Thermal radiation from subwavelength objects and the violation of Planck’s law

Thermal radiation is a ubiquitous physical phenomenon that has been usually described with the help of Planck’s law, but recent developments have proven its limitations. Now, experimental advances have demonstrated that the far-field thermal radiation properties of subwavelength objects drastically violate Planck’s lawJ.C.C. acknowledges financial support from the Spanish MINECO (Contract No. FIS2017–84057-P

Estimating time delays between irregularly sampled time series

The time delay estimation between time series is a real-world problem in gravitational lensing, an area of astrophysics. Lensing is the most direct method of measuring the distribution of matter, which is often dark, and the accurate measurement of time delays set the scale to measure distances over cosmological scales. For our purposes, this means that we have to estimate a time delay between two or more noisy and irregularly sampled time series. Estimations have been made using statistical methods in the astrophysics literature, such as interpolation, dispersion analysis, discrete correlation function, Gaussian processes and Bayesian method, among others. Instead, this thesis proposes a kernel-based approach to estimating the time delay, which is inspired by kernel methods in the context of statistical and machine learning. Moreover, our methodology is evolved to perform model selection, regularisation and time delay estimation globally and simultaneously. Experimental results show that this approach is one of the most accurate methods for gaps (missing data) and distinct noise levels. Results on artificial and real data are shown

Physics and modeling of turbulent advective transport in turbulent boundary layers

\setlength{\baselineskip}{1.5\baselineskip} {Recent studies reveal that at large friction Reynolds number $\delta^+ = u_{\tau}\delta/\nu$ the inertially dominated region of the turbulent boundary layer is composed of large-scale zones of nearly uniform momentum segregated by narrow fissures of concentrated vorticity. Here $u_{\tau} = \sqrt{\tau_{\omega}/\rho}$, where $\tau_{\omega}$ is the shear stress at the wall and $\rho$ the fluid density, $\delta$ is the boundary layer thickness and $\nu$ is the kinematic viscosity of the fluid. The aim of this thesis is better understand the role of this binary structure with respect to the wall-normal transport of momentum and heat in turbulent boundary layers at high $\delta^+$. This is addressed by assuming that the dynamically important processes governing turbulent transport are owed to the interactions between the vorticity $\omega$ field, which quantifies the level of fluid rotation, and the wall-normal velocity $v$. Effectively, it is assumed that turbulent transport is a consequence of the wall-normal motions of concentrated zones of vorticity (or heat). The basis of this assumption is evidenced by the following relation \begin{equation}\label{eq:re_stress_abs} -\frac{\partial \overline{u v}}{\partial y} \cong \overline{v \omega_{z}}-\overline{w \omega_{y}}, \end{equation} where $u$, $v$ and $w$ denote the streamwise, wall-normal and spanwise fluctuating velocity respectively, the subscript on $\omega$ denotes the component of the fluctuating vorticity, and an overbar denotes a correlation. The left-hand side of the equation is the Reynolds stress gradient (responsible for turbulent transport) and the right-hand side of the equation are the velocity-vorticity correlations. The present research is divided into an experimental and numerical study of the $\overline{v \omega_{z}}$ correlation. In the experimental study, the contributions of the $v$ and $\omega_z$ motions to the vorticity transport ($\overline{v\omega_z}$) mechanisms are evaluated at large friction Reynolds numbers $\delta^+$. Here the primary contributions to $v$ and $\omega_z$ are estimated by identifying the peak wavelengths of their streamwise spectra. The magnitudes of these peaks are of the same order, and are shown to exhibit a weak $\delta^+$ dependence. The peak wavelengths of $v$, however, exhibits a strong wall-distance ($y$) dependence, while the peak wavelengths of $\omega_z$ shows only a weak $y$ dependence, and remains almost $O(\sqrt{\delta^+})$ in size throughout the inertial domain. In the numerical study, a simple model that exploits the binary structure of the turbulent boundary layer, i.e., uniform momentum zones (UMZ) separated by vortical fissures (VFs), is developed. First, a master wall-normal profile of streamwise velocity is constructed by placing a discrete number of fissures across the boundary layer. The number of fissures and their wall-normal locations follow scalings informed by analysis of the mean momentum equation. The fissures are then randomly displaced in the wall-normal direction, exchanging momentum as they move, to create an instantaneous velocity profile. This process is repeated to generate ensembles of streamwise velocity profiles from which statistical moments are computed. The modelled statistical profiles are shown to agree remarkably well with those acquired from direct numerical simulations of turbulent channel flow at large . In particular, the model robustly reproduces the empirically observed sub-Gaussian behaviour for the skewness and kurtosis profiles over a large range of input parameters. Encouraged by the success of this simple model with respect to momentum transport, a similar model is developed with respect to the wall-normal transport of a passive scalar (i.e, temperature). Similarly, this model robustly reproduces the statistical moments of the scalar field and the fluctuating streamwise velocity-temperature correlation.

Thermal conductance and thermoelectric figure of merit of C60_{60}-based single-molecule junctions: electrons, phonons, and photons

Motivated by recent experiments, we present here an ab initio study of the impact of the phonon transport on the thermal conductance and thermoelectric figure of merit of C$_{60}$-based single-molecule junctions. To be precise, we combine density functional theory with nonequilibrium Green's function techniques to compute these two quantities in junctions with either a C$_{60}$ monomer or a C$_{60}$ dimer connected to gold electrodes, taking into account the contributions of both electrons and phonons. Our results show that for C$_{60}$ monomer junctions phonon transport plays a minor role in the thermal conductance and, in turn, in the figure of merit, which can reach relatively high values on the order of 0.1, depending on the contact geometry. At the contrary, phonons completely dominate the thermal conductance in C$_{60}$ dimer junctions and strongly reduce the figure of merit as compared to monomer junctions. Thus, claims that by stacking C$_{60}$ molecules one could achieve high thermoelectric performance, which have been made without considering the phonon contribution, are not justified. Moreover, we analyze the relevance of near-field thermal radiation for the figure of merit of these junctions within the framework of fluctuational electrodynamics. We conclude that photon tunneling can be another detrimental factor for the thermoelectric performance, which has been overlooked so far in the field of molecular electronics. Our study illustrates the crucial roles that phonon transport and photon tunneling can play when critically assessing the performance of molecular junctions as potential nanoscale thermoelectric devices

Theory of Microwave-Assisted Supercurrent in Diffusive SNS Junctions

The observation of very large microwave-enhanced critical currents in superconductor-normal metal-superconductor (SNS) junctions at temperatures well below the critical temperature of the electrodes has remained without a satisfactory theoretical explanation for more than three decades. Here we present a theory of the supercurrent in diffusive SNS junctions under microwave irradiation based on the quasiclassical Green's function formalism. We show that the enhancement of the critical current is due to the energy redistribution of the quasiparticles in the normal wire induced by the electromagnetic field. The theory provides predictions across a wide range of temperatures, frequencies, and radiation powers, both for the critical current and the current-phase relationship.Comment: 4 pages, 4 figure

Optical Rectification and Field Enhancement in a Plasmonic Nanogap

Metal nanostructures act as powerful optical antennas[1, 2] because collective modes of the electron fluid in the metal are excited when light strikes the surface of the nanostructure. These excitations, known as plasmons, can have evanescent electromagnetic fields that are orders of magnitude larger than the incident electromagnetic field. The largest field enhancements often occur in nanogaps between plasmonically active nanostructures[3, 4], but it is extremely challenging to measure the fields in such gaps directly. These enhanced fields have applications in surface-enhanced spectroscopies[5-7], nonlinear optics[1, 8-10], and nanophotonics[11-15]. Here we show that nonlinear tunnelling conduction between gold electrodes separated by a subnanometre gap leads to optical rectification, producing a DC photocurrent when the gap is illuminated. Comparing this photocurrent with low frequency conduction measurements, we determine the optical frequency voltage across the tunnelling region of the nanogap, and also the enhancement of the electric field in the tunnelling region, as a function of gap size. The measured field enhancements exceed 1000, consistent with estimates from surface-enhanced Raman measurements[16-18]. Our results highlight the need for more realistic theoretical approaches that are able to model the electromagnetic response of metal nanostructures on scales ranging from the free space wavelength, $\lambda$, down to $\sim \lambda/1000$, and for experiments with new materials, different wavelengths, and different incident polarizations.Comment: 15 pages, 5 figures + 12 pages, 5 figures of supplemental informatio

Shot noise variation within ensembles of gold atomic break junctions at room temperature

Atomic-scale junctions are a powerful tool to study quantum transport, and are frequently examined through the mechanically controllable break junction technique (MCBJ). The junction-to-junction variation of atomic configurations often leads to a statistical approach, with ensemble-averaged properties providing access to the relevant physics. However, the full ensemble contains considerable additional information. We report a new analysis of shot noise over entire ensembles of junction configurations using scanning tunneling microscope (STM)-style gold break junctions at room temperature in ambient conditions, and compare this data with simulations based on molecular dynamics (MD), a sophisticated tight-binding model, and nonequilibrium Green's functions. The experimental data show a suppression in the variation of the noise near conductances dominated by fully transmitting channels, and a surprising participation of multiple channels in the nominal tunneling regime. Comparison with the simulations, which agree well with published work at low temperatures and ultrahigh vacuum (UHV) conditions, suggests that these effects likely result from surface contamination and disorder in the electrodes. We propose additional experiments that can distinguish the relative contributions of these factors.Comment: 21 pages, 6 figures. To appear in J. Phys: Condens. Matt., special issue on break junction

Anisotropic thermal magnetoresistance for an active control of radiative heat transfer

We predict a huge anisotropic thermal magnetoresistance (ATMR) in the near-field radiative heat transfer between magneto-optical particles when the direction of an external magnetic field is changed with respect to the heat current direction. We illustrate this effect with the case of two InSb spherical particles where we find that the ATMR amplitude can reach values of up to 800% for a magnetic field of 5 T, which is many orders of magnitude larger than its spintronic analogue in electronic devices. This thermomagnetic effect could find broad applications in the fields of ultrafast thermal management as well as magnetic and thermal remote sensing.Comment: 6 pages, 4 figure

Thermal discrete dipole approximation for near-field radiative heat transfer in many-body systems with arbitrary nonreciprocal bodies

The theoretical study of many-body effects in the context of near-field radiative heat transfer (NFRHT) has already led to the prediction of a plethora of thermal radiation phenomena. Special attention has been paid to nonreciprocal systems in which the lack of the Lorentz reciprocity has been shown to give rise to unique physical effects. However, most of the theoretical work in this regard has been carried out with the help of approaches that consider either pointlike particles or highly symmetric bodies (such as spheres), which are not easy to realize and explore experimentally. In this work we develop a many-body approach based on the thermal discrete dipole approximation (TDDA) that is able to describe the NFRHT between nonreciprocal objects of arbitrary size and shape. We illustrate the potential and the relevance of this approach with the analysis of two related phenomena, namely the existence of persistent thermal currents and the photon thermal Hall effect, in a system with several magneto-optical bodies. Our many-body TDDA approach paves the way for closing the gap between experiment and theory that is hindering the progress of the topic of NFRHT in many-body systemsJ.C.C. acknowledges funding from the Spanish Ministry of Science and Innovation (Grant No. PID2020-114880GB-I00