Interstitial-induced ferromagnetism in a two-dimensional Wigner crystal

TL;DR

This paper investigates how interstitial defects in a two-dimensional Wigner crystal induce local ferromagnetism at higher energy scales, potentially explaining experimental observations of ferromagnetic insulating states.

Contribution

It introduces a detailed analysis of interstitial defect dynamics in a 2D Wigner crystal, revealing their role in generating high-energy ferromagnetic polarons.

Findings

Interstitial defects produce local ferromagnetism with higher energy scales.

Three dominant hopping processes favor fully polarized ferromagnetic polarons.

Results suggest a link between defect-induced magnetism and the metal-insulator transition.

Abstract

The two-dimensional Wigner crystal (WC) occurs in the strongly interacting regime ($r_s \gg 1$) of the two-dimensional electron gas (2DEG). The magnetism of a pure WC is determined by tunneling processes that induce multi-spin ring-exchange interactions, resulting in fully polarized ferromagnetism for large enough $r_s$. Recently, Hossain et al. [PNAS 117 (51) 32244-32250] reported the occurrence of a fully polarized ferromagnetic insulator at $r_s \gtrsim 35$ in an AlAs quantum well, but at temperatures orders of magnitude larger than the predicted exchange energies for the pure WC. Here, we analyze the large $r_s$ dynamics of an interstitial defect in the WC, and show that it produces local ferromagnetism with much higher energy scales. Three hopping processes are dominant, which favor a large, fully polarized ferromagnetic polaron. Based on the above results, we speculate concerning the phenomenology of the magnetism near the metal-insulator transition of the 2DEG.