Time-resolved photoemission of infinitely periodic atomic arrangements: correlation-dressed excited states of solids

A theory of the time-resolved photoemission spectroscopy (TRPES) is developed, which enables to explore the real-time electron dynamics of infinitely periodic crystalline solids. In the strongly correlated electron systems NiO and CuO, the early-stage dynamics of the valence band edge are found to be sharply contrasted between those in the spectra of TRPES. This provides a new dynamical insight to the Zaanen–Sawatzky–Allen (ZSA) classification scheme of correlated insulators and makes us assert that NiO dynamically behaves like the Mott–Hubbard insulator (MHI) and CuO like the charge transfer insulator (CTI). In the two-dimensional carbon layer graphene, the real-time electron dynamics of quantum-phase-dressed excited states, i.e., due to the Berry phase and the pseudospin correlation, are investigated in an unprecedented way through the time-resolved angle-resolved photoemission spectroscopy (TR-ARPES). In particular, the dephasing dynamics of optically doped electrons and holes in the massless Dirac band, accompanying a field-induced gliding of the Dirac cone, are discovered. © 2020, The Author(s).1

Effect of interlayer interactions on exciton luminescence in atomic-layered MoS2 crystals

The atomic-layered semiconducting materials of transition metal dichalcogenides are considered effective light sources with both potential applications in thin and flexible optoelectronics and novel functionalities. In spite of the great interest in optoelectronic properties of two-dimensional transition metal dichalcogenides, the excitonic properties still need to be addressed, specifically in terms of the interlayer interactions. Here, we report the distinct behavior of the A and B excitons in the presence of interlayer interactions of layered MoS 2 crystals. Micro-photoluminescence spectroscopic studies reveal that on the interlayer interactions in double layer MoS 2 crystals, the emission quantum yield of the A exciton is drastically changed, whereas that of the B exciton remains nearly constant for both single and double layer MoS 2 crystals. First-principles density functional theory calculations confirm that a significant charge redistribution occurs in the double layer MoS 2 due to the interlayer interactions producing a local electric field at the interfacial region. Analogous to the quantum-confined Stark effect, we suggest that the distinct behavior of the A and B excitons can be explained by a simplified band-bending model.1

Optogenetic activation of parvalbumin and somatostatin interneurons selectively restores theta-nested gamma oscillations and oscillation-induced spike timing-dependent long-term potentiation impaired by amyloid β oligomers

BACKGROUND: Abnormal accumulation of amyloid β1-42 oligomers (AβO1-42), a hallmark of Alzheimer's disease, impairs hippocampal theta-nested gamma oscillations and long-term potentiation (LTP) that are believed to underlie learning and memory. Parvalbumin-positive (PV) and somatostatin-positive (SST) interneurons are critically involved in theta-nested gamma oscillogenesis and LTP induction. However, how AβO1-42 affects PV and SST interneuron circuits is unclear. Through optogenetic manipulation of PV and SST interneurons and computational modeling of the hippocampal neural circuits, we dissected the contributions of PV and SST interneuron circuit dysfunctions on AβO1-42-induced impairments of hippocampal theta-nested gamma oscillations and oscillation-induced LTP. RESULTS: Targeted whole-cell patch-clamp recordings and optogenetic manipulations of PV and SST interneurons during in vivo-like, optogenetically induced theta-nested gamma oscillations in vitro revealed that AβO1-42 causes synapse-specific dysfunction in PV and SST interneurons. AβO1-42 selectively disrupted CA1 pyramidal cells (PC)-to-PV interneuron and PV-to-PC synapses to impair theta-nested gamma oscillogenesis. In contrast, while having no effect on PC-to-SST or SST-to-PC synapses, AβO1-42 selectively disrupted SST interneuron-mediated disinhibition to CA1 PC to impair theta-nested gamma oscillation-induced spike timing-dependent LTP (tLTP). Such AβO1-42-induced impairments of gamma oscillogenesis and oscillation-induced tLTP were fully restored by optogenetic activation of PV and SST interneurons, respectively, further supporting synapse-specific dysfunctions in PV and SST interneurons. Finally, computational modeling of hippocampal neural circuits including CA1 PC, PV, and SST interneurons confirmed the experimental observations and further revealed distinct functional roles of PV and SST interneurons in theta-nested gamma oscillations and tLTP induction. CONCLUSIONS: Our results reveal that AβO1-42 causes synapse-specific dysfunctions in PV and SST interneurons and that optogenetic modulations of these interneurons present potential therapeutic targets for restoring hippocampal network oscillations and synaptic plasticity impairments in Alzheimer's disease

Breaking the absorption limit of Si toward SWIR wavelength range via strain engineering

Silicon has been widely used in the microelectronics industry. However, its photonic applications are restricted to visible and partial near-infrared spectral range owing to its fundamental optical bandgap (1.12 eV). With recent advances in strain engineering, material properties, including optical bandgap, can be tailored considerably. This paper reports the strain-induced shrinkage in the Si bandgap, providing photosensing well beyond its fundamental absorption limit in Si nanomembrane (NM) photodetectors (PDs). The Si-NM PD pixels were mechanically stretched (biaxially) by a maximum strain of similar to 3.5% through pneumatic pressure-induced bulging, enhancing photoresponsivity and extending the Si absorption limit up to 1550 nm, which is the essential wavelength range of the lidar sensors for obstacle detection in self-driving vehicles. The development of deformable three-dimensional optoelectronics via gas pressure-induced bulging also facilitated the realization of unique device designs with concave and convex hemispherical architectures, which mimics the electronic prototypes of biological eyes.1

Proposed Valley Valve from Four-Channel Valley Manipulation

Suggesting the AB-stacked bilayer WS2/MoS2 heterostructure as an ideal architecture for Berry curvature engineering, we theoretically demonstrate that it is possible to manipulate valley polarizations at K and -K valleys on each layer in a distinguishable fashion and then attempt an operation using four channels of valley manipulations, that is, four pairwise incorporations of KWS2 and (-K)WS2, KMoS2 and (-K)MoS2,KWS2 and KMoS2, and (-K)WS2 and (-K)MoS2, within a frame of the electro-optic method. Four-channel valley manipulation, which provokes varied anomalous Lorentz effects, conveys a development of multimode inverse valley Hall currents comprising two intralayer and one interlayer modes, each of which has a different nonlocal resistance. This finding proposes an alternative discipline of valley valve of valleytronics to manage a variable (multilevel) nonlocal resistance to the valley-mediated nonlocal charge current, in analogy with the spin valve of spintronics. © 2019 American Physical Society.1

Ultrafast dynamics of phase and topology in Dirac semiconductors

Ultrafast dynamics in the Floquet states of Dirac semiconductors are theoretically explored, which accentuates nonequilibrium dynamical nature of the Floquet state beyond the usually considered quasi static band nature and then gives a missing connection between the two. Phase oscillation of the Fano resonance at the frequency of 2 omega(pump) on the Floquet Dirac cone of graphene is disclosed from the transient absorption spectroscopy (TAS), which is due to the correlation between 2 omega(pump)-absorption paths. This establishes a driving of the petahertz (PHz) quantum oscillation from Dirac materials. On another Floquet Dirac cone of the graphene/h-BN (Gr/h-BN) heterostructure under the circularly polarized pulse pumping, we find a regime that exists between topologically trivial and optically induced nontrivial phases from the time-resolved dichroic photoemission spectroscopy (TRdPES), which we call the fluctuating topological order. The topological order fluctuation is a novel mixed phase, i.e., a dynamical mixture of trivial and nontrivial phases, as a precursor to the transient nontrivial phase. (C) 2021 Published by Elsevier Ltd.1

Subfemtosecond charge driving with correlation-assisted band engineering in a wide-gap semiconductor

First-principles calculations indicate that, before falling into dielectric breakdown, charge transport induced by a strong-intensity few-cycle optical waveform in the subfemtosecond time domain can be precisely controlled depending on band distortion engineered by strain along the [0001] direction in wurtzite-AlN. It is further discovered from a model of electron-hole interaction that the subfemtosecond charge driving with band engineering can be substantially strengthened by excitonic correlation and dynamics. With these findings, we reveal band engineering to be a route to the ultrafast charge control of semiconductors and indeed suggest an unexplored prototype of solid-state petahertz (1015Hz) device. © 2018 American Physical Society.1