Probing quantum geometric nonlinear magnetization via second-harmonic magneto-optical Kerr effect

TL;DR

This paper demonstrates that quantum geometry, specifically the quantum Christoffel symbol, influences nonlinear magnetization in WTe₂, linking fundamental geometric concepts to observable magnetic phenomena via second-harmonic magneto-optical Kerr effect.

Contribution

It provides the first experimental evidence connecting quantum geometric quantities to nonlinear magnetization in a nonmagnetic material.

Findings

Nonlinear SMOKE signal scales quadratically with current.

Signal persists up to 200 K.

Orbital contribution dominates the nonlinear magnetization.

Abstract

Quantum geometry provides an intrinsic framework for characterizing the geometric structure of quantum states. It highlights its relevance to various aspects of fundamental physics. However, its direct implications for magnetic phenomena remain largely unexplored. Here, we report the observation of electric-field-induced nonlinear magnetization in the nonmagnetic semimetal WTe$_2$ by using a second-harmonic magneto-optical Kerr effect (SMOKE) spectroscopy. We observe a robust nonlinear SMOKE signal that scales quadratically with current and persists up to 200 K. Theoretical modeling and scaling analysis indicate that this nonlinear magnetization is dominated by the orbital contribution and is intrinsically linked to the quantum Christoffel symbol. Just as the Christoffel symbol is a fundamental quantity encoding spacetime geometry in Einstein's general relativity, our work establishes a direct link between quantum geometry and nonlinear magnetization, and provides a geometric perspective for designing future orbitronic devices.