[Paper Review] Surface orbitronics: new twists from orbital Rashba physics

Dongwook Go, Jan-Philipp Hanke+6 more|arXiv (Cornell University)|Nov 15, 2016

Topological Materials and PhenomenaPhysics and Astronomy39 references22 citations

TL;DR

This paper proposes a mechanism for the orbital Rashba effect (ORE) in surface alloys like BiAg₂, driven by sp orbital hybridization and surface potential gradients, leading to chiral orbital textures and electric polarization. First-principles calculations demonstrate that Berry phase theory enhances orbital moments significantly near band crossings, enabling giant orbital effects crucial for surface orbitronics.

ABSTRACT

When the inversion symmetry is broken at a surface, spin-orbit interaction gives rise to spin-dependent energy shifts - a phenomenon which is known as the spin Rashba effect. Recently, it has been recognized that an orbital counterpart of the spin Rashba effect - the orbital Rashba effect - can be realized at surfaces even without spin- orbit coupling. Here, we propose a mechanism for the orbital Rashba effect based on sp orbital hybridization, which ultimately leads to the electric polarization of surface states. As a proof of principle, we show from first principles that this effect leads to chiral orbital textures in $\mathbf{k}$-space of the BiAg$_2$ monolayer. In predicting the magnitude of the orbital moment arising from the orbital Rashba effect, we demonstrate the crucial role that the Berry phase theory plays for the magnitude and variation of the orbital textures. As a result, we predict a pronounced manifestation of various orbital effects at surfaces, and proclaim the orbital Rashba effect to be a key platform for surface orbitronics.

Motivation & Objective

• To identify a mechanism for the orbital Rashba effect (ORE) in surface systems without relying on spin-orbit coupling.
• To establish the role of sp orbital hybridization and surface potential gradients in generating chiral orbital textures and electric polarization in surface states.
• To demonstrate that Berry phase theory drastically enhances orbital moment magnitudes compared to standard atomic approximations.
• To predict giant orbital responses—such as orbital Hall effect and gyrotropic magnetic effect—by positioning singularities in orbital moments at the Fermi level.
• To position the orbital Rashba effect as a foundational platform for surface orbitronics, analogous to spin Rashba in spintronics.

Proposed method

• Developed a tight-binding model incorporating sp orbital hybridization and surface potential gradients to describe orbital Rashba physics.
• Performed self-consistent density-functional theory (DFT) calculations using the FLEUR code with FLAPW method and GGA exchange-correlation functional.
• Constructed maximally localized Wannier functions (MLWFs) from DFT bands to enable accurate orbital moment (OM) calculations.
• Calculated orbital moments using two approaches: the atomic-like approximation (ACA) and the Berry phase-based method of Lopez et al.
• Analyzed the k-space dependence of orbital textures and their chiral nature, particularly near band crossings.
• Evaluated the impact of non-local effects via Berry phase theory on the magnitude and spatial variation of orbital moments.

Experimental results

Research questions

• RQ1 Can the orbital Rashba effect be realized in surface systems without spin-orbit coupling, and what physical mechanism drives it?
• RQ2 How does sp orbital hybridization contribute to the formation of chiral orbital textures and electric polarization in surface states?
• RQ3 To what extent does Berry phase theory enhance the predicted magnitude of orbital moments compared to conventional atomic approximations?
• RQ4 What is the role of band crossings in amplifying orbital responses such as the orbital Hall effect or gyrotropic magnetic effect?
• RQ5 Can the orbital Rashba effect serve as a viable platform for engineering novel surface orbitronic phenomena?

Key findings

• The orbital Rashba effect arises from sp orbital hybridization and surface potential gradients in BiAg₂ monolayer, even in the absence of spin-orbit coupling.
• First-principles calculations confirm the presence of chiral orbital textures in k-space, with distinct p₋₁, p₀, and p₊₁ orbital character in the surface states.
• Berry phase theory increases the predicted orbital moment magnitude by several orders of magnitude compared to atomic-like approximations, especially near band crossings.
• The orbital moment singularities near band crossings can be tuned to the Fermi level, leading to giant enhancements in orbital responses such as the orbital Edelstein effect and gyrotropic magnetic effect.
• The system exhibits a valley-dependent orbital magnetoelectric response due to time-reversal symmetry, with opposite orbital moments in opposite valleys.
• The orbital Rashba effect is intrinsically linked to electric polarization of surface states, establishing a direct connection between orbital texture and ferroelectric-like behavior.