# Alumni Dissertations and Theses

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### Dynamics and Manipulation of Nanomagnets

Author:Liufei CaiYear of Dissertation:2014Program:PhysicsAdvisor:Eugene ChudnovskyAbstract:This thesis presents my work on the spin dynamics of nanomagnets and investigates the possibility of manipulating nanomagnets by various means. Most of the work has been published\cite{LC-PRB2010,LC-PRB2012,LC-PRB2013,LC-EPL2014}. Some has been submitted for publication\cite{LC-arxiv2014}. The structure of this thesis is as follows. In Chapter 1, I present the theory of manipulation of a nanomagnet by rotating ac fields whose frequency is time dependent. Theory has been developed that maps the problem onto Landau-Zener problem. For the linear frequency sweep the switching phase diagrams are obtained on the amplitude of the ac field and the frequency sweep rate. Switching conditions have been obtained numerically and analytically. For the nonlinear frequency sweep, the optimal time dependence of the frequency is obtained analytically with account of damping that gives the fastest controllable switching of the magnetization. In Chapter 2, interaction between a nanomagnet and a Josephson junction has been studied. The I-V curve of the Josephson junction in the proximity of a nanomagnet shows Shapiro-like steps due to the ac field generated by the precessing magnetic moment. Possibility of switching of the magnetic moment by a time-linear voltage in the Josephson junction is demonstrated. Realization of the optimal switching is suggested that employs two perpendicular Josephson junctions with time-dependent voltage signals. The result is shown to be robust against voltage noises. Quantum-mechanical coupling between the nanomagnet considered as a two-level system and a Josephson junction has been studied and quantum oscillations of the populations of the spin states have been computed. In Chapter 3, the switching dynamics of a nanomagnet embedded in a torsional oscillator that serves as a conducting wire for a spin current has been investigated. Generalized Slonczewski's equation is derived. The coupling of the nanomagnet, the torsional oscillator and the spin current generates a number of interesting phenomena. The mechanically-assisted magnetization switching is studied, in which the magnetization can be reversed by tilting the torsional oscillator. The effect of the torsional oscillator on the switching of the magnetization in the presence of spin-polarized current is computed. Combined effects of the spin current and a mechanical kick of the torsional oscillator have been studied. In Chapter 4, skyrmion dynamics and interaction of the skyrmion with an electron have been studied. Corrections to the spin texture of the skyrmion due to the crystal lattice have been computed. Due to the lattice effects the skyrmion collapses in clean ferromagnetic and anti-ferromagnetic materials. The lifetime of the skyrmion has been computed numerically and compared with analytical theory. In doped anti-ferromagnetic materials the weak attraction between a skyrmion and an electron may generate a bound state. In Chapter 5, experimental results of the NIST group on magnetic multilayer microcantilevers have been analyzed. Theoretical framework has been suggested that explains the observed strong damping effect of the platinum layer on the mechanical oscillations of Py-Pt bilayer cantilevers. The strong spin-orbit coupling of platinum is shown to impede the motion of the domain wall in permalloy and to dramatically increase the damping of the cantilever motion.

### Characterization of wide band gap semiconductors and multiferroic materials

Author:Bo CaiYear of Dissertation:2014Program:PhysicsAdvisor:Mim NakarmiAbstract:Structural, optical and electrical properties of zinc oxide (ZnO), aluminum nitride (AlN), and lutetium ferrite (LuFe2O4) have been investigated. Temperature dependent Hall Effect measurements were performed between 80 and 800 K for phosphorus (P) and arsenic (As) doped ZnO thin films grown on c-plane sapphire substrate by RF magnetron sputtering. These samples exhibited n-type conductivity throughout the temperature range with carrier concentration of 3.85 × 1016 cm-3 and 3.65 × 1017 cm-3 at room temperature for P-doped and As-doped ZnO films, respectively. The Arrhenius plots of free electron concentration of those doped samples showed double thermal activation processes with a small activation energy of about 0.04 eV due to shallow donors and a large activation energy of about 0.8 eV due to deep donors. The deep donor level could be related to oxygen vacancy. For undoped ZnO layer, growth condition was optimized to use as low background electron buffer layer. Hall Effect measurements showed that the resistivity and background electron concentration of the films decreases as the substrate temperature increases. The film deposited at 900 oC has more than two orders less background electron concentration than that deposited at 300 oC. Based on photoluminescence and Transmission Electron Microscopy (TEM) analysis, the ZnO grown under this condition is formed to be a greatly reduced density of stacking faults. Transmission electron microscopy (TEM) was employed to investigate dislocations in aluminum nitride (AlN) epilayers grown on sapphire substrate using three-step growth method by metal organic chemical vapor deposition (MOCVD). AlN epilayers grown by this method have smooth surfaces, narrow width of X-ray rocking curves, and strong band edge photoluminescence (PL) emissions with low impurity emissions. Transmission electron microscopy revealed that most of the threading dislocations are annihilated within 300 nm. Stacking faults are greatly reduced in the epilayers grown by this method resulting in very low screw type threading dislocation density. Dominant threading dislocations in the AlN epilayers are edge type originated from misfit dislocations (MD). The electro-optical and temperature-dependent electrical-transport properties of LuFe2O4 (LFO) thin films have been investigated. The LFO thin films at 78 K showed the electro-optical effects of size up to 5% near the Fe2+ d to d on-site electronic transition. In the three-dimensional charge-ordered state of LFO, we observed hysteresis in dc voltage-current measurements and nonlinear voltage-current relationship in transient response of voltage under current pulses. The electro-optical and electrical properties of LFO thin films are interpreted in terms of the field-induced changes of the charge-ordered state mediated by the spin-charge-lattice coupling effect. We also discuss possible mechanisms of the complex electrical properties and electro-optical effects in conjunction with the Maxwell-Wagner effects.

### Horava Gravity: Symmetries and Generalized Particle Dynamics

Author:Dario CapassoYear of Dissertation:2011Program:PhysicsAdvisor:Alexios PolychronakosAbstract:In the search for a theory of Quantum Gravity a new proposal was recently made by P. Horava. The main feature of this new proposed theory is that it is power-counting renormalizable by construction, and could prove to be truly renormalizable, although more work is needed in this direction. The renormalizability of the theory is a central issue. Indeed, General Relativity does not have this property, implying that to construct its quantum version we need to "complete" the theory in the UV. Horava suggested a possible way to provide a UV completion of GR by giving up full spacetime reparametrization symmetry, which is one of the fundamental assumptions of GR, and adding appropriate higher order terms in the action. In this Thesis we review Horava's theory and analyze some of the issues related to the breaking of the spacetime structure. Specifically, we derive the general static spherically symmetric solutions for Horava's theory with a nonvanishing radial "shift" field g

_{tr}. Such "hedgehog" configurations are not considered in GR, since g_{tr}can be mapped to zero with an appropriate reparametrization, but they are physically distinct solutions in Horava gravity where the reparametrization is not allowed by the reduced symmetry. These new solutions exhibit specific properties from the particle dynamics point of view and possess an extra gauge symmetry. We also study the deformed kinematics of point particles allowed by the reduced reparametrization symmetry. The main result is that particles can have generalized dispersion relations that include higher even powers of the momentum. We analyze the implications of this and provide some examples that may be converted into possible experimental tests for the deviations of this new theory of gravity from standard GR.### SYNTHESIS AND CHARACTERIZATION OF POLYCRYSTALLINE SEMICONDUCTOR CsSnI3 THIN-FILMS

Author:Zhuo ChenYear of Dissertation:2013Program:PhysicsAdvisor:Kai ShumAbstract:This thesis deals with a virtually unexplored semiconductor material CsSnI3 from material synthesis, structural, optical, and electrical characterization to the fabrication and validation of CsSnI3 thin-film solar cells. We started with synthesizing CsSnI3 thin films based on CsI and SnCl2 (or SnI2) by using an apparatus which consists of e-beam and thermal evaporators. The quality of polycrystalline CsSnI3 thin-films were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). Experimental data on XRD and electron diffraction patterns taking from the synthesized thin-films match very well to the theoretically calculated ones based the first principles calculations, confirming that the synthesized CsSnI3 thin-films have an orthorhombic crystal structure. With the well-defined crystal structure, we theoretically studied the electronic band structure of CsSnI3. Extensive optical characterizations of CsSnI3 thin-films were then carried out revealing many extraordinary properties such as 1) direct band gap energy of 1.32 eV at 300 K with its abnormal temperature dependence, 2) extremely high photoluminescence quantum yield, 3) large exciton binding energy, and 4) strong two-phonon assisted excitonic absorption near band edge. These properties are interpreted in terms of the unique electronic and structural properties of CsSnI3. The value of 1.3 eV for the energy band gap of CsSnI3 suggests a unique application of CsSnI3 thin-films on solar cells. This is because this value is right in the small range of the optimal band gaps for the Shockley-Queisser maximum efficiency limit of a single-junction solar cell. A prototype Schottky solar cell was designed, fabricated, and validated. The measured power conversion efficiency (PCE) is 0.9 % which is presently limited by the series and shunt resistance. The improvement strategy on PCE is given at the end of my thesis. In order to make the CsSnI3 thin-film solar cells cost effective, various low cost materials synthesis methods for CsSnI3 are also described in this thesis. CsSnI3 thin-films can be now inexpensively deposited on to glass or other low-cost substrates. I believe that the CsSnI3 based materials are ideally suited for many applications such as lasers, light-emitting diodes, integrated photonic devices such as infrared electro-optic modulator, solar cells, and even more specialized applications such as spectral solar concentrators.

### Dynamics of Nanoparticles in Fluids and at Interfaces

Author:Weikang ChenYear of Dissertation:2014Program:PhysicsAdvisor:Joel KoplikAbstract:In this thesis, we use molecular dynamics simulation to study three basic behaviors or properties of nanoparticles: deposition during droplets evaporation, slip boundary condition and Brownian motion. These three problems address the need for an in-depth understanding of the dynamics of nanoparticles in fluids and at interfaces. In the first problem, evaporation of the droplets dispersed with particles, we investigated the distribution of evaporative flux, inner flow field, density and temperature. And we use these numerical experiments to check on our hydrodynamic theory of the "coffee ring" phenomenon. The simulations reveal the connection between the particle interactions and the deposit structure, and indicate some limitations in continuum modeling. In the second problem, we explore the slip boundary conditions for curved surfaces, which is one of the desired information in modeling the hydrodynamics of micro-fluidic objects. The conclusion we draw is strong: the slip length, defined in a consistent tensorial manner, depends only on the physical properties of the solid and fluid involved and does not vary with the flow configuration. The final part is devoted to the Brownian motion of Janus particle, where we use a simple model to explain the increase of diffusivity of self-propelling Janus particles. We also show that the hydrodynamic image could be used to account for the self-aligning phenomenon at liquid-solid interfaces. The coupling between the translation and rotation is investigated by Brownian simulation, where we modify the standard Langevin equation with coupling terms which derive from the hydrodynamic interaction with the liquid-solid interfaces. The resultant individual trajectories and their diffusivities are consistent with both the laboratory observations and theoretical calculations.

### SECOND QUANTUM STATE TRANSITION IN GaAs/AlGaAs RESONANT BRAGG STRUCTURE PROBED BY MODULATION REFLECTANCE SPECTROSCOPY

Author:Yuechao ChenYear of Dissertation:2014Program:PhysicsAdvisor:Mim NakarmiAbstract:Modulation spectroscopy, ever since its introduction by B.O. Seraphin in 1964, has been considered and widely used as a sensitive experiment technique for studying and characterizing the properties of varieties of semiconductor materials. Compared to general optical reflectance spectrum which measures the absolute reflection, the modulation spectroscopy evaluates the interpretation of the changes in the optical response from the sample caused by a periodic physical perturbation applied to the sample, such as temperature, electric fields, hydrostatic pressure, uniaxial stress, \emph{etc}. Those modulation spectroscopies with an external electric field perturbation are known as electroreflectance spectroscopy, which provides sharp and derivative-like spectral features in the energy region of excitonic transitions in the semiconductors while suppressing uninteresting background effects that are not affected by the electro-modulation. One interesting phenomenon is that when the excitonic transition energy in a periodic dielectric structure, for example multiple quantum well (MQW) structure, meets the Bragg resonance condition, the reflectance spectrum shows an enlarged responding effect with enhanced reflectivity and broadened transition features. This kind of a structure is known as a resonant Bragg structure (RBS) and the coincidence of the exciton and Bragg resonances is called the double resonance condition. In this thesis, we employed both electroreflectance and optical reflectance spectroscopies to probe excitonic transitions in a GaAs/AlGaAs RBS MQW structure. The sample structure was specially designed and fabricated to tune the double resonance condition around the second state of the heavy-hole exciton x(e2-hh2) transitions by variation of the incident angle and temperature. The sample used in this experiment consists of 60 periods of quantum well structures with GaAs well layer (13 nm) and AlGaAs barrier layer (94 nm), grown by solid source molecular beam expitaxy on a semi-insulating GaAs substrate. We observed a significant enhancement of excitonic features at the x(e2-hh2) exciton transitions around incident angle of 23$^\circ$ in both techniques, revealing the double resonance condition at low temperature. Additionally, heavy-hole and light-hole ground state exciton transitions x(e1-hh1) and x(e1-lh1) were also evaluated. In the temperature dependence of optical reflectance and electroreflectance from the double resonance condition, we observed redshift of the excitonic features. The electric field dependence measurement of electroreflectance exhibited a broadening effect for the x(e2-hh2) exciton transition.

### Novel materials and techniques for renewable energy and biosensing applications

Author:Yongki ChoiYear of Dissertation:2010Program:PhysicsAdvisor:Ramzi KhuriAbstract:Ultrasmall (1 nm and 2.8 nm) colloidal silicon nanoparticles behave as electrocatalysts for the electrooxidation of the renewable energy sources such as ethanol, methanol and glucose. Particle-immobilized electrodes show an onset of electrocatalysis occurring at potentials between -0.4 V and 0.05 V vs. Ag/AgCl at neutral pH. Both the onset potential and the strength of electrocatalysis are dependent on particle size. Tafel measurements show that electrooxidation of the fuels is a first order reaction with the transfer of one electron. The electrocatalytic activity of the particles to the fuels undergoes at least a 50-fold increase under alkaline condition compared to under acidic condition. A significant increase in the electrocatalytic current is obtained when the electrocatalysis is performed in darkness. Prototype single-compartment and double-compartment hybrid fuel cells have been constructed and tested, using the particles as the anode electrocatalyst, in order to demonstrate the potential of the particles in fuel cell applications. Voltage-controlled amplification of the output current of an enzymatic transistor has been demonstrated. By applying external voltage between the gating and the working electrode on which the enzyme glucose oxidase was immobilized, the biocatalytic output current was increased significantly, allowing the detection limit of glucose to be lowered from the milli-molar to the zepto-molar level. The current amplification was reversibly controlled by the applied voltage. Applying this technique to the ethanol-alcohol dehydrogenase system showed similar results. The enzyme's bio-specificity was preserved in the presence of the field. The detector, with its output current controlled by the voltage applied at a third electrode, behaves as a field-effect transistor, whose current-generating mechanism is the conversion of analytes to products using an enzyme as catalyst. In addition, voltage-controlled reaction kinetics of biological catalysis is achieved using the microperoxidase-11 and hydrogen peroxide system. The interfacial electron transfer of the system was manipulated by applying the voltage to the electrode. The manipulated electron transfer causes kinetic parameters of the catalysis to acquire nonlinear dependences on the voltage. The nonlinearity indicates the feasibility of effectively controlling the efficiency of a bio-catalytic reaction or a conversion process using the voltage

### Generalization of the three-term recurrence formula and its applications

Author:Yoon Seok ChounYear of Dissertation:2012Program:PhysicsAdvisor:Sultan CattoAbstract:In an earlier paper we showed development of a bilocal baryon-meson field from two quark-antiquark fields. In the local approximation the hadron field was shown to exhibit supersymmetry which was then extended to hadronic mother trajectories and to inclusion of multiquark states. The Hamiltonian in the case of vanishing quark masses was shown to have a very good agreement with experiments. The theory for vanishing mass was solved using confluent hypergeometric functions. In order to solve the spin-free Hamiltonian with light quark masses we are led to develop a totally new kind of special function theory in mathematics that generalize all existing theories of confluent hypergeometric types. We call it the `Grand Confluent Hypergeometric Function.' Our new solution produces previously unknown extra "hidden" quantum numbers relevant for description of supersymmetry and for generating new mass formulas. Furthermore, we show for the first time how to solve mathematical equations having three term recursion relations and go on producing the exact solutions of some of the well-known special function theories that include Mathieu, Heun, Lame and the Grand Confluent Hypergeometric Function. We hope these new functions and their solutions will produce remarkable new range of applications not only in supersymmetric field theories as is shown here, but in the areas of all different classes of mathematical physics, applied mathematics and in engineering applications.

### The Impact of Context on Learning and Epistemology in Physics

Author:Sebastien CormierYear of Dissertation:2009Program:PhysicsAdvisor:Richard SteinbergAbstract:This dissertation investigates the impact that various contexts have on student learning and epistemology. This is accomplished by analyzing diverse student populations learning various subjects in physics in distinctive classroom environments at City College New York (CCNY). Studies in Physics Education Research (PER) have found that many students lack proper conceptual understanding after instruction in physics and that their epistemology, or approaches to learning and doing science, is different from those of experts. The PER community has used these results to develop models of learning and tools for teaching introductory and modern physics with goals that include improving the conceptual understanding, problem solving abilities, and epistemologies. These curricula are applied in a wide variety of contexts here at CCNY. The student contexts in this dissertation range from high school to graduate school, and the topics range from introductory to modern physics. We apply many tools commonly used in PER, such as multiple-choice surveys, essay questions, and guided interviews to study these classrooms. We find that PER- based curriculum implemented in these different contexts is able to improve both conceptual understanding and epistemology.

### Yang-Mills Theories as Deformations of Massive Integrable Models

Author:Axel Cortes CuberoYear of Dissertation:2014Program:PhysicsAdvisor:Peter OrlandAbstract:Yang Mills theory in 2+1 dimensions can be expressed as an array of coupled (1+1)-dimensional principal chiral sigma models. The SU(N) principal chiral sigma model in 1+1 dimensions is integrable, asymptotically free and has massive excitations. We calculate all the form factors and two- point correlation functions of the Noether current and energy-momentum tensor, in 't Hooft's large-N limit (some form factors can be found even at finite N). We use these new form factors to calculate physical quantities in (2+1)-dimensional Yang-Mills theory, generalizing previous SU(2) by P. Orland to SU(N). The anisotropic gauge theory is related to standard isotropic one by a Wilsonian renormalization group with ellipsoidal cutoffs in momentum. We calculate quantum corrections to the effective action of QED and QCD, as the theory flows from isotropic to anisotropic. The exact principal chiral sigma model S-matrix is used to examine the spectrum of (1+1)-dimensional massive Yang Mills theory.