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

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. 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 is observed even between identical slabs.

Nontrivial retardation effects in dispersion forces: From anomalous distance dependence to novel traps

“…[2,17,19,20,23,30,43]) to a more general case where retarded dispersion forces can reveal a very complex behaviour in media. For this reason, considerations of retardation effects are important for medium-assisted dispersion force experiments, for example Casimir experiments [16,23,43]; medium-assisted optical tweezers [44]; colloidal systems [45]; and in future measurements of the Casimir torque in a medium [46,47]. Beyond these impacts, we have shown that the near-field effect of dispersion forces in colloidal system yields an inhomogeneous particle density distribution near interfaces due to trapping potentials.…”