32,864 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
Scale Invariance vs. Conformal Invariance: Holographic Two-Point Functions in Horndeski Gravity
We consider Einstein-Horndeski gravity with a negative bare constant as a
holographic model to investigate whether a scale invariant quantum field theory
can exist without the full conformal invariance. Einstein-Horndeski gravity can
admit two different AdS vacua. One is conformal, and the holographic two-point
functions of the boundary energy-momentum tensor are the same as the ones
obtained in Einstein gravity. The other AdS vacuum, which arises at some
critical point of the coupling constants, preserves the scale invariance but
not the special conformal invariance due to the logarithmic radial dependence
of the Horndeski scalar. In addition to the transverse and traceless graviton
modes, the theory admits an additional trace/scalar mode in the scale invariant
vacuum. We obtain the two-point functions of the corresponding boundary
operators. We find that the trace/scalar mode gives rise to an non-vanishing
two-point function, which distinguishes the scale invariant theory from the
conformal theory. The two-point function vanishes in , where the full
conformal symmetry is restored. Our results indicate the strongly coupled scale
invariant unitary quantum field theory may exist in without the full
conformal symmetry. The operator that is dual to the bulk trace/scalar mode
however violates the dominant energy condition.Comment: Latex, 28 pages, comments and references adde
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