Abstract
Materials with spin-orbit coupling are of great interest for various spintronics applications due to the efficient electrical generation and detection of spin-polarized electrons. Over the past decade, many materials have been studied, including topological insulators, transition metals, Kondo insulators, semimetals, semiconductors, and oxides; however, there is no unifying physical framework for understanding the physics and therefore designing a material system and devices with the desired properties. We present a model that binds together the experimental data observed on the wide variety of materials in a unified manner. We show that in a material with a given spin-momentum locking, the density of states plays a crucial role in determining the charge-spin interconversion efficiency, and a simple inverse relationship can be obtained. Remarkably, experimental data obtained over the last decade on many different materials closely follow such an inverse relationship. We further deduce two figure-of-merits of great current interest: the spin-orbit torque (SOT) efficiency (for the direct effect) and the inverse Rashba-Edelstein effect length (for the inverse effect), which statistically show good agreement with the existing experimental data on wide varieties of materials. Especially, we identify a scaling law for the SOT efficiency with respect to the carrier concentration in the sample, which agrees with existing data. Such an agreement is intriguing since our transport model includes only Fermi surface contributions and fundamentally different from the conventional views of the SOT efficiency that includes contributions from all the occupied states.
Highlights
We present a model that binds together the charge-spin interconversion observed on the wide variety of materials in a unified manner using only four model parameters: the number of modes or density of states, the strength of spin-momentum locking (SML), mean free path, and interface spin conductance
In a material with a given SML, the number of modes in the channel, which is related to the material density of states around the Fermi energy, plays a crucial role in determining the charge-spin interconversion efficiency
We point out a simple inverse relationship between the interconversion efficiency and the material density of states
Summary
Charge-spin interconversion in various materials exhibiting spin-momentum locking (SML), e.g., topological insulators (TIs) [1,2,3,4,5,6,7], semiconductors [8,9], transition metals [10,11,12,13,14,15,16,17,18], semimetals [19], oxides [20,21,22,23], antiferromagnets [24], and superconductors [25], are growing interest for efficient spintronic applications. We present a model (see Fig. 1) that binds together the charge-spin interconversion observed on the wide variety of materials in a unified manner using only four model parameters: the number of modes or density of states, the strength of SML, mean free path, and interface spin conductance. These four parameters are independently measurable and theoretically well understood.
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