Effect of excess charge carriers and fluid medium on the magnitude and the sign of the Casimir-Lifshitz torque

TL;DR

This paper investigates how excess charge carriers and fluid media influence the magnitude and sign of the Casimir-Lifshitz torque between biaxial black phosphorus slabs, revealing enhancement effects and non-monotonic behaviors with separation distance.

Contribution

It extends a perturbative theory of Casimir-Lifshitz torque to include effects of charge carriers and fluid media, showing their impact on torque magnitude and sign reversal.

Findings

Charge carriers and fluid media enhance torque magnitude.

Enhancement increases with carrier concentration and separation distance.

Non-monotonic torque behavior observed in non-identical slabs.

Abstract

Last year, we reported a perturbative theory of the Casimir-Lifshitz torque between planar biaxially anisotropic materials in the retarded limit [Phys. Rev. Lett. {\bf 120}, 131601 (2018)], which is applied here to study the change of sign and magnitude of the torque with separation distance in biaxial black phosphorus having excess charge carriers. The study is carried out both in vacuum as well as in a background fluid medium. The presence of extra charge carriers and that of an intervening fluid medium are both found to promote enhancement of the magnitude of the torque between identical slabs. The degree of enhancement of the magnitude of torque increases not only with an increased carrier concentration but also with separation distance. In the non-identical case when different planes of anisotropic black phosphorus face each other, owing to the non-monotonic characteristic of the sign-reversal effect of the torque, the enhancement by carrier addition and intervening medium also becomes non-monotonic with distance. In the presence of a background medium, the non-monotonic degree of enhancement of the torque with distance is observed even between identical slabs.