion forms via
spatially localized hole states making this trion less prone to dephasing in
the presence of elevated hole carrier concentrations. Quantum beat signatures
demonstrate coherent coupling between excitons and trions that have a dephasing
time up to 0.6 ps, a two-fold increase over those in previous reports. A key
merit of the prolonged exciton/trion coherences is that they were achieved in a
linear optical experiment, and thus are directly relevant to applications in
nanolasers, coherent control, and on-chip quantum information processing
requiring long photon coherence.Comment: 21 pages, 6 figures, 2 SOI figure
Atomically thin MoS2: A new direct-gap semiconductor
The electronic properties of ultrathin crystals of molybdenum disulfide
consisting of N = 1, 2, ... 6 S-Mo-S monolayers have been investigated by
optical spectroscopy. Through characterization by absorption,
photoluminescence, and photoconductivity spectroscopy, we trace the effect of
quantum confinement on the material's electronic structure. With decreasing
thickness, the indirect band gap, which lies below the direct gap in the bulk
material, shifts upwards in energy by more than 0.6 eV. This leads to a
crossover to a direct-gap material in the limit of the single monolayer. Unlike
the bulk material, the MoS2 monolayer emits light strongly. The freestanding
monolayer exhibits an increase in luminescence quantum efficiency by more than
a factor of 1000 compared with the bulk material.Comment: 15 pages, 4 figure
Tunable and giant valley-selective Hall effect in gapped bilayer graphene
Berry curvature is analogous to magnetic field but in momentum space and is
commonly present in materials with non-trivial quantum geometry. It endows
Bloch electrons with transverse anomalous velocities to produce Hall-like
currents even in the absence of a magnetic field. We report the direct
observation of in situ tunable valley-selective Hall effect (VSHE), where
inversion symmetry, and thus the geometric phase of electrons, is controllable
by an out-of-plane electric field. We use high-quality bilayer graphene with an
intrinsic and tunable bandgap, illuminated by circularly polarized mid-infrared
light and confirm that the observed Hall voltage arises from an
optically-induced valley population. Compared with molybdenum disulfide, we
find orders of magnitude larger VSHE, attributed to the inverse scaling of the
Berry curvature with bandgap. By monitoring the valley-selective Hall
conductivity, we study Berry curvature's evolution with bandgap. This in situ
manipulation of VSHE paves the way for topological and quantum geometric
opto-electronic devices, such as more robust switches and detectors
Fragility of foot process morphology in kidney podocytes arises from chaotic spatial propagation of cytoskeletal instability
Kidney podocytes’ function depends on fingerlike projections (foot processes) that interdigitate with those from neighboring cells to form the glomerular filtration barrier. The integrity of the barrier depends on spatial control of dynamics of actin cytoskeleton in the foot processes. We determined how imbalances in regulation of actin cytoskeletal dynamics could result in pathological morphology. We obtained 3-D electron microscopy images of podocytes and used quantitative features to build dynamical models to investigate how regulation of actin dynamics within foot processes controls local morphology. We find that imbalances in regulation of actin bundling lead to chaotic spatial patterns that could impair the foot process morphology. Simulation results are consistent with experimental observations for cytoskeletal reconfiguration through dysregulated RhoA or Rac1, and they predict compensatory mechanisms for biochemical stability. We conclude that podocyte morphology, optimized for filtration, is intrinsically fragile, whereby local transient biochemical imbalances may lead to permanent morphological changes associated with pathophysiology
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