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Редкоземельные цирконаты со структурой пирохлора R2Zr2O7 (R=La-Lu), обладают разнообразными свойствами, в том числе имеют потенциальное применение как люминофоры, при активации другими редкоземельными ионами. Экспериментальные данные по ИК и КР спектрам редкоземельных цирконатов, по упругим свойствам, по ширине запрещенной щели отрывочны, и имеются в научной для 2-3 представителей ряда. Представляется актуальным в рамках единого ab initio подхода исследовать комплекс свойств ряда редкоземельных цирконатов R2Zr2O7(R=La-Lu). В работе проведен ab initio расчет фононного спектра всего ряда редкоземельных цирконатов, определены частоты и типы ИК и КР-мод, что позволило дополнить экспериментальные данные по ИК и КР спектрам. Выявлены моды, в которых участвуют преимущественно ионы кислорода. Изменение этих мод может говорить об искажении кислородной подрешетки под влиянием внешних воздействий на кристалл. Рассчитаны упругие постоянные, модули упругости и твердость по Виккерсу для ряда R2Zr2O7 (R=La-Lu), что дополнило отсутствующие экспериментальные данные. Показано, что при лантаноидном сжатии усиливается анизотропия упругих свойств. Показано, что в примесных центрах La2Zr2O7:R3+ расстояние «примесный ион-лиганд» изменяется в пределах ~ 0.1 Å, что существенно меньше, чем различие ионных радиусов по Шеннону La и примесного иона R 3+. Показано, что замещение ионов La в кристалле La2Zr2O7 другими редкоземельными ионами R3+ до 25% изменяет упругие модули кристалла в пределах 10%. В работе показана возможность заменять на псевдопотенциал внутренние оболочки редкоземельных ионов по 4f включительно при расчете кристаллической структуры, фононного спектра, упругих свойств 3 редкоземельных цирконатов R2Zr2O7 (R=La-Lu). Такой подход может быть использован и в других кристаллах с РЗ подрешеткой для минимизации затрат компьютерных ресурсов. Результаты работы представлены на международной и всероссийской конференциях: LXVII Международная конференция «Актуальные проблемы прочности» («АПП-2024», Екатеринбург), «XXIII Всероссийская школа-семинар по проблемам физики конденсированного состояния вещества» («СПФКС-23», Екатеринбург). / Rare earth zirconates with a pyrochlore structure R2Zr2O7 (R=La- Lu), have a variety of properties, including potential using as luminophores when activated by others rare earths ions. Experimental data on the IR and Raman spectra of rare earths zirconates, elastic properties, band gap width are poor, and are available in scientific journals for 2-3 crystals of the row. It seems actual to study complex of properties of the all row of rare earth zirconates R2Zr2O7(R=La-Lu) within the framework of ab initio approach. The work carried out an ab initio calculation of the phonon spectrum of the all row of rare earth zirconates. The frequencies and types of IR and Raman modes have been determined, which made it possible to supplement the absent experimental data on the IR and Raman spectra. Phonon modes have been identified in which predominantly oxygen ions participate. A change of these modes may indicate to a distortion of the oxygen sublattice under the influence of external effects on the crystal. Calculated elastic constants, elastic moduli and Vickers hardness for the series R2Zr2O7 (R=La-Lu), which supplemented the missing experimental data. It has been shown that the anisotropy of elastic properties increases by lanthanide pressure. It has been shown that in La2Zr2O7:R3+ impurity centers the distance “impurity ion-ligand” varies within ~ 0.1 Å, which is significant less than the difference between the Shannon ionic radii between La and the impurity ion R3+. It has been shown that the replacement of La ions in the La2Zr2O7 crystal by other rare earth ions R3+ up to 25% changes the elastic moduli of the crystal in within 10%. The work shows the possibility of replacing internal shells of rare earth ions up to 4f inclusive by calculating crystal structure, phonon spectrum, elastic properties of rare earth zirconates R2Zr2O7 (R=La-Lu). This approach could be used in other crystals with rare-earth sublattice to minimize the computer cost. The results of the work were presented at the international and all-Russian conferences: LXVII International Conference “Current Problems strength" ("APP-2024", Ekaterinburg), "XXIII All-Russian School- seminar on problems of condensed matter physics" (“SPFKS-23”, Ekaterinburg).

The structure of a material, whether at the atomic scale or patterned at the nanoscale, is the basis of many of its physical properties—color, emission wavelength, optical nonlinearity, electrical conductivity, thermal conductivity, brittleness, and more. Therefore, one of the most important developments in photonics, electronics, and magnetics is the ability to manipulate the nanostructure of materials as a way to augment their natural qualities and adapt them to greater applications. The cleanroom debuted in the mid-20th century, alongside and followed by an assortment of precision nanofabrication instruments performing photolithography, electron-beam lithography, ion implantation, femtosecond laser machining, etc. While these techniques have demonstrated breakthroughs such as fabricating ever-smaller transistors keeping pace with the famous Moore’s Law, they require cleanroom facilities, multi-step processing, or leave behind debris or residue. Such impurities have an outsize effect on a burgeoning class of materials with desirable optical and electronic properties—two-dimensional (2D) layered van der Waals materials—as their dimensions approach the single-atom limit, leading a desire for additional approaches to material nanostructuring. In this thesis, we describe a novel approach to generating atomically sharp linear nanostructures in hexagonal boron nitride (hBN) via resonant optical phonon pumping with a pulsed mid-infrared laser and detail its development from discovery to a useful technique that complements established approaches to nanopatterning. The femtosecond laser is tuned to the material’s infrared-active transverse optical TO (E1u) phonon, located at ? = 7.3 ?? or 1367 cm-1, and its polarization aligned parallel to the crystal zigzag axis, in the direction of the phonon’s characteristic atomic motion. The optical field coherently drives and amplifies the intrinsic ionic motion toward bond breakage, resulting in a gentle tearing of the hBN flake along the crystal axis at the material damage threshold. All processing is performed in situ at room temperature under ambient conditions, free from cryogenics and vacuum setups, unlike in the conventional nanofabrication methods confined to the cleanroom. This phenomenon is termed “unzipping” to depict the rapid formation and emanation of a crack tens of nanometers wide from a point within the laser-excited area. The generation of these fea- tures is ascribed to the large atomic displacements and localized bond strain produced by strongly driving the crystal at an intrinsic resonance, which is absent under non-resonant irradiation and is greatly sensitive to the relative angle between the crystal orientation and the linear laser polarization. We perform detailed characterization of the unzipped features and their host hBN flakes us- ing atomic force microscopy (AFM) topographic imaging, scanning electron microscopy (SEM), atomic-scale lateral force microscopy (LFM), nanoindentation in the plastic deformation regime, and near-field optical probing (scattering-type scanning near-field optical microscopy, s-SNOM) to reveal their atomically sharp, six-fold symmetric, orientation-selective, defect-seeded nature. Then, we fabricated several nanostructures—gratings, Fabry-Perot resonators, and cleaved and shaped flakes—to demonstrate the technique in useful nanophotonics applications. The preliminary Fabry-Perot resonator, examined in the near-field with nanoscale Fourier-transform infrared spectroscopy (nano-FTIR), exhibited performance that is competitive with similar structures fabricated by cleanroom etching. Our initial approach achieved a quality factor of ? ≈ 70, already on par with ? = 50 to 100 achieved by conventional nanofabrication methods. The cleanliness, sharpness, and directionality of nanostructures fabricated in situ via unzipping, along with the ability to deterministically seed the location of its constituent line defects using nanoindentation, enable vast future applications in patterning hBN and other polar crystals that possess optically-addressable, high-energy optical phonon modes in the mid-infrared.

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.

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This thesis is concerned with studying the fascinating quantum properties of real-world nanostructures embedded in a noisy condensed matter environment. The interaction with light is used for controlling and manipulating the quantum state of the systems considered here. In some instances, laser pulses also provide a way of actively probing and controlling environmental interactions. The first two research chapters assess two different ways of performing all-optical spin qubit gates in self-assembled quantum dots. The principal conclusion is that an `adiabatic' control technique holds the promise of achieving a high fidelity when all primary sources of decoherence are taken into account. In the next chapter, it is shown that an optically driven quantum dot exciton interacting with the phonons of the surrounding lattice acts as a heat pump. Further, a model is developed which predicts the temperature-dependent damping of Rabi oscillations caused by bulk phonons, finding an excellent agreement with experimental data. A different system is studied in the following chapter: two electron spin qubits with no direct interaction, yet both exchange-coupled to an optically active mediator spin. The results of this study show that these general assumptions are sufficient for generating controlled electron spin entanglement over a wide range of parameters, even in the presence of noise. Finally, the Radical Pair model of the avian compass is investigated in the light of recent experimental results, leading to the surprising prediction that the electron spin coherence time in this molecular system seems to approach the millisecond timescale.

The unique properties, such as large direct bandgap, excellent thermal stability, high μH × ns, of III-nitrides make them ideal candidates for both optoelectronic and high-speed electronic devices. In the past decades, great success has been achieved in commercialization of GaN based light emitting diodes (LEDs) and laser diodes (LDs). However, due to the lack of native substrates, thin films grown on sapphire or SiC substrates have high defect densities that degrade the device performance and reliability. Conventional epitaxy lateral overgrowth (ELO) can reduce dislocation densities down to ∼10-6 cm-2 in the lateral growth area, but requires ex situ photolithography steps. Hence, an in situ method using a SiNx interlayer (nano-scale ELOG) has emerged as a promising technique. The GaN templates prepared by this method exhibit a very low dislocation density (low-10-7 cm-2) and excellent optical and electrical properties. As a cost, such high quality GaN templates containing SiN, nanonetworks are not suitable for heterojunction field effect transistor (HFET) applications due to degenerate GaN:Si layer which serves as parallel conduction channel. This dissertation discusses the growth of low dislocation density GaN templates, by using the in situ SiNx nanonetwork for conductive templates, and the AIN buffer for semi-insulating templates. On SiN x nanonetwork templates, double-barrier RTD and superlattice (SL) exhibited negative differential resistances. Moreover, the injection current of Blue LEDs (450 nm) was improved ∼30%. On semi-insulating GaN templates, nearly lattice matched AlInN/AIN/GaN HFETs were successfully demonstrated and exhibited ∼ 1600 cm2/Vs and 17 600 cm2/Vs Hall mobilities at 300 K and 10 K, respectively. Those mobility values are much higher than literature reports and indicate that high quality HFETs can be realized in lattice matched AlInN/AIN/GaN, thereby solving the strain related issue. The attempt to use InGaN as the 2DEG channel has also been successfully implemented. A Hall mobility (1230 cm2/Vs) was achieved in a 12 nm InGaN channel HFET with AlInGaN barrier, which demonstrates the viability of InGaN channel HFETs.