Examinando por Autor "Rodriguez-Lopez, Pablo"

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Casimir energy and entropy between perfect metal spheres
(World Scientific Publishing, 2012-07-28) Rodriguez-Lopez, Pablo
We calculate the Casimir energy and entropy for two perfect metal spheres in the large and short separation limit. We obtain nonmonotonic behavior of the Helmholtz free energy with separation and temperature, leading to parameter ranges with negative entropy, and also nonmonotonic behavior of the entropy with temperature and with the separation between the spheres. The appearance of this anomalous behavior of the entropy is discussed as well as its thermodynamic consequences.
Casimir force phase transitions in the graphene family
(Springer Nature, 2017-03-15) Rodriguez-Lopez, Pablo; Kort-Kamp, Wilton J.M.; Dalvit, Diego A.R.; Woods, Lilia M.
The Casimir force is a universal interaction induced by electromagnetic quantum fluctuations between any types of objects. The expansion of the graphene family by adding silicene, germanene and stanene (2D allotropes of Si, Ge, and Sn), lends itself as a platform to probe Dirac-like physics in honeycomb staggered systems in such a ubiquitous interaction. We discover Casimir force phase transitions between these staggered 2D materials induced by the complex interplay between Dirac physics, spin-orbit coupling and externally applied fields. In particular, we find that the interaction energy experiences different power law distance decays, magnitudes and dependences on characteristic physical constants. Furthermore, due to the topological properties of these materials, repulsive and quantized Casimir interactions become possible.
Casimir interaction between inclined metallic cylinders
(American Physical Society, 2012-03-12) Rodriguez-Lopez, Pablo; Emig, Thorsten
The Casimir interaction between one-dimensional metallic objects (cylinders, wires) displays unconventional features. Here we study the orientation dependence of this interaction by computing the Casimir energy between two inclined cylinders over a wide range of separations. We consider Dirichlet, Neumann, and perfect-metal boundary conditions, both at zero temperature and in the classical high-temperature limit. For all types of boundary conditions, we find that at large distances the interaction decays slowly with distance, similarly to the case of parallel cylinders, and at small distances scales as the interaction of two spheres (but with different numerical coefficients). Our numerical results at intermediate distances agree with our analytical predictions at small and large separations. Experimental implications are discussed.
Composition and stacking dependent topology in bilayers from the graphene family
(American Physical Society, 2019-06-21) Popescu, Adrian; Rodriguez-Lopez, Pablo; Woods, Lilia M
We present a compositional and structural investigation of silicene, germanene, and stanene bilayers from first principles. Due to the staggering of the individual layers, several stacking patterns are possible, most of which are not available to the bilayer graphene. This structural variety, in conjunction with the presence of the spin-orbit coupling, unveils a diversity of the electronic properties, with the appearance of distinct band features, including orbital hybridization and band inversion. We show that for particular cases, the intrinsic spin Hall response exhibits signatures of nontrivial electronic band topology, making these structures promising candidates to probe Dirac-like physics.
Confinement-Induced Nonlocality and Casimir Force in Transdimensional Systems
(The Royal Society of Chemistry, 2023-10-09) Bondarev, Igor V.; Pugh, Michael D.; Rodriguez-Lopez, Pablo; Woods, Lilia M.; Antezza, Mauro
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 ∼1/l 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.
Dispersive interactions between standard and Dirac materials and the role of dimensionality
(IOP Publishing, 2022-05-17) Le, Dai-Nam; Rodriguez-Lopez, Pablo; Woods, Lilia M
The van der Waals (vdW) interaction plays a prominent role between neutral objects at separations where short ranged chemical forces are negligible. This type of dispersive coupling is determined by the interplay between geometry and response properties of the materials making up the objects. Here, we investigate the vdW interaction between 1D, 2D, and 3D standard and Dirac materials within the Random Phase Approximation, which takes into account collective excitations originating from the electronic Coulomb potential. A comprehensive understanding of characteristic functionalities and scaling laws are obtained for systems with parabolic energy dispersion (standard materials) and crossing linear bands (Dirac materials). By comparing the quantum mechanical and thermal limits the onset of thermal fluctuations in the vdW interaction is discussed showing that thermal effects are significantly pronounced at smaller scales in reduced dimensions.
Effect of curvature and confinement on the Casimir-Polder interaction
(American Physical Society, 2015-01-30) Rodriguez-Lopez, Pablo; Emig, Thorsten; Noruzifar, Ehsan; Zandi, Roya
Modifications of Casimir-Polder interactions due to confinement inside a cylindrical cavity and due to curvature in- and outside the cavity are studied. We consider a perfectly conducting cylindrical shell with a single particle (atom or macroscopic sphere) located next to its interior or exterior surface or two atoms placed inside the shell. By employing the scattering approach, we obtain the particle-cavity interaction and the modification of the two-particle interaction due to the cavity. We consider both retardation and thermal effects. While for the atoms a dipole description is sufficient, for the macroscopic sphere we sum (numerically) over many multipole fluctuations to compute the interaction at short separations. In the latter limit we make comparisons to the proximity approximation and a gradient expansion and find agreement. Our results indicate a confinement-induced suppression of the force between atoms. General criteria for suppression and enhancement of Casimir interactions due to confinement are discussed.
Electric conductivity in graphene: Kubo model versus a nonlocal quantum field theory model
(American Physical Society, 2025-03-26) Rodriguez-Lopez, Pablo; Wang, Jian-Sheng; Antezza, Mauro
We compare three models of graphene electric conductivity: a non-local Kubo model, a local model derived by Falkovsky, and finally, a non-local quantum field theory (QFT) polarization-based model. These models are supposed to provide consistent results since they are derived from the same Hamiltonian. While we confirm that the local model is a proper $\textbf{q}\to\textbf{0}$ limit of both the non-local Kubo and the non-local QFT model (once losses are added to this last model), we find hard inconsistencies in the non-local QFT model as derived and currently used in literature. In particular, in the genuine non-local region ($\textbf{q}\neq\textbf{0}$), the available QFT model shows an intrinsic non-physical plasma-like behavior for the interband transversal electric conductivity at low frequencies (even after introducing the unavoidable losses). The Kubo model, instead, shows the expected behavior, i.e., an almost constant electric conductivity as a function of frequency $\omega$ with a gap for frequencies $\hbar\omega < \sqrt{(\hbar v_{F}q)^{2} + 4m^{2}}$. We show that the Kubo and QFT models can be expressed using an identical Polarization operator $\Pi_{\mu\nu}(\omega,\textbf{q})$, but they employ different expressions for the electric conductivity $\sigma_{\mu\nu}(\omega,\textbf{q})$. In particular, the Kubo model uses a standard regularized expression, a direct consequence of Ohm's Law and causality, as we rigorously re-derive. We show that, once the standard regularized expression for $\sigma_{\mu\nu}(\omega,\textbf{q})$ is used in the QFT model, and losses are included, the Kubo and QFT model coincide, and all its anomalies naturally disappear. Our findings show the necessity to appropriately define and regularize the electric conductivity to connect it with the available QFT model. This can be relevant for theory, predictions, and experimental tests in the nanophotonics and Casimir effect communities.
Fluctuations, correlations, and Casimir-like forces in the homogeneous cooling state of a granular gas
(AIP, 2024-01-22) Jiménez Oliva, Jesús David; Rodriguez-Lopez, Pablo; Khalil, Nagi
The fluctuating hydrodynamics by Brey et. al. is analytically solved to get the long-time limit of the fluctuations of the number density, velocity field, and energy density around the homogeneous cooling state of a granular gas, under physical conditions where it keeps stable. Explicit expressions are given for the non-white contributions in the elastic limit. For small dissipation, the latter is shown to be much smaller than the inelastic contributions, in general. The fluctuation-induced Casimir-like forces on the walls of the system are calculated assuming a fluctuating pressure tensor resulting from perturbing its Navier-Stokes expression. This way, the Casimir-like forces emerges as the correlation between the longitudinal velocity and the energy density. Interestingly, the fluctuation-induced forces push/pull the system towards the square or rectangular geometry when they vanish, in good agreement with the event-driven numerical simulations.
Giant anisotropy and Casimir phenomena: The case of carbon nanotube metasurfaces
(American Physical Society, 2024-01-17) Rodriguez-Lopez, Pablo; Le, Dai-Nam; Bondarev, Igor V.; Antezza, Mauro; Woods, Lilia M.
The Casimir interaction and torque are related phenomena originating from the exchange of electromagnetic excitations between objects. While the Casimir force exists between any types of objects, the materials or geometrical anisotropy drives the emergence of the Casimir torque. Here both phenomena are studied theoretically between dielectric films with immersed parallel single wall carbon nanotubes in the dilute limit with their chirality and collective electronic and optical response properties taken into account. It is found that the Casimir interaction is dominated by thermal fluctuations at sub-micron separations, while the torque is primarily determined by quantum mechanical effects. This peculiar quantum vs. thermal separation is attributed to the strong influence of reduced dimensionality and inherent anisotropy of the materials. Our study suggests that nanostructured anisotropic materials can serve as novel platforms to uncover new functionalities in ubiquitous Casimir phenomena.
Materials perspective on Casimir and van der Waals interactions
(American Physical Society, 2016-11-02) Woods, Lilia M.; Dalvit, Diego A. R.; Tkatchenko, Alexandre; Rodriguez-Lopez, Pablo; Rodriguez, Alejandro W.; Podgornik, Rudolf
Interactions induced by electromagnetic fluctuations, such as van der Waals and Casimir forces, are of universal nature present at any length scale between any types of systems. Such interactions are important not only for the fundamental science of materials behavior, but also for the design and improvement of micro- and nanostructured devices. In the past decade, many new materials have become available, which has stimulated the need for understanding their dispersive interactions. The field of van der Waals and Casimir forces has experienced an impetus in terms of developing novel theoretical and computational methods to provide new insights into related phenomena. The understanding of such forces has far reaching consequences as it bridges concepts in materials, atomic and molecular physics, condensed-matter physics, high-energy physics, chemistry, and biology. This review summarizes major breakthroughs and emphasizes the common origin of van der Waals and Casimir interactions. Progress related to novel ab initio modeling approaches and their application in various systems, interactions in materials with Dirac-like spectra, force manipulations through nontrivial boundary conditions, and applications of van der Waals forces in organic and biological matter are examined. The outlook of the review is to give the scientific community a materials perspective of van der Waals and Casimir phenomena and stimulate the development of experimental techniques and applications.
Nematic phase in a two-dimensional Hubbard model at weak coupling and finite temperature
(American Physical Society, 2018-08-14) Slizovskiy, Sergey; Rodriguez-Lopez, Pablo; Betouras, Joseph J.
We apply the self-consistent renormalized perturbation theory to the Hubbard model on the square lattice at finite temperatures to study the evolution of the Fermi surface (FS) as a function of temperature and doping. Previously, a nematic phase for the same model has been reported to appear at weak coupling near a Lifshitz transition from closed to open FS at zero temperature where the self-consistent renormalized perturbation theory was shown to be sensitive to small deformations of the FS. We find that the competition with the superconducting order leads to a maximal nematic order appearing at nonzero temperature. We explicitly observe the two competing phases near the onset of nematic instability, and by comparing the grand canonical potentials, we find that the transitions are first order. We explain the origin of the interaction-driven spontaneous symmetry breaking to a nematic phase in a system with several symmetry-related Van Hove points and discuss the required conditions.
Nonlinear effects in manybody van der Waals interactions
(American Physical Society, 2024-03-15) Le, Dai-Nam; Rodriguez-Lopez, Pablo; Woods, Lilia M
Van der Waals interactions are ubiquitous and they play an important role for the stability of materials. Current understanding of this type of coupling is based on linear response theory, while optical nonlinearities are rarely considered in this context. Many materials, however, exhibit strong optical nonlinear response, which prompts further evaluation of dispersive forces beyond linear response. Here we present a discrete coupled nonlinear dipole approach that takes into account linear and nonlinear properties of all dipolar nanoparticles in a given system. This method is based on a Hamiltonian for nonlinear dipoles, which we apply in different systems uncovering a complex interplay of distance, anisotropy, polarizabilities, and hyperpolarizabilities in the vdW energy. This investigation broadens our basic understanding of dispersive interactions, especially in the context of nonlinear materials.
Nonlocal optical response in topological phase transitions in the graphene family
(American Physical Society, 2018-01-22) Rodriguez-Lopez, Pablo; Kort-Kamp, Wilton J. M.; Dalvit, Diego A. R.; Woods, Lilia M.
We investigate the electromagnetic response of staggered two-dimensional materials of the graphene family, including silicene, germanene, and stanene, as they are driven through various topological phase transitions using external fields. Utilizing Kubo formalism, we compute their optical conductivity tensor taking into account the frequency and wave vector of the electromagnetic excitations, and study its behavior over the full electronic phase diagram of the materials. In particular, we find that the resonant behavior of the nonlocal Hall conductivity is strongly affected by the various topological phases present in these materials. We also consider the plasmon excitations in the graphene family and find that nonlocality in the optical response can affect the plasmon dispersion spectra of the various phases. We find a regime of wave vectors for which the plasmon relations for phases with trivial topology are essentially indistinguishable, while those for phases with nontrivial topology are distinct and are redshifted as the corresponding Chern number increases. The expressions for the conductivity components are valid for the entire graphene family and can be readily used by others.
Nonreciprocal heat flux via synthetic fields in linear quantum systems
(American Physical Society, 2023-10-03) Biehs, Svend-Age; Rodriguez-Lopez, Pablo; Antezza, Mauro; Agarwal, Girish S.
We study the heat transfer between N coupled quantum resonators with applied synthetic electric and magnetic fields realized by changing the resonator parameters by external drivings. To this end we develop two general methods, based on the quantum optical master equation and on the Langevin equation for N coupled oscillators where all quantum oscillators can have their own heat baths. The synthetic electric and magnetic fields are generated by a dynamical modulation of the oscillator resonance with a given phase. Using Floquet theory, we solve the dynamical equations with both methods, which allow us to determine the heat flux spectra and the transferred power. We apply these methods to study the specific case of a linear tight-binding chain of four quantum coupled resonators. We find that, in that case, in addition to a nonreciprocal heat flux spectrum already predicted in previous investigations, the synthetic fields induce here nonreciprocity in the total heat flux, hence realizing a net heat flux rectification.
Phonon-assisted Casimir interactions between piezoelectric materials
(Nature Portfolio, 2024-12-02) Le, Dai-Nam; Rodriguez-Lopez, Pablo; Woods, Lilia M.
The strong coupling between electromagnetic fields and lattice oscillations in piezoelectric materials gives rise to phonon polariton excitations. Such quasiparticles are important in modulating the ubiquitous Casimir force. Here by utilizing the generalized Born-Huang hydrodynamics model exemplified in SiC, three types of phonons are studied: longitudinal optical phonon, transverse optical phonon and phonon polariton. The Fresnel reflection coefficients for the piezoelectric composed of semi-infinite substrates or thin films are then obtained by taking into account the phonon-electromagnetic coupling. The Casimir interaction, calculated via a generalized Lifshitz approach, is examined to highlight the interplay between different types of phonon modes and electromagnetic excitations. Our study shows that piezoelectrics emerge as materials where this ubiquitous force can be controlled via phonon properties. Different types of surface phonon polaritons associated with structural polytypes may also be distinguished through the Casimir interaction.
Radiative heat transfer in 2D Dirac materials
(IOP Publishing, 2015-05-12) Rodriguez-Lopez, Pablo; Tse, Wang-Kong; Dalvit, Diego A. R.
We compute the radiative heat transfer between two sheets of 2D Dirac materials, including topological Chern insulators and graphene, within the framework of the local approximation for the optical response of these materials. In this approximation, which neglects spatial dispersion, we derive both numerically and analytically the short-distance asymptotic of the near-field heat transfer in these systems, and show that it scales as the inverse of the distance between the two sheets. Finally, we discuss the limitations to the validity of this scaling law imposed by spatial dispersion in 2D Dirac materials.
Relativistic quantum optics: The relativistic invariance 3 of the light-matter interaction models
(American Physical Society, 2018-05-29) Martín-Martínez, Eduardo; Rodriguez-Lopez, Pablo
In this article we discuss the invariance under general changes of reference frame of all the physical predictions of particle detector models in quantum field theory in general and, in particular, of those used in quantum optics to model atoms interacting with light. We find explicitly how the light-matter interaction Hamiltonians change under general coordinate transformations, and analyze the subtleties of the Hamiltonians commonly used to describe the light-matter interaction when relativistic motion is taken into account.
Signatures of complex optical response in Casimir interactions of type I and II Weyl semimetals
(Nature Portfolio, 2020-03-26) Rodriguez-Lopez, Pablo; Popescu, Adrian; Fialkovsky, Ignat; Khusnutdinov, Nail; Woods, Lilia M
The Casimir interaction, induced by electromagnetic fluctuations between objects, is strongly dependent upon the electronic and optical properties of the materials making up the objects. Here we investigate this ubiquitous interaction between semi-infinite spaces of topologically nontrivial Weyl semimetals. A comprehensive examination of all components of the bulk conductivity tensor and the surface conductivity due to the Fermi arc states in real and imaginary frequency domains is presented using the Kubo formalism for materials with different degree of tilting of their linear energy cones. The Casimir energy is calculated using a generalized Lifshitz approach, for which electromagnetic boundary conditions for anisotropic materials were derived and used. We find that the interaction between Weyl semimetals is metallic-like and its magnitude and characteristic distance dependence can be modified by the degree of tilting and chemical potential. The nontrivial topology plays a secondary role in the interaction and thermal fluctuations are expected to have similar effects as in metallic systems.
Stochastic quantization and Casimir forces
(IOP Publishing, 2011-11-25) Rodriguez-Lopez, Pablo; Brito, Ricardo; Soto, Rodrigo
In this paper we show how the stochastic quantization method developed by Parisi and Wu can be used to obtain Casimir forces. Both quantum and thermal fluctuations are taken into account by a Langevin equation for the field. The method allows the Casimir force to be obtained directly, derived from the stress tensor instead of the free energy. It only requires the spectral decomposition of the Laplacian operator in the given geometry. The formalism provides also an expression for the fluctuations of the force. As an application we compute the Casimir force on the plates of a finite piston of arbitrary cross-section. Fluctuations of the force are also directly obtained, and it is shown that, in the piston case, the variance of the force is twice the force squared.