180 research outputs found
Many-body effects in nonlinear optical responses of 2D layered semiconductors
We performed ultrafast degenerate pump-probe spectroscopy on monolayer WSe2
near its exciton resonance. The observed differential reflectance signals
exhibit signatures of strong many-body interactions including the
exciton-exciton interaction and free carrier induced band gap renormalization.
The exciton-exciton interaction results in a resonance blue shift which lasts
for the exciton lifetime (several ps), while the band gap renormalization
manifests as a resonance red shift with several tens ps lifetime. Our model
based on the many-body interactions for the nonlinear optical susceptibility
fits well the experimental observations. The power dependence of the spectra
shows that with the increase of pump power, the exciton population increases
linearly and then saturates, while the free carrier density increases
superlinearly, implying that exciton Auger recombination could be the origin of
these free carriers. Our model demonstrates a simple but efficient method for
quantitatively analyzing the spectra, and indicates the important role of
Coulomb interactions in nonlinear optical responses of such 2D materials
Electrical Control of Two-Dimensional Neutral and Charged Excitons in a Monolayer Semiconductor
Monolayer group VI transition metal dichalcogenides have recently emerged as
semiconducting alternatives to graphene in which the true two-dimensionality
(2D) is expected to illuminate new semiconducting physics. Here we investigate
excitons and trions (their singly charged counterparts) which have thus far
been challenging to generate and control in the ultimate 2D limit. Utilizing
high quality monolayer molybdenum diselenide (MoSe2), we report the unambiguous
observation and electrostatic tunability of charging effects in positively
charged (X+), neutral (Xo), and negatively charged (X-) excitons in field
effect transistors via photoluminescence. The trion charging energy is large
(30 meV), enhanced by strong confinement and heavy effective masses, while the
linewidth is narrow (5 meV) at temperatures below 55 K. This is greater
spectral contrast than in any known quasi-2D system. We also find the charging
energies for X+ and X- to be nearly identical implying the same effective mass
for electrons and holes.Comment: 11 pages main text with 4 figures + 7 pages supplemental material
Regional Disparities and Investment-Cash Flow Sensitivity: Evidence from Chinese Listed Firms
In China, regional disparities are important. We examine the difference in the sensitivity of investment to cash flow between firms in inland regions and those in coastal regions. By using the financial data of Chinese listed firms, we found that firms in inland regions rely more on their internal funds in terms of their investment activities than those in coastal regions and that the sensitivity gap between inland and coastal firms widened in the recent contractionary monetary policy period. This suggests that firms in inland regions are harder to obtain outside funds due to unfavorable social and economic environments for inland firms. Our findings suggest that capital markets in China respond rationally to the potential impact of regional disparities on a firm’s performance
Entanglement of single-photons and chiral phonons in atomically thin WSe
Quantum entanglement is a fundamental phenomenon which, on the one hand,
reveals deep connections between quantum mechanics, gravity and the space-time;
on the other hand, has practical applications as a key resource in quantum
information processing. While it is routinely achieved in photon-atom
ensembles, entanglement involving the solid-state or macroscopic objects
remains challenging albeit promising for both fundamental physics and
technological applications. Here, we report entanglement between collective,
chiral vibrations in two-dimensional (2D) WSe host --- chiral phonons (CPs)
--- and single-photons emitted from quantum dots (QDs) present in it. CPs which
carry angular momentum were recently observed in WSe and are a
distinguishing feature of the underlying honeycomb lattice. The entanglement
results from a "which-way" scattering process, involving an optical excitation
in a QD and doubly-degenerate CPs, which takes place via two indistinguishable
paths. Our unveiling of entanglement involving a macroscopic, collective
excitation together with strong interaction between CPs and QDs in 2D materials
opens up ways for phonon-driven entanglement of QDs and engineering chiral or
non-reciprocal interactions at the single-photon level
Magnetic Control of Valley Pseudospin in Monolayer WSe2
Local energy extrema of the bands in momentum space, or valleys, can endow
electrons in solids with pseudo-spin in addition to real spin. In transition
metal dichalcogenides this valley pseudo-spin, like real spin, is associated
with a magnetic moment which underlies the valley-dependent circular dichroism
that allows optical generation of valley polarization, intervalley quantum
coherence, and the valley Hall effect. However, magnetic manipulation of valley
pseudospin via this magnetic moment, analogous to what is possible with real
spin, has not been shown before. Here we report observation of the valley
Zeeman splitting and magnetic tuning of polarization and coherence of the
excitonic valley pseudospin, by performing polarization-resolved
magneto-photoluminescence on monolayer WSe2. Our measurements reveal both the
atomic orbital and lattice contributions to the valley orbital magnetic moment;
demonstrate the deviation of the band edges in the valleys from an exact
massive Dirac fermion model; and reveal a striking difference between the
magnetic responses of neutral and charged valley excitons which is explained by
renormalization of the excitonic spectrum due to strong exchange interactions
TIP47 functions in the biogenesis of lipid droplets
TIP47 (tail-interacting protein of 47 kD) was characterized as a cargo selection device for mannose 6-phosphate receptors (MPRs), directing their transport from endosomes to the trans-Golgi network. In contrast, our current analysis shows that cytosolic TIP47 is not recruited to organelles of the biosynthetic and endocytic pathways. Knockdown of TIP47 expression had no effect on MPR distribution or trafficking and did not affect lysosomal enzyme sorting. Therefore, our data argue against a function of TIP47 as a sorting device. Instead, TIP47 is recruited to lipid droplets (LDs) by an amino-terminal sequence comprising 11-mer repeats. We show that TIP47 has apolipoprotein-like properties and reorganizes liposomes into small lipid discs. Suppression of TIP47 blocked LD maturation and decreased the incorporation of triacylglycerol into LDs. We conclude that TIP47 functions in the biogenesis of LDs
Electrical Tuning of Valley Magnetic Moment via Symmetry Control
Crystal symmetry governs the nature of electronic Bloch states. For example,
in the presence of time reversal symmetry, the orbital magnetic moment and
Berry curvature of the Bloch states must vanish unless inversion symmetry is
broken. In certain 2D electron systems such as bilayer graphene, the intrinsic
inversion symmetry can be broken simply by applying a perpendicular electric
field. In principle, this offers the remarkable possibility of switching on/off
and continuously tuning the magnetic moment and Berry curvature near the Dirac
valleys by reversible electrical control. Here we demonstrate this principle
for the first time using bilayer MoS2, which has the same symmetry as bilayer
graphene but has a bandgap in the visible that allows direct optical probing of
these Berry-phase related properties. We show that the optical circular
dichroism, which reflects the orbital magnetic moment in the valleys, can be
continuously tuned from -15% to 15% as a function of gate voltage in bilayer
MoS2 field-effect transistors. In contrast, the dichroism is gate-independent
in monolayer MoS2, which is structurally non-centrosymmetric. Our work
demonstrates the ability to continuously vary orbital magnetic moments between
positive and negative values via symmetry control. This represents a new
approach to manipulating Berry-phase effects for applications in quantum
electronics associated with 2D electronic materials.Comment: 13 pages main text + 4 pages supplementary material
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