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

Last year, we reported a perturbative theory of the Casimir-Lifshitz torque between planar biaxially anisotropic materials in the retarded limit [Thiyam et al., Phys. Rev. Lett. 120, 131601 (201810.1103/PhysRevLett.120.131601)], which is applied here to study the change in 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 and 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 nonidentical case when different planes of anisotropic black phosphorus face each other, owing to the nonmonotonic characteristic of the sign-reversal effect of the torque, the enhancement by carrier addition and intervening medium also becomes nonmonotonic with distance. In the presence of a background medium, the nonmonotonic degree of enhancement of the torque with distance is observed even between identical slabs.