298 research outputs found
Classical and fluctuation-induced electromagnetic interactions in micronscale systems: designer bonding, antibonding, and Casimir forces
Whether intentionally introduced to exert control over particles and
macroscopic objects, such as for trapping or cooling, or whether arising from
the quantum and thermal fluctuations of charges in otherwise neutral bodies,
leading to unwanted stiction between nearby mechanical parts, electromagnetic
interactions play a fundamental role in many naturally occurring processes and
technologies. In this review, we survey recent progress in the understanding
and experimental observation of optomechanical and quantum-fluctuation forces.
Although both of these effects arise from exchange of electromagnetic momentum,
their dramatically different origins, involving either real or virtual photons,
lead to different physical manifestations and design principles. Specifically,
we describe recent predictions and measurements of attractive and repulsive
optomechanical forces, based on the bonding and antibonding interactions of
evanescent waves, as well as predictions of modified and even repulsive Casimir
forces between nanostructured bodies. Finally, we discuss the potential impact
and interplay of these forces in emerging experimental regimes of
micromechanical devices.Comment: Review to appear on the topical issue "Quantum and Hybrid Mechanical
Systems" in Annalen der Physi
Impact of nuclear vibrations on van der Waals and Casimir interactions at zero and finite temperature
Van der Waals (vdW) and Casimir interactions depend crucially on material
properties and geometry, especially at molecular scales, and temperature can
produce noticeable relative shifts in interaction characteristics. Despite
this, common treatments of these interactions ignore electromagnetic
retardation, atomism, or contributions of collective mechanical vibrations
(phonons) to the infrared response, which can interplay with temperature in
nontrivial ways. We present a theoretical framework for computing
electromagnetic interactions among molecular structures, accounting for their
geometry, electronic delocalization, short-range interatomic correlations,
dissipation, and phonons at atomic scales, along with long-range
electromagnetic interactions among themselves or in the vicinity of continuous
macroscopic bodies. We find that in carbon allotropes, particularly fullerenes,
carbyne wires, and graphene sheets, phonons can couple strongly with long-range
electromagnetic fields, especially at mesoscopic scales (nanometers), to create
delocalized phonon polaritons that significantly modify the infrared molecular
response. These polaritons especially depend on the molecular dimensionality
and dissipation, and in turn affect the vdW interaction free energies of these
bodies above a macroscopic gold surface, producing nonmonotonic power laws and
nontrivial temperature variations at nanometer separations that are within the
reach of current Casimir force experiments.Comment: 11 pages, 4 figures (3 single-column, 1 double-column), 2 appendice
Fundamental limits to attractive and repulsive Casimir--Polder forces
We derive upper and lower bounds on the Casimir--Polder force between an
anisotropic dipolar body and a macroscopic body separated by vacuum via
algebraic properties of Maxwell's equations. These bounds require only a coarse
characterization of the system---the material composition of the macroscopic
object, the polarizability of the dipole, and any convenient partition between
the two objects---to encompass all structuring possibilities. We find that the
attractive Casimir--Polder force between a polarizable dipole and a uniform
planar semi-infinite bulk medium always comes within 10% of the lower bound,
implying that nanostructuring is of limited use for increasing attraction. In
contrast, the possibility of repulsion is observed even for isotropic dipoles,
and is routinely found to be several orders of magnitude larger than any known
design, including recently predicted geometries involving conductors with sharp
edges. Our results have ramifications for the design of surfaces to trap,
suspend, or adsorb ultracold gases.Comment: 6 pages, 3 figure
Confinement-Induced Nonlocality and Casimir Force in Transdimensional Systems
We study within the framework of the Lifshitz theory the long-range Casimir
force for in-plane isotropic and anisotropic free-standing transdimensional
material slabs. In the former case, we show that the confinement-induced
nonlocality not only weakens the attraction of ultrathin slabs but also changes
the distance dependence of the material-dependent correction to the Casimir
force to go as contrary to the dependence of
that of the local Lifshitz force. In the latter case, we use closely packed
array of parallel aligned single-wall carbon nanotubes in a dielectric layer of
finite thickness to demonstrate strong orientational anisotropy and crossover
behavior for the inter-slab attractive force in addition to its reduction with
decreasing slab thickness. We give physical insight as to why such a pair of
ultrathin slabs prefers to stick together in the perpendicularly oriented
manner, rather than in the parallel relative orientation as one would
customarily expect.Comment: 20 pages, 4 figures, 52 reference
Effect of excess charge carriers and fluid medium on the magnitude and the sign of the Casimir-Lifshitz torqueP. Thiyam
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
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