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Register a new userGigantic intrinsic orbital Hall effects in weakly spin-orbit coupled metals
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A recent paper [Go $textit{et al}$., Phys. Rev. Lett. $textbf{121}$, 086602 (2018)] proposed that the intrinsic orbital Hall effect (OHE) can emerge from momentum-space orbital texture in centrosymmetric materials. In searching for real materials with strong OHE, we investigate the intrinsic OHE in metals with small spin-orbit coupling (SOC) in face-centered cubic and body-centered cubic structures (Li, Al, V, Cr, Mn, Ni, and Cu). We find that orbital Hall conductivities (OHCs) in these materials are gigantic $sim 10^3-10^4 (hbar/e)(Omegacdotmathrm{cm})^{-1}$, which are comparable or larger than spin Hall conductivity (SHC) of Pt. Although SHCs in these materials are smaller than OHCs due to small SOC, we found that SHCs are still sizable and the spin Hall angles may be of the order of 0.1. We discuss implications on recent spin-charge interconversion experiments on materials having small SOC.
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We show theoretically that both intrinsic spin Hall effect (SHE) and orbital Hall effect (OHE) can arise in centrosymmetric systems through momentum-space orbital texture, which is ubiquitous even in centrosymmetric systems unlike spin texture. OHE occurs even without spin-orbit coupling (SOC) and is converted into SHE through SOC. The resulting spin Hall conductivity is large (comparable to that of Pt) but depends on the SOC strength in a nonmonotonic way. This mechanism is stable against orbital quenching. This work suggests a path for an ongoing search for materials with stronger SHE. It also calls for experimental efforts to probe orbital degrees of freedom in OHE and SHE. Possible ways for experimental detection are briefly discussed.
Orbital Hall effect (OHE) is the phenomenon of transverse flow of orbital moment in presence of an applied electric field. Solids with broken inversion symmetry are expected to exhibit a strong OHE due to the presence of an intrinsic orbital moment at individual momentum points in the Brillouin zone, which in presence of an applied electric field, flows in different directions causing a net orbital Hall current. Here we provide a comprehensive understanding of the effect and its tunability in the monolayer 2D transition metal dichalcogenides (TMDCs). Both metallic and insulating TMDCs are investigated from full density-functional calculations, effective $d$-band tight-binding models, as well as a minimal four-band model for the valley points that captures the key physics of the system. For the tuning of the OHE, we examine the role of hole doping as well as the change in the band parameters, which, e. g., can be controlled by strain. We demonstrate that the OHE is a more fundamental effect than the spin Hall effect (SHE), with the momentum-space orbital moments inducing a spin moment in the presence of the spin-orbit coupling, leading to the SHE. The physics of the OHE, described here, is relevant for 2D materials with broken inversion symmetry in general, even beyond the TMDCs, providing a broad platform for future research.
We report on the observation of the acoustic spin Hall effect that facilitates lattice motion induced spin current via spin orbit interaction (SOI). Under excitation of surface acoustic wave (SAW), we find a spin current flows orthogonal to the propagation direction of a surface acoustic wave (SAW) in non-magnetic metals. The acoustic spin Hall effect manifests itself in a field-dependent acoustic voltage in non-magnetic metal (NM)/ferromagnetic metal (FM) bilayers. The acoustic voltage takes a maximum when the NM layer thickness is close to its spin diffusion length, vanishes for NM layers with weak SOI and increases linearly with the SAW frequency. To account for these results, we find the spin current must scale with the SOI and the time derivative of the lattice displacement. Such form of spin current can be derived from a Berry electric field associated with time varying Berry curvature and/or an unconventional spin-lattice interaction mediated by SOI. These results, which imply the strong coupling of electron spins with rotating lattices via the SOI, show the potential of lattice dynamics to supply spin current in strong spin orbit metals.
The Rashba spin-orbit coupling arising from structure inversion asymmetry couples spin and momentum degrees of freedom providing a suitable (and very intensively investigated) environment for spintronic effects and devices. Here we show that in the presence of strong disorder, non-homogeneity in the spin-orbit coupling gives rise to a finite spin Hall conductivity in contrast with the corresponding case of a homogeneous linear spin-orbit coupling. In particular, we examine the inhomogeneity arising from a striped structure for a two-dimensional electron gas, affecting both density and Rashba spin-orbit coupling. We suggest that this situation can be realized at oxide interfaces with periodic top gating.
In transition-metal dichalcogenides, electrons in the K-valleys can experience both Ising and Rashba spin-orbit couplings. In this work, we show that the coexistence of Ising and Rashba spin-orbit couplings leads to a special type of valley Hall effect, which we call spin-orbit coupling induced valley Hall effect. Importantly, near the conduction band edge, the valley-dependent Berry curvatures generated by spin-orbit couplings are highly tunable by external gates and dominate over the intrinsic Berry curvatures originating from orbital degrees of freedom under accessible experimental conditions. We show that the spin-orbit coupling induced valley Hall effect is manifested in the gate dependence of the valley Hall conductivity, which can be detected by Kerr effect experiments.
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