Quantized Piezospintronic Effect in Moiré Systems

Abstract

This paper presents a novel approach for generating and controlling spin currents in an antiferromagnetic twisted honeycomb bilayer in response to an elastic deformation. Utilizing a continuum model, closely based upon the seminal Bistritzer-MacDonald model, that captures the essential physics of low-energy moiré bands, we calculate the spin current response to the deformation in terms of the familiar Berry phase formalism. The resulting moiré superlattice potential modulates the electronic band structure, leading to emergent topological phases and novel transport properties such as quantized piezo responses both for spin and charge transport. This approach allows us to tune the system across different topological regimes and to explore the piezo-spintronic responses as a function of the band topology. When inversion symmetry is broken either by a sublattice potential $V$, alignment with an hBN substrate, uniaxial strain, or structural asymmetry present in the moiré superlattice, the system acquires a finite Berry curvature that is opposite in the $K$ and $K'$ valleys (protected by valley time reversal symmetry). In contrast, for strain, the valley-contrasting nature of the pseudo-gauge field ensures that the quantized response is robust and proportional to the sum of the valley Chern numbers. These notable physical properties make these systems promising candidates for groundbreaking spintronic and valleytronic devices.

Figures (3)

• Figure 1: (a) Twisted honeycomb bilayer under uniaxial strain. (b-d) shows the band structure of the system for different values of $\Delta$. (b), (c) and (d) considers $\Delta = 0$, $\Delta = 5$ meV, and $\Delta = 15$ meV respectively. In addition, we consider $V = 10$ meV and $\xi = 1$. The red and blue colors denote the bands associated with spin up and down, respectively. (e) shows the Chern number of the spin-down valence band as a function of $V$ and $\Delta$. Depending on the sign of $V-\Delta$, the valley Chern number of the band can be $+1$ or $-1$.
• Figure 2: Piezoelectric and piezospintronics response as a function of $\Delta$. The gray dot-line denotes the value of the ideal quantized value \ref{['quantization']}. We have considered that V = 10 meV and $\xi = 1$.
• Figure 3: Orbital magnetic moment for differents value of $\Delta$ for the valence band. (a), (b) and (c) considers $\Delta = 0$, $\Delta = 5$ meV and $\Delta = 15$ meV, respectively. We consider that $V = 10$ meV and $\xi = 1$. The color scale indicates the magnitude of the orbital moment (in units of $\mu_B$). (D) shows the total magnetizacion $M_z$ as a function of $\Delta$ showing a clear decrease of $M_z$ as $\Delta$ increases.