<p dir="ltr">The limitations of the traditional von Neumann computing architecture, particularly evident in the slowdown of Moore's law, have spurred the development of alternative domain-specific computing paradigms. This dissertation explores novel materials-physics based solutions for two promising alternatives: quantum computing and probabilistic computing, with a specific focus on leveraging magnetic spin textures and their unique properties. We demonstrate that magnetic spin textures, with their inherent topology and chirality, offer distinctive advantages in addressing key challenges in both computing paradigms. These textures' ability to couple with various degrees of freedom, such as electrical, thermal, mechanical, and optical, makes them particularly suitable for hybrid device implementations. Our work presents four contributions to the field.</p><p dir="ltr">First, we propose a novel approach of using skyrmions --- topologically protected rigid-object like spin textures --- to nucleate and braid Majorana modes in topological superconductor-magnetic multilayer heterostructures. We show analytically and numerically that skyrmion--vortex bound pairs can be braided in experimentally relevant timescales. Inspired by circuit quantum electrodynamics methods, we propose a novel readout scheme based on the dispersive coupling between vortex confinement states and Majorana bound states. This work paves the way for experimentally demonstrating the non-Abelian statistics of Majorana bound states, which might be a crucial step towards the development of fault-tolerant topological quantum computers.</p><p dir="ltr">Second, we study thermal relaxation mechanisms and timescales of spin-split chiral antiferromagnets. The class of spin-split antiferromagnets, including altermagnets, have recently emerged as excellent candidates for ultra-fast and low-energy spintronics applications. Due the lack of dipolar order, they are unaffected by stray fields. However, the spin-split bands still offer electrical control and readout of these antiferromagnets unlike the conventional antiferromagnets. While a lot of promising phenomena in these materials has already been experimentally demonstrated, thermal relaxation mechanisms of such magnets remain unexplored. Using reaction rate theories and statistical physics tools, we study the thermal dynamics of chiral antiferromagnets. We show that these materials thermally relax at ultra-fast picosecond-order timescales. Further, by building on the analogy between XY magnets and current biased Josephson junctions, we propose a novel approach to electrically tune the thermal barrier in chiral antiferromagnets. Although such chiral antiferromagnets may not be suitable for non-volatile memory type of applications, they emerge as promising candidates for the building blocks of probabilistic computers.</p><p dir="ltr">We then turn our attention to the strongly correlated quantum system of quantum spin liquids. We show that spin textures exchange coupled to a Kitaev spin liquid (KSL) can induce emergent gauge fields on the Majorana fermions in the spin liquid. These emergent gauge fields may trap zero energy modes if they are able to thread a net flux through the KSL. We derive analytical expressions for the gauge fields in the presence of spin textures and outline the conditions to obtain a net flux. Zero energy Majorana fermion modes trapped on such spin textures may eventually be used for fault tolerant quantum computing.</p><p dir="ltr">Finally, in the last project, we bring the quantum and probabilistic computing paradigms together by proposing a quantum two level system as a sensor for the building blocks of a probabilistic computer. we show that quantum spin defects such as Nitrogen vacancy centers (NV) can be used as novel probes for characterizing probabilistic bits. We show that various NV sensing protocols can be leveraged to create a complete picture of this nascent magnet based probabilistic bits including their energy barrier and attempt times.</p><p dir="ltr">Our findings suggest that magnetic spin textures, particularly their topological and chiral properties, could provide crucial solutions to current challenges in alternative computing platforms. This work bridges the gap between materials physics, device physics and the applications in alternative computing platforms.</p>

This work investigates the growth dynamic of the reactive molecular beam epitaxy of Fe3O4 films, and its impact on the cation distribution as well as on the magnetic and structural properties at the surface and the interfaces. In order to study the structure and composition of Fe3O4 films during growth, time-resolved high-energy x-ray diffraction (tr-HEXRD) and time-resolved hard x-ray photoelectron spectroscopy (tr-HAXPES) measurements are used to monitor the deposition process of Fe3O4 ultrathin films on SrTiO3(001), MgO(001) and NiO/MgO(001). For Fe3O4\SrTiO3(001) is found that the film first grows in a disordered island structure, between thicknesses of 1.5nm to 3nm in FeO islands and finally in the inverse spinel structure of Fe3O4, displaying (111) nanofacets on the surface. The films on MgO(001) and NiO/MgO(001) show a similar result, with the exception that the films are not disordered in the early growth stage, but form islands which immediately exhibit a crystalline FeO phase up to a thickness of 1nm. After that, the films grown in the inverse spinel structure on both MgO(001) and NiO/MgO(001). Additionally, the tr-HAXPES measurements of Fe3O4/SrTiO3(001) demonstrate that the FeO phase is only stable during the deposition process, but turns into a Fe3O4 phase when the deposition is interrupted. This suggests that this FeO layer is a strictly dynamic property of the growth process, and might not be retained in the as-grown films. In order to characterize the as-grown films, a technique is introduced to extract the cation depth distribution of Fe3O4 films from magnetooptical depth profiles obtained by fitting x-ray resonant magnetic reflectivity (XRMR) curves. To this end, x-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD) spectra are recorded as well as XRMR curves to obtain magnetooptical depth profiles. To attribute these magnetooptical depth profiles to the depth distribution of the cations, multiplet calculations are fitted to the XMCD data. From these calculations, the cation contributions at the three resonant energies of the XMCD spectrum can be evaluated. Recording XRMR curves at those energies allows to resolve the magnetooptical depth profiles of the three iron cation species in Fe3O4. This technique is used to resolve the cation stoichiometry at the surface of Fe3O4/MgO(001) films and at the interfaces of Fe3O4/MgO(001) and Fe3O4/NiO. The first unit cell of the Fe3O4(001) surface shows an excess of Fe3+ cations, likely related to a subsurface cation-vacancy reconstruction of the Fe3O4(001) surface, but the magnetic order of the different cation species appears to be not disturbed in this reconstructed layer. Beyond this layer, the magnetic order of all three iron cation species in Fe3O4/MgO(001) is stable for the entire film with no interlayer or magnetic dead layer at the interface. For Fe3O4/NiO films, we unexpectedly observe a magnetooptical absorption at the Ni L3 edge in the NiO film corresponding to a ferromagnetic order throughout the entire NiO film, which is antiferromagnetic in the bulk. Additionally, the magnetooptical profiles indicate a single intermixed layer containing both Fe2+ and Ni2+ cations.

Most developing countries are faced with drinking water problems, with conditions becoming more severe due to water pollution. Meeting the growing demands for clean water in most countries, there are difficult challenges as the availability and supply of drinkable water are diminishing. Due to economic and environmental concerns, development of additional physical means for the removal of organic compounds from wastewater using permanent magnets, electromagnetic coils, electrodes and ultrasonic pretreatment is desirable. Improving the adsorption and separation process, magnetic field exposure method has progressively drawn consideration. Magnetic field exposure method has demonstrated its capacity for increasing the adsorptive elimination of contaminants from water as static magnetization is suitable, simple and cost-effective. The polypyrrole magnetic nanocomposite use for adsorption experiments influenced by exter-nal magnetic field was prepared using the in-situ polymerization method, which was charac-terized using TEM, SEM, EDX, XRD, BET, FTIR, VSM, and ESR spectrophotometers. The magnetic nanocomposite (PPy/Fe3O4) was observed to have an average particle size of 10 nm with the elementary composition of carbon, oxygen, nitrogen, chloride and iron. The magnetic nanocomposite had a crystalline structure of face-centred cubic lattice of Fe3O4, an adsorption-desorption isotherm shape indicating a typical type-IV mesoporous material with a surface area of 28.77 m2/g. Characteristic peaks of Fe3O4 and PPy were also observed using FTIR spectro-photometer. From the VSM and ESR characterization, the synthesized superparamagnetic ma-terial was shown to have a saturation magnetization of 23 emu/g and an effective g-value of 2.25 g which was attributed to Fe3+ spin interaction. An enhanced removal of Cr(VI), fluoride and congo red dye were observed under the influence of magnetic field, with parameters like pH, adsorbent dosage, the initial concentration of ad-sorbate, magnetic field and magnetic exposure time been varied. The enhanced adsorption of contaminants using magnetic field is attributed to the increase in the magnetic field induced on the particles over a magnetic exposure time, resulting in the rotating particles forming aggre-gates due to the increased magnetic force and torque on the particles from the PSV results. This leads to increase in the chain collision and area of particle interaction with the aqueous solution of hexavalent chromium, fluoride and congo red dye. / Physics / Ph. D. (Physics)

Recent findings of multiferroicity and magnetoelectric effects in rare earth manganites have fuelled research in this class of materials. These multiferroics can be structurally divided into two classes – orthorhombic and hexagonal. Especially attractive are TbMnO3, HoMnO3 and DyMnO3. Since the ionic radius of Dy is at the boundary that separates the orthorhombic and hexagonal RMnO3, DyMnO3 can be synthesized in both the structures using different synthesis conditions. In this thesis, DyMnO3 single crystals (both hexagonal and orthorhombic) prepared using optical floating zone furnace are studied through structural, magnetic and thermal properties. The influence of rare earth ion on the magnetic phase transitions is revealed in magnetisation, ac susceptibility and specific heat studies. Moreover, doping RMnO3 (small R) with alkaline earth ions creates an arena to test the interesting physics of spin-glass-like phenomena in manganites that arises due to quenched disorder. In this regard, 50% strontium diluted DyMnO3 could be an ideal system to study the effects of quenched disorder and structural/magnetic inhomogeneities that govern the magnetic phases in manganites. Structural phase-coexistence and ensuing anomalous magnetism in Pr–based manganite Pr0.6Sr0.4MnO3 are also presented in this thesis. Details of how the thesis is organized into eight chapters and a brief summary of each chapter follows: Chapter 1 is an introduction to the physics of manganites which progresses into multiferroics and eventually discusses the spin-glass-like effects arising due to size mismatch. A discussion on the phase-coexistence and its effect on physical properties are also presented. Eventually, the scope of the thesis is outlined in the last section. Chapter 2 outlines the basic experimental methods employed in this thesis work. Chapter 3 describes the details of crystal growth by optical floating zone method. DyMnO3 crystals in both hexagonal and orthorhombic structures are grown by employing the ambience of argon and air respectively. The crystals in the two crystallographic variants are characterized by X ray diffraction, Energy dispersive X ray analysis and Inductively coupled plasma atomic emission spectroscopy. The crystal structures are refined using Rietveld method with FULLPROF code and found to be P63cm for hexagonal and Pnma for orthorhombic DyMnO3. Details of crystal growth of Dy1−xSrxMnO3 are also presented. The change in ambience has no effect in the crystal structure of this doped manganite. A comparison of the growth of undoped and doped systems is given. In a later section, the crystal growth and structure refinement of Pr0.6Sr0.4MnO3 are discussed and the optimized growth parameters are tabulated for various manganite systems grown in the present work. Chapter 4 deals with the magnetic and thermal characterization of hexagonal and orthorhombic DyMnO3 single crystals. Magnetic measurements reveal the importance of rare earth magnetism in these compounds. The antiferromagnetic transition to a stacked triangular antiferromagnet is discernible from the specific heat studies of hexagonal DyMnO3, which is masked in the bulk magnetisation measurements. Various magnetic transitions pertaining to the antiferromagnetic sinusoidal – spiral – incommensurate magnet, are evident in the magnetisation and specific heat of orthorhombic DyMnO3 which belongs to the class of non-collinear magnets. Chapter 5 deals with basic investigations on the spin-glass-like state in Dy0.5Sr0.5MnO3. Preliminary dc magnetisation shows indication of spin-glass state as a split in field-cooled and zero-field-cooled magnetisation cycles. Further, the failure of scaling of M(T) with H/T indicates the absence of superparamagnetism in Dy0.5Sr0.5MnO3. The dynamic susceptibility and its analysis using the theory of critical slowing down yield exponents pertaining to the spin-glasses. However, a four-order magnitude change is observed in the characteristic spin-flip time. This leads to the assumption that in Dy0.5Sr0.5MnO3 the spin entities are not atomic spins as in canonical spin-glasses but clusters of spins. The specific heat is analysed for signatures of spin-glass state and is found that a linear term in temperature is essential in fitting the observed data. The crystalline electric fields of Dy ion is also analysed attempting multiple Schottky-levels instead of two. Chapter 6 concerns with the aging experiments performed in the spin-glass-like state in Dy0.5Sr0.5MnO3. Striking aging and chaos effects are observed through these measurements. However, owing to the clusters of spins present, deviations from the typical time-dependent behavior seen in canonical spin-glass materials are anticipated in Dy0.5Sr0.5MnO3. In fact, the relaxation measurements indicate that the glassy magnetic properties are due to a cooperative and frustrated dynamics in a heterogeneous or clustered magnetic state. In particular, the microscopic spin flip time obtained from dynamical scaling near the spin-glass transition temperature is four orders of magnitude larger than microscopic times found in atomic spin-glasses. Magnetic viscosity, deduced from the waiting time dependence of the zero field cooled magnetisation, exhibits a peak at a temperature T<Tsg. Waiting time experiments prove that the dynamics is collective and that the observed memory effects are not due to superparamagnetism of separate magnetic entities. Chapter 7 discusses the Electron paramagnetic resonance (EPR) studies on single crystals of DyMnO3 in hexagonal as well as orthorhombic structures. The interesting effect of strontium dilution on the frustrated antiferromagnetism of DyMnO3 is also probed using EPR. The lineshapes are fitted to broad Lorentzian in the case of pure DyMnO3 and to modified Dysonian in the case of Dy0.5Sr0.5MnO3. The linewidth, integrated intensity and geff derived from the signals are analysed as a function of temperature. The EPR results corroborate well with the magnetisation measurements. The study clearly reveals the signature of frustrated magnetism in pure DyMnO3 systems. It is found that antiferromagnetic correlations in these systems persist even above the transition. Moreover, a spinglass-like behaviour in Dy0.5Sr0.5MnO3 is indicated by a step-like feature in the EPR signals at low fields. Chapter 8 deals with the magnetic and electrical properties of Pr0.6Sr0.4MnO3 single crystals. This crystal undergoes two prominent phase transitions – a paramagnetic to ferromagnetic at Tc~300 K and a structural transition at Tstr ~ 64 K. These phase transitions are evident in the static magnetisation as well as in frequency-dependent susceptibility. In these measurements, the structural transition is associated with a sizeable hysteresis typical of a first-order transition. The M–H curves below Tc show clear indication of anomalous magnetism at low temperatures: the virgin curve lies outside the subsequent magnetisation loops. These observations are explained by assuming structural coexistence of a high–temperature orthorhombic and a low–temperature monoclinic ferromagnetic phases. The nature of static magnetisation data is analysed in the critical region. Modified Arrott’s plots yielded perfect straight lines with the isotherm at ~ 300 K passing through the origin. The exponent values thus should be very close to those expected for the universality class of Heisenberg ferromagnets. The temperature dependence of resistivity also shows critical nature with an exponent belonging to the Heisenberg class. The thesis concludes with a chapter on General conclusions and future scope on these systems.

Electronic structure of transition metal oxides has been a subject of intense research since decades due to the wide spectrum of properties that they exhibit, like high temperature superconductivity, metal-insulator transitions (MIT), phase separation etc. Among these, colossal magnetoresistance (CMR), i.e. a sharp drop in the electrical resistance by the application of an external magnetic field, is a property of fundamental and technological importance. In the present study we investigate several of these interesting properties ranging from colossal magnetoresistance, metal-insulator transitions and phase separation phenomena on a wide range of magnetoresistive systems. All these properties originate in transition metal oxides due to a competition between the strong inter-atomic Coulomb interaction strength within the transition metal d electrons and a large hopping interaction strength between the metal d and oxygen 2p states. In this thesis we report the investigation of the electronic and magnetic structures of some magnetoresistive oxides, including various double perovskites and manganites, using various high energy spectroscopies in conjunction with various theoretical approaches. The samples for the present experimental investigation were prepared by different synthetic routes, such as solid state reaction, nitrate method, d.c arc melting and float zone method, and were characterized by x-ray diffraction, four probe resistivity, magnetic susceptibility, optical absorption and energy dispersive analysis of x-rays while some of the samples were supplied by our collaborators. Various spectroscopic techniques like x-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS) , bremsstrahlung isochromat spectroscopy (BIS), x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism spectroscopy (XMCD) , electron energy loss spectroscopy (EELS), spatially resolved photoelectron spectroscopy and M¨ ossbauer spectroscopy were used to probe the samples. Theoretical methods include configuration interaction cluster approach to fit the XAS and XMCD spectra while ab initio band structure calculations along with the least-square fitting procedure was used to fit some of the valence and conduction bands. Following a general introduction in Chapter 1, the details of various experimental and theoretical techniques are discussed in Chapter 2 of this thesis. Recently, a double perovskite, Sr2FeMoO6, belonging to a general family of halfmetallic ferromagnetic oxides, has shown a spectacularly large magnetoresistance even at the room temperature and at relatively small applied magnetic fields compared to the extensively investigated class of magnetoresistive manganites. Physical properties of this compound is strongly influenced by the Fe -Mo ordering. We hence synthesized Sr2FeMoO6 sample, both with high and low degree of Fe/Mo ordering. Spectroscopic investigations of these samples suggest the presence of Fe rich and Mo rich domains of the type Sr2Fe1+xMo1−xO6 in disordered Sr2FeMoO6 at times. This prompted us to prepare bulk samples of Sr2Fe1+xMo1−xO6. In Chapter 3 we address various issues related to Fe/Mo ordering like saturation magnetization, variation of TC, and CMR as well as oxidation state of Fe and Mo in Sr2FeMoO6using this new series, ”Sr2Fe1+xMo1−xO6” as it offers a better control on the Fe/Mo bonds by controlling x. On the basis of the electron spectroscopic studies in conjunction with a configuration interaction cluster calculation model coupled with the conduction band, we claim that Fe remains in 3+oxidation state throughout the series, where as Mo changes its valency to maintain the charge neutrality. An analysis of the magnetic momentas a function of x suggests that Fe at the ”wrong” crystallographic site is coupled anti-parallel to the Fe moments at the ”correct” site. Additionally, Mo depolarizes to the extend proportional to the number of Mo sites in the near-neighbor co-ordination shell. Continuing with the double perovskites in Chapter 4 we investigate the electronic and magnetic structure of Sr2FeMoO6, Ca2FeMoO6 and Ba2FeMoO6using XAS and XMCD studies. We find that the conventional XAS and XMCD calculations based on configuration interaction of a typical fragment, FeO6in this case, is insufficient to reproduce the experimental spectrum as the compounds considered here are metallic. In order to include the non local charge transfer, we coupled FeO6 octahedra to a conduction band which mimics the Mo band. Within this model we obtained a good fit to the experimental spectrum. Chapter 5 deals with another series of double perovskite (Sr1−yCay)2FeReO6which exhibits a rich phase diagram since it undergoes a metal insulator transition (MIT) with composition at low temperatures. This system becomes more interesting due to the presence of a temperature driven MIT for higher y compositions. We find that the MIT is not related to the change in valency of Fe and Re. Analysis of the near Fermi edge valence band spectra suggests opening up of a soft gap. The main reason for MIT in this system is most likely the presence of strong electron-electron correlation between multiple electrons at the Re site, which is caused by the mismatch of the Re ionic radius and change in the crystal structure across MIT. Another issue which has been extensively investigated in this thesis is phase separation in manganites presented in Chapter 6. We use a spatially resolved, direct spectroscopic probe for electronic structure with an additional unique sensitivity to chemical compositions, to investigate high quality single crystal samples of La1/4Pr3/8Ca3/8MnO3 in the first section. This unique probe establishes the formation of distinct insulating domains embedded in the metallic host at low temperatures, significantly in the absence of any perceptible chemical inhomogeneity, with the domain-size at least an order of magnitude larger than the previous largest estimate. We also provide compelling evidence of memory effects in such domain formation and morphology, suggesting an intimate connection between these electronic domains and long-range strains, often thought to be an important ingredient in the physics of doped manganites. In second part of this chapter we discuss another system namely Eu0.5Y0.5MnO3 which undergoes a chemical phase separation forming alternate stripes of Eu rich (Y deficient) orthorhombic phase and Y rich (Eu deficient) hexagonal phases. These stripes are amazingly straight and run parallel over millimeters. One more system that we investigated is a mixture of ferromagnetic La5/8Sr3/8MnO3and insulating ferroelectric LuMnO3 taken in ratio 3:7, here too the attempt to make a single crystal resulted into a chemical phase separation forming strips of metallic La5/8Sr3/8MnO3and insulating LuMnO3 throughout the sample surface. Preliminary studies suggests that strain between the chemically and crystallographically different species may result into such interesting morphology. In Chapter 7 we study pseudo-one dimensional compounds Sr3CuIrO6 and Sr3ZnIrO6 using photo electron spectroscopy. The experimental results were fitted using band structure calculations with Full Potential Linearized Augmented Plane Wave (FP-LAPW) method.

Verbindungen mit Spinellstruktur im quasiternären System &amp;quot;LiO0.5-MnOx-FeOx&amp;quot; finden industriell als keramische Werkstoffe in der Elektrotechnik und Elektronik Verwendung. So werden Lithium-Mangan-Spinelloxide der Form Li1+xMn2-xO4 (x =&amp;gt; 0) als Kathodenmaterial für wiederaufladbare Lithiumbatterien untersucht. Sowohl Lithium- als auch Manganferrit finden Einsatz als steuerbare Komponenten in der Mikrowellentechnik und Manganferrite als Leistungsüberträger in Spulen und Transformatoren der Hochfrequenztechnik. Für einen solchen technischen Einsatz sind die Kenntnisse der Bedingungen für die Synthese phasenreiner Spinelle und deren Struktur unerlässlich. Die Darstellung der Spinelle erfolgte im Rahmen dieser Arbeit aus gefriergetrockneten Lithium-Mangan-Eisenformiaten. Diese Precursoren zeichnen sich durch hohe Reaktivität und exakte Metallionenstöchiometrie aus. Der Zersetzungsablauf von gefriergetrockneten Li-Mn(II)-Fe(III)-Formiaten unter Argon wurde mittels thermischer Analyse, gekoppelt mit der Massenspektroskopie, sowie durch Röntgenpulveraufnahmen der Zwischenprodukte untersucht. Aus den vorzersetzten Precursoren gewünschter Zusammensetzung wurden unter kontrollierten Temperatur- und Sauerstoffpartialdruckbedingungen einphasige Spinelloxide dargestellt. Die so erhaltenen Verbindungen mit Spinellstruktur wurden mittels Röntgenbeugung und Strukturverfeinerung sowie XANES- und Mößbauerspektroskopie und magnetischen Messungen untersucht. Aus der Kombination dieser Methoden konnten Schlussfolgerungen bezüglich der Struktur, Kationenverteilung und Eigenschaften der jeweiligen Spinelle gewonnen werden. Im Mittelpunkt der Arbeit steht die in der Literatur nicht beschriebene Mischkristallreihe LixMn1+xFe2?2xO4, die Mn(II) und Mn(III) oder Mn(III) und Mn(IV) für x &amp;lt; 0.5 oder x &amp;gt; 0.5 enthält. Mit zunehmendem x-Wert vergrößert sich der Anteil von Lithiumionen auf Tetraeder-plätzen. Bei einem Wert x = 4/7 erreicht dieser Anteil 100%. Unter Einbeziehung der Ergebnisse der Mößbaueruntersuchungen ergeben sich für die Spinellverbindungen mit x = 2/7, 3/7 und 4/7 die folgenden Kationenverteilungen: (Li1.04Mn2+2.81-[delta]Fe3+3.15Mn3+[delta])A[Li0.96Fe3+6.85Mn3+6-[delta]Mn2+0.19+[delta]]BO28 (Li2.37Mn2+1.0-*Fe3+2.98Mn3+0.65+*)A[Li0.63Fe3+5.02Mn3+8.35-*Mn2+*]BO28 (Li4.0Fe3+2.37Mn3+0.63)A[Fe3+3.63Mn3+9.37Mn4+1.0]BO28. Eine theoretisch vorhersehbare Zunahme der Sättigungsmagnetisierung bei kleinen x-Werten wird durch Abnahme der kooperativen Kopplungseffekte mit Abnahme des Eisengehaltes nicht beobachtet. Zusammenfassend kann festgehalten werden, dass die Darstellung phasenreiner Spinelloxide aus den vorzersetzten gefriergetrockneten Li-Mn-Fe-Formiaten im gesamten Bereich zwischen den bekannten quasibinären Spinellverbindungen MnFe2O4, Li0.5Fe2.5O4, LiMn2O4 und Li4/3Mn5/3O4 im quaternären System Li-Mn-Fe-O unter jeweils definierten pO2/T-Bedingungen möglich ist. Die Synthesetemperaturen sind teilweise um 100°C bis 200°C niedriger als bei vergleichbaren Proben aus den Festkörpereaktionen. Manganreiche Spinelle außerhalb dieses Bereiches konnten nicht synthetisiert werden.

Both cuprates and manganites belong to the transition metal oxides. The physics of these compounds is characterized by a dualism of local electron interaction and itinerant charge motion. In the present work, several key issues of metallic cuprates and manganites are addressed on a theoretical level, while close connection to recent experimental work is kept. The work is based on the notion of spin and orbital liquids, representing elegant tools to handle the strongly correlated nature of the metallic state in an efficient and transparent manner. A concise introduction to the physics of cuprates and manganites as well as to the methods employed is presented at the beginning of the work. In a subsequent part, we show that the peculiar magnetic response of metallic cuprates upon impurity doping can be successfully explained within a spin-liquid picture. The remainder of the work is devoted to the metallic state of manganites. Elaborating on the notion of an orbital liquid, the interplay of electron correlations, orbital degeneracy, and double exchange is studied. Thereby, the unconventionally large incoherent optical spectrum of metallic manganites and the pronounced softening of the magnon spectrum observed in experiment can be explained. Finally, a theory of the metal-insulator transition of manganites is presented which is based upon the newly introduced notion of orbital polarons. In general, we believe the close agreement of our results with experiment to strongly support the validity of our approach, giving new insight into the spectacular and sometimes as-tonishing physics of transition metal oxides.

No description available.

No description available.

Στην εργασία αυτή μελετάμε μια σειρά από αντιπροσωπευτικά νανοφασικά συστήματα πυριτίου, στο πλαίσιο κοινών ιδιοτήτων και αρχών που θα βοηθήσουν σε μελλοντικές εφαρμογές σχεδιασμού μοριακών υλικών βασισμένων σε αυτά τα συστήματα. Οι κατηγορίες των συστημάτων με τα οποία ασχολούμαστε είναι (α) υδρογονωμένα και μη-υδρογονωμένα νανοσυσσωματώματα και νανοκρυσταλλικά συστήματα πυριτίου με ή χωρίς ενσωματωμένα μέταλλα μετάπτωσης, που αποτελούν χαρακτηριστικά μοντέλα ενδοεπιφάνειας μετάλλου−ημιαγωγού, (β) υπέρλεπτα υδρογονωμένα νανοσύρματα πυριτίου και (γ) οργανομεταλλικά πολλαπλών στρώσεων (multidecker-sandwiches) πυριτίου−άνθρακα. Εκτός από τη μελέτη των δομικών, ηλεκτρονικών, οπτικών, δονητικών και μαγνητικών ιδιοτήτων των συστημάτων, εστιάζουμε στην αναζήτηση μηχανισμών σταθεροποίησης και την εύρεση και καθορισμό κανόνων που μπορούν να λειτουργήσουν ως «εργαλεία μοριακού σχεδιασμού» με τη γενικότερη δυνατή ισχύ. Τα συσσωματώματα πυριτίου σταθεροποιούνται μέσου των μετάλλων μετάπτωσης σε δομές κλωβού και χαρακτηρίζονται συχνά από υψηλή συμμετρία και μεγάλα ενεργειακά χάσματα, ιδιότητες επιθυμητές για εφαρμογές στην οπτοηλεκτρονική και νανοηλεκτρονική. Στη μελέτη των νανοσυρμάτων πυριτίου συγκρίνουμε την σταθερότητα μεταξύ νανοσυρμάτων με διαφορετικές επιφανειακές δομές ενώ διατυπώνουμε κανόνα «μαγικότητας» νανοσυρμάτων με τον οποίο ερμηνεύουμε την σταθερότητά τους και την συνδέουμε με την ελαστικότητα και την κατανομή υδρογόνου στην επιφάνεια τους. Τέλος, βασιζόμενοι στην ισολοβική αρχή the boron connection, σχεδιάζουμε και μελετάμε μια νέα κατηγορία νανοδομών τύπου multidecker sandwiches οργανοπυριτίου. Στη διάρκεια εκπόνησης αυτής της διατριβής δημοσιεύτηκαν συνολικά 19 εργασίες σε διεθνή περιοδικά και σε πρακτικά συνεδρίων. / In this work we study a series of representative nanoscale systems based on silicon, in the context of common properties and principles which will assist in future applications in designing molecular materials based on these systems. The categories of the systems which we work on are (a) hydrogenated and non-hydrogenated silicon nanoclusters and nanocrystallic systems with or without embedded transition metals, which constitute models of metal−semiconductor interfaces, (b) ultrathin hydrogenated silicon nanowires and (c) organometallic silicon−carbon multidecker-sandwiches. In addition to the study of structural, electronic, optical, vibrational and magnetic properties of these systems, we focus on a search for stabilizing mechanisms and in finding and defining rules that can function as “molecular designing tools” with the broadest possible validity. The silicon nanoclusters are stabilized to cage-like structures by the insertion of transition metals and are characterized by high symmetry and large energy gaps, desirable properties for applications in optoelectronics and nanoelectronics. In the study of silicon nanowires we compare the stability between nanowires with different surface structures while we formulate “magicity” rules for nanowires with which we interpret their stability and associate it with their elasticity and the distribution of the surface hydrogen. Finally, based on the isolobal principle the boron connection, we design and study a new class of organometallic multidecker-sandwich type nanostructures. During the elaboration of this dissertation we published overall 19 papers in international scientific journals and conferences’ proceedings.