Repulsive Casimir Forces Research Articles

Large Casimir Flipping Torque in Quantum Trap.

Casimir torque between parallel plates, a macroscopic quantum electrodynamics effect, is known to be induced by dielectric anisotropy and related to the rotational degree of freedom. We here reveal a different type of Casimir torque generated on a Au plate suspended in a quantum trap without recourse to materials anisotropy. As the Au plate deflects from the equilibrium plane with a nonzero flipping angle, the regions departing from and approaching the Teflon-coated Au substrate are subjected to attractive and repulsive Casimir forces, respectively, resulting in a restoring torque about the axis of flipping. For a quantum trap with an equilibrium separation of ∼10 nm, the stiffness per unit area of the Casimir flipping torque can be an order of magnitude larger than those of previously reported dielectric anisotropy-induced rotational torques at the same separation. The large Casimir flipping torque provides the possibility of designing a mechanical oscillator completely dominated by quantum and thermal fluctuations.

Interplay between chiral media and perfect electromagnetic conductor plates: Repulsive vs. attractive Casimir force transitions

We determine the Casimir energies and forces in a variety of potentially experimentally viable setups, consisting of parallel plates made of perfect electromagnetic conductors (PEMCs), which generalize perfect electric conductors (PECs) and perfect magnetic conductors (PMCs), and Weyl semimetals (WSMs). Where comparison is possible, our results agree with the Casimir forces calculated elsewhere in the literature, albeit with different methods. We find a multitude of known but also new cases where repulsive Casimir forces are in principle possible, but restricting the setup to PECs combined with the aforementioned WSM geometry, results in purely attractive Casimir forces.

Enhanced repulsive Casimir forces between gold and thin magnetodielectric plates

Enhanced repulsive Casimir forces between gold and thin magnetodielectric plates

Tunable critical Casimir forces counteract Casimir–Lifshitz attraction

In developing micro- and nanodevices, stiction between their parts, that is, static friction preventing surfaces in contact from moving, is a well-known problem. It is caused by the finite-temperature analogue of the quantum electrodynamical Casimir–Lifshitz forces, which are normally attractive. Repulsive Casimir–Lifshitz forces have been realized experimentally, but their reliance on specialized materials severely limits their applicability and prevents their dynamic control. Here we demonstrate that repulsive critical Casimir forces, which emerge in a critical binary liquid mixture upon approaching the critical temperature, can be used to counteract stiction due to Casimir–Lifshitz forces and actively control microscopic and nanoscopic objects with nanometre precision. Our experiment is conducted on a microscopic gold flake suspended above a flat gold-coated substrate immersed in a critical binary liquid mixture. This may stimulate the development of micro- and nanodevices by preventing stiction as well as by providing active control and precise tunability of the forces acting between their constituent parts.

Nonlinear actuation of micromechanical Casimir oscillators with topological insulator materials toward chaotic motion: Sensitivity on magnetization and dielectric properties.

We have investigated the dynamical actuation of micro-electromechanical systems under the influence of attractive and repulsive Casimir forces between topological insulator plates as a function of their dielectric function and coating magnetization. The analysis of the Casimir force in the limit of strong and weak magnetization shows that the attractive force, which is produced for plate magnetizations in the same direction, is greater than the repulsive force that is produced for opposite magnetizations. However, both forces remain comparable for intermediate magnetizations. Moreover, for weak magnetization, the attractive force becomes stronger for an increasing dielectric function, while the opposite occurs for the repulsive force. On the other hand, increasing magnetization decreases the influence of the dielectric function on both the repulsive and attractive forces. Furthermore, for conservative systems, bifurcation and phase portrait analysis revealed that increasing magnetization decreases the regime of stable operation for devices with attractive forces, while their operation remains always stable under the presence of repulsive forces. Finally, for non-conservative periodically driven systems, the Melnikov function and Poincaré portrait analysis show that for magnetizations in the same direction leading to strong attractive Casimir forces, chaotic motion toward stiction is highly likely to occur preventing the long-term prediction of actuating dynamics. A remedy for this situation is obtained by the application of any magnetization in opposite directions between the interacting surfaces since the repulsive force makes it possible to prevent stiction.

Thermal hysteresis of Casimir suspensions enabled by vanadium dioxide

In this work, the Casimir suspensions of a nanoplate (gold or Teflon) in a liquid environment are theoretically investigated. The substrate of the system is a three-layer structure of Teflon/vanadium dioxide(VO2)/Teflon. It is found that stable suspensions of nanoplates can be achieved due to the balances of repulsive Casimir forces, gravity and buoyancy. Remarkably, the suspension positions of gold and Teflon nanoplates are strongly associated with the preceding thermal history of VO2, i.e., heating or cooling. The differences in the suspension positions between the heating and cooling can reach tens of nanometers. Finally, a thermal hysteresis of Casimir suspensions via the temperature is shown for gold and Teflon nanoplates. Our findings offer the possibility of designing conceptually new devices in micro and nanoelectromechanical systems (MEMS and NEMS).

Origin of the Repulsive Casimir Force in Giant Polarization-Interconversion Materials

Achieving strong repulsive Casimir forces through engineered coatings can pave the way for micro- and nano-electromechanical applications where adhesive forces currently cause reliability issues. Here, we exploit Lifshitz theory to identify the requirements for repulsive Casimir forces in gyrotropic media for two limiting cases (ultra-strong gyroelectric and non-gyroelectric). We show that the origin of repulsive force in media with strong gyrotropy such as Weyl semi-metals arises from the giant interconversion of polarization of vacuum fluctuations.

Comparing Classical and Quantum Electromagnetics: Part 3 [Special Series: Guest Editorial

Quantum technology is becoming increasingly important with the recent advancements in quantum computers. The article “Casimir Force: Vacuum Fluctuation, Zero-Point Energy, and Computational Electromagnetics,” by Xia et al., summarizes how to calculate the Casimir force in the simplest fashion. It gives a brief overview of quantum theory and quantum weirdness. It then explains the role of quantum theory in electromagnetics and how the Casimir energy is equal to the sum of the zero-point energies of different modes in a system. The zero-point energy gives rise to vacuum fluctuations that persist down to zero temperature. Next, the article shows how to find the electromagnetic modes of an electromagnetic system using computational electromagnetics. The authors discuss the use of the argument principle as a way of finding this sum of zero-point energies indirectly. Moreover, a broadband fast multipole algorithm is discussed that is error controllable from statics to microwave frequencies. As a result, both attractive and repulsive Casimir forces are demonstrated with some complex structures.

Repulsive Casimir forces

Repulsive Casimir forces

Thermal Casimir effect with general boundary conditions

In this paper we study the system of a scalar quantum field confined between two plane, isotropic, and homogeneous parallel plates at thermal equilibrium. We represent the plates by the most general lossless and frequency-independent boundary conditions that satisfy the conditions of isotropy and homogeneity and are compatible with the unitarity of the quantum field theory. Under these conditions we compute the thermal correction to the quantum vacuum energy as a function of the temperature and the parameters encoding the boundary condition. The latter enables us to obtain similar results for the pressure between plates and the quantum thermal correction to the entropy. We find out that our system is thermodynamically stable for any boundary conditions, and we identify a critical temperature below which certain boundary conditions yield attractive, repulsive, and null Casimir forces.

Chiral Casimir forces: Repulsive, enhanced, tunable

Both theoretical interest and practical significance attach to the sign and strength of Casimir forces. A famous, discouraging no-go theorem states that "The Casimir force between two bodies with reflection symmetry is always attractive." Here we identify a loophole in the reasoning, and propose a universal way to realize repulsive Casimir forces. We show that the sign and strength of Casimir forces can be adjusted by inserting optically active or gyrotropic media between bodies, and modulated by external fields.

Conditions for repulsive Casimir forces between identical birefringent materials

Repulsive Casimir-Lifshitz forces are known to exist between two dissimilar materials if a third material, whose dielectric response is intermediate, separates them. However, the force between two identical materials is almost always attractive. Here we show that the force between two identical, semi-infinite birefringent slabs can be repulsive for particular orientations and compare the conditions for repulsion in this system to those of isotropic materials. We examine the dependence of the Casimir-Lifshitz force on retardation and relative orientation in this system and discuss situations in which the force can be changed from attractive to repulsive as a function of both distance and rotation angle.

Measurement of non-monotonic Casimir forces between silicon nanostructures

Casimir forces are of fundamental interest because they originate from quantum fluctuations of the electromagnetic field. Apart from controlling the Casimir force via the optical properties of the materials, a number of novel geometries have been proposed to generate repulsive and/or non-monotonic Casimir forces between bodies separated by vacuum gaps. Experimental realization of these geometries, however, is hindered by the difficulties in alignment when the bodies are brought into close proximity. Here, using an on-chip platform with integrated force sensors and actuators, we circumvent the alignment problem and measure the Casimir force between two surfaces with nanoscale protrusions. We demonstrate that the Casimir force depends non-monotonically on the displacement. At some displacements, the Casimir force leads to an effective stiffening of the nanomechanical spring. Our findings pave the way for exploiting the Casimir force in nanomechanical systems using structures of complex and non-conventional shapes.

Repulsive Casimir forces at quantum criticality

We study the Casimir effect in the vicinity of a quantum-critical point. As a prototypical system we analyze the d-dimensional imperfect (mean-field) Bose gas enclosed in a slab of extension and subject to periodic boundary conditions. The thermodynamic state is adjusted so that , where is the thermal de Broglie length, and denotes microscopic lengthscales. Our exact analysis indicates that the Casimir force in the above-specified regime is generically repulsive and decays either algebraically or exponentially, with a non-universal amplitude.

Classical and fluctuation‐induced electromagnetic interactions in micron‐scale 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.

Attractive and repulsive Casimir vacuum energy with general boundary conditions

The infrared behaviour of quantum field theories confined in bounded domains is strongly dependent on the shape and structure of space boundaries. The most significant physical effect arises in the behaviour of the vacuum energy. The Casimir energy can be attractive or repulsive depending on the nature of the boundary. We calculate the vacuum energy for a massless scalar field confined between two homogeneous parallel plates with the most general type of boundary conditions depending on four parameters. The analysis provides a powerful method to identify which boundary conditions generate attractive or repulsive Casimir forces between the plates. In the interface between both regimes we find a very interesting family of boundary conditions which do not induce any type of Casimir force. We also show that the attractive regime holds far beyond identical boundary conditions for the two plates required by the Kenneth–Klich theorem and that the strongest attractive Casimir force appears for periodic boundary conditions whereas the strongest repulsive Casimir force corresponds to anti-periodic boundary conditions. Most of the analysed boundary conditions are new and some of them can be physically implemented with metamaterials.

Coherent control of Casimir force in a chiral medium

Chirality has been previously reported as a way to observe both attractive and repulsive Casimir forces. Here we propose the coherent control of the Casimir force between two identical atomic chiral media. A magnetic field is applied to a specific example system to split the detuning as well as the refractive indices for the two circularly polarizations, which leads to chirality. It is shown that, by controlling the strength of an external magnetic field, the Casimir force can switch between attractive and repulsive forces.

Repulsive and attractive Casimir forces between magnetodielectric slabs

The Casimir force between two parallel magnetodielectric slabs is investigated by means of Casimir–Lifshitz Theory. For two magnetodielectric slabs, one is permittivity-negative, while the other is permeability-negative in the real frequency space. Numerical results show that when the separation between these two slabs is small (or large), the Casimir force is repulsive, while for the intermediate separation, the Casimir force is attractive. As a consequence, there are two equilibria with zero Casimir force, and a repulsive–attractive–repulsive transition takes place with increasing the separation. Therefore, if the separation between two interacting slabs is manipulated in the small (or large) separation region, it is possible to overcome the stiction in micromechanical and nanomechanical systems.

REPULSIVE CASIMIR EFFECTS

Like Casimir's original force between conducting plates in vacuum, Casimir forces are usually attractive. But repulsive Casimir forces can be achieved in special circumstances. These might prove useful in nanotechnology. We give examples of when repulsive quantum vacuum forces can arise with conducting materials.

Superconducting phase-dependent force in SNS junctions with mobile scatterers

We calculate the quantum (Casimir-like) superconducting phase-dependent force acting on mobile scatterers in superconductor–normal metal–superconductor (SNS) junctions. Repulsive Casimir forces are predicted in short SNS junctions with nonequilibrium (inverse) Andreev level populations. An anomalous (nonmonotonic) temperature dependence of the quantum force is found in long SNS junctions.