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In this article we use an exact method to resolve the fields scattered by a spherical magneto-optical particle and calculate the optical forces exerted on it. The resulting force and the contributing components, i.e. magneto-optical gradient force and magneto-optical extinction force, are presented in an analytical form. We also derive analytical expressions for the scattering and extinction cross sections of a magneto-optical particle, expressions which intuitively demonstrate the effect of circular dichroism in magneto-optical scattering and forces. Finally, we demonstrate that the magneto-optical extinction force is the result of circular dichroism in magneto-optical scattering. We show that it is possible to completely cancel the scattering in the forward or in the backward direction, when the incident field is composed of a circularly-polarized reflected beam. Moreover, the directional scattering is interrelated to the direction of the force exerted on the particle., Comunidad de Madrid (SI1/PJI/2019-00052); Ministerio de Ciencia e Innovación (CEX2018-000805-M, PGC2018-095777-B-C21, PGC2018-095777-B-C22, PID2019-109905GA-C22).

A physical system experiences a phase transition when it goes from an ordered configuration (low temperature) to a disordered state (higher temperature). An order parameter is related to the corres...

Lanthanide-based upconverting nanoparticles (UCNPs) are trustworthy workhorses in luminescent nanothermometry. The use of UCNPs-based nanothermometers has enabled the determination of the thermal properties of cell membranes and monitoring of in vivo thermal therapies in real time. However, UCNPs boast low thermal sensitivity and brightness, which, along with the difficulty in controlling individual UCNP remotely, make them less than ideal nanothermometers at the single-particle level. In this work, it is shown how these problems can be elegantly solved using a thermoresponsive polymeric coating. Upon decorating the surface of NaYF4:Er3+,Yb3+ UCNPs with poly(N-isopropylacrylamide) (PNIPAM), a >10-fold enhancement in optical forces is observed, allowing stable trapping and manipulation of a single UCNP in the physiological temperature range (20–45 °C). This optical force improvement is accompanied by a significant enhancement of the thermal sensitivity— a maximum value of 8% °C+1 at 32 °C induced by the collapse of PNIPAM. Numerical simulations reveal that the enhancement in thermal sensitivity mainly stems from the high-refractive-index polymeric coating that behaves as a nanolens of high numerical aperture. The results in this work demonstrate how UCNP nanothermometers can be further improved by an adequate surface decoration and open a new avenue toward highly sensitive single-particle nanothermometry, This work was supported by the Ministerio de Ciencia e Innovación de España (PID2019-106211RB-I00 PID2019-105195RA-I00 and MAT2017- 83111R), by the Comunidad de Madrid (S2017/BMD-3867 RENIM-CM), co-financed by European Structural and Investment Fund and by the Universidad Autónoma de Madrid and Comunidad Autónoma de Madrid (SI1/PJI/2019-00052 and PR38/21-36 ANTICIPA-CM). D.L. acknowledges a scholarship from the China Scholarship Council (201808350097). J.R.B. acknowledges the support from Carl Tryggers Foundation (CTS18:229). M.I.M acknowledges financial support from the Spanish Ministerio de Ciencia e Innovación, through the “María de Maeztu” Programme for Units of Excellence in R&D (CEX2018-000805-M) and the MELODIA PGC2018-095777-B-C22 projec

Active particles are capable of extracting energy from their environment and converting it into direct motion. This direct motion is usually attained by asymmetries, which induce a net force in the particle. In this article we show how a dimer made up of two nanoparticles can obtain this energy supply from the nonreciprocal force induced by an homogeneous and isotropic random electromagnetic field. We explicitly obtain the value of the force and find it to be different from zero only for sets of different particles in which at least one of them is absorbent. In addition, we prove that the nonreciprocity of the force is the condition for the interaction dynamics to be nonconservative. We further show how this mechanism can be used to induce active Brownian motion with persistent random walks, effective diffusion coefficients, and noticeable Péclet numbers., This work has been supported by the Spanish Ministerio de Ciencia e Innovacion (MELODIA PGC2018-095777-B-C-22), ́ the UAM-CAM project (SI1/PJI/2019−00052), and the “Marí a de Maeztu” Programme for Units of Excellence in R and D (CEX2018-000805-M)

In this paper we show analytically and numerically the formation of a near-field stable optical binding between two identical plasmonic particles, induced by an incident plane wave. The equilibrium binding distance is controlled by the angle between the polarization plane of the incoming field and the dimer axis, for which we have calculated an explicit formula. We have found that the condition to achieve stable binding depends on the particle’s dielectric function and happens near the frequency of the dipole plasmonic resonance. The binding stiffness of this stable attaching interaction is four orders of magnitude larger than the usual far-field optical binding and is formed orthogonal to the propagation direction of the incident beam (transverse binding). The binding distance can be further manipulated considering the magneto-optical effect and an equation relating the desired equilibrium distance with the required external magnetic field is obtained. Finally, the effect induced by the proposed binding method is tested using molecular dynamics simulations. Our study paves the way to achieve complete control of near-field binding forces between plasmonic nanoparticles., This work has been supported by the Spanish Ministerio de Ciencia e Innovación (MELODIA PGC2018-095777-B-C21 and C-22) and UAM-CAM project (SI1/PJI/2019-00052). MIM acknowledges also financial support from the Spanish Ministerio de Ciencia e Innovación, through the “María de Maeztu” Programme for Units of Excellence in R& D (CEX2018-000805-M). AGM acknowledges support from the Spanish Ministerio de Ciencia e Innovación through Grant No. PID2019-109905GA-C22.

Optical tweezers are tools made of light that enable contactless pushing, trapping, and manipulation of objects ranging from atoms to space light sails. Since the pioneering work by Arthur Ashkin in the 1970s, optical tweezers have evolved into sophisticated instruments and have been employed in a broad range of applications in life sciences, physics, and engineering. These include accurate force and torque measurement at the femtonewton level, microrheology of complex fluids, single micro- and nanoparticle spectroscopy, single-cell analysis, and statistical-physics experiments. This roadmap provides insights into current investigations involving optical forces and optical tweezers from their theoretical foundations to designs and setups. It also offers perspectives for applications to a wide range of research fields, from biophysics to space exploration., Comment: 181 pages, 61 figures

11 pags., 4 figs., In recent years, optical forces and torques have been investigated in sub-wavelength evanescent fields yielding a rich phenomenology of fundamental and applied interest. Here we demonstrate analytically that guided modes carrying transverse spin density induce optical torques depending on the character, either electric or magnetic, of the dipolar particles. The existence of a nonzero longitudinal extraordinary linear spin momentum suitable to manipulate optical forces and torques modifies optical forces either enhancing or inhibiting radiation pressure. Hybrid modes supported by cylindrical waveguides also exhibit intrinsic helicity that leads to a rich distribution of longitudinal optical torques. Finally, we show that chiral dipolar particles also undergo lateral forces induced by transverse spin density, amenable to chiral particle sorting. These properties are revealed in configurations on achiral and chiral dipolar particles within confined geometries throughout the electromagnetic spectra., Ministerio de Ciencia, Innovación y Universidades (CEX2018-000805-M, FPU15/03566, PGC2018-095777- B-C21, PGC2018-095777-B-C22, UAM-CAM project SI1/PJI/2019-00052).

The linear momentum of electromagnetic radiation is proportional to the Poynting vector. However, when light interacts with a dipole, the radiation pressure generated is no longer proportional to the Poynting vector: the so-called Belinfante’s momentum or equivalently, the curl of the spin density of the light field, must be considered. In this paper, we propose an optical configuration, based on two evanescent counter-propagating waves, perpendicularly polarized, capable of detecting Belinfante’s mechanical action. The two beams interact with a high-refractive-index particle like silicon. The direction of the radiation pressure exerted on the particle, proportional only to the curl of the spin density, depends on the electric and magnetic response of the particle and changes sign at the radiation wavelengths corresponding with the Kerker conditions.

Focused laser beams allow controlling the mechanical motion of objects and can serve as a tool for assembling micro and nanostructures in space. While small particles mainly experience attractive gradient forces and repulsive radiation pressure, introducing additional flexibility suggests approaching new capabilities. Herein, optical forces acting on a high refractive index sphere in a focused Gaussian beam are analyzed and new regimes are revealed. Multipolar analysis allows separating an optical force into interception and recoil components, resulting in different mechanical actions. In particular, interplaying interception radial forces and multipolar resonances within a particle can lead to either trapping or antitrapping, depending on the system parameters. At the same time, the recoil force generates a significant azimuthal component along with an angulardependent radial force. Those contributions enable enhancing either trapping or antitrapping and also introduce bending reactions. These effects are linked to the far-field multipole interference and, specifically, to asymmetric scattering patterns. The latter approach is extremely useful, as it allows assessing the nature of optomechanical motion by observing far-fields. Multipolar engineering of optical forces, being quite a general approach, is not necessarily linked to simple spherical shapes and paves a way to new possibilities in microfluidic applications, including sorting and microassembly

An optical system based on a photonic nanojet created by an optically trapped microsphere is proposed for enhanced trapping and remote manipulation of upconverting nanoparticles. The nanojet-induced enhancement in the optical forces acting on a 8 nm single upconverting nanoparticle are compared to those achievable by the traditional approach based on the use of high numerical aperture optics. The results presented in this work prove that this is a simple and effective method to remotely trap, manipulate, and detect single upconverting nanoparticles. This is an innovative approach to overcome the current challenges in the remote manipulation of probes in biosensing or in single particle studies.

Curie-Weiss temperature dependence of the dielectric permittivity has been shown to take place in water. A bilinear change at T*=60 oC has been reported recently. In this work an effective field st...

“This document is the Accepted Manuscript version of a Published Work that appeared in final form in Nano Letters copyright © American Chemical Society after peer review and technical editing by publisher. To acces final work see Optical Forces at the Nanoscale: Size and Electrostatic Effects, Nano Letters, 18.1 (2018), pags. 602-609. http://doi.org/10.1021/acs.nanolett.7b04804", The reduced magnitude of the optical trapping forces exerted over sub-200 nm dielectric nanoparticles complicates their optical manipulation, hindering the development of techniques and studies based on it. Improvement of trapping capabilities for such tiny objects requires a deep understanding of the mechanisms beneath them. Traditionally, the optical forces acting on dielectric nanoparticles have been only correlated with their volume, and the size has been traditionally identified as a key parameter. However, the most recently published research results have shown that the electrostatic characteristics of a sub-100 nm dielectric particle could also play a significant role. Indeed, at present it is not clear what optical forces depend. In this work, we designed a set of experiments in order to elucidate the different mechanism and properties (i.e., size and/or electrostatic properties) that governs the magnitude of optical forces. The comparison between experimental data and numerical simulations have shown that the double layer induced at nanoparticle’s surface, not considered in the classical description of nanoparticle’s polarizability, plays a relevant role determining the magnitude of the optical forces. Here, the presented results constitute the first step toward the development of the dielectric nanoparticle over which enhanced optical forces could be exerted, enabling their optical manipulation for multiples purposes ranging from fundamental to applied studies, This work has been supported by the Spanish Ministerio de Economia y Competitividad (project Nr.MAT2016-75362-C3-1-R), FIS2015-69295-C3-3-P and the “María de Maeztu” Program Ref: MDM-2014-0377. P.R.S. thanks MINECO and the Fondo Social Europeo (FSE) for the “Promoción del talento y su Empleabilidad en I+D+i” statal program (BES-2014-069410). K.P. acknowledges financial support from the National Science Center Poland (NCN) under the ETIUDA doctoral scholarship on the basis of decision number DEC-2014/12/T/ST5/00646. A.B. acknowledges the statutory financial support from ILT&SR PAS. P.H.G. thanks MINECO for the Juan de la Cierva program (IJCI-2015- 24551). The European Upconversion Network (COST Action CM1403) is acknowledged

The interaction forces between identical resonant molecules or nanoparticles, optically induced by a quasimonochromatic isotropic random light field, are theoretically analyzed. In general, the interaction force exhibits a far-field oscillatory behavior at separation distances larger than the light wavelength. However, we show that the oscillations disappear when the frequency of the random field is tuned to an absorption Fröhlich resonance, at which the real part of the particle's electric polarizability is zero. At the resonant condition, the interaction forces follow a long-range gravitylike inverse square distance law which holds for both near- and far-field separation distances.

In this paper we calculate the optical forces and torques caused by the presence of a sizable magneto-optical effect. We find a conservative force proportional to the gradient of the spin density of the light field and an extinction force proportional to the helicity of the light field. The conservative interaction allows for a spin-selective, magnetic field based Stern-Gerlach experiment, capable of differentiating between right and left circular polarizations. We also prove that by using a spinless linearly polarized plane wave, the magneto-optical effect allows for the existence of a permanent nonreciprocal torque, proportional to the intensity of the light field., This research was supported by the Spanish MICINN and European Regional Development Fund (ERDF) through Projects No. FIS2015-69295-C3-1-P, No. FIS2015-69295-C3-3-P, No. PGC2018-095777-B-C21, No. PGC2018-095777-B-C22, the Basque Departamento de Educación through Project No. PI-2016-1-0041, and the María de Maeztu Program No. MDM-2014-0377

Upconverting nanoparticles (UCNPs) have been used as optical probes in a great variety of scenarios ranging from cells to animal models. When optically trapped, a single UCNP can be remotely manipulated making possible, for instance, thermal scanning in the surroundings of a living cell. When conventional optics is used, the stability of an optically trapped UCNP is very limited. Its reduced size leads to optical potentials comparable to thermal energy, and up to now, stable optical trapping of a UCNP has been demonstrated only close to room temperature. This fact limits their use above room temperature, for instance, the use to investigate protein denaturalization that occurs in the 40-50 °C range. In this work, stable optical trapping of a single UCNP in the 20-90 °C range has been demonstrated by using a photonic nanojet. The use of an optically trapped microsphere makes it possible to overcome the diffraction limit producing another optical trap of smaller size and enhanced strength. This simple strategy leads not only to an improvement in the thermal stability of the optical trap but also to an enhancement of the emission intensity generated by the optically trapped UCNP.

The diffusion of a dimer made out of two resonant dipolar scatters in an optical lattice is theoretically analyzed. When a small particle diffuses through an optically induced potential landscape, its Brownian motion can be strongly suppressed by gradient forces, proportional to the particle’s polarizability. For a single lossless monomer at resonance, the gradient force vanishes and the particle diffuses as in absence of external fields. However, we show that when two monomers link in a dimer, the multiple scattering among the monomers induces both a torque and a net force on the dimer’s center of mass. This “self-induced back-action” force leads to an effective potential energy landscape, entirely dominated by the mutual interaction between monomers, which strongly influences the dynamics of the dimer. Under appropriate illumination, single monomers in a colloidal suspension freely diffuse while dimers become trapped. Our theoretical predictions are tested against extensive Langevin molecular dynamics s...

Gold nanoparticles moving in an aqueous solution under an optical vortex lattice are shown to present a complex collective optofluidic dynamics. Above a critical field intensity and concentration, the system presents a spontaneous transition toward synchronized motion, driven by the interplay between nonconservative optical forces, thermal fluctuations, and hydrodynamic pairing. Above criticality, the system exhibits swarm behavior with strong unidirectional currents of nanoparticles reaching speeds of centimeters per second. This relatively simple optofluidic setup offers an alternative way to control nanoparticle transport in aqueous environments.

An expression for the electromagnetic drag force experienced by an electric dipole in a light field consisting of a monochromatic plane wave with polarization and phase randomly fluctuating is obtained. The expression explicitly considers the transformations of the field and frequency due to the Doppler shift and the change of the polarizability response of the electric dipole. The conditions to be fulfilled by the polarizability of the dipole in order to obtain a positive, a null, and a negative drag coefficient are analytically determined and checked against numerical simulations for the dynamics of a silver nanoparticle. The theoretically predicted diffusive, superdiffusive, and exponentially accelerated dynamical regimes are numerically confirmed.

Recently, the paraelectric response of water was investigated in the range 0–100 °C. It showed an almost perfect Curie–Weiss behaviour up to 60 °C, but a slight change in slope of 1/ed versus T at 60 °C was overlooked. In this work, we report optical extinction measurements on metallic (gold and silver) nanoparticles dispersed in water, annealed at various temperatures in the range from 20 to 90 °C. An anomalous response at 60 °C is clearly detectable, which we associate to a subtle structural transformation in the water molecules at that temperature. This water anomaly is also manifested by means of a blue shift in the longitudinal surface plasmon resonance of the metallic nanoparticles for the solutions annealed at temperatures higher than about 60 °C. A reanalysis of 1/ed (T) for water in the whole temperature range leads us to conclude that the water molecule undergoes a subtle transformation from a low temperature (0–60 °C) configuration with a dipole moment μ1 = 2.18 D (close to the molecular dipole...

Paloma Rodríguez-Sevilla, Yuhai Zhang, Nuno de Sousa, Manuel I. Marqués, Francisco Sanz-Rodríguez, Daniel Jaque, Xiaogang Liu, Patricia Haro-González, "Microrheometric upconversion-based techniques for intracellular viscosity measurements", Optical Trapping and Optical Micromanipulation XIV, Proc. SPIE 10347, 103471S (25 August 2017); doi: 10.1117/12.2275944. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited, Proceedings Volume XIV Optical Trapping and Optical Micromanipulation Conference (San Diego, California, United States), Rheological parameters (viscosity, creep compliance and elasticity) play an important role in cell function and viability. For this reason different strategies have been developed for their study. In this work, two new microrheometric techniques are presented. Both methods take advantage of the analysis of the polarized emission of an upconverting particle to determine its orientation inside the optical trap. Upconverting particles are optical materials that are able to convert infrared radiation into visible light. Their usefulness has been further boosted by the recent demonstration of their three-dimensional control and tracking by single beam infrared optical traps. In this work it is demonstrated that optical torques are responsible of the stable orientation of the upconverting particle inside the trap. Moreover, numerical calculations and experimental data allowed to use the rotation dynamics of the optically trapped upconverting particle for environmental sensing. In particular, the cytoplasm viscosity could be measured by using the rotation time and thermal fluctuations of an intracellular optically trapped upconverting particle, by means of the two previously mentioned microrheometric techniques, This work was supported by the Spanish Ministerio de Educación y Ciencia (MAT2016-75362-C3-1-R) and by COST Action 1403. P.H.G. thanks the Spanish Ministerio de Economía y Competitividad (MINECO) for the Juan de la Cierva- Incorporación program. P.R.S. thanks MINECO and the Fondo Social Europeo (FSE) for the “Promoción del talento y su Empleabilidad en I+D+i” statal program (BES-2014-069410). M.I.M. acknowledges the financial support from MINECO Grant FIS2015-69295-C3-3-P and from the María de Maeztu Programme for Units of Excellence in R&D (Grant MDM-2014-0377). N.d.S. thanks MINECO for grants FIS2012-36113-C03, FIS2015-69295-C3-3-P, and MAT2014-58860-P and the Comunidad de Madrid (Contract No. S2013/MIT-2740). J. R. Procopio and M. T. Sevilla are acknowledged for material support. A. García-Martín is gratefully acknowledged for supporting computing facilities

This work was supported by the Spanish Ministerio de Educación y Ciencia (MAT2013-47395-C4-1-R and MAT2016-75362-C3-1- R) and by the COST Action CM1403. L.L.-P. thanks the Universidad Autónoma de Madrid for the “Formación de Personal Investigador (FPI-UAM)” program. P.H.-G. thanks the Spanish Ministerio de Economia y Competitividad (MINECO) for the Juan de la Cierva program. The authors from the University of Belgrade acknowledge the financial support of the Ministry of Education, Science and Technological Development of the Republic of Serbia (Project Nos. 45020 and 172056). M.I.M. thanks financial support from the Spanish Ministerio de Economía y Competitividad (MICINN) Project FIS2015-69295-C3-3-P and the “María de Maeztu” Program Ref: MDM-2014-0377. The work of K.S. was supported by the Latvian National Research Program IMIS2 (Grant No. 302/2012)., Nowadays a large variety of applications are based on solid nanoparticles dispersed in liquids—so called nanofluids. The interaction between the fluid and the nanoparticles plays a decisive role in the physical properties of the nanofluid. A novel approach based on the nonradiative energy transfer between two small luminescent nanocrystals (GdVO4:Nd3+ and GdVO4:Yb3+) dispersed in water is used in this work to investigate how temperature affects both the processes of interaction between nanoparticles and the effect of the fluid on the nanoparticles. From a systematic analysis of the effect of temperature on the GdVO4:Nd3+ → GdVO4:Yb3+ interparticle energy transfer, it can be concluded that a dramatic increase in the energy transfer efficiency occurs for temperatures above 45 °C. This change is properly explained by taking into account a crossover existing in diverse water properties that occurs at about this temperature. The obtained results allow elucidation on the molecular arrangement of water molecules below and above this crossover temperature. In addition, it is observed that an energy transfer process is produced as a result of interparticle collisions that induce irreversible ion exchange between the interacting nanoparticles., Spanish Ministerio de Economia y Competitividad; Universidad Autónoma de Madrid; Spanish Ministerio de Educación y Ciencia MDM-2014-0377,FIS2015-69295-C3-3-P, 302/2012; Spanish Ministerio de Ciencia Tecnología y Telecomunicaciones MAT2013-47395-C4-1-R,MAT2016-75362-C3-1-R; Ministarstvo Prosvete, Nauke i TehnoloÅ¡kog Razvoja 45020,172056; European Cooperation in Science and Technology CM1403; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²

Water exhibits unique physical and chemicals properties and its low frequency dielectric response is not an exception. The large directionable dipole moment of the water molecule results in a relatively high dielectric constant whose temperature dependence above 0°C follows well a Curie-Weiss law (ɛs-ɛei)−1 = (1/C)(T-TC), with ɛei ≅ 6.6, C = 1.49 × 104 and TC = 90.4 K. The frequency dependence of the permittivity of pure water has recently been measured at various temperatures with high accuracy in the frequency range of 0 – 25 THz, showing a well-defined Debye-type relaxation behaviour, which can be described reasonably well by means of a paraelectric-like temperature dependence relaxation time given by τ−1 = υD e−ϕ/kBT [(T-TC)/T], where υD = 1.877 × 1014 and ϕ = 0.180 eV. The combined pressure and temperature dependence of the dielectric constant is described approximately by means of a generalized Curie-Weiss state-law.

In surface science, water adsorption on hydrophilic samples is usually invoked, addressing their nanoscale experimental effects in scanning probe microscopy, especially when water condensates between tip and sample. Here we study by means of a numerical hybrid method the effect of water bridge formation in near field imaging. We show how this nanometric water neck plays an important role not only in the optical image, producing a high contrast at hydrophilic patches, but also in the tip-sample distance control. This work contributes with a new methodology able to retrieve the original application of SNOM, using it as an instrument to study the optical properties of matter overcoming the diffraction limit. It extends the application of SNOM to study the hydrophilic character of polymeric and biological samples, taking advantage of ubiquitous effect of humidity when operating in ambient condition. © 2013 Elsevier B.V., This work has been funded through projects FIS2009-13403-C02-01(MICINN), S2009/MAT-1467(CAM) and CSD2010-00024(MICINN).

An experimental study of the effects of low energy (1–2 keV) argon ion (Ar+) irradiation on Triglycine Sulfate (TGS) has been performed. Ferroelectric parameters, such as the Curie temperature TC determined from the dielectric constant peaks ɛ(T), or the remnant polarization Pr, and coercive field Ec, obtained from the hysteresis loops, show interesting differences between samples irradiated in ferroelectric and paraelectric phases, respectively. The radiation damage seems to be superficial, as observed by AFM microscope, and the surface alteration in both phases becomes eventually notorious when the radiation dosage increases.

Precise knowledge and control over the orientation of individual upconverting particles is extremely important for full exploiting their capabilities as multifunctional bioprobes for interdisciplinary applications. In this work, we report on how time-resolved, single particle polarized spectroscopy can be used to determine the orientation dynamics of a single upconverting particle when entering into an optical trap. Experimental results have unequivocally evidenced the existence of a unique stable configuration. Numerical simulations and simple numerical calculations have demonstrated that the dipole magnetic interactions between the upconverting particle and trapping radiation are the main mechanisms responsible of the optical torques that drive the upconverting particle to its stable orientation. Finally, how a proper analysis of the rotation dynamics of a single upconverting particle within an optical trap can provide valuable information about the properties of the medium in which it is suspended is demonstrated. A proof of concept is given in which the laser driven intracellular rotation of upconverting particles is used to successfully determine the intracellular dynamic viscosity by a passive and an active method.

The dynamics of an electric dipole in a light field consisting of electromagnetic plane waves with polarizations randomly distributed and fluctuating phases is theoretically analyzed. The expression for the optical random-force fluctuations is derived and found to be proportional to the scattering cross section and to the square of the intensity divided by the frequency of the electromagnetic field. Under these fluctuations, and in the absence of damping, the dipole behaves like a super-diffusive particle with a kinetic energy growing linearly with time. The analytic predictions are tested against numerical simulations for the particular case of a resonant dipole.

Monte Carlo investigations of magnetization versus field, Mc(H), at the critical temperature provide direct accurate results on the critical exponent δ−1 for one, two, three and four-dimensional lattices: δ−1 1D =0, δ−1 2D =0.0666(2)≈1/15, δ−1 3D =0.1997(4)≈1/5, δ−1 4D =0.332(5)≈1/3. This type of Monte Carlo data on δ, which is not easily found in studies of Ising lattices in the current literature, as far as we know, defines extremely well the numerical value of this exponent within very stringent limits.

Strontium Titanate (SrTiO3), the most studied and well known quantum paraelectric since sixty years ago, results in a typical ferroelectric when substituting 16O by 18O. An effective field approach including quantum considerations of zero point energy explains satisfactorily the behavior of different ferroelectric-quantum paraelectric mixed systems. The isotopic strontium titanate SrTi (16O1-x 18Ox)3 can then be modelled as a mixed system with a critical composition (xc = 0.33) at which a crossover between ferroelectric and quantum paraelectric behavior takes place. A discussion is made in terms of dipole interaction strength for both isotopes.

The polarization in a model for a relaxor placed between two metallic electrodes is studied by Monte Carlo numerical simulations. We consider different thicknesses and different values of the compositional charge disorder density. The depolarization field is found to be attenuated for large values of the number density of charge carriers and, in such a case, a homogeneous ferroelectric ground state is obtained.

The behavior of the polarization in a microscopic statistical model for a thin film relaxor placed between two metallic electrodes is studied by numerical simulations. Depolarization fields different from zero, due to a non-perfect compensation of surface charges at the metallic electrodes, are taken into account. Different thicknesses and different values of the compositional charge disorder density for the relaxor are considered. Depolarization field is found to be extremely attenuated for large values of the number density of charge carriers in the relaxor. In such a case, field attenuation allows for the existence of a homogeneous ferroelectric ground state.

Monte Carlo simulation with Metropolis algorithm has been performed on a cubic Ising lattice with four-spin couplings. Temperature dependence of the spin average and average energy has been obtained. We found that the phase transition is of first order when the strength of four-spin coupling is stronger a critical value, and this critical value is of lattice size dependence. Binder cumulants still can be used to determine the second order phase transition temperature. For first order phase transition, the spin average disappears abruptly at the temperature of the average energy being a third of the ground state energy.

By using a lattice-gas model, we report numerical simulation for the action of capillary forces of water confined at the nanoscale during desiccation of viruses. Results are compared with structural effects of desiccation measured by Atomic Force Microscopy on individual viruses of the bacteriophage ϕ 29 and the minute virus of mice (MVM). Structural integrity is found for theMVM, but not for the ϕ 29. Numerical simulations show that in the desiccation process, the meniscus shape formed inside the capsids strongly depends on the virus symmetry. This suggests that capillary forces could play a key role on the explanation of the different measured collapse processes (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We study the diffusion of a metal nanoparticle in the nonconservative force field of an optical vortex lattice. Radiation pressure in the vortex array is shown to induce a giant enhancement over the free thermal diffusion. Langevin dynamics simulations show that the diffusion coefficient of (50 nm radius) gold particles at room temperature is enhanced by 2 orders of magnitude at power densities of the order or smaller than those used to trap nanoparticles with optical tweezers.

We present an investigation of water menisci confined in closed geometries by studying the structural effects of their capillary forces on viruses during the final stage of desiccation. We used individual particles of the bacteriophage ϕ29 and the minute virus of mice. In both cases the genomic DNA was ejected from the capsid. However, although the structural integrity of the minute virus of mice was essentially preserved, the ϕ29 capsid underwent a wall-to-wall collapse. We provide evidence that the capillary forces of water confined inside the viruses are mainly responsible for these effects. Moreover, by performing theoretical simulations with a lattice gas model, we found that some structural differences between these 2 viruses may be crucial to explain the different ways in which they are affected by water menisci forces confined at the nanoscale.

The transition temperature T C (x) for non-stoichiometric lithium niobate, LiNbO 3 , here denoted by (Li 2 O) x (Nb 2 O 5 ) 1 − x where 0.465 < x < 0.500, evolves from T C (0.465) ≅ 1290 K to T C (0.50) ≅ 1485 K. Using an effective field approach, as done in previous works for isomorphous ferroelectrics solid solutions, we investigate the composition dependence of the critical temperature in this narrow range for ferroelectric lithium niobate. Experimental data are fitted by the function T C (x), deduced from the effective field theory when considering the short composition range of LiNbO 3 as a mixed system with increasing stoichiometry.

The dielectric permittivity in Pb 0.85 La 0.10 Zr 0.6 Ti 0.4 O 3 (PLZT 10/60/40) relaxor ferroelectrics is studied for different degrees of poling. We find that the half width of the dielectric permittivity and the transition temperature dependence on frequency becomes smaller by increasing the degree of poling. In order to explain such behavior we consider a model, previously used to study uniaxial relaxor ferroelectrics, where the nanoregions inside the sample are redistributed by applying an external electric field. The redistribution consists in a decrease on the dispersion of the random fields affecting to each nanoregion. This decrease on the dispersion decimates both, the broadening of the dielectric peak and the dependence with frequency of the transition temperature.

Experimental data of spontaneous polarization in pure and deuterated crystals of the triglycine sulfate (TGS) family have been obtained by means of pyroelectric charge measurements. The critical exponent β, that indicates the temperature dependence of the spontaneous polarization P S ∝ (T C −T)β, decreases with deuteration in triglycine sulfate (TGS) family crystals, specially in the case of deuterated triglycine selenate (DTGSe). Monte Carlo simulations of the equivalent neighbour Ising model are performed to analyse to what extent the experimental behaviour can be explained by a change in the interaction range between dipoles. From the simulations we find a possible way to explain the experimental results based on a short range dominated dipole interaction. However fitting to experimental results is not perfect, so we propose some other possible mechanisms like an increase in the degree of freedom of individual dipoles.

Monte Carlo investigations of the D∗ value for Ising strips D × L (D ≪ L) with periodic boundary conditions have been performed. The apparent critical temperature of Ising strips was determined by means of the Binder Cumulant Method crossing data for D × L with data for D × 2L. The results show that D∗ is 4.25 ± 0.05, i.e., the critical temperature of Ising strips is T C (D) = 0 for D < D∗ = 4.25 ± 0.05 and 0 < TC (D) < TC (d = 2) = 2.269185 for D ≥ D∗ = 4.25 ± 0.05.

An electric field induced phase transition in first order ferroelectrics with very large zero point energy is studied on the framework of the effective field approach. It is well known that when the zero point energy of a system is relatively large, the ferroelectric behaviour is depressed and no phase transition can be observed. The critical value Ω cf of zero point energy for whom the phase transition disappears turns out to be dependant on the order of transition. For zero point energies larger than this critical value, a phase transition may be induced applying an external electric field. This temperature dependence of the induced polarization shows a discontinuous step when the applied electric field is weak, but becoming a continuous one at a strong applied electric field. Another critical value of zero point energy Ω cp > Ω cf is deduced for which no phase transition at all can be attained.

In this paper we calculate the constant pressure specific heat response of a water model with a liquid–liquid critical point. We show how, due to the existence of the critical point, there is a secondary maximum in the specific heat at some temperature T * > T h for any pressure P > P c , being T h the first order transition temperature between the high and the low density liquid phases and P c the pressure of the critical point. This secondary maximum does not correspond to any long range correlated phase transition and does not show up in the temperature dependence of the isothermal compressibility.

The paired electrons of a formed Cooper pair, orbiting each other, behave like tiny magnets. This suggests a simple and straightforward effective field approach to superconductivity, at least for the simplest superconductors: some metallic elements in the periodic table. For temperatures below the transition temperature TC, one may assume a spontaneous magnetic field BS= 4π NS μ, where NS is the number of bound pairs, and μ the elementary dipole moment of a Cooper pair. Simple statistical mechanics allows us to obtain the effective field expression for BS as a function of temperature for T ≤ TC. Only two experimental pieces of information, the critical temperature TC and BS0, the critical magnetic field al 0 K, are then necessary to describe the superconductive behaviour. Groups of elements such us [Ti, Zr, and Hf (hcp)], [V, Nb and Ta (bcc)], and [Cr, Mo and W (bcc)] are examined, as well as groups of alloys with relatively high T C values.

A simple model for a thin film memory based on a first neighbor interacting model is studied in detail. We have found that the minimum possible value for the thickness (D) as a function of the lateral size of the memory (L), the screening of the charges at the substrate (S) and the strength of the ferroelectric interaction (J), in order to obtain spontaneous polarization is D = SL/2J. We propose a new mechanism to obtain miniaturization of thin film memories to a single layer based on the use of relaxor ferroelectrics instead of regular ferroelectrics. Under the hypothesis of an internal organization of the random fields inside the nanofilm we show analytically how it should be possible to miniaturize the memory to a width as small as D = 1 for any value of L, J and S.

The effects of dilution on the transition temperature of two dimensional Ising systems (monolayers) have been investigated in lattices with nearest neighbours interactions (neff = 4) and for the equivalent-neighbor model with neff = 8,12. The dependence of T c with neff is found to be approximately linear for c ≤ 0.4

The behavior of the local mode's potential energy in the unit cell of BaTiO 3 is studied by effective Hamiltonian numerical simulations. The study is focused on the evolution of the potential's minimum felt by the Ti ion. At the paraelectric phase, the single instantaneous minimum is located at one of eight off-centered regions at the body diagonals of the unit cell. The study shows graphically how the soft mode behavior of the perovskite comes from the average response of the Ti ion to the potential changes in the unit cell, showing that the single minimum of the time-averaged potential is centered at the paraelectric phase and off-centered at the ferroelectric phase.

The composition dependence of the Curie temperature in mixed Ising systems with different coupling constants has been studied by means of Monte Carlo numerical simulations. The results obtained are useful to get a better understanding of the deviations from Vegard's law observed in mixed ferroelectric-ferroelectric solid solutions. Deviations from linearity are obtained for non-homogeneous mixed solutions of Ising systems with coupling constants Ja and Jb and with a coupling constant among the two species Jab different from (Ja+ Jb)/2. Two cases are studied: (a) Jab > (Ja+ Jb)/2 (critical temperature above the linear interpolation) and (b) Jab < (Ja+ Jb)/2 (critical temperature below the linear interpolation).