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Title: Effects Of Stuffing On The Atomic And Electronic Structure Of The Pyrochlore Yb2ti2o7.
Creator: Ghosh, Soham S., Manousakis, Efstratios
Abstract/Description: There are reasons to believe that the ground state of the magnetic rare-earth pyrochlore Yb(2)Ti2O(7) is on the boundary between competing ground states. We have carried out ab initio density functional calculations to determine the most stable chemical formula as a function of the oxygen chemical potential and the likely location of the oxygen atoms in the unit cell of the "stuffed" system. We find that it is energetically favorable in the stuffed crystal (with an Yb replacement on a Ti site...
Show moreThere are reasons to believe that the ground state of the magnetic rare-earth pyrochlore Yb(2)Ti2O(7) is on the boundary between competing ground states. We have carried out ab initio density functional calculations to determine the most stable chemical formula as a function of the oxygen chemical potential and the likely location of the oxygen atoms in the unit cell of the "stuffed" system. We find that it is energetically favorable in the stuffed crystal (with an Yb replacement on a Ti site) to contain oxygen vacancies which dope the Yb 4f orbitals and qualitatively change the electronic properties of the system. In addition, with the inclusion of the contribution of spin-orbit coupling (SOC) on top of the GGA + U approach, we investigated the electronic structure and the magnetic moments of the most stable stuffed system. In our determined stuffed structure the valence bands as compared to those of the pure system are pushed down and a change in hybridization between the O 2p orbitals and the metal ion states is found. Our first-principle findings should form a foundation for effective models describing the low-temperature properties of this material whose true ground state remains controversial.
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Date Issued: 2018-06-11
Identifier: FSU_libsubv1_wos_000434762500006, 10.1103/PhysRevB.97.245117
Format: Citation

Title: Structure And Ferromagnetic Instability Of The Oxygen-deficient Srtio3 Surface.
Creator: Ghosh, Soham S., Manousakis, Efstratios
Abstract/Description: SrTiO3 (STO) is the substrate of choice to grow oxide thin films and oxide heterojunctions, which can form quasi-two-dimensional electronic phases that exhibit a wealth of phenomena, and thus a workhorse in the emerging field of metal-oxide electronics. Hence, it is of great importance to know the exact character of the STO surface itself under various oxygen environments. Using density functional theory within the spin generalized gradient approximation we have investigated the structural,...
Show moreSrTiO3 (STO) is the substrate of choice to grow oxide thin films and oxide heterojunctions, which can form quasi-two-dimensional electronic phases that exhibit a wealth of phenomena, and thus a workhorse in the emerging field of metal-oxide electronics. Hence, it is of great importance to know the exact character of the STO surface itself under various oxygen environments. Using density functional theory within the spin generalized gradient approximation we have investigated the structural, electronic, and magnetic properties of the oxygen-deficient STO surface. We find that the surface oxygen vacancies order in periodic arrays giving rise to surface magnetic moments and a quasi-two-dimensional electron gas in the occupied Ti 3d orbitals. The surface confinement, the oxygen-vacancy ordering, and the octahedra distortions give rise to spin-polarized t(2g) dispersive subbands; their energy split near the Brillouin zone center acts as an effective Zeeman term, which, when we turn on a Rashba interaction, produces bands with momentum-spin correlations similar to those recently discovered on oxygen-deficient STO surface.
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Date Issued: 2016-08-24
Identifier: FSU_libsubv1_wos_000381890100001, 10.1103/PhysRevB.94.085141
Format: Citation

Title: Optoelectronic switch having a photovoltaic response and associated method of use.
Creator: Balicas, Luis, Pradhan, Nihar R., Manousakis, Efstratios
Abstract/Description: A light-induced diode-like response in multi-layered MoSe₂field-effect transistors resulting from a difference in the size of the Schottky barriers between drain and source contacts, wherein each barrier can be modeled as a Schottky diode but with opposite senses of current rectification, wherein the diode response results from the light induced promotion of photo-generated carriers across the smaller barrier. The sense of current rectification can be controlled by the gate voltage which is...
Show moreA light-induced diode-like response in multi-layered MoSe₂field-effect transistors resulting from a difference in the size of the Schottky barriers between drain and source contacts, wherein each barrier can be modeled as a Schottky diode but with opposite senses of current rectification, wherein the diode response results from the light induced promotion of photo-generated carriers across the smaller barrier. The sense of current rectification can be controlled by the gate voltage which is able to modulate the relative amplitude between both barriers, yielding a photovoltaic response.
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Date Issued: 2016-09-27
Identifier: FSU_uspto_9455365, 9455365, 3225697, 15/054829, 6c9256d97d167832cbab694dc904bd27
Format: Citation

Title: SO(10) Supersymmetric Grand Unified Theories: from Cosmology to Colliders.
Creator: Summy, Heaya Ann, Baer, Howard, Sussman, Mark, Reina, Laura, Wahl, Horst, Manousakis, Efstratios, Department of Physics, Florida State University
Abstract/Description: Simple SUSY GUT models based on the gauge group SO(10) require t-b-t Yukawa coupling unification, in addition to gauge coupling and matter unification. The Yukawa coupling unification places a severe constraint on the expected spectrum of superpartners, with scalar masses ~ 10 TeV while gaugino masses are quite light. For Yukawa-unified models with μ > 0, the spectrum is characterized by three mass scales: i). first and second generation scalars in the multi-TeV range, ii). third generation...
Show moreSimple SUSY GUT models based on the gauge group SO(10) require t-b-t Yukawa coupling unification, in addition to gauge coupling and matter unification. The Yukawa coupling unification places a severe constraint on the expected spectrum of superpartners, with scalar masses ~ 10 TeV while gaugino masses are quite light. For Yukawa-unified models with μ > 0, the spectrum is characterized by three mass scales: i). first and second generation scalars in the multi-TeV range, ii). third generation scalars, μ and mA in the few-TeV range and iii). gluinos in the ~ 350−500 GeV range with chargino masses around 100−160 GeV. In such a scenario, gluino pair production should occur at large rates at the CERN LHC, followed by gluino three-body decays into neutralinos or charginos. Discovery of Yukawa-unified SUSY at the LHC should hence be possible with only 1 fb−1 of integrated luminosity, by tagging multi-jet events with 2–3 isolated leptons, without relying on missing ET . A characteristic dilepton mass edge should easily be apparent above Standard Model background. Combining dileptons with b-jets, along with the gluino pair production cross section information, should allow for gluino and neutralino mass reconstruction. A secondary corroborative signal should be visible at higher integrated luminosity in the X1±1X20 → 3l channel, and should exhibit the same dilepton mass edge as in the gluino cascade decay signal. A problem generic to all supergravity models comes from overproduction of gravitinos in the early universe: if gravitinos are unstable, then their late decays may destroy the predictions of Big Bang nucleosynthesis. We also present a Yukawa-unified SO(10) SUSY GUT scenario which avoids the gravitino problem, gives rise to the correct matter-antimatter asymmetry via non-thermal leptogenesis, and is consistent with the WMAP-measured abundance of cold dark matter due to the presence of an axino LSP. To maintain a consistent cosmology for Yukawa-unified SUSY models, we require a re-heat temperature TR ~ 106−107 GeV, an axino mass around 0.1−10 MeV, and a Peccei-Quinn breaking scale fa ~ 1012 GeV.
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Date Issued: 2008
Identifier: FSU_migr_etd-0405
Format: Thesis

Title: Measurements of the Differential Cross Sections for the Inclusive Production of a Photon and Heavy Flavor Jet.
Creator: Duggan, Daniel J., Wahl, Horst, Schatschneider, Chris, Adams, Todd, III, Joseph Owens, Manousakis, Efstratios, Department of Physics, Florida State University
Abstract/Description: This thesis presents the first measurement of the differential production cross section of a heavy flavor (bottom or charm) jet and direct photon at the Fermilab Tevatron. These measurements were performed using data recorded with the D0 detector from proton-antiproton collisions at a center of mass energy of sqrt(s) = 1.96 TeV. These results probe a kinematic range for the photon transverse momentum of 30 to 150 GeV and photon rapidity |y|
Date Issued: 2009
Identifier: FSU_migr_etd-0649
Format: Thesis

Title: An Analysis of Two Dimensional Materials: Monolayer and Bulk.
Creator: Murray, Andy, Mendoza-Cortes, Jose L., Manousakis, Efstratios, Huang, Chen, Florida State University, Graduate School, Program in Materials Science
Abstract/Description: Two Dimensional Materials has been the focus of much research in the past decade. We review 145 stable two dimensional materials in both bulk and monolayer. We compare their final electronic properties and discuss the results. Specifically, we discuss notable materials that have transitions between bulk and monolayer. Additionally, we use both the bulk and monolayer data to search for structural trends that may be corralated with the electronic properties using machine learning techniques. We...
Show moreTwo Dimensional Materials has been the focus of much research in the past decade. We review 145 stable two dimensional materials in both bulk and monolayer. We compare their final electronic properties and discuss the results. Specifically, we discuss notable materials that have transitions between bulk and monolayer. Additionally, we use both the bulk and monolayer data to search for structural trends that may be corralated with the electronic properties using machine learning techniques. We find that our machine was able to produce results that predict the basic electronic properties with approximately 65% accuracy.
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Date Issued: 2018
Identifier: 2018_Su_Murray_fsu_0071N_14806
Format: Thesis

Title: Coulomb Liquids: from Electron to Biology Systems.
Creator: Pramudya, Yohanes H. R., Dobrosavljevic, Vladimir, Dalal, Naresh, Manousakis, Efstratios, Berg, Bernd A., Engel, Lloyd, Department of Physics, Florida State University
Abstract/Description: We study the unusual transport behavior driven mostly by the correlations between the charge carriers in lattice and continuum systems. The strongly correlated systems are studied in the classical limit where the Coulomb interaction dominates the systems and brings the systems into the nearly frozen state that we call Coulomb Liquids. Theoretical description of the materials with the strongly interacting electron systems in the lattice or solid state physics and ions in biological systems or...
Show moreWe study the unusual transport behavior driven mostly by the correlations between the charge carriers in lattice and continuum systems. The strongly correlated systems are studied in the classical limit where the Coulomb interaction dominates the systems and brings the systems into the nearly frozen state that we call Coulomb Liquids. Theoretical description of the materials with the strongly interacting electron systems in the lattice or solid state physics and ions in biological systems or solutions are constantly challenging problems in condensed matter physics and biophysics. First we investigate the non-activated behavior of transport that observed near Wigner crystallization in two dimensions electron gas (2DEG) that is observed experimentally. The effect of Coulomb interaction in the system is studied with Monte Carlo (MC) simulation and extended dynamical mean field theory (EDMFT) in hypercubic lattice and pseudogap phase is found as a robust feature driven by long-range interaction. In the second part of the thesis we study the effect of geometrical frustration in pyrochlore lattice which competes with the interaction-driven frustration. The MC and EDMFT methods give pseudo-gap phase as found in cubic lattice that has no geometrical frustration and agree with the unusual transport property of ancient magnet/magnetite above the Verwey transition. The third part discusses the effect of strongly-correlated liquid (SCL) model by Shklovskii and the mean-field approach of Poisson-Boltzmann equation (PBE) describing the screening of macro-ions in solution or biological system. We compare those approximations with MC simulation and explain which approximation is good at different limit of temperature or ion density. And the last part describes the visualization for different type of interactions and the effect of filling, disordered, etc. from the study of Coulomb liquids.
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Date Issued: 2013
Identifier: FSU_migr_etd-8715
Format: Thesis

Title: Helium Atom Scattering Study of the Surface Structure and Dynamics of KCl(001) and Potassium Tantalate (KTaO₃) Doped with Lithium and Niobium.
Creator: Fatema, Rifat, Van Winkle, David H., Safron, Sanford A., Flaherty, Francis A., Manousakis, Efstratios, Rogachev, Grigory, Department of Physics, Florida State University
Abstract/Description: The structure and dynamics of the (001) surface of potassium tantalate (KTaO3), doped with lithium (KTL) and niobium (KTN) have been investigated using high resolution helium atom diffraction and single-phonon creation and annihilation, inelastic scattering experiments. This work is the first to investigate the surface structure and dynamics of KTL samples. These experiments are an extension of earlier work done with pure KTaO3 and KTN crystals. As in the previous experiments broad half-order...
Show moreThe structure and dynamics of the (001) surface of potassium tantalate (KTaO3), doped with lithium (KTL) and niobium (KTN) have been investigated using high resolution helium atom diffraction and single-phonon creation and annihilation, inelastic scattering experiments. This work is the first to investigate the surface structure and dynamics of KTL samples. These experiments are an extension of earlier work done with pure KTaO3 and KTN crystals. As in the previous experiments broad half-order diffraction peaks were observed in the <100> azimuth, which may be related to the formation of polar nanoregions that have been reported for bulk KTaO3. However, the condition for the formation of these regions differ somewhat from those reported previously. Drift spectra for these samples, which reveal the step-height distribution of terraces at the surface, show the expected oscillatory behavior for lightly doped KTaO3. Significantly, they also confirm the unusual behavior reported in earlier experiments for the 30% KTN samples. Surface dispersion curves in the two high symmetry directions, namely, <100> and <110>, have been obtained for both KTL and KTN samples. This work compares the above phenomena observed in KTL and KTN to similar phenomena previously observed in pure KTaO3 and KTN. Surface dynamics of KCl(001) is also investigated in this work and compared with reported model calculations. The agreement is generally good for the low-energy Rayleigh mode and for a crossing mode. However, a high energy dispersionless mode, found in the <100> high symmetry direction lies in the region of a shear-horizontal Lucas mode, that cannot in theory be observed by helium atom scattering in our experimental configuration. This disagreement between theory and experiment suggests that further theoretical work is necessary to understand the dynamics of simple ionic-insulator systems like KCl.
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Date Issued: 2009
Identifier: FSU_migr_etd-4495
Format: Thesis

Title: The Dynamics of DNA Electrophoresis in Lyotropic Polymer Liquid Crystals.
Creator: You, Seungyong, Van Winkle, David H., Shanbhag, Sachin, Brooks, James, Manousakis, Efstratios, Wiedenhover, Ingo, Department of Physics, Florida State University
Abstract/Description: Electrophoresis is one of the most powerful tools used to separate biological molecules by size in a supporting medium under the influence of an electric field. Pluronic F127 gel, introduced as a new sieving medium for electrophoresis in the late 1990s, has potential promise, but the transport and separation of DNA molecules in Pluronic gel are not fully understood among scientists. This dissertation research conducts three experiments to investigate the dynamics of DNA electrophoresis in...
Show moreElectrophoresis is one of the most powerful tools used to separate biological molecules by size in a supporting medium under the influence of an electric field. Pluronic F127 gel, introduced as a new sieving medium for electrophoresis in the late 1990s, has potential promise, but the transport and separation of DNA molecules in Pluronic gel are not fully understood among scientists. This dissertation research conducts three experiments to investigate the dynamics of DNA electrophoresis in Pluronic gel. First, a direct observation study examines how DNA molecules move through the Pluronic gel on a microscopic scale using fluorescence microscopy. Evidence is presented that in some cases DNA molecules electrophorese directly through gel crystallites and in other cases along grain boundaries between gel crystallites. Next, two dimensional electrophoresis is performed in order to understand the size dependence of the mobility of DNA molecules ranging from 20 bp to 3500 bp in Pluronic gels. Small DNA molecules (<125 >bp) became slower as their molecular size decreased, while large DNA molecules (>175 bp) became exponentially slower as their molecular size increased. DNA molecules of intermediate sizes showed a sinusoidal pattern in their mobility. Based on the experimental results, we developed a phenomenological model to fit the size dependence of the DNA mobility, and discussed the separation mechanisms of the DNA molecules in the Pluronic gels. In the final experiment, DNA molecules of the same length, which moved together and formed a single band in the agarose gel, were split into two bands in the Pluronic gel. This indicates that the Pluronic gel is possibly able to separate the DNA molecules according to their sequence. We expect that this study will contribute to an enhanced understanding of the dynamics of DNA molecules in Pluronic gel and facilitation of Pluronic gel as an alternative sieving medium for electrophoresis in the fields of molecular biology and biotechnology.
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Date Issued: 2009
Identifier: FSU_migr_etd-0863
Format: Thesis

Title: Anomalous Metallic Behavior in Strongly Correlated Electron Systems with Disorder.
Creator: Tanaskovic, Darko, Dobrosavljevic, Vladimir, Dalal, Naresh, Manousakis, Efstratios, Capstick, Simon, Xiong, Peng, Department of Physics, Florida State University
Abstract/Description: We study several aspects of the behavior of strongly correlated electron systems with disorder. First we examine the influence of strong electron-electron interactions on the impurity dominated resistivity. In the weak-coupling limit, the resistivity is reduced by the screening effect which is determined by the charge compressibility, which is proportional to the inverse screening length. We show that when strong correlations are present, although the compressibility is reduced, the screening...
Show moreWe study several aspects of the behavior of strongly correlated electron systems with disorder. First we examine the influence of strong electron-electron interactions on the impurity dominated resistivity. In the weak-coupling limit, the resistivity is reduced by the screening effect which is determined by the charge compressibility, which is proportional to the inverse screening length. We show that when strong correlations are present, although the compressibility is reduced, the screening effect is nevertheless strongly enhanced. This phenomenon is traced to the non-perturbative Kondo-like processes captured by dynamical mean field theory, but which are absent in weak coupling approaches. We discuss a possible relevance of our results for the physics of high mobility MOSFETs. In the second part of the thesis we study possible mechanisms of disorder-driven non-Fermi liquid behavior in heavy fermion systems. We present simple analytical arguments explaining the universal emergence of electronic Griffiths phases as precursors of disorder-driven metal-insulator transitions in correlated electronic systems. Then we examine the interplay of the Kondo effect and the RKKY interactions in electronic Griffiths phases using extended dynamical mean-field theory methods. We find that sub-Ohmic dissipation is generated for sufficiently strong disorder, leading to suppression of Kondo screening on a finite fraction of spins, and giving rise to universal spin-liquid behavior.
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Date Issued: 2005
Identifier: FSU_migr_etd-1718
Format: Thesis

Title: Variational Study of the Nematic State of the Two Dimensional Electron Gas in a Magnetic Field.
Creator: Doan, Quoc M., Manousakis, Efstratios, Mascagni, Michael, Yang, Kun, Piekerawicz, Jorge, Xiong, Peng, Department of Physics, Florida State University
Abstract/Description: We have studied the nematic state of the two dimensional electron gas (2DEG) at halffilled Landau level (LL). Our motivation comes from experiments in which anisotropic transport in the 2DEG under high magnetic field and at low temperature was observed. Based on a model of the nematic state proposed by Oganesyan, Fradkin and Kivelson, we investigate this state and compare it with other competing states proposed by other groups. First, we investigate at what LL the nematic state becomes...
Show moreWe have studied the nematic state of the two dimensional electron gas (2DEG) at halffilled Landau level (LL). Our motivation comes from experiments in which anisotropic transport in the 2DEG under high magnetic field and at low temperature was observed. Based on a model of the nematic state proposed by Oganesyan, Fradkin and Kivelson, we investigate this state and compare it with other competing states proposed by other groups. First, we investigate at what LL the nematic state becomes energetically favorable as compared to the isotropic state. Our studies indicate that this occurs at the second excited LL. Moreover, we compare the energy of the nematic state with that of the stripe state obtained within the Hartree-Fock approximation (HF) and we conclude that, for the samples studied experimentally, the nematic state might be more stable. In our study we have used two different methods which both have advantages and disadvantages. The first method used is the Fermi-hyper-netted chain (FHNC) which provides results valid in the thermodynamic limit (infinite size system), however, it is accurate only for a low density system. The second method used is the Monte Carlo method (MC) which can be used on a finite-size system and, thus, the issue arises of how to extrapolate in the thermodynamic limit (finite-size effect). The results obtained from both methods are in good agreement and indicate that the nematic state might be a viable candidate to explain the experimental findings. In order to compare our results for the nematic state to those obtained for the stripe state, we needed to include the kinetic energy contribution beyond the familiar hw_c/2 term in the case of the nematic state which comes from the deformed geometry of the Fermi sea. For the stripe state we have carried out a HF calculation for a more realistic potential for a 2DEG which includes the effects of the finite confinement of the electron wave function along the z-direction.
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Date Issued: 2008
Identifier: FSU_migr_etd-1196
Format: Thesis

Title: Role of Defects in Possible Superfluidity of Spatially Ordered Helium.
Creator: Wierschem, Keola, Manousakis, Efstratios, Yang, Wei, Bonesteel, Nicholas, Van Winkle, David, Riley, Mark, Department of Physics, Florida State University
Abstract/Description: A path integral Monte Carlo investigation of the role of defects in ordered 4He systems has been conducted. We find that interstitial defects in two- and three-dimensional solid 4He can lead to a small but non-zero superfluid fraction, while still maintaining the original solid ordering. Furthermore, a 3He impurity atom initially placed as an interstitial defect is found to relax onto the solid lattice through the promotion of a 4He atom to the interstitial space. We also study the melting...
Show moreA path integral Monte Carlo investigation of the role of defects in ordered 4He systems has been conducted. We find that interstitial defects in two- and three-dimensional solid 4He can lead to a small but non-zero superfluid fraction, while still maintaining the original solid ordering. Furthermore, a 3He impurity atom initially placed as an interstitial defect is found to relax onto the solid lattice through the promotion of a 4He atom to the interstitial space. We also study the melting transition of pure 4He in two dimensions, where thermally excited defects give rise to unique phases with quasi-long-range order. Finite-size scaling techniques, initially applied to a classical system of Lennard-Jones particles, are found to be able to distinguish two separate melting temperatures. Additionally, coexistence of super fluid and diagonal order was observed in several of our finite-sized samples, raising the possibility that such real- and momentum-space ordering could be held simultaneously by quantum systems in two dimensions
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Date Issued: 2010
Identifier: FSU_migr_etd-1039
Format: Thesis

Title: Analysis and Approximation of a Two-Band Ginzburg-Landau Model of Superconductivity.
Creator: Chan, Wan-Kan, Gunzburger, Max, Peterson, Janet, Manousakis, Efstratios, Wang, Xiaoming, Department of Mathematics, Florida State University
Abstract/Description: In 2001, the discovery of the intermetallic compound superconductor MgB2 having a critical temperature of 39K stirred up great interest in using a generalization of the Ginzburg-Landau model, namely the two-band time-dependent Ginzburg-Landau (2B-TDGL) equations, to model the phenomena of two-band superconductivity. In this work, various mathematical and numerical aspects of the two-dimensional, isothermal, isotropic 2B-TDGL equations in the presence of a time-dependent applied magnetic field...
Show moreIn 2001, the discovery of the intermetallic compound superconductor MgB2 having a critical temperature of 39K stirred up great interest in using a generalization of the Ginzburg-Landau model, namely the two-band time-dependent Ginzburg-Landau (2B-TDGL) equations, to model the phenomena of two-band superconductivity. In this work, various mathematical and numerical aspects of the two-dimensional, isothermal, isotropic 2B-TDGL equations in the presence of a time-dependent applied magnetic field and a time-dependent applied current are investigated. A new gauge is proposed to facilitate the inclusion of a time-dependent current into the model. There are three parts in this work. First, the 2B-TDGL model which includes a time-dependent applied current is derived. Then, assuming sufficient smoothness of the boundary of the domain, the applied magnetic field, and the applied current, the global existence, uniqueness and boundedness of weak solutions of the 2B-TDGL equations are proved. Second, the existence, uniqueness, and stability of finite element approximations of the solutions are shown and error estimates are derived. Third, numerical experiments are presented and compared to some known results which are related to MgB2 or general two-band superconductivity. Some novel behaviors are also identified.
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Date Issued: 2007
Identifier: FSU_migr_etd-3923
Format: Thesis

Title: Coulomb Pseudogaps and Their Role at Metal-Insulator Transitions.
Creator: Mahmoudian, Samiyeh, Dobrosavljević, Vladimir, Dalal, Naresh S., Balicas, Luis, Manousakis, Efstratios, Capstick, Simon, Florida State University, College of Arts and Sciences,...
Show moreMahmoudian, Samiyeh, Dobrosavljević, Vladimir, Dalal, Naresh S., Balicas, Luis, Manousakis, Efstratios, Capstick, Simon, Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: We investigate various fundamental aspects of the metal insulator transition (MIT) by employing several theoretical approaches. Describing the novel features of interacting electronic systems remains one of the principle challenges to theoretical condensed matter physics. Therefore, any effort to provide a deep insight into understanding the nature of MITs opens a new avenue for development of modern technology. First, we present a complete analytical and numerical solution of the Typical...
Show moreWe investigate various fundamental aspects of the metal insulator transition (MIT) by employing several theoretical approaches. Describing the novel features of interacting electronic systems remains one of the principle challenges to theoretical condensed matter physics. Therefore, any effort to provide a deep insight into understanding the nature of MITs opens a new avenue for development of modern technology. First, we present a complete analytical and numerical solution of the Typical Medium Theory (TMT) for the metal-insulator transition. In this theory, we self-consistently calculate the typical amplitude of the electron wave-functions, representing the conceptually simplest order-parameter for the Anderson transition. We classify all possible universality classes for the critical behavior, that can be found within such a mean-field approach. This provides insights into how interaction-induced renormalizations of the disorder potential may produce qualitative modifications of critical behavior. We also formulate a simple description of the leading critical behavior of varies quantities, and then obtain an effective Landau theory for Anderson localization. We also develop an efficient numerical algorithm, "Cluster Typical Medium Theory" (CTMT) to capture both non-local effects and localization in disordered electronic systems. Our formalism utilizes the momentum-resolved typical density of states to characterize the localization transition. We apply this approach to the Anderson model of localization in one and two-dimensions. In one dimension, we find that the critical disorder strength scales inversely with the linear cluster size with a power-law, W[subscript c] ∼ (1/L[subscript c])[superscript 1/v]; whereas in two dimensions, the critical disorder strength decreases logarithmically with the linear cluster size. Our results are in agreement with the one-parameter scaling theory. Furthermore, we show how spatial correlations can also be captured analytically within such a self-consistent theory, by utilizing the standard Landau method of allowing for (slow) spatial fluctuations of the order parameter, and performing an appropriate gradient expansion. Our theoretical results provide insight into recent STM experiments, which were used to visualize the spatially fluctuating electronic wave functions near the metal insulator transition in Ga[subscript 1-x]Mn[subscript x]As.$ We show that, within our theory, all features of the experiment can be accounted for by considering a model of disorder renormalized by long-range Coulomb interactions. This includes the pseudogap formation, the C(R) ∼ 1/R form of the LDOS autocorrelations function, and the ζ ∼ 1/E energy dependence of the correlation length at criticality. In the second part of my Thesis, we show that introducing long-range Coulomb interactions immediately lifts the massive ground state degeneracy induced by geometric frustration for electrons on quarter-filled triangular lattices in the classical limit. Important consequences include the stabilization of a stripe-ordered crystalline (global) ground state, but also the emergence of very many low-lying metastable states with amorphous "stripe-glass" spatial structures. Melting of the stripe order thus leads to a frustrated Coulomb liquid at intermediate temperatures, showing remarkably slow (viscous) dynamics, with very long relaxation times growing in Arrhenius fashion upon cooling, as typical of strong glass formers. On shorter time scales, the system falls out of equilibrium and displays the aging phenomena characteristic of supercooled liquids above the glass transition. Our results show remarkable similarity with the recent observations of charge-glass behavior in ultra-clean triangular organic materials of the θ-(BEDT-TTF)₂ family.
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Date Issued: 2015
Identifier: FSU_2015fall_Mahmoudian_fsu_0071E_12817
Format: Thesis

Title: Structural Stability and Emergent Phases in Oxygen Deficient Complex Transition Metal Oxides.
Creator: Ghosh, Soham S., Manousakis, Efstratios, Shatruk, Mykhailo, Bonesteel, N. E., Roberts, Winston, Van Winkle, David, Flaherty, Francis A., Florida State University, College of...
Show moreGhosh, Soham S., Manousakis, Efstratios, Shatruk, Mykhailo, Bonesteel, N. E., Roberts, Winston, Van Winkle, David, Flaherty, Francis A., Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: This dissertation is a theoretical and computational examination of structural, electronic and magnetic properties of complex transition metal oxide structures. Our work is motivated by experimental observations that transition metal oxides manifest novel properties at surfaces and interfaces that are absent in bulk, and that there exist competing ground states driven by off-stoichiometry, oxygen vacancy and reduction of symmetry. We examine these properties using density functional theory ...
Show moreThis dissertation is a theoretical and computational examination of structural, electronic and magnetic properties of complex transition metal oxide structures. Our work is motivated by experimental observations that transition metal oxides manifest novel properties at surfaces and interfaces that are absent in bulk, and that there exist competing ground states driven by off-stoichiometry, oxygen vacancy and reduction of symmetry. We examine these properties using density functional theory (DFT) within the spin-generalized gradient approximation (Spin-GGA) along with the application of a Hubbard U (GGA + U). We present our detailed results for the following systems: oxygen deficient strontium titanate surface, strontium ruthenate interfaced with ruthenium metal inclusions, and ytterbium titanate with Yb "stuffing". In the course of our work, we cover materials with 3d, 4d and 4f band characters, each of which have different band masses, electron-electron correlations and spin-orbit coupling (SOC) strength. We investigate the role of surface termination, oxygen vacancy doping and cation "stuffing" defects in these metal-oxides and show the emergence of novel properties consistent with experimentally acquired information and possible applications. We conclude by presenting implications for further work.
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Date Issued: 2017
Identifier: FSU_SUMMER2017_Ghosh_fsu_0071E_13962
Format: Thesis

Title: Mott Transition in Strongly Correlated Materials: Many-Body Methods and Realistic Materials Simulations.
Creator: Lee, Tsung-Han, Dobrosavljević, Vladimir, Dalal, Naresh S., Manousakis, Efstratios, Balicas, Luis, Piekarewicz, Jorge, Florida State University, College of Arts and Sciences,...
Show moreLee, Tsung-Han, Dobrosavljević, Vladimir, Dalal, Naresh S., Manousakis, Efstratios, Balicas, Luis, Piekarewicz, Jorge, Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: Strongly correlated materials are a class of materials that cannot be properly described by the Density Functional Theory (DFT), which is a single-particle approximation to the original many-body electronic Hamiltonian. These systems contain d or f orbital electrons, i.e., transition metals, actinides, and lanthanides compounds, for which the electron-electron interaction (correlation) effects are too strong to be described by the single-particle approximation of DFT. Therefore, complementary...
Show moreStrongly correlated materials are a class of materials that cannot be properly described by the Density Functional Theory (DFT), which is a single-particle approximation to the original many-body electronic Hamiltonian. These systems contain d or f orbital electrons, i.e., transition metals, actinides, and lanthanides compounds, for which the electron-electron interaction (correlation) effects are too strong to be described by the single-particle approximation of DFT. Therefore, complementary many-body methods have been developed, at the model Hamiltonians level, to describe these strong correlation effects. Dynamical Mean Field Theory (DMFT) and Rotationally Invariant Slave-Boson (RISB) approaches are two successful methods that can capture the correlation effects for a broad interaction strength. However, these many-body methods, as applied to model Hamiltonians, treat the electronic structure of realistic materials in a phenomenological fashion, which only allow to describe their properties qualitatively. Consequently, the combination of DFT and many body methods, e.g., Local Density Approximation augmented by RISB and DMFT (LDA+RISB and LDA+DMFT), have been recently proposed to combine the advantages of both methods into a quantitative tool to analyze strongly correlated systems. In this dissertation, we studied the possible improvements of these approaches, and tested their accuracy on realistic materials. This dissertation is separated into two parts. In the first part, we studied the extension of DMFT and RISB in three directions. First, we extended DMFT framework to investigate the behavior of the domain wall structure in metal-Mott insulator coexistence regime by studying the unstable solution describing the domain wall. We found that this solution, differing qualitatively from both the metallic and the insulating solutions, displays an insulating-like behavior in resistivity while carrying a weak metallic character in its electronic structure. Second, we improved DMFT to describe a Mott insulator containing spin-propagating and chargeless fermionic excitations, spinons. We found the spinon Fermi-liquid, in the Mott insulating phase, is immiscible to the electron Fermi-liquid, in the metallic phase, due to the strong scattering between spinons in a metal. Third, we proposed a new approach within the slave-boson (Gutzwiller) framework that allows to describe both the low energy quasiparticle excitation and the high energy Hubbard excitation, which cannot be captured within the original slave-boson framework. In the second part, we applied LDA+RISB to realistic materials modeling. First, we tested the accuracy of LDA+RISB on predicting the structure of transition metal compounds, CrO, MnO, FeO, CoO, CoS, and CoSe. Our results display remarkable agreements with the experimental observations. Second, we applied LDA+RISB to analyze the nature of the Am-O chemical bonding in the CsAm(CrO_4)_2 crystal. Our results indicate the Am-O bonding has strongly covalent character, and they also address the importance of the correlation effects to describe the experimentally observed electronic structure. In summary, we proposed three extensions within DMFT and RISB framework, which allow to investigate the domain wall structure in metal-Mott insulator coexistence regime, the metal-to-Mott-insulator transition with spinons excitation in the Mott-insulating phase, and the Hubbard excitation within RISB approach. Furthermore, we demonstrated that LDA+RISB is a reliable approximation to the strongly correlated materials by applying it to the transition metal compounds and the Americian chromate compounds.
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Date Issued: 2017
Identifier: FSU_SUMMER2017_Lee_fsu_0071E_13983
Format: Thesis

Title: Lateral P-N Junctions Based on 2-D Materials.
Creator: Memaran, Shahriar, Balicas, Luis, Rikvold, Per Arne, Shatruk, Mykhailo, Manousakis, Efstratios, Cao, Jianming, Almaraz-Calderon, Sergio J., Florida State University, College of...
Show moreMemaran, Shahriar, Balicas, Luis, Rikvold, Per Arne, Shatruk, Mykhailo, Manousakis, Efstratios, Cao, Jianming, Almaraz-Calderon, Sergio J., Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: The discovery of graphene marked a turning point in research and interest towards 2 -D materials. Among them, Transition Metal Dichalcogenides (TMDs) and Metal Monochalcogenides (MM) have seen an upturn in interest owing to their versatile properties. Although, they have been studied for many years in bulk form, recent advances in nano-technology enabled new opportunities to study the role of atomically thin materials. In recent years much work has been dedicated to development of their...
Show moreThe discovery of graphene marked a turning point in research and interest towards 2 -D materials. Among them, Transition Metal Dichalcogenides (TMDs) and Metal Monochalcogenides (MM) have seen an upturn in interest owing to their versatile properties. Although, they have been studied for many years in bulk form, recent advances in nano-technology enabled new opportunities to study the role of atomically thin materials. In recent years much work has been dedicated to development of their application for the next generation of electronic and optoelectronic devices, and we are witnessing the dawn of the exploration of their properties. In Chapter 1 a brief introduction of highlighted properties of the newly emerged 2 -D materials and their heterostructures is provided. Chapter 2 focuses on field-effect transistor response of few atomic layers of MoSe2, MoTe2 and WSe2. In contrast to previous reports on MoSe2 FETs electrically contacted with Ni, MoSe2 FETs electrically contacted with Ti display ambipolar behavior with current ON to OFF ratios up to 10^6 for both hole and electron channels when applying a small excitation voltage. For both channels the Hall effect indicates Hall mobilities H = 250 cm^2/V.s. Our MoTe2 field-effect transistors are observed to be hole-doped, displaying ON/OFF ratios surpassing 10^6 and typical subthreshold swings of ~140mV per decade. Both field-effect and Hall mobilities indicate maximum values approaching or surpassing 10 cm^2/V.s, which are comparable to figures previously reported for single or bilayer MoS2 and/or for MoSe2 exfoliated onto SiO2 at room temperature and without the use of dielectric engineering. Temperature dependent comparison between field-effect and Hall mobilities in field effect transistors based on few-layered WSe2 exfoliated onto SiO2 is also reported. We observe maximum hole mobilities approaching 350 cm^2/V.s at T = 300 K. The hole Hall mobility reaches a maximum value of 650 cm^2/V.s as T is lowered below ~150 K, indicating that insofar WSe2- based field-effect transistors (FETs) display the largest Hall mobilities among the transition metal dichalcogenides. Chapter 3 evaluates electrostatically gated p-n junctions based on MoSe2 and the photovoltaic response of electrostatically generated p-n junctions composed of approximately 10 atomic layers of MoSe2 stacked onto dielectric h-BN is presented. In addition to ideal diode-like response, we find that these junctions can yield photovoltaic effciencies exceeding 14% under standard solar simulator spectrum with fill factors values of about 70 %. Chapter 4 presents electrical and optical characterization of monolayer and bilayer lateral heterostructures of MoS2-WS2 and MoSe2-WSe2, grown by a one-pot chemical vapor deposition (CVD) synthesis approach, using a single heterogeneous solid source, a newly developed CVD growth method that eliminates the need for the exchange of multiple sources which leads to sample air exposure. The structures show a diode like response which is enhanced under optical illumination. Additionally, bilayer lateral heterostructures exhibit a clear photovoltaic response to optical excitation.
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Date Issued: 2018
Identifier: 2018_Sp_Memaran_fsu_0071E_14363
Format: Thesis

Title: Topological Materials and Their Interfaces.
Creator: Aryal, Niraj, Manousakis, Efstratios, Dalal, Naresh S., Bonesteel, N. E., Berg, Bernd A., Balicas, Luis, Florida State University, College of Arts and Sciences, Department of...
Show moreAryal, Niraj, Manousakis, Efstratios, Dalal, Naresh S., Bonesteel, N. E., Berg, Bernd A., Balicas, Luis, Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: This dissertation is a theoretical and computational examination of electronic properties of topological materials, such as topological insulators and Weyl semimetals. Our work is motivated by various experimental observations and theoretical predictions about the presence of exotic electronic properties and transport phenomena in different topological materials. These materials have been a subject of intense research since last decade because their low-energy dispersion can be described by...
Show moreThis dissertation is a theoretical and computational examination of electronic properties of topological materials, such as topological insulators and Weyl semimetals. Our work is motivated by various experimental observations and theoretical predictions about the presence of exotic electronic properties and transport phenomena in different topological materials. These materials have been a subject of intense research since last decade because their low-energy dispersion can be described by Dirac and Weyl equations and they are predicted to have many exciting properties of both fundamental and practical significance. In this dissertation, we examine theoretical predictions and experimental measurements using density functional theory (DFT) based methods and using calculations based on a model Hamiltonian. In the first half of this dissertation, we present our detailed results for the Weyl semimetal candidate T[subscript d]-MoTe₂ using DFT and DFT+U methods while making careful comparison to different experiments in order to validate our results. We also address the fate of the Weyl fermions as a function of the Hubbard U using both DFT and a model Hamiltonian approach. Moreover, from our calculations, we predict that the system could be in close vicinity of a Lifshitz transition. Such a prediction can be experimentally verified by means of doping or electrostatic gating. In the second half of this dissertation, we present our results for the surface and interface states of the prototypical topological insulator material Bi₂Se₃. We study interfaces of the topological insulator with different conventional insulators using both DFT and model Hamiltonian calculations and examine the fate of the topological states at the interface. Moreover, we predict the occurrence of a topological phase transition in the interface geometry from our calculations and point towards a few directions where some of these predictions could be verified experimentally. We conclude by presenting implications for further work.
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Date Issued: 2019
Identifier: 2019_Summer_Aryal_fsu_0071E_15363
Format: Thesis

Title: Role of Phonons, Doping and Domainwalls in Hole Propagation in Two-Dimensional Quantum Antiferromagnets.
Creator: Kar, Satyaki, Manousakis, Efstratios, Hellstrom, Eric, Schlottmann, Pedro, Van Winkle, David H., Hoeﬂich, Peter, Department of Physics, Florida State University
Abstract/Description: We study the two-dimensional (2D) t−J model (and its extensions) in order to understand the hole dynamics in the 2D CuO2 plane of the cuprate superconductors. Within the linear spin wave approximation (LSW) of the Hamiltonian and the non-crossing approximation (NCA) for the hole self energies, thermal broadening of the hole spectral peaks is investigated with and without the contribution of optical phonons. We find the string excitations [15] to survive even for relatively strong electron...
Show moreWe study the two-dimensional (2D) t−J model (and its extensions) in order to understand the hole dynamics in the 2D CuO2 plane of the cuprate superconductors. Within the linear spin wave approximation (LSW) of the Hamiltonian and the non-crossing approximation (NCA) for the hole self energies, thermal broadening of the hole spectral peaks is investigated with and without the contribution of optical phonons. We find the string excitations [15] to survive even for relatively strong electron-phonon coupling. Experimental angle-resolved photoemission spectroscopy (ARPES) results compare well with our calculations at finite temperature when we use strong electron-phonon coupling (ã). With vertex correction the agreement with experiment becomes possible even at a moderate value of ã. Finite hole doping in a 2D t−J model and its extensions at T = 0 is studied using NCA. Dressed hole and magnon Green's functions are obtained to analyze the hole energy bands/ Fermi surface topology and the magnon broadening and softening for the doped system. The doping-dependent staggered magnetization of the system is computed and the doping fraction up to which the staggered magnetization is non-zero is indicated. Contribution of the anomalous magnons has been reported as well. We also study the dynamics of a hole in a 2D lattice in a stripe-ordered background. Within the same LSW and NCA treatment to the t − J Hamiltonian we obtain the different spin wave modes and hole Green's functions of the superlattice structure. The hole spectra indicates the preference for the hole to be in the anti-phase domain wall rather than being within the antiferromagnetic block.
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Date Issued: 2010
Identifier: FSU_migr_etd-3338
Format: Thesis

Title: A Configuration Space Monte Carlo Algorithm for Solving the Nuclear Pairing Problem.
Creator: Lingle, Mark, Volya, Alexander, Kopriva, David A., Capstick, Simon, Wiedenhöver, Ingo, Manousakis, Efstratios, Florida State University, College of Arts and Sciences, Department...
Show moreLingle, Mark, Volya, Alexander, Kopriva, David A., Capstick, Simon, Wiedenhöver, Ingo, Manousakis, Efstratios, Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: Nuclear pairing correlations using Quantum Monte Carlo are studied in this dissertation. We start by defining the nuclear pairing problem and discussing several historical methods developed to solve this problem, paying special attention to the applicability of such methods. A numerical example discussing pairing correlations in several calcium isotopes using the BCS and Exact Pairing solutions are presented. The ground state energies, correlation energies, and occupation numbers are compared...
Show moreNuclear pairing correlations using Quantum Monte Carlo are studied in this dissertation. We start by defining the nuclear pairing problem and discussing several historical methods developed to solve this problem, paying special attention to the applicability of such methods. A numerical example discussing pairing correlations in several calcium isotopes using the BCS and Exact Pairing solutions are presented. The ground state energies, correlation energies, and occupation numbers are compared to determine the applicability of each approach to realistic cases. Next we discuss some generalities related to the theory of Markov Chains and Quantum Monte Carlo in regards to nuclear structure. Finally we present our configuration space Monte Carlo algorithm starting from a discussion of a path integral approach by the authors [2, 3]. Some general features of the Pairing Hamiltonian that boost the effectiveness of a configuration space Monte Carlo approach are mentioned. The full details of our method are presented and special attention is paid to convergence and error control. We present a series of examples illustrating the effectiveness of our approach. These include situations with non-constant pairing strengths, limits when pairing correlations are weak, the computation of excited states, and problems when the relevant configuration space is large. We conclude with a chapter examining some of the effects of continuum states in 24O.
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Date Issued: 2015
Identifier: FSU_migr_etd-9383
Format: Thesis

Title: Photo-Excited Multiple Excitons in Strongly Correlated Insulators.
Creator: Coulter, John E., Manousakis, Efstratios, Siegrist, Theo M., Berg, Bernd A., Schlottmann, P., Beekman, Christianne, Florida State University, College of Arts and Sciences,...
Show moreCoulter, John E., Manousakis, Efstratios, Siegrist, Theo M., Berg, Bernd A., Schlottmann, P., Beekman, Christianne, Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: This dissertation examines the theoretical and computational methods needed to calcu- late opto-electronic multiple-carrier excitation rates in strongly correlated insulators (SCI). Through several studies, we find that we are able to calculate the exciton-exciton decay rate, also called impact ionization rate (IIR) in real materials, and describe the the methods required to do so accurately. In the course of our search for a useful method, we investigate several levels of approximation, from...
Show moreThis dissertation examines the theoretical and computational methods needed to calcu- late opto-electronic multiple-carrier excitation rates in strongly correlated insulators (SCI). Through several studies, we find that we are able to calculate the exciton-exciton decay rate, also called impact ionization rate (IIR) in real materials, and describe the the methods required to do so accurately. In the course of our search for a useful method, we investigate several levels of approximation, from simple density-functional theory (DFT), to so-called hy- brid functionals, and finally we add many-body corrections to the DFT scheme. We present results for the materials vanadium dioxide (VO 2 ), nickel oxide (NiO), lanthanum vanadate (LaVO 3 ) and yttrium titanate (YTIO 3 ) along the way. The first two have been used as sim- ple test materials, while we have chosen the latter two as interesting and complex strongly correlated insulators. We present conclusions on the results, and discuss the implications for further experimental work.
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Date Issued: 2015
Identifier: FSU_migr_etd-9315
Format: Thesis

Title: Quantum Oscillations in Two Dimensional Dirac and Weyl Semimetals.
Creator: Das, Suvadip, Manousakis, Efstratios, Bonesteel, N. E., Balicas, Luis, Piekarewicz, Jorge, Florida State University, College of Arts and Sciences, Department of Physics
Abstract/Description: Since the discovery of the exotic properties of graphene, two dimensional materials such as metal chalcogenides, transition metal oxides and other 2D compounds have gained renewed interest. Graphene, silicene, germanene, graphyne and boron allotropes form a rare class of 2D Dirac materials. The presence of such Dirac points near the Fermi level provides us the option to switch between two carrier types by slightly doping the material and could lead to potential optoelectronic devices....
Show moreSince the discovery of the exotic properties of graphene, two dimensional materials such as metal chalcogenides, transition metal oxides and other 2D compounds have gained renewed interest. Graphene, silicene, germanene, graphyne and boron allotropes form a rare class of 2D Dirac materials. The presence of such Dirac points near the Fermi level provides us the option to switch between two carrier types by slightly doping the material and could lead to potential optoelectronic devices. Recently discovered sister compounds WTe₂[2] and MoTe₂[62] have gained significant impetus for extremely pronounced nonsaturating magnetroresistance and topological semimetal hosting type II Weyl points. Further, a new class of two dimensional materials with multiple Dirac cones were discovered following the compound Zr₂Te₂P , and include the compounds Hf₂Te₂P , Zr₂Te₂As and Ti₂Te₂P. Quantum oscillation studies were performed to investigate the detailed Fermi surfaces and the topological properties such as Berry phase were obtained for the newly discovered two dimensional materials at the National High Magnetic Field Laboratory. In this thesis we will address the electronic structure, density of states and Fermi surface in all of these materials using Density Functional Theory and related methodology. The low energy (near Fermi energy) physics of all the materials studied are characterized by Dirac-like or Weyl- like electronic structure. Namely, the electrons obey Dirac-like or Weyl-like equations where the speed of light is replaced by the Fermi velocity. Furthermore, these materials share fundamental topological properties at the electronic low energy spectrum. Therefore, we wish to undertake the task of studying such fundamental properties from first principles.
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Date Issued: 2016
Identifier: FSU_FA2016_Das_fsu_0071E_13587
Format: Thesis

Title: Magnetothermal Transport and Elastoresistive Properties of Low-Dimensional Magnetoelectrics and Dichalcogenides.
Creator: Benjamin, Shermane Mark, Choi, Eun Sang, Manousakis, Efstratios, Oates, William, Reina, Laura, Beekman, Christianne, Florida State University, College of Arts and Sciences,...
Show moreBenjamin, Shermane Mark, Choi, Eun Sang, Manousakis, Efstratios, Oates, William, Reina, Laura, Beekman, Christianne, Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: Co₄Nb₂O₉ is a quasi two dimensional material that has been known to exhibit magnetoelectric behavior with an antiferromagnetic transition (T[subscript N]) at ∼ 27 K. Our findings reveal strong evidence of magnon heat transport below T[subscript N] through magnetothermal conductivity measurements. Magnetothermal coupling is strongest below 2 K around 0.2 T suggesting presence of the thermal heat-valve effect causing an increase in its thermal conductivity by nearly 4-fold. Independent of field...
Show moreCo₄Nb₂O₉ is a quasi two dimensional material that has been known to exhibit magnetoelectric behavior with an antiferromagnetic transition (T[subscript N]) at ∼ 27 K. Our findings reveal strong evidence of magnon heat transport below T[subscript N] through magnetothermal conductivity measurements. Magnetothermal coupling is strongest below 2 K around 0.2 T suggesting presence of the thermal heat-valve effect causing an increase in its thermal conductivity by nearly 4-fold. Independent of field, we not only see the presence of phonon-magnon resonant scattering but also strong phonon-spin coupling; giving rise to an anisotropic thermal conductivity where within the plane of spins conductivity is greater than inter-plane. Thermal measurements were also carried out on Ba₃Cr₂O₈ which has been shown to undergo Bose-Einstein condensation of magnetic excitations. Through intercalation with copper and palladium atoms between the layers of the transition metal dichalcogenides titanium diselenide (TiSe₂), it has previously been shown that where the intrinsic charge density wave’s (CDW) temperature dependent resistivity peak anomaly occurs (T[subscript cdw] ∼165 K) decreases with increased intercalation and/or hydrostatic pressure. To mimic the chemical pressure caused by intercalation, uniaxial physical pressure (both compression and expansion) is used along with the Poisson effect. In the experiment with compressive pressure perpendicular to the layers (c-axis), resistivity data shows a shift in the peak (T[subscript cdw]) towards lower temperatures; expected for increasing dimensionality. For uniaxial pressure parallel to the layers, the Poisson effect should separate the layers, providing a separation of the layers similar to that of intercalation. Preliminary results show T[subscript cdw] increases initially, and then decreases. A prototype to induce uniaxial tension perpendicular to the layers has recently been developed and preliminary results will be reported. Electrical transport measurements were also executed for the first time on the first FE(II)-TCNQ spin cross-over (SCO) system ever synthesized. In fact, our material is the first structurally defined magnetically bistable semiconductor to be constructed from TCNQ radical ions. With the aid of spider silk fibers (detailed in further sections), our findings show it is a narrow band-gap semiconductor where its activation energy changes from 110 meV to 10 meV as it crosses the SCO transition temperature to lower temperature.
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Date Issued: 2016
Identifier: FSU_FA2016_Benjamin_fsu_0071E_13618
Format: Thesis

Title: Magnetic Phase Diagram of Triangular Lattice Antiferromagnet Ba₃MNb₂O9 (M = Co, Mn) and Its Multiferroicity.
Creator: Lee, Minseong, Choi, Eun Sang, Manousakis, Efstratios, Dalal, Naresh S., Chiorescu, Irinel, Crede, Volker, Florida State University, College of Arts and Sciences, Department of...
Show moreLee, Minseong, Choi, Eun Sang, Manousakis, Efstratios, Dalal, Naresh S., Chiorescu, Irinel, Crede, Volker, Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: This dissertation mainly focuses on the investigation of the magnetic phase diagram of quasi two di- mensional triangular lattice antiferromagnets (TLAFs) Ba₃MNb₂O9 (M = Co2+ (S = 1/2), Mn2+ (S = 5/2)), and their multiferroic properties. Both compounds show a two-step magnetic phase transition at TN1 and TN2 upon cooling from paramagnetic to up-up-down(uud) phase due to the easy-axis anisotropy, and 120 degree ordered phase at zero field. This feature is dissimilar to that of sister a...
Show moreThis dissertation mainly focuses on the investigation of the magnetic phase diagram of quasi two di- mensional triangular lattice antiferromagnets (TLAFs) Ba₃MNb₂O9 (M = Co2+ (S = 1/2), Mn2+ (S = 5/2)), and their multiferroic properties. Both compounds show a two-step magnetic phase transition at TN1 and TN2 upon cooling from paramagnetic to up-up-down(uud) phase due to the easy-axis anisotropy, and 120 degree ordered phase at zero field. This feature is dissimilar to that of sister a compound Ba₃MNb₂O9 (Ni2+, (S = 1)), in which a single magnetic phase transition occurs due to the easy-plane anisotropy at zero field. Moreover, at low temperature below TN1, successive magnetic phase transitions were observed in both compounds. However, in case of Co compounds, the range of magnetic field where the uud phase stabilizes becomes wider at lower temperature whereas becomes narrower in case of Mn compounds. This different behavior is originated from the nature of the fluctuations that stabilize the uud phase, that is, quantum and/or classical fluctuations. We also found that the spin magnitude and spin structure play a crucial role in stabilizing the multiferroic ground state. Multiferroicity appears in all magnetically ordered phase in the small-spin-system Co compound but only in 120 degree ordered state in the large-spin-system Mn compound. The systematic studies on these compounds provide a highly valuable playground in the investigate of the effect of spin varied from 1/2 to 5/2 in frustrated magnets and multiferroics, attract many interests and in the field.
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Date Issued: 2016
Identifier: FSU_2016SP_Lee_fsu_0071E_13120
Format: Thesis

Title: Evidence For Impact Ionization In Vanadium Dioxide.
Creator: Holleman, Joshua, Bishop, Michael M., Garcia, Carlos, Winfred, J. S. R. Vellore, Lee, Shinbuhm, Lee, Ho Nyung, Beekman, Christianne, Manousakis, Efstratios, McGill, Stephen A.
Abstract/Description: Pump-probe optical spectroscopy was used to investigate proposed charge-carrier multiplication via impact ionization in the M-1 insulating phase of VO2. By comparing the transient reflectivities of the film when pumped at less than and then more than twice the band-gap energy, we observed a larger ultrafast response with the higher energy pump color while the film was still transiently in the insulating phase. We additionally identified multiple time scales within the charge dynamics and...
Show morePump-probe optical spectroscopy was used to investigate proposed charge-carrier multiplication via impact ionization in the M-1 insulating phase of VO2. By comparing the transient reflectivities of the film when pumped at less than and then more than twice the band-gap energy, we observed a larger ultrafast response with the higher energy pump color while the film was still transiently in the insulating phase. We additionally identified multiple time scales within the charge dynamics and analyzed how these changed when the pump and probe wavelengths were varied. This experiment provided evidence that a fast carrier multiplication process, i.e., impact ionization, acts efficiently in this prototypical strongly correlated insulator, as was recently predicted by theoretical calculations.
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Date Issued: 2016-10-17
Identifier: FSU_libsubv1_wos_000386097100004, 10.1103/PhysRevB.94.155129
Format: Citation

Title: De Haas-Van Alphen Measurements in Topological Metals and Semimetals.
Creator: Chen, Kuan-Wen, Baumbach, Ryan E., Balicas, Luis, Greene, Laura H., Shatruk, Mykhailo, Manousakis, Efstratios, Graf, David E. (David Earl), Collins, David C. (David Christopher)...
Show moreChen, Kuan-Wen, Baumbach, Ryan E., Balicas, Luis, Greene, Laura H., Shatruk, Mykhailo, Manousakis, Efstratios, Graf, David E. (David Earl), Collins, David C. (David Christopher), Florida State University, College of Arts and Sciences, Department of Physics
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Abstract/Description: This thesis studied the electronic structure at the Fermi level and the topological character of topological semimetals via torque magnetometry. Torque magnetometry measures the anisotropic magnetization of the sample in a tilted magnetic field. In our measurements, the magnetic field is up to 35 T and the temperature is down to 300 mK. The oscillatory signal of the magnetization is detected, which is the so called "de Hass van Alphen (dHvA)" effect. By using dHvA effect, many important...
Show moreThis thesis studied the electronic structure at the Fermi level and the topological character of topological semimetals via torque magnetometry. Torque magnetometry measures the anisotropic magnetization of the sample in a tilted magnetic field. In our measurements, the magnetic field is up to 35 T and the temperature is down to 300 mK. The oscillatory signal of the magnetization is detected, which is the so called "de Hass van Alphen (dHvA)" effect. By using dHvA effect, many important parameters such as the geometry of Fermi surfaces, effective masses, quantum mobilities, Land ́e g factors and Berry's phases. It is especially important for the Berry's phase extraction. It is known that if there is a cyclotron orbit encircling a Dirac node, a non-trivial Berry's phase π can be extracted and a trivial Berry's phase 0 is expected for a conventional parabolic band. In the study of MAl3, we provided a detailed study of the dHvA oscillations and provided a comparison with the calculated band structures. The angular dependence of their Fermi surface cross-sectional areas reveals a remarkably good agreement with our first-principles calculations. dHvA supports the existence of tilted Dirac cones with Dirac type-II nodes located at 100, 230 and 250 meV above the Fermi level EF for VAl3,NbAl3 and TaAl3 respectively, in agreement with the prediction of broken Lorentz invariance in these compounds. However, for all three compounds we find that the cyclotron orbits on their FSs, including an orbit nearly enclosing the Dirac type-II node, yield trivial Berry phases. We showed that if one would like to derive a convincing Berry's phase from quantum oscillations one has to take into account the spin dephasing term in the LK formalism, and the precise location between the cyclotron orbit and the Dirac node. M2Te2X is studied via both torque magnetometry and angle-resolved photoemission spectroscopy (ARPES). Bulk two-dimensional Fermi surfaces are well-described by the dHvA oscillations and first principles calculations. Intriguingly, slab electronic structure calculations predict Dirac-like surface states at different locations within the Brillouin zone, which is consistent with ARPES observations.
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Date Issued: 2019
Identifier: 2019_Summer_Chen_fsu_0071E_15394
Format: Thesis

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