arXiv subjects

Bing-Sui Lu

Publications and source records attributed to Bing-Sui Lu.

At least 19 recordsLinked to original sources

arXiv2026

Fluctuation force induced by quenched random polarizations

We investigate the zero-temperature behavior of the fluctuation force induced by random electric dipoles that are frozen into a material and incapable of fluctuating thermally. Examples of such materials are relaxor ferroelectrics. In terms of the setup and geometry, we focus on a layered system comprising two coplanar semi-infinite slabs separated by a distance $\ell$ as well as a system comprising a neutral atom in the vacuum located at a distance $\ell$ above the surface of a semi-infinite slab. For both systems, we consider the cases where the quenched random dipolar disorder occurs inside the bulk as well as on the surface of the slabs. In all of these cases, we find that the bulk (surface) dipolar disorder-induced fluctuation force grows with the mean square quenched electric dipole moment per unit volume (area). The bulk (surface) dipolar disorder-induced fluctuation pressure between two semi-infinite single-layered slabs decays with $\ell^{-3}$ ($\ell^{-4}$), whereas the bulk (surface) dipolar disorder-induced fluctuation force on an atom in the vacuum near a slab containing the disorder decays with $\ell^{-4}$ ($\ell^{-5}$). We also find that the quenched dipolar disorder-induced fluctuation force between two coplanar slabs can be repulsive and serve to enhance the ``nanolevitation effect" in a three-layered dielectric system that obeys the Dzyaloshinskii-Lifshitz-Pitaevskii condition.

cond-mat.dis-nn↗

arXiv2025

Atom-surface interaction induced by quenched monopolar charge disorder

We study the modification to the energy level shifts of an atom induced by the quenched monopolar charge disorder inside the bulk of neighboring dielectric slabs as well as their surfaces. By assuming that the charge disorder follows Gaussian statistics with a zero mean, we find that the disorder generally results in a downward shift of the energy levels, which corresponds to an attractive force that can compete with and overcome the nonresonant Casimir-Polder force for sufficiently large atom-surface separations $z_0$. For an atom near a single semi-infinite slab with bulk (surface) charge disorder, the shift decays as $z_0^{-1}$ ($z_0^{-2}$). For both surface and bulk disorder, the shift is proportional to the variance of the charge disorder density. In addition, we investigate the behavior of the charge disorder-induced energy level shift for an atom confined to a vacuum gap between two coplanar and semi-infinite slabs of the same dielectric material, finding that the position of net zero disorder-induced force occurs closer to the surface of the slab with the smaller charge disorder variance.

cond-mat.mes-hall↗

arXiv2025

Nonresonant Casimir-Polder repulsion with a monolayer topological insulator

We investigate the behavior of the nonresonant Casimir-Polder force acting on a metastable polarized state of a two-level atomic system with a right-circularly polarized electric dipole transition in the presence of a monolayer topological insulator inhabiting an anomalous quantum Hall insulator state with a negative Chern number, finding that the force can be repulsive for a certain range within the far-field region. By considering stanene irradiated by circularly polarized monochromatic laser light as a material example of a monolayer topological insulator, we study the effect of the frequency dispersion in the conductivity tensor on the force behavior, finding that the dispersion generally leads to a slight downward shift of the force relative to that predicted by a nondispersive approximation, as well as a slight reduction of the range over which the force is repulsive.

cond-mat.mes-hall↗

arXiv2024

Casimir-Polder interaction between an atom and a Chern insulator: topological signature and long-range repulsion

We consider the Casimir-Polder interaction between a two-level atomic system and a Chern insulator for both the resonant and nonresonant channels. For a right circularly polarized excited atomic state near a Chern insulator with a negative Chern number $C$, the resonant Casimir-Polder force can be monotonically repulsive over a large range of separations. In the presence of the same Chern insulator, a right circularly polarized metastable atomic state is expected to experience a repulsive nonresonant Casimir-Polder force over a certain range of atom-surface separations in the far-field region. At still greater separations, the nonresonant Casimir-Polder force is expected to become attractive and exhibit a topological signature, being proportional to $(Cα)^2/(1+(Cα)^2)$, where $α$ is the fine-structure constant.

quant-ph↗

arXiv2022

An excited atom interacting with a Chern insulator: towards a far-field resonant Casimir-Polder repulsion

We consider the resonant Casimir-Polder interaction of an excited alkali-type atom which has a single (electric dipole) transition with a Chern insulator. The Chern insulator has a nonzero, time reversal symmetry breaking Hall conductance, leading to an additional contribution to the resonant Casimir-Polder interaction which depends on the coupling between the Hall conductance and the circular polarization state of the atomic transition. We find that the resonant Casimir-Polder shift can be significantly enhanced for de-excitation frequencies near values associated with the van Hove singularities of the Chern insulator. Furthermore, we find that the resonant Casimir-Polder force can become monotonically decaying and repulsive for a relatively large atom-surface distance, if the atomic dipole transition is right circularly polarized and the Chern number of the Chern insulator is $-1$ or the atomic dipole transition is left circularly polarized and the Chern number is $1$.

quant-ph↗

arXiv2021

Atom-surface physics: A review

An atom in front of a surface is one of the simplest and fundamental problem in physics. Yet, it allows testing quantum electrodynamics, while providing potential platforms and interfaces for quantum technologies. Despite, its simplicity, combined with strong scientific and technological interests, atom-surface physics, at its fundamental level, remains largely unexplored mainly because of challenges associated with precise control of the atom-surface distance. Nevertheless, substantial breakthroughs have been made over the last two decades. With the development of cold and quantum atomic gases, one has gained further control on atom-surface position, naturally leading to improved precision in the Casimir-Polder interaction measurement. Advances have also been reported in finding experimental knobs to tune and even reverse the Casimir-Polder interaction strength. So far, this has only been achieved for atoms in short-lived excited states, however, the rapid progresses in material sciences, e.g. metamaterials and topological materials have inspired new ideas for controlling the atom-surface interaction in long-lived states. In addition, combining nano-photonic and atom-surface physics is now envisioned for applications in quantum information processing. The first purpose of this review is to give a general overview on the latest experimental developments in atom-surface physics. The second main objective is to sketch a vision of the future of the field, mainly inspired by the abundant theoretical works and proposals available now in the literature.

physics.atom-ph↗

arXiv2021

The Casimir effect in topological matter

We give an overview of the work done during the past ten years on the Casimir interaction in electronic topological materials, our focus being solids which possess surface or bulk electronic band structures with nontrivial topologies, which can be evinced through optical properties that are characterizable in terms of nonzero topological invariants. The examples we review are three-dimensional magnetic topological insulators, two-dimensional Chern insulators, graphene monolayers exhibiting the relativistic quantum Hall effect, and time reversal symmetry-broken Weyl semimetals, which are fascinating systems in the context of Casimir physics, firstly for the reason that they possess electromagnetic properties characterizable by axial vectors (because of time reversal symmetry breaking), and depending on the mutual orientation of a pair of such axial vectors, two systems can experience a repulsive Casimir-Lifshitz force even though they may be dielectrically identical. Secondly, the repulsion thus generated is potentially robust against weak disorder, as such repulsion is associated with a Hall conductivity which is topologically protected in the zero-frequency limit. Finally, the far-field low-temperature behavior of the Casimir force of such systems can provide signatures of topological quantization.

cond-mat.mes-hall↗

arXiv2021

Qualitative chirality effects on the Casimir-Lifshitz torque with liquid crystals

We model a cholesteric liquid crystal as a planar uniaxial multilayer system, where the orientation of each layer differs slightly from that of the adjacent one. This allows us to analytically simplify the otherwise acutely complicated calculation of the Casimir-Lifshitz torque. Numerical results differ appreciably from the case of nematic liquid crystals, which can be treated like bloc birefringent media. In particular, we find that the torque deviates considerably from its usual sinusoidal behavior as a function of the misalignment angle. In the case of a birefringent crystal faced with a cholesteric liquid one, the Casimir-Lifshitz torque decreases more slowly as a function of distance than in the nematic case. In the case of two cholesteric liquid crystals, either in the homochiral or in the heterochiral configuration, the angular dependence changes qualitatively as a function of distance. In all considered cases, finite pitch length effects are most pronounced at distances of about 10 nm.

cond-mat.soft↗

arXiv2020

Spontaneous emission of a quantum emitter near a Chern insulator: interplay of time reversal symmetry breaking and van Hove singularity

We consider the generic problem of a two-level quantum emitter near a two-dimensional Chern insulator in the dipole approximation, and study how the frequency-dependent response and electronic density of states of the insulator modifies the transition rate of the emitter between the ground and excited levels. To this end, we obtain the full real-frequency behavior of the conductivity tensor by performing a tight-binding calculation based on the Qi-Wu-Zhang model and using a Kubo formula, and derive the full electromagnetic Green tensor of the system, which breaks Onsager reciprocity. This enables us to find that for frequencies smaller than the maximum band gap, the system is sensitive to time reversal symmetry-breaking, whereas for much larger frequencies the system becomes insensitive, with implications for the discrimination of the state of a circularly polarised dipole emitter. We also study the impact of a van Hove singularity on the surface-induced correction to the transition rate, finding that it can enhance its amplitude by a few orders of magnitude compared to the case where the conductivity is set to its static value. By considering configurations in which the dipole is circularly polarised or parallel with the surface of the Chern insulator, we find that the surface correction to the transition rate can exhibit a novel decay with sine integral-like oscillations.

cond-mat.mes-hall↗

arXiv2019

Maxwell Eigenmode approach to the Casimir-Lifshitz Torque

More than forty years ago, Barash published a calculation of the full retarded Casimir-Lifshitz torque for planar birefringent media with arbitrary degrees of anisotropy. An independent theoretical confirmation has been lacking since. We report a systematic and transparent derivation of the torque between two media with both electric and magnetic birefringence. Our approach, based on an eigenmode decomposition of Maxwell's equations, generalizes Barash's result for electrically birefringent materials, and can be generalized to a wide range of anisotropic materials and finite thickness effects.

cond-mat.mes-hall↗

arXiv2018

Van der Waals torque and force between anisotropic topological insulator slabs

We investigate the character of the van der Waals (vdW) torque and force between two coplanar and dielectrically anisotropic topological insulator (TI) slabs separated by a vacuum gap in the non-retardation regime, where the optic axes of the slabs are each perpendicular to the normal direction to the slab-gap interface and also generally differently oriented from each other. We find that in addition to the magnetoelectric coupling strength, the anisotropy can also influence the sign of the vdW force, viz., a repulsive vdW force can become attractive if the anistropy is increased sufficiently. In addition, the vdW force oscillates as a function of the angular difference between the optic axes of the TI slabs, being most repulsive/least attractive (least repulsive/most attractive) for angular differences that are integer (half-integer) multiples of $π$. Our third finding is that the vdW torque for TI slabs is generally weaker than that for ordinary dielectric slabs. Our work provides the first instance in which the vector potential appears in a calculation of the vdW interaction for which the limit is non-retarded or static.

cond-mat.mes-hall↗

arXiv2018

A multiscale approach to liquid crystal nematics via statistical field theory

We propose an approach to a multiscale problem in the theory of thermotropic uniaxial nematics based on the method of statistical field theory. This approach enables us to relate the coefficients $A$, $B$, $C$, $L_1$ and $L_2$ of the Landau-de Gennes free energy for the isotropic-nematic phase transition to the parameters of a molecular model of uniaxial nematics, which we take to be a lattice gas model of nematogenic molecules interacting via a short-ranged potential. We obtain general constraints on the temperature and volume fraction of nematogens for the Landau-de Gennes theory to be stable against molecular orientation fluctuations at quartic order. In particular, for the case of a fully occupied lattice, we compute the values of the isotropic-nematic transition temperature and the order parameter discontinuity predicted by (i) a continuum approximation of the nearest-neighbor Lebwohl-Lasher model and (ii) a Lebwohl-Lasher-type model with a nematogenic interaction of finite range. We find that the predictions of (i) are in reasonably good agreement with known results of MC simulation.

cond-mat.stat-mech↗

arXiv2016

Modulation of elasticity and interactions in charged lipid multibilayers: monovalent salt solutions

We have studied the electrostatic screening effect of NaCl solutions on the interactions between anionic lipid bilayers in the fluid lamellar phase using a Poisson-Boltzmann based mean-field approach with constant charge and constant potential limiting charge regulation boundary conditions. The full DLVO potential, including the electrostatic, hydration and van der Waals interactions, was coupled to thermal bending fluctuations of the membranes via a variational Gaussian Ansatz. This allowed us to analyze the coupling between the osmotic pressure and the fluctuation amplitudes and compare them both simultaneously with the measured dependence on the bilayer separation, determined by the small-angle X-ray scattering experiments. High-structural resolution analysis of the scattering data revealed no significant changes of membrane structure as a function of salt concentration. Parsimonious description of our results is consistent with the constant charge limit of the general charge regulation phenomenology, with fully dissociated lipid charge groups, together with a four-fold reduction of the membranes' bending rigidity upon increasing NaCl concentration.

cond-mat.soft↗

arXiv2016

Van der Waals torque and force between dielectrically anisotropic layered media

We analyse van der Waals interactions between a pair of dielectrically anisotropic plane-layered media interacting across a dielectrically isotropic solvent medium. We develop a general formalism based on transfer matrices to investigate the van der Waals torque and force in the limit of weak birefringence and dielectric matching between the ordinary axes of the anisotropic layers and the solvent. We apply this formalism to study the following systems: (i) a pair of single anisotropic layers, (ii) a single anisotropic layer interacting with a multilayered slab consisting of alternating anisotropic and isotropic layers, and (iii) a pair of multilayered slabs each consisting of alternating anisotropic and isotropic layers, looking at the cases where the optic axes lie parallel and/or perpendicular to the plane of the layers. For the first case, the optic axes of the oppositely facing anisotropic layers of the two interacting slabs generally possess an angular mismatch, and within each multilayered slab the optic axes may either be the same, or undergo constant angular increments across the anisotropic layers. In particular, we examine how the behaviors of the van der Waals torque and force can be "tuned" by adjusting the layer thicknesses, the relative angular increment within each slab, and the angular mismatch between the slabs.

cond-mat.soft↗

arXiv2016

Nonequilibrium Tuning of the Thermal Casimir Effect

In net-neutral systems correlations between charge fluctuations generate strong attractive thermal Casimir forces and engineering these forces to optimize nanodevice performance is an important challenge. We show how the normal and lateral thermal Casimir forces between two plates containing Brownian charges can be modulated by decorrelating the system through the application of an electric field, which generates a nonequilibrium steady state with a constant current in one or both plates, reducing the ensuing fluctuation-generated normal force while at the same time generating a lateral drag force. This hypothesis is confirmed by detailed numerical simulations as well as an analytical approach based on stochastic density functional theory.

cond-mat.stat-mech↗

arXiv2016

Pseudo-Casimir stresses and elasticity of a confined elastomer film

Investigations of the elastic behavior of bulk elastomers have traditionally proceeded on the basis of classical rubber elasticity, which regards chains as thermally fluctuating but disregards the thermal fluctuations of the cross-links. Here, we consider an incompressible and flat elastomer film of an axisymmetric shape confined between two large hard co-planar substrates, with the axis of the film perpendicular to the substrates. We address the impact that thermal fluctuations of the cross-links have on the free energy of elastic deformation of the system, subject to the requirement that the fluctuating elastomer cannot detach from the substrates. We examine the behavior of the deformation free energy for one case where a rigid pinning boundary condition is applied to a class of elastic fluctuations at the confining surfaces, and another case where the same elastic fluctuations are subjected to soft "gluing" potentials. We find that there can be significant departures (both quantitative and qualitative) from the prediction of classical rubber elasticity theory when elastic fluctuations are included. Finally, we compare the character of the attractive part of the elastic fluctuation-induced, or pseudo-Casimir, stress with the standard thermal Casimir stress in confined but non-elastomeric systems, finding the same power law decay behavior when a rigid pinning boundary condition is applied, for the case of the gluing potential, we find that the leading order correction to the attractive part of the fluctuation stress decays inversely with the inter-substrate separation.

cond-mat.soft↗

arXiv2016

Charge Renormalization and Charge Oscillation in Asymmetric Primitive Model

The Debye charging method is generalized to study the linear response properties of the asymmetric primitive model for electrolytes. Analytic results are obtained for the effective charge distributions of constituent ions inside the electrolyte, from which all static linear response properties of system follow. It is found that, as the ion density increases, both the screening length and the dielectric constant receive substantial renormalization due to ionic correlations. Furthermore, the valence of larger ion is substantially renormalized upwards by ionic correlations, whilst that of smaller ions remains approximately the same. For sufficiently high density, the system exhibit charge oscillations. The threshold ion density for charge oscillation is much lower than the corresponding value for symmetric electrolytes. Our results agree well with large scale Monte Carlo simulations.

cond-mat.soft↗

arXiv2015

Hydrodynamic fluctuation-induced forces in confined fluids

We study thermal, fluctuation-induced hydrodynamic interaction forces in a classical, compressible, viscous fluid confined between two rigid, planar walls with no-slip boundary conditions. We calculate hydrodynamic fluctuations using the linearized, stochastic Navier-Stokes formalism of Landau and Lifshitz. The mean fluctuation-induced force acting on the fluid boundaries vanishes in this system, so we evaluate the two-point, time-dependent force correlations. The equal-time correlation function of the forces acting on a single wall gives the force variance, which we show to be finite and independent of the plate separation at large inter-plate distances. The equal-time, cross-plate force correlation, on the other hand, decays with the inverse inter-plate distance and is independent of the fluid viscosity at large distances; it turns out to be negative over the whole range of plate separations, indicating that the two bounding plates are subjected to counter-phase correlations. We show that the time-dependent force correlations exhibit damped temporal oscillations for small plate separations and a more irregular oscillatory behavior at large separations. The long-range hydrodynamic correlations reported here represent a "secondary Casimir effect", because the mean fluctuation-induced force, which represents the primary Casimir effect, is absent.

cond-mat.stat-mech↗