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Thesis (Ph. D.)--Florida State University, 2003. / Advisor: Dr. L.P. Gorʹkov, Florida State University, Dept. of Physics. Title and description from dissertation home page (viewed Nov. 24, 2003). Includes bibliographical references.

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Orientador: Dayse Iara dos Santos / Banca: Adilson Jesus Aparecido de Oliveira / Banca: Raquel de Almeida Ribeiro / Banca: Antonio Renato Bigansolli / Banca: José Humberto Dias da Silva / Resumo: O supercondutor (SC) YBa2Cu3O7 (YBCO) foi descoberto em 1986, e desde então tem sido amplamente estudado em suas diversas formas e propriedades. Dentre suas várias características, sua alta temperatura crítica é uma das mais importantes na ciência e na tecnologia. Atualmente existem vários dispositivos baseados no YBCO. As heteroestruturas do tipo SC/FM, onde FM representa uma camada ferromagnética, criadas nas últimas décadas, constituem uma das mais interessantes formas de aplicação deste material. Estas são altamente promissoras para o desenvolvimento de dispositivos spintrônicos que exploram as propriedades dos spins dos elétrons. Neste trabalho o foco foi estudar os efeitos das interações entre a camada de YBCO e a do ferromagneto La2/3Ca1/3MnO3 em super-redes intermediadas por camadas de diferentes espessuras de determinados materiais isolantes. Os materiais isolantes testados foram o SrTiO3 (STO), o CeO3 e o PrBa2Cu3O. As super-redes foram fabricadas pela técnica PLD com parâmetros optimizados. Algumas das características foram determinadas por meio de medidas de DRX, MEV, EDS, de magnetização e de transporte. Estruturalmente, as super-redes apresentaram integridade de composição e cristalinidade com crescimento preferencial na direção c. As imagens de MEV confirmaram o crescimento apitaxial das camadas, e o EDS sua composição. As medidas de magnetização mostraram o grau de qualidade e o comportamento da super-rede que resulta da interação ou desacoplamento eletrônico das camadas SC e FM quando uma camada isolante é inserida entre elas. A mesma análise foi feita nas medidas de transporte. As super-redes (YBCO20nm/ISOdl/LCMO10nm)x20 apresentaram comportamento predominantemente supercondutor. Enquanto as super-redes (YBCO20nm/ISOdl/LCMO20nm)x20 apresentaram oscilações das temperaturas crítica e de Curie assim como magnetização e resistividade não usuais dependendo do isolante e de sua espessura... / Abstract: The superconducting (SC) YBa2Cu3O7 (YBCO) was discovered in 1986, and since than it has been widely studied in its various forms and properties. Among its many features, its high critical temperature is one of the most important in science and technology fields. Currently there are several devices based on YBCO. The SC/FM heterostructures, where FM is a ferromagnetic layer, created in recent decades, is one of the most interesting ways of applying this material. These are highly promising for the development of spintronic devices exploding the properties of electron spins. In this work the focus was to study the effects of interactions between the YBCO layer and the ferromagnet La2/3Ca1/3MnO3 in superlattices mediated by layers of differente thicknesses of certain insulating materials. The tested insulating material were SrTiO3 (STO), CeO2 and PrBa2Cu3O6. The superlattices were fabricated by PLD technique with optimized parameters. Some of the features were determined by measurements of XRD, SEM, EDS, magnetization and electric transport. Structurally, the superlattices showed integrity of composition and crystallinity with preferencial growth in the c-direction. The SEM images confirmed the epitaxial growth of the layers, and the EDS confirmed the composition. The magnetization measurements showed the quality and behavior of the superlattices which results from the interaction or uncoupling of SC and FM layers when an insulating layer is inserted between them. The same analysis was done in transport measurements. The superlattices (YBCO20mn/ISOdl/LCMO10mn)x20 showed predominantly superconducting behavior. While (YBCO20nm/ISOdl?LCMO20mn)x20 superlattices showed oscillations on critical and Curie temperatures as well as unusual magnetization and resistivity which depended on the insulation and its thickness. The STO acted as an efficient insulator between the SC and FM layers, resulting in superlattices modulated by the superconducting... / Doutor

Made available in DSpace on 2014-12-02T11:16:42Z (GMT). No. of bitstreams: 0 Previous issue date: 2014-05-29Bitstream added on 2014-12-02T11:20:59Z : No. of bitstreams: 1 000795868.pdf: 4956316 bytes, checksum: b119bd205d170de63c73fc175a957677 (MD5) / O supercondutor (SC) YBa2Cu3O7 (YBCO) foi descoberto em 1986, e desde então tem sido amplamente estudado em suas diversas formas e propriedades. Dentre suas várias características, sua alta temperatura crítica é uma das mais importantes na ciência e na tecnologia. Atualmente existem vários dispositivos baseados no YBCO. As heteroestruturas do tipo SC/FM, onde FM representa uma camada ferromagnética, criadas nas últimas décadas, constituem uma das mais interessantes formas de aplicação deste material. Estas são altamente promissoras para o desenvolvimento de dispositivos spintrônicos que exploram as propriedades dos spins dos elétrons. Neste trabalho o foco foi estudar os efeitos das interações entre a camada de YBCO e a do ferromagneto La2/3Ca1/3MnO3 em super-redes intermediadas por camadas de diferentes espessuras de determinados materiais isolantes. Os materiais isolantes testados foram o SrTiO3 (STO), o CeO3 e o PrBa2Cu3O. As super-redes foram fabricadas pela técnica PLD com parâmetros optimizados. Algumas das características foram determinadas por meio de medidas de DRX, MEV, EDS, de magnetização e de transporte. Estruturalmente, as super-redes apresentaram integridade de composição e cristalinidade com crescimento preferencial na direção c. As imagens de MEV confirmaram o crescimento apitaxial das camadas, e o EDS sua composição. As medidas de magnetização mostraram o grau de qualidade e o comportamento da super-rede que resulta da interação ou desacoplamento eletrônico das camadas SC e FM quando uma camada isolante é inserida entre elas. A mesma análise foi feita nas medidas de transporte. As super-redes (YBCO20nm/ISOdl/LCMO10nm)x20 apresentaram comportamento predominantemente supercondutor. Enquanto as super-redes (YBCO20nm/ISOdl/LCMO20nm)x20 apresentaram oscilações das temperaturas crítica e de Curie assim como magnetização e resistividade não usuais dependendo do isolante e de sua espessura... / The superconducting (SC) YBa2Cu3O7 (YBCO) was discovered in 1986, and since than it has been widely studied in its various forms and properties. Among its many features, its high critical temperature is one of the most important in science and technology fields. Currently there are several devices based on YBCO. The SC/FM heterostructures, where FM is a ferromagnetic layer, created in recent decades, is one of the most interesting ways of applying this material. These are highly promising for the development of spintronic devices exploding the properties of electron spins. In this work the focus was to study the effects of interactions between the YBCO layer and the ferromagnet La2/3Ca1/3MnO3 in superlattices mediated by layers of differente thicknesses of certain insulating materials. The tested insulating material were SrTiO3 (STO), CeO2 and PrBa2Cu3O6. The superlattices were fabricated by PLD technique with optimized parameters. Some of the features were determined by measurements of XRD, SEM, EDS, magnetization and electric transport. Structurally, the superlattices showed integrity of composition and crystallinity with preferencial growth in the c-direction. The SEM images confirmed the epitaxial growth of the layers, and the EDS confirmed the composition. The magnetization measurements showed the quality and behavior of the superlattices which results from the interaction or uncoupling of SC and FM layers when an insulating layer is inserted between them. The same analysis was done in transport measurements. The superlattices (YBCO20mn/ISOdl/LCMO10mn)x20 showed predominantly superconducting behavior. While (YBCO20nm/ISOdl?LCMO20mn)x20 superlattices showed oscillations on critical and Curie temperatures as well as unusual magnetization and resistivity which depended on the insulation and its thickness. The STO acted as an efficient insulator between the SC and FM layers, resulting in superlattices modulated by the superconducting...

Thesis advisor: Kenneth Burch / In quantum materials, quantum mechanical effects survive over a wide range of temperatures and length scales, which leads to fascinating phenomena, such as charge density waves, spin liquids, and magnetism. These remarkable properties originate from a delicate balance among many interacting degrees of freedom. Even small perturbations such as temperature and doping can induce many diverse phases and colossal changes in the material's functionalities. It remains a mystery how to detect these novel phenomena without complicated fabrication and extreme conditions. This thesis contributed to this field by studying collective modes in quantum materials via inelastic light scattering. After fabricating the devices in a glovebox, we directly transfer them to the measurement platform under a high vacuum. Using Raman spectroscopy, a new quasi-particles--axial Higgs mode in charge density wave systems was discovered and further symmetry breaking accompanied with it suggests the charge density wave is unconventional. In addition to symmetry analysis breakthroughs, this thesis also provided direct evidence of the fractional nature as well as the energy and temperature limits of Kitaev interactions in quantum spin liquid candidate systems, which is the building block for topological quantum computers. Not limited to 2D Kitaev materials, the non-Fluery-Loudon single magnon scattering process was detected in a 3D Kitaev system. Other than using Raman to probe the fundamental nature, we also employed it to reveal, for the first time, a clean way to realize modulation doping in 2D materials, where the acceptor carrier density has reached 10$^{14}$ cm$^{-2}$. This method can be applied to dope magnetic materials or twisted heterostructures to find new phases. / Thesis (PhD) — Boston College, 2022. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

The discovery of strongly correlated electronic phases in twisted bilayer graphene has led to an enormous interest in twisted van der Waals (vdW) heterostructures. While twisting vdW layers provides a new control knob and never before seen functionalities, it also leads to large spatial variations in the electronic properties. Scanning probe experiments are therefore necessary to fully understand the properties of twisted vdW heterostructures. In this thesis, we studied twisted bilayer graphene (TBG) with two scanning probe techniques at two twist angle regimes. At small twist angles, our nano-infrared images resolved the spatial variations of the electronic structure occurring within a Moiré unit cell and uncovered a quantum photonic crystal. Meanwhile, with nano-photocurrent experiments, we resolved DC Seebeck coefficient changes occurring in domain walls on nanometer length scales. At larger twist angles, we mapped the twist angle variations naturally occurring in our device with a combination of nano-photocurrent and nano-infrared imaging. Finally, we also investigated different materials for use as nano-optics compatible top gates in future experiments on TBG. Our results demonstrate the power of nano-optics techniques in uncovering the rich, spatially inhomogeneous physics of twisted vdW heterostructures.

The present research work focuses on the fabrication and characterization of epitaxial heterostructures of 0.7 Pb(Mg1/3N2/3)O3 – 0.3 PbTiO3 (PMN-PT) and La0.6Sr0.4MnO3 (LSMO) using multi target pulsed laser ablation technique. Different heterostructures such as bilayered thin films with different individual layer thickness; symmetric and asymmetric superlattices of different periodicities were fabricated. Roles of individual layer thickness, elastic strain and interfaces between PMN-PT and LSMO layers on different physical properties were studied. An attempt has been made to understand the influence of the charge depleted interface states in addition to the probable strain mediated elastic coupling effect on the observed magneto-dielectric response in these engineered heterostructures. Chapter 1 provides a brief introduction to the multiferroic materials, occurrence of magnetoelectric (ME) coupling in them, their possible technological applications and the challenges involved. A short historical account of the multiferroic research is discussed to emphasize the importance of artificial multiferroics, particularly the engineered thin film heterostructures. Finally the specific objectives of the current research are outlined. Chapter 2 deals with the various experimental studies carried out in this research work. It gives the details of the experimental set up and the basic operation principles of various structural and physical characterizations of the materials prepared. A brief explanation of material fabrication, structural, micro structural and physical property measurements is discussed. Chapter 3 addresses the phase formation, structural and microstructural features of the engineered heterostructures fabricated epitaxially on single crystalline LaAlO3 (100) substrates. A thin layer of LaNiO3 used as the bottom electrode material for electrical characterizations was grown on the bare substrate prior to the fabrication of the PMN-PT/LSMO heterostructures. The structural and microstructural features of different individual layers were also studied by fabricating single layer thin films of the materials. The effects of individual layer thicknesses on the surface roughness, grain size and lattice strain of the heterostructures are discussed. Chapter 4 deals with the ferroelectric studies of the PMN-PT/LSMO epitaxial heterostructures. Polarization hysteresis (P-E), capacitance – voltage (C-V) and pulsed polarization (PUND) measurements were carried out as functions of applied voltage, frequency and delay time to characterize the ferroelectric properties of the heterostructures. All the bilayered heterostructures exhibited robust ferroelectric response and contribution of non – remnant components to their polarization behaviour were observed from the P-E studies. The symmetric superlattices did not exhibit any ferroelectricity due to high leakage current conduction. After optimizing the LSMO and PMN-PT layer thicknesses ferroelectricity was observed in the asymmetric superlattices accompanied by substantial reduction in the leakage current conduction. The P-E loops were found to be asymmetrically shifted along the electric field axis in all the superlattices indicating the presence of dielectric passive layers and strong depolarizing fields at the interfaces between PMN-PT and LSMO. Chapter 5 deals with the ferromagnetic studies of the PMN-PT/LSMO heterostructures. All the heterostructures exhibited ferromagnetic behaviour in the temperature range of 10 K – 300 K with an in plane magnetic easy axis ([100]) compared to the out of plane ([001]) direction. The magnetization behaviour of the bilayers and superlattices as a function of magnetic field strength, temperature and different individual layer thickness of PMN-PT and LSMO are discussed in terms of the oxygen deficiency, magnetic dead layers and lattice strain effects in these engineered epitaxial heterostructures. Chapter 6 addresses the magneto-dielectric response, dielectric properties and ac conduction properties of the engineered biferroic heterostructures. In order to investigate the manifestation of strain mediated ME coupling in these heterostructures their dielectric response as a function of ac electric signal frequency have been studied under different static magnetic fields over a wide range of temperatures. The appearance of magneto-capacitance and its dependence on magnetic field strength and temperature along with the magnetoresistive characteristics of the heterostructures suggested that the charge depleted interfaces between PMN-PT and LSMO can have an effect on the observed dielectric response in addition to the probable strain mediated ME coupling. Dielectric characterization of the heterostructures performed over a wide range of temperature indicated a Maxwell-Wagner type relaxation mechanism. The manifestation of Maxwell-Wagner effect and the very low activation energy of ac conductivity obtained from the ac conduction studies revealed the strong influence of the charge depleted interfaces between PMN-PT and LSMO on the dielectric properties of the heterostructures. Chapter 7 deals with the dc leakage current conduction characteristics of the heterostructures. The leakage current characterization was performed over a wide range of temperature and analyzed in the framework of different models to investigate the leakage mechanism. All the heterostructures were found to obey the power law I∝Vα over the entire range of temperature with different values of α at different applied voltages. The bilayered heterostructures exhibited ohmic conduction in the lower electric field region and space charge limited conduction was observed at higher electric fields. On the other hand the low field dc conduction behaviour of the superlattices could not be attributed unambiguously to a single mechanism. Depending on the superlattice periodicity the low field conduction behaviour was dominated by either Poole-Frenkel (PF) emission or a combined contribution from the PF effect and ohmic conduction. At higher electric fields all the superlattices exhibited space charge limited conduction. Chapter 8 gives the summary and conclusions of the present study and also discusses about the future work that could give more insight into the understanding of the engineered epitaxial biferroic heterostructures.

Thermal transport across three-dimensional Lennard-Jones superlattices, two-dimensional heterostructures of graphene and hexagonal boron nitride (hBN), and in C60 molecular crystals is studied atomistically. The first two systems are studied as finite junctions placed between bulk leads, while the molecular crystal is studied as a bulk. Two computational methods are used: molecular dynamics (MD) simulations and harmonic lattice dynamics calculations in conjunction with the scattering boundary method (SBM). In Lennard-Jones superlattice junctions with a superlattice period of four atomic monolayers at low temperatures, those with mass-mismatched leads have a greater thermal conductance than those with mass-matched leads. We attribute this lead effect to interference between and the ballistic transport of emergent junction vibrational modes. The lead effect diminishes when the temperature is increased, when the superlattice period is increased, and when interfacial disorder is introduced, and is reversed in the harmonic limit. In graphene-hBN heterostructure junctions, the thermal conductance is dominated by acoustic phonon modes near the Brillouin zone center that have high group velocity, population, and transmission coefficient. Out-of-plane modes make their most significant contributions at low frequencies, whereas in-plane modes contribute across the frequency spectrum. Finite-length superlattice junctions between graphene and hBN leads have a lower thermal conductance than comparable junctions between two graphene leads due to lack of transmission in the hBN phonon band gap. The thermal conductances of bilayer systems differ by less that 10% from their single-layer counterparts on a per area basis, in contrast to the strong thermal conductivity reduction when moving to from single- to multi-layer graphene. We model C60 molecules using the polymer consistent force-field and compute the single molecule vibrational spectrum and heat capacity. In the face-center cubic C60 molecular crystal at a temperature of 300 K, we find three frequency peaks in the center-of-mass translations at 20, 30 and 38 cm􀀀1, agreeing with the average frequencies of the three acoustic branches of the frozen phonon model of the same system and suggesting that a phonon description of center-of-mass translations. We use both direct method and Green- Kubo MD simulations to predict the thermal conductivity of the molecular crystals at a temperature of 300 K. We find that the thermal conductivity of the molecular crystal is 20 to 50% lower than that of a reduced order model where only molecular center-ofmass translations are present, suggesting that molecular vibrations and rotations act as significant scattering sources for the center-of-mass phonons.

Ha Kwong-leung = 不同的生長處理對II-VI/III-V外延層的表面、結構和光學特性的影響 / 夏廣良. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2001. / Includes bibliographical references (leaves 76-79). / Text in English; abstracts in English and Chinese. / Ha Kwong-leung = Bu tong de sheng chang chu li dui II-VI/III-V wai yan ceng de biao mian, jie gou he guang xue te xing de ying xiang / Xia Guangliang. / Acknowledgments --- p.i / Abstract --- p.ii / Chapter Chapter 1 --- Introduction / Chapter 1.1 --- Brief introduction --- p.1 / Chapter 1.2 --- Background of the epitaxial growth --- p.4 / Chapter 1.3 --- Our work --- p.6 / Chapter Chapter 2 --- Experimental procedures / Chapter 2.1 --- Substrate preparation --- p.7 / Chapter 2.2 --- Metal-organic sources --- p.9 / Chapter 2.3 --- Growth conditions --- p.10 / Chapter Chapter 3 --- Characterization methods / Chapter 3.1 --- Surface morphology --- p.13 / Chapter 3.2 --- X-ray diffraction --- p.14 / Chapter 3.3 --- Optical properties / Chapter 3.3.1 --- Photoluminescence --- p.17 / Chapter 3.3.2 --- General properties of PL spectrum of ZnSe/GaAs --- p.18 / Chapter 3.4 --- Other techniques --- p.21 / Chapter Chapter 4 --- Influence on the properties of epilayers by the thickness of the low-temperature buffer layer --- p.22 / Chapter 4.1 --- Effects on the surface morphology --- p.23 / Chapter 4.2 --- Effects on the structural properties --- p.31 / Chapter 4.3 --- Effects on the optical properties --- p.35 / Chapter Chapter 5 --- Influence of thickness of epilayers on their properties --- p.39 / Chapter 5.1 --- Effects on the surface morphology --- p.40 / Chapter 5.2 --- Effects on the structural properties --- p.45 / Chapter 5.3 --- Effects on the optical properties --- p.50 / Chapter Chapter 6 --- Influence on the epilayers by passivating with different MO sources and different durations of interruption --- p.53 / Part I: Passivation by different MO sources / Chapter 6.1 --- Effects on the surface morphology --- p.55 / Chapter 6.2 --- Effects on the structural properties --- p.58 / Chapter 6.3 --- Effects on the optical properties --- p.61 / Part II: Different durations of interruption / Chapter 6.4 --- Effects on the surface morphology --- p.63 / Chapter 6.5 --- Effects on the structural properties --- p.66 / Chapter 6.6 --- Effects on the optical properties --- p.70 / Summary --- p.73 / Conclusions --- p.74 / References --- p.76