[Paper Review] Higher-dimensional spin selectivity in chiral crystals

Yinong Zhou, Dmitri Leo Mesoza Cordova+3 more|arXiv (Cornell University)|May 29, 2023

Heusler alloys: electronic and magnetic propertiesMaterials Science3 citations

TL;DR

This study demonstrates higher-dimensional spin selectivity in chiral crystals like InSeI, where chiral-induced spin-orbit coupling (CISOC) along the screw axis and antisymmetric spin-orbit coupling (ASOC) in the normal plane jointly enable spin-polarized transport in all three dimensions. First-principles simulations and model fitting reveal tunable spin textures via symmetry breaking through surface cleavage, thickness control, or strain, enabling 3D spin filtering and potential for multi-axial spintronic devices with quaternary or higher information encoding.

ABSTRACT

This study aims to investigate the interplay between chiral-induced spin-orbit coupling along the screw axis and antisymmetric spin-orbit coupling (ASOC) in the normal plane within a chiral crystal, using both general model analysis and first-principles simulations of InSeI, a chiral van der Waals crystal. While chiral molecules of light atoms typically exhibit spin selectivity only along the screw axis, chiral crystals with heavier atoms can have strong ASOC effects that influence spin-momentum locking in all directions. The resulting phase diagram of spin texture shows the potential for controlling phase transition and flipping spin by reducing symmetry through surface cleavage, thickness reduction or strain. We also experimentally synthesized high-quality InSeI crystals of the thermodynamically stable achiral analogue which showed exposed (110) facets corresponding to single-handed helices to demonstrate the potential of material realization for higher-dimensional spin selectivity in the development of spintronic devices.

Motivation & Objective

• To investigate the interplay between chiral-induced spin-orbit coupling (CISOC) along the screw axis and antisymmetric spin-orbit coupling (ASOC) in the normal plane in chiral crystals.
• To explore how symmetry reduction via surface cleavage, thickness reduction, or strain can control spin texture and enable multidirectional spin filtering.
• To demonstrate experimentally the feasibility of higher-dimensional spin selectivity using high-quality InSeI crystals with exposed (110) facets corresponding to single-handed helices.
• To propose a multi-axial spintronic device architecture capable of detecting spin polarization in three orthogonal directions for high-density information processing.

Proposed method

• First-principles density functional theory (DFT) calculations were performed on bulk and monolayer InSeI to compute electronic band structures and spin textures.
• A general model Hamiltonian incorporating both CISOC (λ_C k_z σ_z) and ASOC terms was derived and fitted to DFT results to analyze spin-momentum locking in 3D momentum space.
• Spin texture maps were generated in k_x-k_y, k_z-k_x, and k_z-k_y planes for valence and conduction bands to visualize 3D spin polarization.
• Symmetry reduction was simulated by applying uniaxial strain along the x-direction, reducing C_4 symmetry to C_2 and altering spin texture topology.
• High-quality InSeI crystals were synthesized via melt growth and characterized using SEM, EDS, and HRTEM to confirm structural chirality and (110) facet exposure.
• A hypothetical spintronic device with three independent source-voltage pairs (S1–S3, V1–V3) was proposed to detect spin selectivity in three dimensions using the inverse spin Hall effect in tungsten electrodes.

Experimental results

Research questions

• RQ1 How does antisymmetric spin-orbit coupling (ASOC) in the normal plane of chiral crystals interact with chiral-induced spin-orbit coupling (CISOC) along the screw axis to enable 3D spin selectivity?
• RQ2 Can symmetry breaking via surface cleavage, thickness control, or strain engineering be used to tune the spin texture and induce phase transitions in spin-momentum locking?
• RQ3 What is the role of heavy atoms and large twist angles (e.g., 135°) in enhancing ASOC and enabling multidirectional spin filtering in chiral van der Waals crystals like InSeI?
• RQ4 Can experimentally synthesized InSeI crystals with (110) facets exhibit single-handed helical nanochains suitable for higher-dimensional spintronic applications?
• RQ5 Can a multi-axial device architecture detect spin polarization in three orthogonal directions, enabling quaternary or higher information encoding in spintronic systems?

Key findings

• First-principles calculations show that in InSeI, ASOC in the normal plane (k_x-k_y plane) is significant due to heavy atoms and large twist angle, leading to complex 3D spin textures beyond the screw axis.
• The spin texture in the k_x-k_y plane exhibits C_4 symmetry in bulk InSeI at 0% strain, which reduces to C_2 symmetry under 5% and 10% uniaxial strain, enabling symmetry-driven phase transitions.
• Model fitting confirms that the spin texture in the conduction and valence bands is well described by a Hamiltonian combining CISOC (λ_C k_z σ_z) and ASOC terms, enabling full 3D spin-momentum locking.
• Monolayer (110) surfaces of InSeI exhibit distinct spin textures with left- and right-handed nanochains, confirmed by HRTEM and FFT indexing, supporting chiral spin filtering in 2D.
• The proposed device architecture with three independent source-voltage pairs (S1–S3, V1–V3) enables detection of spin polarization in three orthogonal directions, suggesting potential for quaternary or octal logic systems.
• Experimental synthesis of InSeI yielded high-quality crystals with exposed (110) facets, confirming the feasibility of realizing single-handed helical structures for 3D spintronic applications.