# Alumni Dissertations and Theses

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### 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.

### Growth and Characterization of Type-II Submonolayer ZnCdTe/ZnCdSe Quantum Dot Superlattices for Efficient Intermediate Band Solar Cells

Author:Siddharth DhomkarYear of Dissertation:2015Program:PhysicsAdvisor:Igor KuskovkyAbstract:In this thesis, we discuss the growth procedure and the characterization results obtained for epitaxially grown submonolayer type-II quantum dot superlattices made of II-VI semiconductors. The goal behind this study is to show the feasibility of this novel material system in fabricating an efficient intermediate band solar cell. Intermediate band solar cells can potentially have an efficiency of 63.2% under full solar concentration, but the material systems investigated until now are far from optimum and are fraught with growth related issues including low quantum dot densities, presence of wetting layers, strain driven dislocations etc. Here, we have investigated a novel material system grown via migration enhanced epitaxy with stacked type-II ZnCdTe submonolayer quantum dots embedded in ZnCdSe matrix and having close to the optimal material parameters required for an IB material. Upon optimizing growth conditions for ZnTe/ZnSe multilayer quantum dot systems, the growth parameters were modified so as to obtain various ZnCdTe/ZnCdSe samples grown on InP substrates. An extensive characterization has been performed to investigate structural, optical as well as electrical properties of these multilayered structures. Finally, a preliminary device fabrication has been performed, which will provide definite guidelines towards optimization of an actual intermediate band solar cell structure. To restate, the objective of this thesis is to demonstrate successful growth and characterization of multilayer structures with embedded submonolayer type-II quantum dots in order to explore the possibility of employing them as an intermediate band material, with the goal of engineering an ultra-efficient intermediate band solar cell.

### A Static and Dynamic Investigation of Quantum Nonlinear Transport in Highly Dense and Mobile 2D Electron Systems

Author:Scott DietrichYear of Dissertation:2015Program:PhysicsAdvisor:Sergey VitkalovAbstract:Heterostructures made of semiconductor materials may be one of most versatile environments for the study of the physics of electron transport in two dimensions. These systems are highly customizable and demonstrate a wide range of interesting physical phenomena. In response to both microwave radiation and DC excitations, strongly nonlinear transport that gives rise to non-equilibrium electron states has been reported and investigated. We have studied GaAs quantum wells with a high density of high mobility two-dimensional electrons placed in a quantizing magnetic field. This study presents the observation of several nonlinear transport mechanisms produced by the quantum nature of these materials. The quantum scattering rate, $1/\tau_q$, is an important parameter in these systems, defining the width of the quantized energy levels. Traditional methods of extracting $1/\tau_q$ involve studying the amplitude of Shubnikov-de Haas oscillations. We analyze the quantum positive magnetoresistance due to the cyclotron motion of electrons in a magnetic field. This method gives $1/\tau_q$ and has the additional benefit of providing access to the strength of electron-electron interactions, which is not possible by conventional techniques. The temperature dependence of the quantum scattering rate is found to be proportional to the square of the temperature and is in very good agreement with theory that considers electron-electron interactions in 2D systems. In quantum wells with a small scattering rate - which corresponds to well-defined Landau levels - quantum oscillations of nonlinear resistance that are independent of magnetic field strength have been observed. These oscillations are periodic in applied bias current and are connected to quantum oscillations of resistance at zero bias: either Shubnikov-de Haas oscillations for single subband systems or magnetointersubband oscillations for two subband systems. The bias-induced oscillations can be explained by a spatial variation of electron density across the sample. The theoretical model predicts the period of these oscillations to depend on the total electron density, which has been confirmed by controlling the density through a voltage top-gate on the sample. The peculiar nonlinear mechanism of quantal heating has garned much attention recently. This bulk phenomenon is a quantum manifestation of Joule heating where an applied bias current causes selective flattening in the electron distribution function but conserves overall broadening. This produces a highly non-equilibrium distribution of electrons that drastically effects the transport properties of the system. Recent studies have proposed contributions from edge states and/or skipping orbitals. We have shown that these contributions are minimal by studying the transition to the zero differential conductance state and comparing results between Hall and Corbino geometries. This demonstrated quantal heating as the dominant nonlinear mechanism in these systems. To study the dynamics of quantal heating, we applied microwave radiation simultaneously from two sources at frequencies $f_1$ and $f_2$ and measured the response of the system at the difference frequency, $f=\left|f_1-f_2\right|$. This provides direct access to the rate of inelastic scattering processes, $1/\tau_{in}$, that tend to bring the electron distribution back to thermal equilibrium. While conventional measurements of the temperature dependence indicate that $1/\tau_{in}$ is proportional to temperature, recent DC investigations and our new dynamic measurements show either $T^2$ or $T^3$ dependence in different magnetic fields. Our microwave experiment is the first $direct$ access to the inelastic relaxation rate and confirms the non-linear temperature dependence.