Intrinsic spin Hall effect in monolayers of group-VI dichalcogenides: A first-principles study

Using first-principles calculations within density functional theory, we investigate the intrinsic spin Hall effect in monolayers of group-VI transition-metal dichalcogenides MX2 (M = Mo, W and X = S, Se). MX2 monolayers are direct band-gap semiconductors with two degenerate valleys located at the corners of the hexagonal Brillouin zone. Because of the inversion symmetry breaking and the strong spin-orbit coupling, charge carriers in opposite valleys carry opposite Berry curvature and spin moment, giving rise to both a valley- and a spin-Hall effect. The intrinsic spin Hall conductivity (ISHC) in p-doped samples is found to be much larger than the ISHC in n-doped samples due to the large spin-splitting at the valence band maximum. We also show that the ISHC in inversion-symmetric bulk dichalcogenides is an order of magnitude smaller compared to monolayers. Our result demonstrates monolayer dichalcogenides as an ideal platform for the integration of valleytronics and spintronics.Comment: published version (7 pages, 6 figures

First-principles calculation of topological invariants Z2 within the FP-LAPW formalism

In this paper, we report the implementation of first-principles calculations of topological invariants Z2 within the full-potential linearized augmented plane-wave (FP-LAPW) formalism. In systems with both time-reversal and spatial inversion symmetry (centrosymmetric), one can use the parity analysis of Bloch functions at time-reversal invariant momenta to determine the Z2 invariants. In systems without spatial inversion symmetry (noncentrosymmetric), however, a more complex and systematic method in terms of the Berry gauge potential and the Berry curvature is required to identify the band topology. We show in detail how both methods are implemented in FP-LAPW formalism and applied to several classes of materials including centrosymmetric compounds Bi2Se3 and Sb2Se3 and noncentrosymmetric compounds LuPtBi, AuTlS2 and CdSnAs2. Our work provides an accurate and effective implementation of first-principles calculations to speed up the search of new topological insulators

Optical bulk-boundary dichotomy in a quantum spin Hall insulator

The bulk-boundary correspondence is a key concept in topological quantum materials. For instance, a quantum spin Hall insulator features a bulk insulating gap with gapless helical boundary states protected by the underlying Z2 topology. However, the bulk-boundary dichotomy and distinction are rarely explored in optical experiments, which can provide unique information about topological charge carriers beyond transport and electronic spectroscopy techniques. Here, we utilize mid-infrared absorption micro-spectroscopy and pump-probe micro-spectroscopy to elucidate the bulk-boundary optical responses of Bi4Br4, a recently discovered room-temperature quantum spin Hall insulator. Benefiting from the low energy of infrared photons and the high spatial resolution, we unambiguously resolve a strong absorption from the boundary states while the bulk absorption is suppressed by its insulating gap. Moreover, the boundary absorption exhibits a strong polarization anisotropy, consistent with the one-dimensional nature of the topological boundary states. Our infrared pump-probe microscopy further measures a substantially increased carrier lifetime for the boundary states, which reaches one nanosecond scale. The nanosecond lifetime is about one to two orders longer than that of most topological materials and can be attributed to the linear dispersion nature of the helical boundary states. Our findings demonstrate the optical bulk-boundary dichotomy in a topological material and provide a proof-of-principal methodology for studying topological optoelectronics.Comment: 26 pages, 4 figure

2018 NSF-SI2 Lightning Talk: PERTURBO

Charge carriers are scattered by quasiparticles and excitations in materials (e.g. phonons, defects, photons, electrons, holes, etc.). PERTURBO is a robust platform to study these electron scattering processes using first-principles calculations and many-body perturbation theory. PERTURBO fills a void in the software ecosystem to design advanced materials, and will foster scientific and technological innovation

Depletion of lncRNA MEG3 Ameliorates Imatinib-Induced Injury of Cardiomyocytes via Regulating miR-129-5p/HMGB1 Axis

Imatinib is a classical targeted drug to treat chronic myeloid leukemia (CML). However, it shows cardiotoxicity, which limits its clinical application. Long noncoding RNA (lncRNA) maternally expressed gene 3 (MEG3) shows proapoptotic properties in human cells. This study is performed to investigate whether targeting MEG3 can attenuate imatinib-mediated cardiotoxicity to cardiomyocytes. In this work, H9c2 cells were divided into four groups: control group, hypoxia group, hypoxia + imatinib, and hypoxia + imatinib + MEG3 knockdown group. MEG3 and microRNA-129-5p (miR-129-5p) expression levels were detected by the quantitative real-time PCR (qRT-PCR). The viability and apoptosis of H9c2 cells were then evaluated by cell counting kit-8 (CCK-8), flow cytometry, and TUNEL assays. The targeting relationships between MEG3 and miR-129-5p, between miR-129-5p and high-mobility group box 1 (HMBG1), were validated by dual-luciferase reporter assay and RNA Immunoprecipitation (RIP) assay. The protein expression level of HMGB1 was detected by western blot. It was revealed that, Imatinib-inhibited cell viability and aggravated the apoptosis of H9c2 cells cultured in hypoxic condition, and MEG3 knockdown significantly counteracted this effect. MiR-129-5p was a downstream target of MEG3 and it directly targeted HMGB1, and knockdown of MEG3 inhibited HMGB1 expression in H9c2 cells. In conclusion, targeting MEG3 ameliorates imatinib-induced injury of cardiomyocytes via regulating miR-129-5p/HMGB1 axis