In absence of external fields, vibrational modes of periodic systems are usually considered as linearly polarized and, as such, they do not carry angular momentum. Our work proves that non-adiabatic effects due to the electron-phonon coupling are time-reversal symmetry breaking interactions for the vibrational field in systems with non-collinear magnetism and large spin-orbit coupling. Since in these systems the deformation potential matrix elements are necessarily complex, a nonzero synthetic gauge field (Berry curvature) arises in the dynamic equations of the ionic motion. As a result, phonon modes are elliptically polarized in the non-adiabatic framework and intrinsic vibrational angular momenta occur even for non-degenerate modes and without external probes. These results are validated by performing fully relativistic ab-initio calculations on two insulating platinum clusters and a metallic manganese compound, with non-collinear magnetism. In both cases, non-adiabatic vibrational modes carry sizeable angular momenta comparable to the orbital electronic ones in itinerant ferromagnets.

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Phonons and electrons are two types of excitations which are responsible for many properties of condensed matter materials. The interaction between them plays an important role in condensed matter physics. In this thesis we present some theoretical investigations of the effects due to the interactions between phonons and electrons interactions. We show evidence that a structural martensitic transition is related to significant changes in the electronic structure, as revealed in thermodynamic measurements made in high magnetic fields. The effect of the magnetic field is considered unusual, as many influential investigations of martensitic transitions have emphasized that the structural transitions are primarily lattice dynamical and are driven by the entropy due to the phonons. We provide a theoretical frame-work which can be used to describe the effect of a magnetic field on the lattice dynamics in which the field dependence originates from the dielectric constant. The temperature-dependence of the phonon spectrum of alpha-uranium has recently been measured by Manley et al. using inelastic neutron scattering and x-ray scattering techniques. Although there is scant evidence of anharmonic interactions, the phonons were reported to show some softening of the optic modes at the zone boundary. The same group of authors later reported that an extra vibrational mode was observed to form at a temperature above 450 K. The existence of the proposed new mode is inconsistent with the usual theory of harmonic phonons, as applied to a structure composed of a monoclinic Bravais lattice with a two-atom basis. We investigate the effect that the f electron-phonon interaction has on the phonon spectrum and its role on the possible formation of a breathing mode of mixed electronic and phonon character. We examine the model by using Green’s function techniques to obtain the phonon spectral density. Some materials undergo phase transitions from a high temperature state with periodic translational invariance to a state in which the electronic charge density is modulated periodically. The wave vector of the modulation may be either commensurate or incommensurate with the reciprocal lattice vectors of the high temperature structure. In the case of an incommensurate charge density wave, the system supports phason excitation. For an incommensurate state, the new ground state has a lower symmetry than the high temperature state since the charge density does not have long-ranged periodic translational order. If the metal is ideal (with no impurities), a charge density wave should be able to slide throughout the crystal without resistance, resulting in current flow similar to that of a superconductor. The phason is an excitation of the charge density wave which is related to the collective motion of electrons. We estimate the phason density of states, and the phason contribution to the specific heat. Angle-resolved photoemission experiments have been performed on USb2, and very narrow quasiparticle peaks have been observed in a band which local spin-density approximation (LSDA) predicts to osculate the Fermi energy. The observed band is found to be depressed by 17 meV below the Fermi energy. The experimentally observed quasiparticle dispersion relation for this band exhibits a kink at an energy of about 23 meV below the Fermi energy. The kink is not found in LSDA calculations and, therefore, is attributable to a change in the quasiparticle mass renormalization by a factor of approximately 2. The existence of a kink in the quasiparticle dispersion relation of a band which does not cross the Fermi energy is unprecedented. The kink in the quasiparticle dispersion relation is attributed to the effect of the interband self-energy involving transitions from the osculating band into a band that does cross the Fermi energy. / Physics

Cette thèse est consacrée à l’étude de la dynamique et du transport des phonons via la résolution de l’équation de transport de Boltzmann (ETB) pour les Phonons. Un ‘solveur’ Monte Carlo dédié à la résolution de l’ETB des phonons dans l’espace réciproque, prenant en compte tous les processus d’interactions Normaux et Umklapp à trois-phonons, est proposé. Une prise en compte rigoureuse des lois de conservation de l’énergie et de la quantité de mouvement est entreprise. Des relations de dispersion réalistes, intégrant tous les modes de polarisations, sont considérées. Le calcul des taux d’interactions à trois-phonons de tous les processus Normaux et Umklapp est effectué en utilisant l’approche théorique due à Ridley qui ne nécessite qu’un unique paramètre semi-ajustable pour chaque mode de polarisation, nommément : le coefficient de couplage anharmonique représenté par les constantes de Grüneisen. Les taux d’interactions ainsi calculés ne servent pas uniquement à la résolution de l’ETB des phonons, mais ont permis aussi une analyse complète des canaux de relaxation des phonons longitudinaux optiques de centre de zone. Cette analyse a montré que le canal de Vallée-Bogani est négligeable dans le GaAs, et que vraisemblablement les temps de vie des phonons LO de centre de zone dans l’InAs et le GaSb rapportés dans la littérature sont fortement sous-estimés. Pour la première fois à notre connaissance, un couplage de deux solveurs Monte Carlo indépendants l’un dédié aux porteurs de charges (Thèse E. Tea) et l’autre dédié aux phonons, est effectué. Cela permet d’étudier l’effet des phonons chauds sur le transport des porteurs de charges. Cette étude a montré que l’approximation de temps de relaxation surestime souvent l’effet bottleneck des phonons. Le ‘solveur’ Monte Carlo est étendu pour résoudre l’ETB des phonons dans l’espace réel (en plus de l’espace réciproque), cela a permet d’étudier le transport des phonons et ainsi de la chaleur. La théorie généralisée de Ridley est toujours utilisée avec des particules de simulations qui interagissent les unes avec les autres directement. Les règles de conservation de l’énergie et de la quantité de mouvement sont rigoureusement respectées. L’effet des processus Umklapp sur la quantité de mouvement totale des phonons est fidèlement traduit; tout comme l’effet des interactions sur les directions des phonons, grâce à une procédure prenant en compte les directions vectorielles respectives lors d’une interaction, au lieu, de la distribution aléatoire usuellement utilisée. Les résultats préliminaires montrent la limite de l’équation analytique de conduction de la chaleur. / This work is dedicated to the study of phonon transport and dynamics via the solution of Boltzmann Transport Equation (BTE) for phonons. The Monte Carlo stochastic method is used to solve the phonon BTE. A solution scheme taking into account all the different individual types of Normal and Umklapp processes which respect energy and momentum conservation rules is presented. The use of the common relaxation time approximation is thus avoided. A generalized Ridley theoretical scheme is used instead to calculate three-phonon scattering rates, with the Grüneisen constant as the only adjustable parameter. A method for deriving adequate adjustable anharmonic coupling coefficients is presented. Polarization branches with real nonlinear dispersion relations for transverse or longitudinal optical and acoustic phonons are considered. Zone-center longitudinal optical (LO) phonon lifetimes are extracted from the MC simulations for GaAs, InP, InAs, and GaSb. Decay channels contributions to zone-center LO phonon lifetimes are investigated using the calculated scattering rates. Vallée-Bogani’s channel is found to have a negligible contribution in all studied materials, notably GaAs. A comparison of phonons behavior between the different materials indicates that the previously reported LO phonon lifetimes in InAs and GaSb were quite underestimated in the literature. For the first time, to our knowledge, a coupling of two independent Monte Carlo solvers, one for charge carriers [PhD manuscript, E. TEA], and one for phonons, is undertaken. Hot phonon effect on charge carrier dynamics is studied. It is shown that the relaxation time approximation overestimates the phonon bottleneck effect. The phonon MC solver is extended to solve the phonon’s BTE in real space simultaneously with the reciprocal space, to study phonon and heat transport. Ridley’s generalized theoretical scheme is utilized again with simulation particles interacting directly together. Energy and momentum conservation laws are rigorously implemented. Umklapp processes effect on the total phonon momentum is thoroughly reproduced, as for the anharmonic interactions effect on resulting phonon directions. This is thanks to a procedure taking in consideration the respective vector directions during an interaction, instead of the randomization procedure usually used in literature. Our preliminary results show the limit of the analytic macroscopic heat conduction equation.

Gallium nitride (GaN) has been targeted for use in high power (>30 W/mm) and high frequency (>160 GHz) application due to its wide band gap and its large break down field. One of the most significant advances in GaN devices has evolved from the AlGaN/GaN high electron mobility transistor (HEMT). As a result of the large power densities being applied to these devices there can develop intense hot spots near areas of highest electric field. The hot spot phenomenon has been linked to a decrease in device reliability through a range of degradation mechanisms. In order to minimize the effect that hot spot temperatures have on device reliability a detailed understanding of relevant transport mechanisms must be developed. This study focuses on two main aspects of phonon transport within GaN devices. The first area of focus was to establish an understanding of phonon relaxation times within bulk GaN. These relaxation times were calculated from an application of Fermi's Golden Rule and explicitly conserve energy and crystal momentum. This analysis gives insight into the details behind the macroscopic thermal conductivity parameter. Once relaxation times for GaN were established a multiscale phonon transport modeling methodology was developed that allowed the Boltzmann Transport Equation to be coupled to the energy equation. This coupling overcomes some computational limits and allows for nanoscale phenomena to be resolved within a macroscopic domain. Results of the transport modeling were focused on benchmarking the coupling method as well as calculating the temperature distribution within an operating 6 finger HEMT.

Recently developed two-dimensional colloidal semiconductor nanocrystals, or nanoplatelets (NPLs), extend the palette of solution-processable free-standing 2D nanomaterials of high performance. Growing CdSe and CdS parts subsequently in either side-by-side or stacked manner results in core–crown or core/shell structures, respectively. Both kinds of heterogeneous NPLs find efficient applications and represent interesting materials to study the electronic and lattice excitations and interaction between them under strong one-directional confinement. Here, we investigated by Raman and infrared spectroscopy the phonon spectra and electron–phonon coupling in CdSe/CdS core/shell and core–crown NPLs. A number of distinct spectral features of the two NPL morphologies are observed, which are further modified by tuning the laser excitation energy Eexc between in- and off-resonant conditions. The general difference is the larger number of phonon modes in core/shell NPLs and their spectral shifts with increasing shell thickness, as well as with Eexc. This behaviour is explained by strong mutual influence of the core and shell and formation of combined phonon modes. In the core–crown structure, the CdSe and CdS modes preserve more independent behaviour with only interface modes forming the phonon overtones with phonons of the core. / Dieser Beitrag ist aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich.

A computational analysis of a metamaterial window design is presented for the purpose of increasing the energy efficiency of buildings in seasonal or cold climates. Commercial low-emissivity windows use nanometer-scale Ag films to reflect infrared energy, while retaining most transmission of optical wavelengths for functionality. An opportunity exists to further increase efficiency through a variable emissivity implementation of Ag thin-film structures. 3-D finite-difference time-domain simulations predict non-linear absorption of near-infrared energy, providing the means to capture a substantial portion of solar energy during cold periods. The effect of various configuration parameters is quantified, with prediction of the net sustainability advantage. Metamaterial window glass technology can be realized as a modification to current, commercial low-emissivity windows through the application of nano-manufactured films, creating the opportunity for both new and after-market sustainable construction. / text

Le but de ce projet de maîtrise est d’explorer la dynamique des jonctions Supraconducteur/métal Normal/Supraconducteur (SNS) diffusives dans un régime de fréquence de l’ordre du temps électron-phonon, soit l’échelle de temps sur laquelle un électron diffuse sur les phonons. Les jonctions utilisées possèdent des bornes en niobium tandis que le métal normal est de l’aluminium. Les mesures sont réalisées à des températures supérieures à 1.4 K, soit au-dessus de la température critique de l’aluminium (1.20 K) et largement en dessous de celle du niobium (9.26 K). On étudie ces jonctions en mesurant leurs caractéristiques courant-tension lorsqu’elles sont excitées avec un signal radio-fréquence (rf). Pour une demi-période d’excitation sinusoïdale (demi-cycle), on observe deux possibilités : soit la jonction reste dans l’état supraconducteur, soit elle transite vers l’état normal. En variant la fréquence et l’amplitude de l’excitation, on relève la statistique de ces transitions. On met ainsi en évidence des changements dans le comportement dynamique de la jonction lorsqu’on varie les paramètres de l’excitation. Si l’on fixe la fréquence d’excitation à une valeur suffisamment basse et qu’on augmente progressivement l’amplitude de cette excitation, on observe une variation graduelle du pourcentage de cycles (ou demi-cycles) d’excitation sinusoïdale où la jonction transite de l’état supraconducteur à l’état normal. Pour les très basses fréquences, cette variation se fait sur un intervalle d’amplitude d’excitation qui coïncide avec la largeur de la distribution du courant critique de la jonction. En augmentant la fréquence d’excitation, cette variation est de plus en plus abrupte et devient un saut discontinu au-delà d’une fréquence critique [florin]*. Ce saut discontinu à haute fréquence s’accompagne d’une hystérèse. Pour un régime de fréquence intermédiaire, les cycles présentant une transition vers l’état normal se regroupent dans le temps et forment des blocs de transitions vers l’état normal. On a montré que la durée moyenne de ces regroupements croît exponentiellement avec la fréquence. Par exemple, pour une température de 1.4 K, la durée moyenne de ces regroupements peut varier de sept ordres de grandeur sur la plage de radio-fréquence étudiée. Enfin, ces comportements dynamiques sont fortement dépendants de la température. En caractérisant ces changements de comportement en fonction des divers paramètres d’excitation et de la température, on met en évidence que le bruit thermique associé au bain de phonons fait partie intégrante du mécanisme causant les regroupements dans le régime de fréquence intermédiaire. À suffisamment basse fréquence (ou à toutes les fréquences dans l’approximation que la dynamique n’affecte pas le « bain »), la caractérisation de ce régime transitoire constitue en soi une « mesure » de l’occurrence d’événements rares, ici les fluctuations thermiques menant la jonction à transiter vers l’état normal.

GaN-based HFETs demonstrate ubiquitous high power and high frequency performance and attract tremendous research efforts. Even though significant advances have been achieved, there still exist some critical issues needed to be investigated and solved. In particular, high defect densities due to inhomogeneous growth and operation under high power conditions bring many unique problems which are not so critical in the conventional Si and GaAs materials systems. In order to reduce the defect density and heat dissipation of GaN-based HFETs, research work on the realization of GaN-based HFETs on bulk GaN substrate has been carried out and the key problems have been identified and solved. Hot phonon scattering is the bottleneck which limits the enhancement of electron velocity in the GaN 2DEG channel. It is found that the plasmon-phonon coupling is the mechanism for converting of hot phonons into high group velocity acoustic phonons. In order to push more electrons into the GaN 2DEG channel in the plasmon-phonon coupling regime and to further reduce the hot phonon lifetime, a novel AlGaN/GaN dual channel HFET structure has been proposed. The growth, fabrication and characterization of such a AlGaN/GaN dual channel HFET structure has been carried out. Conventionally GaN-based light emitting diodes and laser diodes are grown and fabricated using the c-plane III-nitride expitaxy layers. In c-plane III-nitride epi-layers, the polarization-induced electric field introduces spatial separation of electron and hole wave functions in quantum wells (QW)s used LEDs and laser diodes LDs and degrades quantum efficiency. As well, blueshift in the emission wavelength becomes inevitable with increasing injection current unless very thin QWs are employed. The use of nonpolar orientations, namely, m-plane or a-plane GaN, would solve this problem. So far, m-plane GaN has been obtained on LiAlO2 (100), m-plane SiC substrates, and m-plane bulk GaN, which all have limited availability and/or high cost. Silicon substrates are very attractive for the growth of GaN due to their high quality, good thermal conductivity, low cost, availability in large size, and ease with which they can be selectively removed before packaging for better light extraction and heat transfer when needed To realize the low cost and improve the internal quantum efficiency of GaN based light emitting diodes, the process for m-plane GaN growth on Si (112) substrates has been studied and optimized. The continuous m-plane GaN film is successfully grown on Si (112) substrates.

The spin dynamics of electrons in inorganic and organic semiconducting nanostructures has become an area of interest in recent years. The controlled manipulation of an electron’s spin, and in particular its phase, is the primary requirement for applications in quantum information processing. The phase decoheres in a time known as the transverse relaxation time or T2 time. We have carried out a measurement of the ensemble-averaged transverse spin relaxation time (T2*) in bulk and few molecules of the organic semiconductor tris-(8-hydroxyquinolinolato aluminum) or Alq3. The Alq3 system exhibits two characteristic T2* times: the longer of which is temperature independent and the shorter is temperature dependent, indicating that the latter is most likely limited by spin-phonon interaction. Based on the measured data, we infer that the single-particle T2 time in Alq3 is probably long enough to meet Knill's criterion for fault-tolerant quantum computing even at room temperature. Alq3 is also an optically active organic, and we propose a simple optical scheme for spin qubit readout. Moreover, we found that the temperature-dependent T2* time is considerably shorter in bulk Alq3 powder than in few molecules confined in 1–2-nm-sized cavities. Because carriers in organic molecules are localized over individual molecules or atoms but the phonons are delocalized, we believe that this feature is caused by a phonon bottleneck effect. Organic fluorophore molecules, electrosprayed within nanometer sized pores of an anodic alumina film, exhibit unusually large molecule-specific red- or blue-shifts in the fluorescence peak. This molecular specificity allows us to resolve different constituents in a mixture optically, providing a unique new technology for bio- and chemical sensing. We have also observed that the fluorescence efficiency progressively increases with decreasing pore diameter. This trend cannot be explained by the usual photo carrier confinement model since the photo carriers are localized over individual molecules (or atoms) which are much smaller than the pore diameter. A more likely explanation is the metal enhanced fluorescence caused by the plasmon resonance of nanotextured aluminum lying at the bottom of the pores.