Vacuum Torque Without Anisotropy: Switchable Casimir Torque Between Altermagnets

TL;DR

This paper introduces a novel mechanism for Casimir torque generation using magnetic fields in altermagnets, which can induce and control vacuum-induced rotational forces without material anisotropy or geometric asymmetry.

Contribution

It demonstrates that axially symmetric magnetic fields can produce and reverse Casimir torque in altermagnets, revealing a new way to engineer vacuum quantum fluctuation effects.

Findings

Torque scales quadratically with magnetic field strength.

Temperature and distance dependence differ from uniaxial bulk materials.

Magnetic fields can control the sign and magnitude of Casimir torque.

Abstract

Casimir torque is conventionally associated with explicit breaking of rotational symmetry, arising from material dielectric anisotropy, geometric asymmetry, or externally applied fields that themselves break rotational invariance. Here we demonstrate a fundamentally different mechanism: an axially symmetric magnetic field can generate a Casimir torque by inducing an axially asymmetric Casimir energy - and can even reverse the torque's sign. Focusing on two-dimensional altermagnets, we show that a magnetic field applied perpendicular to the plane - while preserving in-plane rotational symmetry - activates an orientation-dependent vacuum interaction through the combined crystalline symmetry $\mathrm{C_n T}$ inherent to altermagnetic order. The resulting torque emerges continuously and scales quadratically with the magnetic field strength. We further analyze its temperature and distance dependence, revealing scaling behaviors that are qualitatively different from those found in uniaxial bulk materials. Our results identify time-reversal symmetry breaking as a powerful route for engineering both the sign and strength of Casimir torque and establish altermagnets as an exciting platform for exploring phenomena driven by vacuum quantum fluctuations.