Academic literature on the topic 'Electric dipole spin resonance'

The tunneling and spin dynamics is studied for the hole states in a GaAs-based double quantum dot in the presence of strong spin-orbit coupling and periodic electric field. The regimes of tunneling with the spin flip are considered for the "slow" evolution when the field frequency is lower than the other energy parameters of the stationary part of the Hamiltonian. It is found that the under such conditions the spin flip tunneling may take place at both resonant and non-resonant regimes with respect to the Zeeman level splitting. In the latter case the driving frequency may be lower compared to the resonance one, and the system dynamics resembles the Landau-Zener-Stuckelberg-Majorana interference effects arising during the dynamic level passage in isolated quantum dots. Keywords: double quantum dot, spin-orbit interaction, Zeeman splitting, tunneling, electric dipole spin resonance.

The effect of the appearance of an electric current induced by the electromagnetic radiation at the interface of a ferromagnet and a non-magnetic material is calculated theoretically, taking into account the Rashba spin-orbit coupling. It is shown that the electric dipole transitions between the spin subbands of the conduction electrons of a ferromagnet due to the Rashba interaction lead to a photocurrent. This current has a resonance at a frequency corresponding to the energy of the exchange splitting of spin subbands. The resonance width is determined by the spin-orbit interaction constant. The estimates made show the possibility of experimental observation of this effect in specially prepared multilayer systems.

The effect of the appearance of an electric current induced by the electromagnetic radiation at the interface of a ferromagnet and a non-magnetic material is calculated theoretically, taking into account the Rashba spin-orbit coupling. It is shown that the electric dipole transitions between the spin subbands of the conduction electrons of a ferromagnet due to the Rashba interaction lead to a photocurrent. This current has a resonance at a frequency corresponding to the energy of the exchange splitting of spin subbands. The resonance width is determined by the spin-orbit interaction constant. The estimates show the possibility of experimental observation of this effect in specially prepared multilayer systems. Keywords: Ferromagnet, exchange coupling, Rashba spin-orbit coupling, photovoltaic effect.

The magnetic proximity effect is significant for atomically thin layers of two-dimensional materials. In this paper, we study the mechanisms of photogeneration spin-polarized carriers in graphene on a magnetic insulator. The magnetic proximity effect and lowered symmetry at the interface enhance the spin response of graphene in the alternating electric field of the incident light. The first leads to spin splitting of the linear spectrum of Dirac electrons. The second increases the role of the spin-orbit interaction. The main mechanisms of photogenerated spin polarization have been considered, including spin flip intersubband and interband transitions, and their contribution to the absorption coefficient of graphene. Keywords: Dirac electrons, graphene, electric dipole spin resonance, spin-orbit coupling, spin generation.

Alors qu’âgée d’à peine plus d’un siècle, la physique quantique est devenue technologique. Son représentant le plus omniprésent est le transistor, en tant que brique élémentaire des appareils électroniques. Les progrès en fabrication à l’échelle nanométrique ont engendré une croissance exponentielle de sa densité dans les circuits microélectroniques. Une fois au nanomètre, des effets quantiques inévitables empêchent de miniaturiser davantage. Des modèles alternatifs sont étudiés pour contourner cet obstacle, dont la plateforme « complementary metal-oxide semiconductor fully depleted silicon-on-insulator” (CMOS FD-SOI).En parallèle de ces développements, la physique quantique a donné naissance à une nouvelle génération d’innovation technologique, grâce à la capacité de contrôler la matière à l’échelle de la particule unique. Isoler une particule dans un environnement suffisamment calme donne accès à ses propriétés de superposition et d’intrication. Exploiter ces phénomènes pour le traitement de l’information conduirait au changement de paradigme du calcul quantique, qui promet de résoudre des problèmes classiquement intractables. Plusieurs acteurs concourent à la meilleure implémentation d’un bit quantique (ou « qubit ») et tous se heurtent au défi de passer de quelques qubits académiques à un processeur industriel. Parmi eux, les électrons piégés dans des structures silicium sont prometteurs du fait de leur exposition réduite aux noyaux magnétiques et au couplage spin-orbite, et de la possibilité de les purifier des noyaux de spin non nul. De plus, leur compatibilité avec le savoir-faire microélectronique donne l’espoir du passage à l’échelle. Dans cette thèse, nous étudions des électrons uniques piégés dans des boites quantiques définies dans des transistors CMOS FD-SOI. Nous nous intéressons en particulier aux aspects de « bruit » dans leur contrôle et leur lecture.Tout d’abord, nous démontrons la manipulation cohérente d’un spin d’électron CMOS par résonance magnétique médiée électriquement. Un microaimant déposé sur la puce CMOS génère un champ magnétique inhomogène. Exciter le déplacement de l’électron dans ce gradient par les tensions de grille lui fait sentir un champ magnétique oscillant, permettant des rotations du spin avec une fréquence Rabi de 1MHz et un temps de déphasage de 500ns. Nous attribuons ces performances limitées à un nombre fini de fluctuateurs à deux niveaux et aux dimensions réduites des boites quantiques en CMOS. L’enveloppe Rabi et le déphasage rapide sont caractéristiques de l’interaction avec les spins nucléaires. Cependant, découpler dynamiquement l’électron de cette gamme de fréquence offre des temps de cohérence à l’état de l’art, limités par le bruit de charge, en accord avec de simples mesures électriques à basse fréquence. Ces résultats suggèrent la pertinence de la purification isotopique pour s’affranchir du bruit hyperfin.Ensuite, nous nous intéressons au bruit de lecture de ces électrons. L’objectif était d’évaluer la pertinence d’un amplificateur paramétrique à ondes progressives (TWPA) dans la chaine de lecture radiofréquence des dispositifs. Fabriquer des résonateurs sur la puce CMOS a permis de réduire leur capacité parasite et de réaliser des mesures par réflectométrie dans la gamme 3-4GHz, plus près du régime habituel du TWPA. Même pompé loin de son gap, le TWPA montre des figures de mérite nominales, et une résilience à un champ magnétique typique des expériences de qubits de spin. Son haut point de compression à -100dBm, sa bande passante large (2GHz) et réglable et son bruit ajouté proche de la limite quantique permettent plus de 10dB d’amélioration du rapport signal-sur-bruit dans la lecture de transitions de charge interdot, et une lecture multiplexée dans un dispositif à six grilles. Cette compatibilité entre un amplificateur supraconducteur à large bande et des dispositifs CMOS FD-SOI multi-grilles est encourageante en vue d’expériences à plus grande échelle
While being a bit more than a century old, quantum physics have become technological. The most ubiquitous instance of the use of quantum physics is the transistor, as the building block of modern computing devices. Progress in nanoscale fabrication has fostered an exponential increase of transistor density In microelectronics circuits. Once in the nanometer range, unavoidable quantum effects tamper further miniaturization. Alternative transistor designs are being developed to mitigate this showstopper. The complementary metal-oxide-semiconductor (CMOS) fully-depleted silicon-on-insulator (FD-SOI) platform is one of them.In parallel to these developments, quantum physics spawned a new generation of technological innovation, thanks to the ability to control matter at the single particle level. Isolating elementary particles in a quiet environment gives access to their superposition and entanglement properties. Using these to process information would realize the quantum computing paradigm shift, where classically intractable problems are promised to come at reach. Many candidates are racing for the best implementation of a quantum bit (or “qubit”) and all of them are facing the challenge to up-scale their architecture from a few lab qubits to an industrial processor. Among them, electrons trapped in silicon structures offer promising prospects thanks to their reduced exposure to magnetic nuclei and spin-orbit interaction, and to the possibility to purify away non-zero nuclear spins. Moreover, the expected compatibility of silicon structures with microelectronics know-how gives hopes for scalability. In this thesis, we study single electrons trapped in gate-defined quantum dots formed in CMOS FD-SOI transistors. We investigate on how to improve their use as qubits, focusing on experimental noise aspects.First, we demonstrate coherent manipulation of a single CMOS electron spin with electrically driven spin resonance. A micromagnet is patterned directly on top of the CMOS chip, creating an inhomogeneous magnetic field. Driving the electron motion inside this gradient with the available electric gates makes it feel an effective oscillating magnetic field, and enables single qubit operations, with a relatively low 1 MHz Rabi frequency and short 500 ns dephasing time. This limited performance is attributed to a finite number of two-level fluctuators and smaller quantum dot sizes compared to other silicon architectures. The shape of the Rabi decay and the sub-µs dephasing time are characteristic of hyperfine interaction with spinful nuclei. However, dynamically decoupling the electron spin from this frequency range showed state-of-the-art coherence times and performance limited by charge noise, in accordance with simple charge sensor measurements at low frequencies. These results point towards the relevance of isotopic purification to avoid hyperfine-induced dephasing in CMOS FD-SOI devices.After focusing on qubit control, a second part of this thesis deals with readout noise. The objective was to demonstrate the use of a traveling-wave parametric amplifier (TWPA) in the amplification chain of radio-frequency readout of CMOS devices. Patterning inductors on the CMOS chip reduced the parasitic capacitance of our resonators and enabled to perform lumped-element reflectometry in the 3-4 GHz range, closer to usual TWPA operating regimes. Even when being pumped far from its gap, the TWPA shows nominal figures of merit, and a resilience to magnetic fields typical for spin qubit experiments. Its high -100dBm compression point, wide and tunable 2 GHz bandwidth and quantum-limited added noise enabled to get more than 10dB signal-to-noise ratio improvement on interdot charge transitions in our devices, and to multiplex interdot readout in a 6-gate device. This compatibility between a large bandwidth superconducting amplifier and multi-gate CMOS FD-SOI quantum devices is promising towards CMOS electron spin qubit experiments at larger scale

Quantum information processing (QIP) has the potential to reduce the complexity of many classically ‘hard’ computational problems. To implement quantum information algorithms, a suitable physical quantum computer architecture must be identified. One approach is to store quantum information in the electron spins of an array of paramagnetic N@C₆₀ endohedral fullerene molecules, using the electron-electron dipolar interaction to permit the formation of the entangled quantum states needed to implement QIP. This thesis explores two different chemical methods to create two-spin centre arrays that contain N@C₆₀. The first method uses a double 2,3 dipolar cycloaddition reaction to a dibenzaldehyde-terminated oligo-p-phenylene polyethynylene (OPE) unit , to create an (S_3/2, S_3/2) N@C₆₀-N@C₆₀ dimer with a fixed spin centre separation of 2.7 nm. The second approach is via a self-assembly scheme in which a Lewis base functionalised N@C₆₀ molecule coordinates to an antiferromagnetic metallic ring magnet to form a (S_3/2, S_3/2) two-spin centre N@C₆₀-Cr₇Ni system with an inter-spin separation of 1.4 nm. In both systems, a significant perturbation of the electron spin transition energies is observed using CW ESR, this perturbation is shown to be well accounted for by the inclusion of an electron-electron dipolar coupling term in the electron spin Hamiltonians. To create entanglement in an ensemble of two-spin centre molecules, the dipolar coupling interaction must lie within a narrow distribution. To achieve this not only the separation but also the orientation of the inter-spin axis with respect to the applied magnetic field must be controlled for which a method of macroscopic alignment is required. The potential of using a uniaxially drawn liquid crystal elastomer to exert uniaxial order on fullerene dimers is tested, finding that the degree of alignment is insufficient, possibly a result of the propensity for the fullerene molecules to phase separate from the elastomer. This phase separation is shown to restrict N@C₆₀ phase coherence lifetime to 1.4 µs at 40 K due to instantaneous spin diffusion. The electron spin environment of both N@C₆₀ and an N@C₆₀-C₆₀ dimer in a polymer matrix is examined using polystyrene as the host matrix. By deuteration of the polystyrene matrix, a maximum phase coherence lifetimes of 48 µs and 21 µs are measured for the N@C₆₀ and N@C₆₀-C₆₀ dimer, respectively. The concept of reading out the electron spin state of N@C₆₀ molecules by coupling it to a spin system that can be probed using optically detected magnetic resonance (ODMR) such as an NV- centre has been previously suggested. To this end, the photostability of N@C₆₀ under 637 nm laser illumination has been examined in solution. The effect of the presence of an atmospheric concentration of oxygen is striking, affording a 57-fold retardation in the photodecomposition of N@C₆₀ compared to a degassed solution. When ambient oxygen is present, the average number of excitations that are required to cause decomposition is ≈60000. Finally, for future UV photophysics experiments involving N@C₆₀, the best solvent to use was found to be decalin, finding that it significantly slowed decomposition of N@C₆₀ in both ambient and degassed solutions. The conclusions of this work make a significant contribution to the field of QIP with N@C₆₀, showing that there is a bright future for N@C₆₀.

The biological function of macromolecules such as protein, DNA, and RNA depends on their folding and on the relative movements of domains with dimensions of a few nanometers. This length scale can be accessed by distance measurements between paramagnetic spin centers employing Electron Paramagnetic Resonance (EPR) pulsed dipolar spectroscopy (PDS) techniques. In order to use this spectroscopic methods, the biomolecule has to contain either stable or transient paramagnetic centers, which can be metal ions or clusters, amino acid radicals, or organic cofactor radicals. If the biomolecule is diamagnetic, it can be spin-labeled with nitroxides or a diamagnetic metal may be substituted with a paramagnetic one. Nitroxides are the most employed spin probes in PDS, especially for structural studies in proteins were they can be attached to specific sites following a protocol of mutagenesis and site-directed spin labeling (SDSL) on cysteine residues. However, the introduction of a spin label can modify the structure around the labeling site and in some regions it may even interfere with the correct folding. For this reason, exploiting endogenous probes i.e. paramagnetic centers which are naturally present in the protein, represents an primary task in PDS. Indeed, PSD has been entatively performed on various classes of proteins naturally containing metal base-prosthetic groups such as and low-spin ferric heme centers, iron-sulfur cluster, Mn clusters for which mainly the ¢mS Æ §1/2 transition can be selected. Utilizing endogenous probes for EPR detection only causes minimal functional perturbation to the macromolecules. Another advantage is that they are firmly anchored in the protein and, therefore, are not fraught with the problem of flexible linkers as the commonly used spin labels. In recent years photoexcited triplet state of porphyrin has been introduced in the selection of spin labels for PDS applications. In their ground state, these chromophores are diamagnetic and thus EPR-silent, but, upon laser photoexcitation, their triplet state can be populated via intersystem crossing from the lowest excited singlet state, generating in this way the paramagnetic center. The inter-system crossing mechanism makes the population of the triplet sublevels different from the Boltzmann distribution, significantly enhancing the intensity of their EPR signals. Moreover porphyrin-derivative groups are suitable to be exploited as endogenous probes because are present in numerous systems such as heme-protein and photosynthetic proteins. The orthogonal labeling method, based on the use of spectroscopically nonidentical labels which can be addressed selectively in the EPR experiment, is attracting increasing interest in the spectroscopic community. Triplet states work very efficiently as orthogonal labels, adding to the spectroscopic selectivity the advantage of behaving as photoinduced spin probes. This feature allows to perform PDS in the presence of light excitation to measure intramolecular triplet-nitroxide distances or in the absence of light excitation revealing intermolecular nitroxide-nitroxide interactions. While the feasibility of the PDS experiment had already been demonstrated for a photoexcited porphyrin moiety interacting with a nitroxide radical [Di Valentin, M.; Albertini, M.; Zurlo, E.; Gobbo, M. and Carbonera, D. J. Am. Chem. Soc., 2014, 136, 6582 -6585], the accuracy of the new labeling approach for distance determination, and the theoretical frame describing the behavior of polarized high-spin systems for application in dipolar techniques were still laking. In this thesis work a complete spectroscopic and theoretical characterization of photoexcited triplet state probes has been carried out. The reliability and versatility of such spin labels has been tested employing different dipolar pulse schemes and exploiting diverse chromophores for the photogeneration of the paramagnetic center, both in peptide-based model systems and in protein belonging to different classes. The study has been completed with an exhaustive theoretical description. The reliability and the accuracy of the new labeling approach has been demonstrated by measuring the dipolar traces of a spectroscopic ruler composed by a-helix peptides of increasing length, labeled with a porphyrin chromophore, that upon photoexcitation gives the EPR-active species, and a nitroxide artificial amino acid. The good correlation between the distances obtained by experimental PDS data and calculation, is used to asses the accuracy of the new labeling approach. In PDS, there are different pulse sequences that exploits diverse mechanisms to induce the dipolar oscillations. Such pulse schemes have been tested on the triplet state in order to classify the performances of the various PDS techniques with the novel labeling approach. The availability of different light-induced PDS sequences increases the versatility of triplet state probes allowing to select case by case the pulse scheme that guarantees the best signal-to-noise ratio. The new labeling approach has been extended to two paradigmatic proteins: the light-harvesting complex Peridinin-Chlorophyll a-Protein fromAmphidinium Cartarae and the human Neuroglobin belonging to the globins family where the endogenous prosthetic groups have been exploited to photo-generated the triplet state. In the photosynthetic protein the dipolar trace arising from the interaction between the triplet state of one of the carotenoids in the photoactive site and a nitroxide, introduced via site-directed spin labeling, have been measured. This allowed to identify the pigment involved in the photoprotective mechanism and demonstrated that, not only porphyrin-derivatives, but also other chromophores can be used as spin probes. In human neuroglobin the Zn-substitution of the heme has allowed to populate the triplet state of the Zn protoporphyrin IX and successfully measure the dipolar trace proving the applicability of this labeling procedure on the class of hemeproteins. The full characterization of triplet state probes has been completed with a theoretical study based on the density matrix formalism. First, the analytic formula describing the modulation of the dipolar trace for a simplified radical-triplet state system has been obtained, highlighting a time an analogous dependence to the radical-radical case. Subsequently, a program for time-domain numerical calculation of radical-triplet state dipolar traces has been implemented and employed for a quantitative characterization of triplet state probes in PDS.
Il ruolo di molte macromolecole di interesse biologico come ad esempio proteine ed acidi nucleici, dipende dalla loro struttura tridimensionale e da movimenti di domini dell’ordine di pochi nanometri. La spettroscopia paramagnetica elettronica (EPR) ed in particolare le tecniche di spettroscopia impulsata dipolare (PDS) costituiscono lo strumento ideale per studiare sistemi di quest’ordine di grandezza. Tuttavia, per poter utilizzare tecniche PDS nella caratterizzazione di bio-macromolecole, queste devono contenere centri paramagnetici come ad esempio ioni o cluster metallici, oppure centri radicalici. Nel caso in cui il sistema sia diamagnetico è necessario quindi inserire delle sonde paramagnetiche o sostituire eventuali metalli diamagnetici con altri metalli EPR-attivi. I radicali nitrossidi sono le sonde di spin più comunemente impiegate nella spettroscopia dipolare, soprattutto per studi in proteina in cui, per introdurre di tali sonde, è possibile seguire un protocollo di mutagenesi sito-specifica seguita da spin labeling diretto alle cisteine. L’inserimento di sonde di spin tuttavia può causare forti modifiche strutturali alla macromolecola o addirittura interferire con il suo corretto folding. Per questo motivo, quando possibile si tenta di sfruttare centri paramagnetici che siano naturalmente presenti in proteina. Sono stati infatti effettuati diversi studi di spettroscopia dipolare in metallo-proteine sfruttando la transizione ¢mS Æ §1/2 del gruppo prostetico contenente il centro metallico. L’utilizzo di tali gruppi prostetici non causa alcuna alterazione strutturale alla molecola, inoltre, diversamente da molte sonde endogene, questi sono strettamente ancorati all’intorno proteico e forniscono quindi informazioni strutturali più accurate. La porfirina in stato di tripletto fotoeccitato è stata di recente introdotta tra la collezione di sonde di spin utilizzabili nelle tecniche PDS. Nel loro stato fondamentale le porfirine sono diamagnetiche e pertanto EPR silenti, ma in seguito a fotoeccitazione laser possono popolare tramite inter-system crossing lo stato di tripletto eccitato a più bassa energia, divetando in tal modo EPR-attive. Il popolamento tramite inter-system crossing fa si che la popolazione dei sottolivelli di tripletto devii dalla distribuzione di Boltzmann, aumentando enormemente l’intensità del segnale EPR di tale specie che vengono per questo motivo definite "polarizzate". Inoltre derivati porfirinici sono presenti in numerosi sistemi naturali, come ad esempio le emoproteine o le proteine conivolte in processi fotosintetici, e ciò li rende particolarmente interessanti per l’utilizzo in spettroscopia dipolare in quanto possono essere sfruttati come sonde endogene. Il labeling ortogonale, basato sull’impiego di sonde di spin spettroscopicalmente distinte che possono essere eccitate selettivamente durante un esperimento EPR, rappresenta un approccio particolarmente vantaggioso nelle tecniche PDS. Gli stati di tripletto fotoeccitato hanno un valore aggiunto come sonde ortogonali perchè aggiungono alla selezione spettrale il fatto di essere sonde foto-indotte. Questa caratteristica fa si che sia possibile misurare distanze intarmolecolari tripletto-nitrossido, applicando la fotoeccitazione laser, e distanze ntermolecolari nitrossido-nitrossido spegnedo invece la fotoeccitazione. Mentre la fattibilità di esperimenti di spettroscopia dipolare applicati a stati di tripletto fotoeccitati era già stata dimostrata precedentemente a questo lavoro di tesi [Di Valentin, M.; Albertini, M.; Zurlo, E.; Gobbo, M. and Carbonera, D. J. Am. Chem. Soc., 2014, 136, 6582 -6585], mancavano completemente indagini in grado di stabilire l’affidabilità e l’accuratezza del nuovo sistema di labeling e un inquadramento teorico in grado di escrivere il comportamento di tali sistemi polarizzati ad alto spin durante l’esperimento PDS. Il lavoro alla base della presente tesi è consistito nella completa caratterizzazione spettroscopica e teorica di questi sistemi di spin. Per verificare l’affidabilità del nuovo approccio, le sonde di tripletto sono state testate con diverse tecniche PDS, e sono stati inoltre utilizzati vari cromofori per la foto-generazione del centro paramagnetico, effettuando l’analisi sia su sistemi modello che in proteina. Lo studio è stato completato con un esaustivo trattamento teorico dei sistemi tripletto-radicale in spettroscopia dipolare. La precisione e l’accuratezza del metodo sono state verificate misurando le tracce dipolari di un righello spettroscopico costituito da una serie di perptidi in ®-elica di lunghezza crescente, ognuno marcato con un cromoforo porfirinico e un radicale nitrossido. L’ottima correlazione trovata tra le distanze ottenute analizzando le tracce sperimentali e i dati strutturali derivanti dai calcoli ha permesso di dimostrare l’affidabilità delle sonde di tripletto nelle tecniche PDS. Attualmente sono disponibili diverse sequenze PDS che sfruttano diversi meccanismi per indurre l’oscillazione dipolare nelle tracce sperimentali. Molte di queste sequenze sono state quindi testate sulla sonda di tripletto in modo da verificarne le prestazioni con le diverse tecniche. La disponibilità di molteplici sequenze PDS e il loro buon funzionamento su sistemi fotoindotti permette di selezionare, a seconda dei casi, lo schema di impulsi che garantisce le migliori prestazioni in termini di rapporto segnale-rumore e ciò dimostra la versitilità delle sonde di tripletto. La nuova metodologia è stata estesa anche a studi in proteina utilizzando come sistemi modello la Peridinin-Chlorophyill a-Protein, appartenente alla classe delle proteine fotosintetiche, e neuroglobina umana, facente parte della famiglia dellle globine. In Peridinin-Chlorophyill a-Protein è stata misurata l’interazione dipolare tra uno dei carotenoidi presenti nel sito attivo e un nitrossido inserito tramite spin labeling, permettendo non solo l’individuazione del pigmento coinvolto nel meccanismo di fotoprotezione, ma espendendo anche l’applicabilità dell’esperimento a cromofori diversi dai derivati porfirinici. Nella neuroglobina umana invece la zinco-sostituzione dell’eme ha permesso di popolare lo stato di tripletto nel gruppo endogeno e dimostrando che la tecnica dipolare fotoindotta può essere utilizzata anche nello studio strutturale di proteine (macromolecole) apparteneneti alla classe delle emoproteine. Infine è stata effettuata anche un’esaustiva caratterizzazione teorica delle sonde di tripletto basata sul formalismo della matrice densità. E’ stata ricavata l’espressione che descrive la modulazione delle tracce dipolari in sistemi tripletto-radicale, che è risultata essere analoga a quella ottenuta per sistemi di due radicali interagenti. Successivamente è stato implementato un programma per il calcolo numerico di tracce dipolari che ha permesso una descrizione quantitativa diversi sistemi tripletto-radicale.

This thesis is concerned with the design and realisation of components for a new state of the art 94GHz Electron Spin Resonance (ESR) spectrometer capable of operating in both pulsed and CW modes. The complete spectrometer is designed to provide phase coherent 1kW peak power sub-nanosecond π/2 pulses having variable duration and repetition rate. The mm-wave response of a paramagnetic sample to these pulses is detected with a superheterodyne detector. Such a system would offer a step change in performance, promising unprecedented resolution and sensitivity. These aims should be compared with the performance of commercial (Bruker) instruments capable of delivering 200mW 30ns π/2 pulses. For this type of system, both the long term (thermal) and short term (phase) stability of oscillators and sources employed are extremely important. Consideration of phase noise, frequency, tunability and power output shows that multiplied sources offer substantial benefits compared to fundamental sources. A delay line discriminator method of phase noise measurement, suitable for use with the low frequency oscillators is described and implemented. This is extended to 94GHz using a down convertor with a quasi-optically stabilised Gunn oscillator. These tools are used to select an optimum oscillator-multiplier combination to produce a low noise 94GHz source. Anew method of pulse generation, which has produced +23dBm peak power 250ps rectangular and 115ps Gaussian envelope phase coherent pulses, is described. These are believed to be the shortest phase coherent pulses at 94GHz available. This system will be used to provide ns pulses suitable for amplification to 1kW using a Klystron amplifier. A heterodyne detector has been constructed which employs the same oscillator/multiplier techniques identified above to produce the required local oscillator signal. It is demonstrated that by careful consideration of multiplication factors a system employing one variable and one fixed oscillator allows all the signals required in the spectrometer to maintain phase coherence. It is demonstrated that the complete demodulator responds to pulses on a ns time scale and has a noise temperature of 737K.

The bis 1,4,7-trithiacyclononane (1,4,7-TTCN) complexes of iron, cobalt, nickel and copper are reported in this work. Their properties have been examined using computer-controlled electrochemical and spectroscopic techniques. These TTCN complexes form readily, are unusually symmetrical and support electron transfer reactions at the metal center. The cobalt(II) complex is octahedral, low spin and symmetrical. Four oxidation states of cobalt-TTCN complex are observed; two one-electron transfer processes are reversible. Copper (II) bis 1,4,7-TTCN is unusually symmetrical evidenced by both solid phase and ambient temperature aqueous phase electron paramagnetic resonance spectra. An unusually high redox potential for the copper complex indicates extraordinary stability of the Cu(I) oxidation state but evidently not at the expense of Cu(II) stability. The complex also has a high formation constant compared to other copper-thioether complexes. This unusual strength of thioether donor is attributed to ligand geometry. The 1,4,7-TTCN molecule is the only known cyclic polythioether to have all sulfur atoms endodentate. This structure contributes to thermodynamic stability of complexes as the ground state configuration of the free ligand is maintained in the complex.

Thesis (Ph. D.)--Ohio State University, 2005.
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Orientador: Pascoal José Giglio Pagliuso
Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin
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Resumo: A ideia de topologia na Física da Matéria da Condensada, apesar de ter surgido com o efeito Hall quântico inteiro, só voltou a ser explorada recentemente na busca de novas fases da matéria depois do surgimento dos Isolantes Topológicos (ITs) 2D. Após a previsão teórica, e a descoberta experimental, foi proposto que esta nova fase poderia ser generalizada para sistemas 3D, em que o volume do material seria isolante com estados metálicos de superfície que possuiriam canais de spin polarizados. Apesar de diversos experimentos e o surgimento de outras fases topológicas da matéria subsequentes, ainda há dúvidas sobre a natureza dos ITs 3D. Os efeitos topológicos mais claros ainda não foram observados de forma inequívoca e reprodutível experimentalmente e ainda seria de extrema valia encontrar técnicas experimentais que possam complementar os mais discutidos experimentos de ARPES. Nesta dissertação foram estudadas duas famílias distintas de materiais propostas como possíveis ITs 3D: os binários Bi2Se3 e Sb2Te3 e o half-Heusler YPdBi. Utilizando a técnica de auto-fluxo e a fusão estequiométrica, os sistemas foram sintetizados dopados com os terras-rara Gd3+, Nd3+ e Er3+ para realizar estudos de ressonância de spin eletrônico (RSE) e do papel dos efeitos de campo cristalino (CEF) - no caso do half-Heusler. Para o ternário YPdBi foram feitos dois estudos. Na família dos half-Heuslers, a ordem topológica surge da relação entre o acoplamento spin-órbita e a hibridização, que está ligada com a mudança do parâmetro de rede, então os efeitos de CEF poderiam estar refletindo a transição entre a trivialidade e a não-trivialidade. A partir das medidas de susceptibilidade magnética em função da temperatura das amostras dopadas com Nd3+ e Er3+ combinadas com os estudos de RSE, foi possível extrair os parâmetros de campo cristalino (CFP) de quarta e sexta ordem. Comparando esses dados com resultados anteriores para o material, supostamente, não-trivial YPtBi, observou-se uma mudança sistemática no sinal dos CFP. Resultados prévios para as amostras de YPtBi dopadas com Nd3+ mostram uma evolução não usual para uma forma de linha difusiva com a potência de micro-onda. Neste trabalho também foi realizado um estudo da forma de linha em função da potência. Apenas a ressonância do Nd3+ para os monocristais de 10% de Nd em YPdBi mostrou uma forma de linha difusiva que evolui com a potência da micro-onda. No caso dos binários Bi2Se3 e Sb2Te3, o objetivo era otimizar a rampa de tratamento térmico para obter monocristais melhores que poderiam permitir a observação de um espectro totalmente resolvido do Gd3+. Após mudanças no crescimento dos monocristais, o espectro totalmente resolvido foi obtido para as amostras de Bi2Se3. No caso do Sb2Te3 apenas uma linha central com a estrutura fina colapsada foi observada. Acompanhando o deslocamento g e a evolução da largura de linha dH da RSE do Gd3+ com a temperatura, o comportamento negativo do deslocamento g para toda a faixa de temperatura indica que elétrons do tipo p são os grandes responsáveis pela formação da superfície de Fermi residual destes sistemas. Um aumento no coeficiente angular de dH em função da temperatura, a taxa Korringa b, foi observado em baixas temperaturas, logo diferentes concentrações de Gd3+ foram utilizadas para estudar este comportamento. Novamente observou-se um comportamento anômalo em baixas temperaturas, o que poderia estar relacionado com a evolução dos CFP com a temperatura. Todos esses resultados foram discutidos levando-se em conta a possibilidade de existência de topologia não-trivial na estrutura eletrônica desses materiais, com foco particular na relação da interação spin-órbita e os efeitos de campo cristalino com a manifestação da topologia não trivial nesses sistemas
Abstract: The idea of topological systems in Condensed Matter Physics, although already explored in the Quantum Hall Effect, has recently become a topic of intense scientific investigation. In particular, great efforts have been dedicated to the search for new quantum phases since the proposal of the Topological Insulators (TIs) in 2D. After the theoretical prediction and the experimental discovery of the TIs in the 2D case, the existence of the Quantum Hall Spin Effect in 3D, 3D TIs, was proposed, where an insulator bulk and metallic surface states with spin polarized channels could be experimentally realized. Although many experiments have been performed, and some groups claimed the direct observation of such new topological phases, there is still a lot of controversy about the nature of the 3D TIs and about the actual microscopic origin of the metallic states on the surface of the studied materials. Other signatures of the topological phases have not been unambiguously and repeatedly measured yet and there is an obvious lack of a supplementary lab technique to be compared to the most used technique to probe these states, which is ARPES. In this work we have studied two different classes of 3D TIs: the binaries Bi2Se3 and Sb2Te3 and the half-Heusler YPdBi. We have been able to grow single crystals of these materials pure and rare-earth doped with Gd3+, Nd3+ and Er3+ using the self-flux technique and the stoichiometric melting. The aim was to use these crystals to study Electron Spin Resonance (ESR) as a potential probe to investigate the existence of the metallic surface states and to explore the possible of the crystalline electrical field (CEF) effects on the formation of the non-trivial electronic structure of these materials. Regarding the YPdBi, our ESR and magnetization studies have revealed that, in the half-Heusler family, the topological order emerges from the interplay between spin-orbit coupling and the hybridization, which is connected with the changes on the lattice parameter. Thus, the CEF effects could reflect the transition from trivial to nontrivial topology. From the magnetic susceptibility data as a function of temperature from the Nd3+ and Er3+ doped samples combined with the ESR studies, it was possible to extract the fourth and sixth order crystal field parameters (CFP). Comparing our data with the previous results from YPtBi, which is a putative non-trivial material, a systematic change in the sign of the CFP was observed. Previous results with the YPtBi Nd-doped samples show an unusual evolution of the Nd3+ ESR line to a diusive-like line shape as a function of the microwave power. In this work we have performed a similar study of the Nd3+ ESR line shape as a function of the microwave power. Only for the single crystal of 10% Nd in YPdBi resonance shows a diffusive-like line shape that evolves with the microwave power. In the case of the binaries Bi2Se3 e Sb2Te3, the aim of this work was to optimize the heat treatment used in previous works of our group to obtain better single crystals that could allow the observation of the full resolved spectra from Gd3+. After many changes in the single crystal growth method, we were able to observe fully resolved Gd3+ ESR spectra in the Bi2Se3 samples. Regarding the Sb2Te3 single crystals, only a single Gd3+ Dysonian ESR line was observed. Following the Gd3+ ESR dg and dH as a function of temperature, the observed negative behavior of dg, in the whole temperature range studied, indicates that p-type electrons are the main source for the formation of the small the Fermi surface of these materials. An increase of the angular coefficient of dH as a function of temperature, the Korringa rate b, at low temperatures was observed and different concentrations of Gd3+ were required to investigate this anomaly. Again this anomalous behavior at low temperatures was observed for the all Gd-doped samples, which could be related to an evolution of CFP with temperature. We discuss our results taking into account the existence of non-trivial topological states in our samples and the role of spin-orbit and CEF effects might have in the formation of such states
Mestrado
Física
Mestre em Física
132653/2015-0
CNPQ
CAPES
FAPESP

This chapter describes an experiment on the inverse spin Hall effect (ISHE) induced by spin pumping. Spin pumping is the generation of spin currents as a result of magnetization M(t) precession; in a ferromagnetic/paramagnetic bilayer system, a conduction-electron spin current is pumped out of the ferromagnetic layer into the paramagnetic conduction layer in a ferromagnetic resonance condition. The sample used in the experiment is a Ni81Fe19/Pt bilayer film comprising a 10-nm-thick ferromagnetic Ni81Fe19layer and a 10-nm-thick paramagnetic Pt layer. For the measurement, the sample system is placed near the centre of a TE011 microwave cavity at which the magnetic-field component of the microwave mode is maximized while the electric-field component is minimized.

AbstractUltralight bosonic dark matter (UBDM), such as axions and axionlike particles (ALPs), can interact with Standard Model particles via a variety of portals. One type of portal induces electric dipole moments (EDMs) of nuclei and electrons and another type generates torques on nuclear and electronic spins. Several experiments search for interactions of spins with the galactic dark matter background via these portals, comprising a new class of dark matter haloscopes based on magnetic resonance.

AbstractNumerical simulations are inescapable steps in spin dynamics studies and in the design of polarized beam accelerators and optical components. An integral part of the Summer 2021 USPAS Spin Class teachings, under the form of a 2-week miniworkshop, this chapter is also an initiation to the field, “hands on”: in a first Section, numerical simulation exercises are proposed which cover many of the theoretical aspects of hadron and electron spin dynamics addressed during the lectures, including resonant depolarization; preservation methods such as harmonic orbit correction, tune jump, the use of an ac dipole, or snakes; the effect of synchrotron radiation; spin diffusion and its suppression; spin matching. A second Section is dedicated to detailed solutions of these simulation exercises and includes tight comparisons of numerical outcomes and theoretical expectations from the lectures.

Quadrupole relaxation enhancement is a phenomenon observed in field-cycling relaxometry applications to systems containing dipole nuclei dipolarly coupled to quadrupole nuclei. As typical examples, 14N and 2H nuclei interacting with protons in biological systems, including living leeches, are considered. Since the protons are in their high-field limit, crossings of the proton resonance with the low-field resonances of the quadrupole nuclei can be scanned using the field-cycling technique. Due to the strong coupling of quadrupole moments to the electric field gradients in the molecules, the spins of the quadrupole nuclei can be assumed to be permanently in equilibrium. Thus, proton spin–lattice relaxation is enhanced by spin energy exchange mediated by flip-flop transitions with the quadrupole nuclei at the resonance crossings.

Quadrupole relaxation enhancement is a phenomenon observed in field-cycling relaxometry applications to systems containing dipole nuclei dipolarly coupled to quadrupole nuclei. As typical examples, 14N and 2H nuclei interacting with protons in biological systems, including living leeches, are considered. Since the protons are in their high-field limit, crossings of the proton resonance with the low-field resonances of the quadrupole nuclei can be scanned using the field-cycling technique. Due to the strong coupling of quadrupole moments to the electric field gradients in the molecules, the spins of the quadrupole nuclei can be assumed to be permanently in equilibrium. Thus, proton spin–lattice relaxation is enhanced by spin energy exchange mediated by flip-flop transitions with the quadrupole nuclei at the resonance crossings.

Photon echo measurements made at 2 °K on the 3H4 - 1D2 transition in Pr3+:LaF3 show that magnetic dipolar couplings between the Pr and F nuclei account for the 56 kHz homogeneous linewidth of this transition.1 The homogeneous broadening arises from the enhanced 141Pr nuclear moment (I=5/2) interacting with the local field fluctuations of the 19F nuclear moments undergoing mutual spin flip transitions. Such resonant fluctuations should, in the absence of a fluorine spin diffusion barrier, produce a homogeneous linewidth of about 200 kHz which is, in fact, roughly what is observed for the inhomogeneous broadening of the Pr3+ hyperfine levels and is considerably broader than that obtained by the photon echo measurements. Shelby et al2 proposed a simple model analogous to the spin diffusion barriers responsible for narrowing the homogeneous lines in certain electron paramagnetic resonance transitions3. In such systems, the field produced by the electron magnetic dipole moment (2-3 orders of magnitude larger than the enhanced nuclear moment associated with the ground state of Pr3+ in LaF3) de-tunes the nearest neighbors from each other, prohibiting mutual spin flips among them. Thus, the fields produced by the neighboring spins are static and their interaction with the paramagnetic ion contributes to the inhomogeneous linewidth and not to homogeneous broadening. The de-tuned neighbors are referred to as the "frozen core." Application of this picture to the case of Pr3+:LaF3 is supported by the calculation of Devoe et al.4 Their Monte Carlo calculation of the Pr3+ optical dephasing times shows that the observed homogeneous linewidth could be explained by deleting the nearest neighbor F spins from the lattice of rapidly fluctuating fluorine moments, in effect, treating them as part of a frozen core.