Visualizing Orbital Content of Electronic Bands in Anisotropic 2D Semiconducting ReSe2

Many properties of layered materials change as they are thinned from their bulk forms down to single layers, with examples including indirect-to-direct band gap transition in 2H semiconducting transition metal dichalcogenides as well as thickness-dependent changes in the valence band structure in post-transition metal monochalcogenides and black phosphorus. Here, we use angle-resolved photoemission spectroscopy to study the electronic band structure of monolayer ReSe$_{2}$, a semiconductor with a distorted 1T structure and in-plane anisotropy. By changing the polarization of incoming photons, we demonstrate that for ReSe$_{2}$, in contrast to the 2H materials, the out-of-plane transition metal $d_{z^{2}}$ and chalcogen $p_{z}$ orbitals do not contribute significantly to the top of the valence band which explains the reported weak changes in the electronic structure of this compound as a function of layer number. We estimate a band gap of 1.7 eV in pristine ReSe$_{2}$ using scanning tunneling spectroscopy and explore the implications on the gap following surface-doping with potassium. A lower bound of 1.4 eV is estimated for the gap in the fully doped case, suggesting that doping-dependent many-body effects significantly affect the electronic properties of ReSe$_{2}$. Our results, supported by density functional theory calculations, provide insight into the mechanisms behind polarization-dependent optical properties of rhenium dichalcogenides and highlight their place amongst two-dimensional crystals.Comment: 37 pages (including Supporting Information), 7 figures in the main tex

Negative Thermal Expansion Coefficient of Graphene Measured by Raman Spectroscopy

The thermal expansion coefficient (TEC) of single-layer graphene is estimated with temperature-dependent Raman spectroscopy in the temperature range between 200 and 400 K. It is found to be strongly dependent on temperature but remains negative in the whole temperature range, with a room temperature value of -8.0x10^{-6} K^{-1}. The strain caused by the TEC mismatch between graphene and the substrate plays a crucial role in determining the physical properties of graphene, and hence its effect must be accounted for in the interpretation of experimental data taken at cryogenic or elevated temperatures.Comment: 17 pagese, 3 figures, and supporting information (4 pages, 3 figures); Nano Letters, 201

Two-color picosecond and continuous-wave experiments on anti-Stokes and Stokes carrier-transfer phenomena in GaAslAl(x)Ga(1-x)As and InGaP2/AlxGa1-xAs heterostructures

We present direct evidence of the two-step absorption process in anti-Stokes photoluminescence in both GaAs/AlxGa1-xAs and InGaP2/AlxGa1-xAs heterostructures using two-color picosecond and continuous-wave photoluminescence experiments. We show information about the lifetime of the defect states that participate in the two-step absorption process. As a result, we conclude that the long-lived states rather than excitons play the dominant role in the two-step absorption process. We also study the possible contribution of the two-step absorption process to Stokes carrier transfer in GaAs/AlxGa1-xAs asymmetric double quantum well structuresclos

Observation of an optical switching effect in GaAs/AlGaAs quantum wells under hydrostatic pressure

We have observed extremely sharp (\u394 \u2dc 0.01 meV) discontinuities in the photoluminescence excitation (PLE) spectra of GaAs/Al\u2093Ga\u2081\u208b\u2093As single quantum wells under hydrostatic pressure at low temperatures. These discontinuities are found to originate from abrupt changes in the photoluminescence (PL) spectrum as a function of the excitation energy. Near each discontinuity, the PL spectrum shows a doublet for excitation energies on one side of the discontinuity, and only a single peak for excitation energies on the other side. The lower energy peak of the doublet is completely quenched on the other side of the discontinuity, which means that by changing the excitation photon energy slightly, the PL mechanism responsible for the lower energy peak can be switched on and off. The dependence of this phenomenon on pressure, temperature, and the excitation laser intensity has been investigated for a 70 \uc5 quantum well. The energies of these discontinuities in the PLE spectrum have similar pressure coefficients to the GaAs band gap energy, and are observed only in the pressure range of \u2dc17\u201325 kbar, at low temperature ( 7227 K), and at a moderately high (>4 W/cm2) excitation laser intensity.NRC publication: Ye

Ballistic transport effects in a sub-micron InSb quantum well cross structure

We report the low temperature magnetoresistance properties of an InSb quantum well sub-micron Hall cross with lateral dimensions of 477 nm and active device dimensions of the order of 340 nm. Ballistic transport anomalies are observed in the low field regime including negative bend resistance. The use of such devices as high spatial resolution low field sensors is addressed

Giant Magnetic Anisotropy in the Atomically Thin van der Waals Antiferromagnet FePS3

Abstract Van der Waals (vdW) magnets are an ideal platform for tailoring 2D magnetism with immense potential for spintronics applications and are intensively investigated. However, little is known about the microscopic origin of magnetic order in these antiferromagnetic systems. X‐ray photoemission electron microscopy is used to address the electronic and magnetic properties of the vdW antiferromagnet FePS3 down to the monolayer. The experiments reveal a giant out‐of‐plane magnetic anisotropy of 22 meV per Fe ion, accompanied by unquenched magnetic orbital moments. Moreover, the calculations suggest that the Ising magnetism in FePS3 is a visible manifestation of spin–orbit entanglement of the Fe 3d electron system