Intrinsic Spin and Orbital Hall Effects from Orbital Texture (1804.02118v2)

Abstract: 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.

Overview of Intrinsic Spin and Orbital Hall Effects from Orbital Texture

The paper "Intrinsic Spin and Orbital Hall Effects from Orbital Texture" by Dongwook Go and colleagues brings forward a theoretical framework for understanding the origin of intrinsic spin Hall effect (SHE) and orbital Hall effect (OHE) in centrosymmetric systems. The paper posits that momentum-space orbital texture, which is commonly present even in centrosymmetric systems, serves as a fundamental mechanism for generating OHE and subsequently SHE. This paper contributes to the current understanding of spintronics, particularly in the search for materials exhibiting large SHE without depending on impurity scattering mechanisms.

Key Findings and Theoretical Approach

The authors present a compelling argument that OHE can occur in the absence of spin-orbit coupling (SOC) through dynamical generation of orbital angular momentum (OAM) values in momentum space. In systems where SOC is present, this pre-existing OHE translates into SHE. Specifically, they demonstrate that in centrosymmetric materials, the orbital texture naturally arises from inter-orbital hybridization processes—such as $sp$ hybridization—leading to significant OHE and SHE. By constructing a simple cubic tight-binding model with $s$, $p_x$, $p_y$, and $p_z$ orbitals, the authors illustrate how these orbital textures appear and lead to the discussed phenomena.

The simulation results highlight that the spin Hall conductivity (SHC) generated from this mechanism can be comparable in magnitude to well-known materials like platinum, thereby underscoring its efficacy. Notably, SHC, in this mechanism, does not monotonically increase with SOC strength, a finding that challenges prevalent assumptions and implies the potential for optimizing SHE by tuning SOC rather than maximizing it.

Implications and Future Research Directions

The implications of this work are substantial for the development of spintronic devices. The findings suggest that efficient spin current generation could be attained in materials traditionally considered unsuitable due to the lack of appropriate crystal symmetry or strong SOC. This opens avenues for the exploration of a broader class of materials beyond the $5d$ transition metals like platinum, currently favored for their strong SHE.

The paper encourages experimental efforts to probe the orbital contributions to electronic transport phenomena, which have historically been neglected compared to spin contributions. Future directions for this research may involve experimental validation of the proposed mechanisms in real materials, particularly in novel centrosymmetric materials systems with substantial inter-orbital hybridization.

Moreover, the research points toward optimizing material properties beyond simply seeking strong SOC, suggesting that controlling the orbital texture through chemical composition or engineered strain could be a productive strategy.

In corollary, a deeper exploration into the quantum interference effects highlighted by the authors could yield additional insights into related phenomena in topological materials and advance the understanding of orbital-driven transport properties. The knowledge gained could facilitate the rational design of spintronics devices with enhanced efficiency and performance using less conventional materials.