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TIME RESOLVED OPTICAL STUDIES OF SPIN AND QUASIPARTICLE DYNAMICS IN FERROMAGNETIC THIN FILMS AND SUPERCONDUCTORS
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This thesis presents the studies of spin and quasiparticle dynamics in ferromagnetic thin films and iron based superconductors by ferromagnetic resonance (FMR) and time-resolved pump-probe optical techniques. First, the FMR spectroscopies were applied to study the spin dynamics both in frequency and time domains for the epitaxially grown Fe/GaAs thin films and FeCoB/Cr/FeCoB multilayer structures. In the single layer Fe/GaAs thin films, magnetization precessions were studied to characterize the magnetic dynamical parameters. Our results show that the magnetic crystalline anisotropy is dominative and the magnetic damping is strongly dependent on the in-plane magnetic field orientations. In FeCoB/Cr/FeCoB multilayer films, both the acoustic and the optical spin wave modes were identified in the FMR spectra. We reveal that the adjacent magnetic layers in the trilayer structures are antiferromagnetic coupled with an effective interlayer coupling constant Jeff. The magnetic dynamical parameters can be accurately optimized by the interlayer coupling constant Jeff. Second, we employed the time-resolved pump-probe magneto-optical Kerr effect (MOKE) spectroscopy to study the spin dynamics in the Fe/GaAs thin films at picosecond time scale. The time-resolved MOKE results were combined with static magnetic hysteresis loops at various time delays to understand the ultrafast demagnetization dynamics. The ultrafast demagnetization process is faster than the time required for the electron-phonon equilibration and therefore the spin-orbital coupling has to be included with the conventional electron thermalization model to understand our results. Moreover, we show that the ultrafast magnetization excitation and reorientation can be coherently controlled by varying the polarization of the pump beam. The magnetization excitation and reorientation are attributed to the laser induced effective magnetic field in the sample. Third, the quasiparticle relaxation dynamics were studied in electron-doped BaFe1.9Ni0.1As2 and BaFe1.85Co0.15As2 superconductors by time-resolved pump-probe optical spectroscopy. Two distinct relaxation components observed in the transient reflectivity spectra are attributed to the quasiparticle recombination in the superconducting state and quasiparticle relaxations from the higher excited band due to the multiband excitation. The results show the multi-gap characteristic in BaFe1.9Ni0.1As2 and BaFe1.85Co0.15As2 superconductors. Moreover, the estimated electron-phonon coupling constant and the Coulomb pseudopotential indicate that the electron-phonon interaction is not large enough to induce the SC transition. A spin mediated pairing mechanism is necessary to understand the SC phase transition in the iron based superconductors.
THEORY OF BCS-BEC CROSSOVER IN ULTRACOLD ATOMIC GASES
Year of Dissertation:
In ultracold atomic fermions, the sign and the magnitude of pairing interactions can be controlled by using the magnetically-tuned Feshbach resonances to achieve a continuos transition between Cooper pairs of dilute fermi gas to BEC of diatomic molecules, which is known as the "BCS-BEC crossover". At present, although several models have been proposed, there is still no exact analytical solution of the many-body problem of BCS-BEC crossover region. The standard BCS mean field theory of superconductivity was used [1-3] to describe the whole crossover resulting a useful approximation. In our studies, we investigated solvable models for the best variational analytical solution for BCS-BEC crossover at T= 0.
Planar Waveguide Structures for Post-EDFA Broadband Near Infrared Optical Amplifiers
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This thesis reports on optical gain of up to 5.7 dB from a planar waveguide with core made of tetravalent chromium-doped calcium germanate single crystal.
Photonic Structures Based on Hybrid Nanocomposites
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In this thesis, photonic structures embedded with two types of nanomaterials, (i) quantum dots and (ii) metal nanoparticles are studied. Both of these exhibit optical and electronic properties different from their bulk counterpart due to their nanoscale physical structure. By integrating these nanomaterials into photonic structures, in which the electromagnetic field can be confined and controlled via modification of geometry and composition, we can enhance their linear and nonlinear optical properties to realize functional photonic structures. Before embedding quantum dots into photonic structures, we study the effect of various host matrices and fabrication techniques on the optical properties of the colloidal quantum dots. The two host matrices of interest are SU8 and PMMA. It is shown that the emission properties of the quantum dots are significantly altered in these host matrices (especially SU8) and this is attributed to a high rate of nonradiative quenching of the dots. Furthermore, the effects of fabrication techniques on the optical properties of quantum dots are also investigated. Finally a microdisk resonator embedded with quantum dots is fabricated using soft lithography and luminescence from the quantum dots in the disk is observed. We investigate the absorption and effective index properties of silver nanocomposite films. It is shown that by varying the fill factor of the metal nanoparticles and fabrication parameters such as heating time, we can manipulate the optical properties of the metal nanocomposite. Optimizing these parameters, a silver nanocomposite film with a 7% fill factor is prepared. A one-dimensional photonic crystal consisting of alternating layers of the silver nanocomposite and a polymer (Polymethyl methacrylate) is fabricated using spin coating and its linear and nonlinear optical properties are investigated. Using reflectivity measurements we demonstrate that the one-dimensional silver-nanocomposite-dielectric photonic crystal exhibits a 200% enhancement of the reflection band which is attributed to the interplay between the plasmon resonance of the silver nanoparticles and the Bloch modes of the photonic crystal. Nonlinear optical studies on this one-dimensional silver-nanocomposite-dielectric structure using z-scan measurements are conducted. These measurements indicate a three-fold enhancement in the nonlinear absorption coefficient when compared to a single film of comparable metal composite thickness.
TRANSPORT AND OPTICAL PROPERTIES OF LOW-DIMENSIONAL COMPLEX SYSTEMS
Year of Dissertation:
Over the last five years of my research work, I, my research was mainly concerned with certain crucial tunneling, transport and optical properties of novel low-dimensional graphitic and carbon-based materials as well as topological insulators. Both single-electron and many-body problems were addressed. We investigated the Dirac electrons transmission through a potential barrier in the presence of circularly polarized light. An anomalous photon-assisted enhanced transmission is predicted and explained in a comparison with the well-known Klein paradox. It is demonstrated that the perfect transmission for nearly-head-on collision in an infinite graphene is suppressed in gapped dressed states of electrons, which is further accompanied by shift of peaks as a function of the incident angle away from the head-on collision. We calculate the energy bands for graphene monolayers when electrons move through a periodic electrostatic potential in the presence of a uniform perpendicular magnetic field. We clearly demonstrate the quantum fractal nature of the energy bands at reasonably low magnetic fields. We present results for the energy bands as functions of both wave number and magnetic flux through the unit cells of the resulting moir´e superlattice. This feature is also observed at extremely high magnetic fields. We have discovered a novel feature in the plasmon excitations for a pair of Coulomb-coupled non-concentric spherical two-dimensional electron gases (S2DEGs). Our results show that the plasmon excitations for such pairs depend on the orientation with respect to the external electromagnetic probe field. The origin of this anisotropy of the inter-sphere Coulomb interaction is due to the directional asymmetry of the electrostatic coupling of electrons in excited states which depend on both the angular momentum quantum number L and its projection M on the axis of quantization taken as the probe E-field direction. Such an effect from the plasmon spatial correlation is expected to be experimentally observable by employing circularly-polarized light or a helical light beam for incidence. The S2DEG serves as a simple model for fullerenes as well as metallic dimers, when the energy bands are far apart. Magnetoplasmons in gapped graphene have been investigated and the exchange energy dependence on magnetic field is presented.
Quantum Physics of Molecular Magnets
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In this thesis we focus on various aspects of quantum physics in molecular magnets, in particular, in Mn12-acetate. This thesis is divided into three parts. In the first part, we present a review on molecular magnets. Since Mn12-acetate has a large spin (equal to 10), the theory of tunneling of a large spin is discussed as well as the early experiments that were performed two decades ago and which has shown spin tunneling, in particular, the ones that were performed on &gamma -Fe2-O3- and on antiferromagnetic ferritin. Then, the first experiments that presented evidence on spin tunneling in Mn12--acetate are outlined in detail. Magnetic hysteresis curves are shown and Landau-Zener effect in molecular magnets is discussed. Quantum classical crossover between thermally assisted and pure quantum tunneling regimes is described. Finally, magnetic avalanches are introduced: they are another feature of the magnetization curve in Mn12-Acetate where there is a sudden reversal in the magnetization. We exploit the first two experiments performed to elucidate the nature of magnetic avalanches in Mn12--acetate and the theory developed as a result of these experiments. In the second part of this thesis, we focus on three of my publications on quantum magneto-mechanical effects. First, a recent experiment on Einstein-de Haas effect in a NiFe film deposited on a microcantilever is discussed. The cantilever was placed inside a coil that generated an ac magnetic field. Oscillation of the cantilever was measured by a fiber-optic interferometer positioned above the tip of the cantilever. When the frequency of the ac field matched the resonance frequency of the cantilever the amplitude of the oscillations was about 3 nm. The data were analyzed within a model that replaced the mechanical torque due to change in the magnetization with the effect of the periodic force acting on the fictitious point mass at the free end of the cantilever so this model did not account for the microscopic dynamics of the Einstein-de Haas effect. This motivated us to develop a more rigorous theoretical framework for the description of the dynamics of the Einstein-de Haas effect that we applied to the problem of the magnetic cantilever. We then study the quantum dynamics of a magnetic molecule deposited on a microcantilever. Amplitude and frequencies of the coupled magneto-mechanical oscillations have been computed. We show that oscillations of the spin and the cantilever occur independently at frequencies &Delta/&hbar and &omegan respectively, unless these two frequencies come very close to each other. The results show that the splitting &delta has no free parameters and that for a given resonance, &Delta=&hbar&omegan the relative splitting &delta depends only on the position of the molecule on the cantilever. We then show that existing experimental techniques permit observation of the driven coupled oscillations of the spin and the cantilever, as well as of the splitting of the mechanical modes of the cantilever caused by spin tunneling. Finally, the dynamics of a magnetic molecule bridged between two conducting leads is investigated. We start by reviewing various experiments performed when there is a weak coupling between the molecule and the leads and when there is a strong coupling which results in the Kondo effect. Experimental efforts were mainly motivated to measure the electronic current through a single molecule. We study the dynamics of the total angular momentum that couples spin tunneling to the mechanical rotations. We show that the Landau-Zener spin transition produced by the time-dependent magnetic field generates a unique pattern of mechanical oscillations that can be detected by measuring the electronic tunneling current through the molecule. In the last and final part, we present our numerical work to describe quantum magnetic deflagration in Mn12-acetate. This part is related to magnetic deflagration as discussed in part I of this thesis. The focus is on the quantum features of magnetic deflagration which are exhibited by the maxima in the speed of deflagration front as a function of the applied magnetic field. We review recent work on the effect of the dipolar field in forming self-organized fronts of spin tunneling, and present our enhanced computational work on the calculation of the relaxation rate. Previously, spin relaxation rates were calculated using a simple Arrhenius exponent. In this thesis we calculate the relaxation rate as a function of both the external field and temperature using the density matrix formalism and use them to study the effect of the transverse field on the front speed of deflagration.
Solid State NMR Studies of Energy Conversion and Storage Materials
Sohan Roshel De Silva Jankuru Hennadige
Year of Dissertation:
NMR (Nuclear magnetic resonance) spectroscopy is utilized to study energy conversion and storage materials. Different types of NMR techniques including Magic Angle Spinning, Cross-polarization and relaxation measurement experiments were employed. Four different projects are discussed in this dissertation. First, three types of CFx battery materials were investigated. Electrochemical studies have demonstrated different electrochemical performances by one type, delivering superior performance over the other two. 13C and 19F MAS NMR techniques are employed to identify the atomic/molecular structural factors that might account for differences in electrochemical performance among different types. Next as the second project, layered polymer dielectrics were investigated by NMR. Previous studies have shown that thin film capacitors are improved by using alternate layers of two polymers with complementary properties: one with a high breakdown strength and one with high dielectric constant as opposed to monolithic layers. 13C to 1H cross-polarization techniques were used to investigate any inter-layer properties that may cause the increase in the dielectric strength. The third project was to study two types of thermoelectric materials. These samples were made of heavily doped phosphorous and boron in silicon by two different methods: ball-milled and annealed. These samples were investigated by NMR to determine the degree of disorder and obtain insight into the doping efficiency. The last ongoing project is on a lithium-ion battery system. The nature of passivating layers or the solid electrolyte interphase (SEI) formed on the electrodes surface is important because of the direct correlation between the SEI and the battery life time/durability. Multinuclear (7Li, 19F, 31P) techniques are employed to identify the composition of the SEI formation of both positive and negative electrodes.
STATISTICAL MECHANICS OF JAMMED PACKINGS OF SPHERES
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The problem of finding the most efficient way to pack spheres has an illustrious history, dating back to the crystalline arrays conjectured by Kepler and the random geometries explored by Bernal in the 60's. There are presently numerous experiments showing that randomly packing spheres of equal size into a container consistently results in a static configuration with a density of 0.64. The ubiquity of random close packing (RCP) rather than the equilibrium crystalline array at 0.74 begs a new statistical framework. Here we introduce a general volume ensemble statistical approach for jammed packings of spheres. This approach provides a thermodynamic definition of RCP: RCP can be interpreted as a manifestation of a thermodynamic singularity, which defines it as the ``freezing point'' in a first-order phase transition between ordered and disordered packing phases. We generalize the theory to jammed packings of high dimensional and different size spheres. The asymptotic high-dimensional scaling of the RCP density is consistent with that of other approaches, such as replica theory and density functional theory. The theory predicts the density of random close packing and random loose packing (RLP) of polydisperse systems for a given distribution of sphere size. The present mean-field approach may help to treat packing problems of non-spherical particles, and could serve as a starting point to understand the higher-order correlations present in jammed packings.
Geometry and Statistics of Jammed Granular Matter
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We use simulations of soft bidisperse disks to determine the properties of jammed packings and investigate the statistical mechanics of these systems. We have created a novel method for the classification of structural subunits of a packing and to cal- culate relevant physical quantities. The classification scheme is based on a 20 type decomposition of the Delaunay triangles extracted from the centers of the particles. Subunit frequencies are determined from geometrical properties and used to calculate the important macroscopic system quantities co-ordination number, packing fraction, and pressure. These relationships suggest that microscopic particle geometry plays an important role in observed macroscopic behavior. In addition, we investigate the contact network evolution during elastic perturbation. We predict the fraction of time a contact will be broken from the the inter-particle potential before perturbation. We explore the energy regions, below particle rearrangement, where our prediction is valid and discuss a physical mechanism for this behavior based on the exchange of potential and kinetic energy between particles.
NMR Studies of Ionic Liquids and Polymer Membrane Electrolytes for Batteries and Fuel Cells
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In this thesis, NMR (Nuclear Magnetic Resonance) spectroscopic techniques are used to study ionic liquids (ILs) and polymer membranes for advanced Li-ion batteries and fuel cells. Besides structural measurements, dynamics and motional properties were also measured using pulsed field gradient spin echo and relaxation experiments. Five projects are described in this thesis. The first two projects involve 1H, 19F, 7Li and 13C NMR transport studies of two different ILs. The third project is a study of Nafion ionomer aggregations where diffusion and viscosity measurements are made. The fourth project involves the structural study of lithium triflate:PEO3 polymer membranes via 7Li static NMR. The fifth and final project described in the thesis involves 27Al NMR studies of a chloroaluminate (AlCl3) IL and the assessment of the tetrahedral Al species content.