11,823 research outputs found
Spin Hall effect in spin-valley coupled monolayer transition-metal dichalcogenides
We study both the intrinsic and extrinsic spin Hall effect in spin-valley
coupled monolayers of transition metal dichalcogenides. We find that whereas
the skew-scattering contribution is suppressed by the large band gap, the
side-jump contribution is comparable to the intrinsic one with opposite sign in
the presence of scalar and magnetic scattering. Intervalley scattering tends to
suppress the side-jump contribution due to the loss of coherence. By tuning the
ratio of intra- to intervalley scattering, the spin Hall conductivity shows a
sign change in hole-doped samples. Multiband effect in other doping regime is
considered, and it is found that the sign change exists in the heavily
hole-doped regime, but not in the electron-doped regime
Berry phase modification to the energy spectrum of excitons
By quantizing the semiclassical motion of excitons, we show that the Berry
curvature can cause an energy splitting between exciton states with opposite
angular momentum. This splitting is determined by the Berry curvature flux
through the -space area spanned by the relative motion of the
electron-hole pair in the exciton wave function. Using the gapped
two-dimensional Dirac equation as a model, we show that this splitting can be
understood as an effective spin-orbit coupling effect. In addition, there is
also an energy shift caused by other "relativistic" terms. Our result reveals
the limitation of the venerable hydrogenic model of excitons, and highlights
the importance of the Berry curvature in the effective mass approximation.Comment: 4.5 pages, 2 figures, reference updated and minor change
How do credit ratings affect corporate investment efficiency?
This study examines the impact of credit ratings on the efficiency of firms' investments. Using a large sample of US firms, we find a positive relationship between the existence of credit ratings and investment efficiency. The cross‐sectional analyses show the positive relationship is more pronounced for firms with greater information asymmetry and weaker corporate governance. Our results are robust to different methods to address potential endogeneity concerns, alternative measures of key variables, and the inclusion of additional control variables. Overall, the findings support the notion that credit rating agencies enhance information transparency and external monitoring, thereby allowing rated firms to promote investment efficiency. The findings contribute to our understanding of the significant role played by credit rating agencies in shaping firms' investment behaviour and efficiency
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