PhD Defenses - UMD Physics

Noah Sennett - December 13, 2019

Dissertation Title: Probing fundamental physics with gravitational waves from inspiraling binary systems

Date and Time: Friday, December 13, 2:30 pm

Location: PSC 3150

Dissertation Committee Chair: Prof. Alessandra Buonanno

Committee:

Prof. Theodore Jacobson
Prof. Julie McEnery
Prof. Peter Shawhan
Prof. Raman Sundrum
Prof. Massimo Ricotti, Dean’s Representative

Abstract:

The first observations of gravitational waves from the mergers of black holes and/or neutron stars with Advanced LIGO and Virgo have opened a new window to the cosmos. This thesis examines how the gravitational-wave signal produced during the inspiral---the earliest phase of a binary system’s coalescence---can better inform our understanding of the highly dynamical, strong-curvature regime of gravity.

My work addressing this topic is comprised of two major components. First, I examine the behavior of binary black-hole and neutron-star systems in various possible extensions of General Relativity, constructing analytic models of their orbital motion and gravitational-wave production during their inspiral. Particular attention is devoted to alternative theories that admit scalarization, a second-order phase transition that occurs in compact bodies or binary systems that can manifest as non-perturbative phenomenology in a gravitational-wave signal.

The other component of this thesis is the development of a statistical infrastructure suitable for testing General Relativity using gravitational-wave observations. This framework is more flexible and modular approach than existing alternatives, allowing this infrastructure to be immediately employed with a wide range of waveform models. In work done in conjunction with the LIGO Scientific and Virgo Collaborations, I use this statistical framework to place bounds on phenomenological deviations from General Relativity using the binary black-hole and neutron-star events detected during LIGO's first and second observing runs. I also use this infrastructure to constrain certain specific alternative theories of gravity, including Brans-Dicke gravity.

Andrew Glaudell - December 9, 2019

Dissertation Title: Quantum Compiling Methods for Fault-Tolerant Gate Sets of Dimension Greater than Two

Date and Time: Monday, December 9, 12:15 pm

Location: PSC 3150

Dissertation Committee Chair: Andrew Childs

Committee:

Jacob M. Taylor (Advisor)
Norbert Linke
Michael Hicks
Larry Washington

Abstract:

Fault-tolerant gate sets whose generators belong to the Clifford hierarchy form the basis of many protocols for scalable quantum computing architectures. At the beginning of the decade, number-theoretic techniques were employed to analyze circuits over these gate sets on single qubits, providing the basis for a number of state-of-the-art quantum compiling algorithms. In this dissertation, I further this program by employing number-theoretic techniques for higher-dimensional gate sets on both qudit and multi-qubit circuits.

First, I introduce canonical forms for single qutrit Clifford+T circuits and prove that every single-qutrit Clifford+T operator admits a unique such canonical form. I show that these canonical forms are T-optimal and describe an algorithm which takes as input a Clifford+T circuit and outputs the canonical form for that operator. The algorithm runs in time linear in the number of gates of the circuit. Our results provide a higher-dimensional generalization of prior work by Matsumoto and Amano who introduced similar canonical forms for single-qubit Clifford+T circuits. Finally, we show that a similar extension of these normal forms to higher dimensions exists, but do not establish uniqueness.

Moving to multi-qubit circuits, I provide number-theoretic characterizations for certain restricted Clifford+T circuits by considering unitary matrices over subrings of Z[1/√2, i]. We focus on the subrings Z[1/2], Z[1/√2], Z[1/√−2], and Z[1/2, i], and we prove that unitary matrices with entries in these rings correspond to circuits over well-known universal gate sets. In each case, the desired gate set is obtained by extending the set of classical reversible gates {X, CX, CCX} with an analogue of the Hadamard gate and an optional phase gate.

I then establish the existence and uniqueness of a normal form for one of these gate sets, the two-qubit gate set of Clifford+Controlled Phase gate CS. This normal form is optimal in the number of CS gates, making it the first normal form that is non-Clifford optimal for a fault tolerant universal multi-qubit gate set. We provide a synthesis algorithm that runs in a time linear in the gate count and outputs the equivalent normal form. In proving the existence and uniqueness of the normal form, we likewise establish the generators and relations for the two-qubit Clifford+CS group. Finally, we demonstrate that a lower bound of 5 log₂(1/ε) + O(1) CS gates are required to ε-approximate any 4 × 4 unitary matrix.

Lastly, using the characterization of circuits over the Clifford+CS gate set and the existence of an optimal normal form, I provide an optimal ancilla-free inexact synthesis algorithm for two-qubit unitaries using the Clifford+SC gate set for Pauli-rotations. These operators require 6 log₂(1/ε) + O(1) CS gates to synthesize in the typical case and 8 log₂(1/ε) + O(1) in the worst case.

Andrew Smith - December 5, 2019

Dissertation Title: Studies in Nonequilibrium Quantum Thermodynamics

Date and Time: Thursday, December 5, 3:00 pm

Location: IPST Building Room 1116

Dissertation Committee Chair: Christopher Jarzynski

Committee:

Victor Yakovenko
Norbert Linke
Theodore Einstein
Sebastian Deffner

Abstract:

The first part of this thesis focuses on verifying the quantum nonequilibrium work relation in the presence of decoherence. The nonequilibrium work relation is a generalization of the second law of thermodynamics that links nonequilibrium work measurements to equilibrium free energies via an equality. Despite being well established for classical systems, a quantum work relation is conceptually difficult to construct for systems that interact with their environment. We argue that for a quantum system which undergoes decoherence but not dissipation, these conceptual difficulties do not arise and the work relation can be proven similarly to the case of an isolated system. This result is accompanied by an experimental demonstration using trapped ions.

The second part of this thesis examines the relationship between quantum work and coherence by constructing analogous quantities in classical physics. It has recently been shown that quantum coherence can function as a resource for work extraction. Furthermore, it has been suggested that this property could be a truly quantum aspect of thermodynamics with no classical analog. We examine this assertion within the framework of classical Hamiltonian mechanics and canonical quantization. For classical states we define a so called non-uniformity measure and show that it is a resource for work extraction similar to quantum coherence. Additionally, we show that work extracted from non-uniformity and coherence agree in the classical limit. This calls into question the idea that coherence qualitatively separates classical and quantum thermodynamics.

The final part of this thesis explores the connection between decoherence and adiabatic (quasistatic) driving. This topic is inspired by an experiment where it was seen that strong dephasing suppressed energy level transitions. Using a perturbative method we investigate this mechanism in the regime of small to moderate decoherence rate and ask if decoherence can help suppress energy transitions when compared with an adiabatic process without decoherence. We find that strategies that include decoherence are inferior to those where decoherence is absent.

While all of these topics will be touched upon in the defense presentation, the talk will focus on work, quantum coherence, and classical non-uniformity.

AnaValdés-Curiel - December 2, 2019

Dissertation Title: Topological dispersion relations in spin-orbit coupled Bose gases

Date and Time: Monday, December 2, 2:00 pm

Location: PSC 2136

Dissertation Committee Chair: Dr. Alicia Kollár

Committee:

Dr. Ian Spielman
Dr. Mohammad Hafezi
Dr. Norbert Linke
Dr. Gretchen Campbell
Dr. Trey Porto

Abstract:

Quantum degenerate gases have proven to be an ideal platform for the simulation of complex systems. Due to their high level of control it is possible to readily design and implement systems with effective Hamiltonians in the laboratory. This thesis presents new tools for the characterization and control of engineered quantum systems and describes one application in the engineering and characterization of a topological system with Rashba-type spin-orbit coupling.

The underlying properties of these engineered systems depend on their single particle energies. I describe a Fourier transform spectroscopy technique for characterizing the single particle spectrum of a quantum system. We tested Fourier spectroscopy by measuring the dispersion relation of a spin-1 spin-orbit coupled Bose-Einstein condensate (BEC) and found good agreement with our predictions.

Decoherence due to uncontrolled fluctuations of the environment presents fundamental obstacles in quantum science. I describe an implementation of continuous dynamical decoupling (CDD) in a spin-1 BEC. We applied a strong radio-frequency magnetic field to the ground state hyperfine manifold of Rubidium-87 atoms, generating a dynamically protected dressed system that was first-order insensitive to changes in magnetic field. The CDD states constitute effective clock states and we observed a reduction in sensitivity to magnetic field of up to four orders of magnitude. I show that the CDD states can be coupled in a fully connected geometry and thus enable the implementation of new models not possible using the bare atomic states.

Finally, I describe the engineering of Rashba-type SOC using Raman coupled CDD states. Our system had non-trivial topology but no underlying crystalline structure that yields integer valued Chern numbers in conventional materials. We validated our procedure using Fourier transform spectroscopy to measure the full dispersion relation containing only a single Dirac point. We measured the quantum geometry underlying the dispersion relation and obtained the topological index using matter-wave interferometry. In contrast to crystalline materials, where topological indices take on integer values, our continuum system reveals an unconventional half-integer Chern number.

Ali Hamed Moosavian - November 19, 2019

Jeremy Young - November 6, 2019

Dissertation Title: Nonequilibrium dynamics in open quantum systems

Date and Time: Wednesday, November 6, 3:00 pm

Location: PSC 1136

Dissertation Committee Chair: Professor Steven Rolston

Committee:

Professor Alexey Gorshkov (Advisor)
Professor Mohammad Maghrebi
Professor Mohammad Hafezi
Professor Maissam Barkeshli

Abstract:

Due to the variety of tools possible to control atomic, molecular, and optical (AMO) systems, they provide a versatile platform for studying many-body physics, quantum simulation, and quantum computation. Although extensive efforts are employed to reduce coupling between the system and the environment, the effects of the environment can never fully be avoided, so it is important to develop a comprehensive understanding of open quantum systems. The system-environment coupling often leads to loss via dissipation, which can be countered by a coherent drive. Open quantum systems subject to dissipation and drive are known as driven-dissipative systems, and they provide an excellent platform for studying many-body nonequilibrium physics.

The first part of this dissertation will focus on Rydberg atoms. In particular, we study how the spontaneous generation of contaminant Rydberg states drastically modifies the behavior of a driven-dissipative Rydberg system due to the resultant dipole-dipole interactions. These interactions lead to a complicated competition of both blockade and antiblockade effects, resulting in strongly enhanced Rydberg populations for far-detuned drive and reduced Rydberg populations for resonant drive.

The second part of this dissertation will focus on driven-dissipative phase transitions. In spite of the nonequilibrium nature of these systems, the corresponding phase transitions tend to exhibit emergent equilibrium behavior. However, we will show that in the vicinity of a multicritical point where multiple phase transitions intersect, genuinely nonequilibrium criticality can emerge, even though the individual phase transitions on their own exhibit equilibrium criticality. These nonequilibrium multicritical points can exhibit a variety of exotic phenomena not possible in their equilibrium counterparts, including the emergence of complex critical exponents, which lead to discrete scale invariance and spiraling phase boundaries. Furthermore, the Liouvillian gap can take on complex values, and the fluctuation-dissipation theorem is violated, corresponding to an effective “temperature” which is scale-dependent.

Sanjukta Krishnagopal - November 4, 2019

Dissertation Title: Machine Learning and Network Science to Unravel Patterns in Complex Temporal Data

Date and Time: Monday, November 4, 12:30 pm

Location: IREAP Seminar room ERF 1207

Dissertation Committee Chair: Dr. Michelle Girvan

Committee:

Dr. Brian Hunt
Dr. Edward Ott
Dr. Raj Roy
Dr. James Reggia

Abstract:

We uncover underlying patterns in complex data and forecast their temporal evolution through machine learning and network science. With the availability of more data, machine learning for data analysis has advanced rapidly. We primarily use a dynamical network architecture called reservoir computing to solve the ‘chaos’ version of the cocktail party problem, i.e., where a party-goer hears several signals but is interested in separating out only one signal. We separate superimposed signals from the chaotic Lorenz system and forecast the separated signals. We assume no knowledge of the system that produce the signals, and require only that the training data consist of finite time samples of the component signals. We find that our method significantly outperforms the optimal linear solution to the separation problem, the Wiener filter.

While machine learning is a useful data processing tool, often systems with multiple interacting components are best modeled as a network. Tools that identify properties on networks of temporal multi-variate data (such as disease data) are limited in literature. We close this gap by introducing a novel data-driven, network-based Trajectory Profile Clustering (TPC) algorithm for 1) identification of disease subtypes and 2) early prediction of subtype/disease progression patterns. TPC identifies subtypes by clustering patients with similar disease trajectory profiles derived from bipartite patient-variable networks. For the Parkinson’s dataset, we show that TPC identifies 3 distinct disease subtypes and predicts subtype in test patients 4 years in advance with 74% accuracy. While we implement it on Parkinson's datasets to identify patient subtypes and predict disease progression in new patients, this network medicine approach can be extended to any complex dataset.

Jaideep Pathak - October 18, 2019

Dissertation Title: Machine Learning Approaches for Data-Driven Analysis and Forecasting of High-Dimensional Chaotic Systems

Date and Time: Friday, October 18, 10:30 am

Location: A.V. Williams Building, Room 2460 (ECE Conference Room)

Dissertation Committee Chair: Dr. Edward Ott

Committee:

Dr. Michelle Girvan
Dr. Brian Hunt
Dr. Rajarshi Roy
Dr. Thomas Antonsen

Abstract:

We consider problems in the forecasting of large, complex, spatiotemporal chaotic systems and the possibility that machine learning might be a useful tool for significant improvement of such forecasts. Focusing on weather forecasting as perhaps the most important example of such systems, we note that physics-based weather models have substantial error due to various factors including imperfect modeling of subgrid-scale dynamics and incomplete knowledge of physical processes. In this thesis, we ask if machine learning can potentially correct for such knowledge deficits.

First, we demonstrate the effectiveness of using machine learning for model-free prediction of spatiotemporally chaotic systems of arbitrarily large spatial extent and attractor dimension purely from observations of the system's past evolution. We present a parallel scheme with an example implementation based on the reservoir computing paradigm and demonstrate the scalability of our scheme using the Kuramoto-Sivashinsky equation as an example of a spatiotemporally chaotic system.

We then demonstrate the use of machine learning for inferring fundamental properties of dynamical systems, namely the Lyapunov exponents, purely from observed data. We obtain results of unprecedented fidelity with our novel technique, making it possible to find the Lyapunov exponents of large systems where previously known techniques have failed.

Next, we propose a general method that combines a physics-informed knowledge-based model and a machine learning technique to build a hybrid forecasting scheme. We further extend our hybrid forecasting approach to the difficult case where only partial measurements of the state of the dynamical system are available. For this purpose, we propose a novel technique that combines machine learning with a data assimilation method called an Ensemble Transform Kalman Filter (ETKF).

We conclude by describing our ongoing collaboration with atmospheric science researchers on applying the above proposed techniques to a real-world weather forecasting scenario and also remark on the potential of machine learning assisted numerical weather prediction.

Aaron Ostrander - October 17, 2019

Dissertation Title: Quantum Algorithms for Differential Equations

Date and Time: Thursday, October 16, 10:00 am

Location: PSC 1136

Dissertation Committee Chair: Prof. Andrew Childs (Advisor)

Committee:

Prof. Chris Monroe (Co-Advisor)
Prof. Alexey Gorshkov
Prof. Mohammad Hafezi
Prof. Xiaodi Wu

Abstract:

This thesis describes quantum algorithms for Hamiltonian simulation, ordinary differential equations (ODEs), and partial differential equations (PDEs).

Product formulas are used to simulate Hamiltonians which can be expressed as a sum of terms which can each be simulated individually. By simulating each of these terms in sequence, the net effect approximately simulates the total Hamiltonian. We find that the error of product formulas can be improved by randomizing over the order in which the Hamiltonian terms are simulated. We prove that this approach is asymptotically better than ordinary product formulas and present numerical comparisons for small numbers of qubits.

The ODE algorithm applies to the initial value problem for time-independent first order linear ODEs. We approximate the propagator of the ODE by a truncated Taylor series, and we encode the initial value problem in a large linear system. We solve this linear system with a quantum linear system algorithm (QLSA) whose output we perform a post-selective measurement on. The resulting state encodes the solution to the initial value problem. We prove that our algorithm is asymptotically optimal with respect to several system parameters.

The PDE algorithms apply the finite difference method (FDM) to Poisson's equation, the wave equation, and the Klein-Gordon equation. We use high order FDM approximations of the Laplacian operator to develop linear systems for Poisson's equation in cubic volumes under periodic, Neumann, and Dirichlet boundary conditions. Using QLSAs, we output states encoding solutions to Poisson's equation. We prove that our algorithm is exponentially faster with respect to the spatial dimension than analogous classical algorithms. We also consider how high order Laplacian approximations can be used for simulating the wave and Klein-Gordon equations. We consider under what conditions it suffices to use Hamiltonian simulation for time evolution, and we propose an algorithm for these cases that uses QLSAs for state preparation and post-processing.

Daniel Campbell - October 16, 2019

Dissertation Title: Electronic and Magnetic Properties of MnP-Type Binary Compounds

Date and Time: Wednesday, October 16, 1:00 pm

Location: PSC 2126

Dissertation Committee Chair: Prof. Johnpierre Paglione

Committee:

Prof. Richard L. Greene
Prof. Nicholas P. Butch
Prof. James R. Williams
Prof. Efrain Rodriguez (Dean's Representative)

Abstract:

The interactions between electrons, and the resulting impact on physical properties, are at the heart of present-day materials science. This thesis looks at this idea through the lens of several compounds from a single family: the MnP-type transition metal pnictides. FeAs and FeP show long range magnetic order with some similarities to the high temperature, unconventional iron-based superconductors. CoAs lies on the border of magnetism, with strong fluctuations but no stable ordered state. CoP, in contrast, shows no strong magnetic fluctuations but serves as a useful baseline in determining the origin (from composition, structure, or magnetic order) of behavior in the other materials.

For this work, single crystals were grown with two different techniques: solvent flux and chemical vapor transport. In the case of FeAs the flux method resulted in the highest quality crystals yet produced. Extensive work was then performed on these samples at the University of Maryland and the National High Magnetic Field Laboratory. Quantum oscillations observed in high magnetic fields, in combination with density functional theory calculations, give insight into the Fermi surfaces of these materials. Large magnetoresistance in the phosphides, but not the arsenides, demonstrates differences in the choice of pnictogen atom that cannot be simply a product of electron count. Angle-dependent linear magnetoresistance in FeP is a sign of a possible Dirac dispersion and topological physics, as has been hinted it in other MnP-type materials. Ultimately, it is possible to examine results for all four compounds and draw conclusions on the role of each of the two elements in the formula, which can be extended to other members of this family.

Harvey Kaplan - September 25, 2019

Connor Roncaioli - September 10, 2019

Donggeun Tak - August 29, 2019

Zachary Schutz Smith - August 8, 2019

Megan Smith - July 23, 2019

Qile Zhang - July 19, 2019

Dissertation Title: PARTICLE HEATING AND ENERGY PARTITION IN RECONNECTION WITH A GUIDE FIELD

Date and Time: Friday, July 19, 10:00 am

Location: AVW 2460

Dissertation Committee Chair: Dr. James Drake

Committee:

Dr. Marc Swisdak
Dr. Thomas Antonsen
Dr. Adil Hassam
Dr. Dennis Papadopoulos

Abstract:

Kinetic Riemann simulations have been completed to explore particle heating during reconnection with a guide field in the low-beta environment of the inner heliosphere and the solar corona. The reconnection exhaust is bounded by two rotational discontinuities (RDs) with two slow shocks (SSs) that form within the exhaust as in magnetohydrodynamic (MHD) models. At the RDs, ions are accelerated by the magnetic field tension to drive the reconnection outflow as well as flows in the out-of-plane direction. The out-of-plane flows stream toward the midplane and meet to drive the SSs. The turbulence at the SSs is weak so the shocks are laminar and produce little dissipation, which differs greatly from the MHD treatment. Downstream of the SSs the counter-streaming ion beams lead to higher density and therefore to a positive potential between the SSs that confines the downstream electrons to maintain charge neutrality. The potential accelerates electrons from upstream of the SSs to downstream and traps a small fraction but only produces modest electron heating. In the low-beta limit the released magnetic energy is split between bulk flow and ion heating with little energy going to electrons.

To firmly establish the laminar nature of reconnection exhausts, we explore the role of instabilities and turbulence in the dynamics. Two-dimensional reconnection and Riemann simulations reveal that the exhaust develops large-amplitude striations resulting from electron-beam-driven ion cyclotron waves. The electron beams driving the instability are injected into the exhaust from one of the RDs. However, in 3D Riemann simulations, the additional dimension results in a strong Buneman instability at the RD, which suppresses electron beam formation. The 3D simulation does reveal a weak ion-ion streaming instability within the exhaust. All these instabilities become weaker with higher ion-to-electron mass ratio due to higher electron thermal speed. We also use a kinetic dispersion relation solver to show that the ion-ion instability will become stable in conditions expected under lower upstream beta. The results suggest that in realistic reconnection exhausts, which have three dimensions and real mass ratio, the kinetic-scale turbulence that develops will be too weak to play a significant role in energy conversion.

Israel Martinez Castellanos - July 10, 2019

Dissertation Title: Search for gamma-ray counterparts of gravitational wave events and other transient signals with HAWC

Date and Time: Wednesday, July 10, 10:00 am

Location: PSC 3150

Dissertation Committee Chair: Dr. Jordan A. Goodman

Committee:

Dr. Peter S. Shawhan
Dr. Julie McEnery
Dr. Gregory Sullivan
Dr. Suvi Gezari

Abstract:

In recent years we have seen major advances in multi-messenger astronomy. A milestone was achieved by identifying the electromagnetic counterpart of the gravitational wave event GW170817 detected by LIGO and Virgo. Similar efforts led to a set of neutrinos detected by IceCube to be associated with the blazar TXS 0506+056. Both demonstrate the potential of using multiple types of probes to study an astrophysical source.

The High-Altitude Water Cherenkov Observatory (HAWC), located in the state of Puebla, Mexico, is a wide field instrument (~2 sr) sensitive to very-high-energy gamma rays (~0.1-100 TeV) which can operate with a large duty cycle (>95%). These characteristics make it well suited to look for transient events correlated with other astronomical messengers. In this work we present a maximum likelihood analysis framework developed to search and analyze signals in HAWC data of arbitrary timescales.

We apply this method to search for very-high-energy gamma-ray counterparts of gravitational waves in short timescales (0.3-1000 s). We show that we would be able to either detect or meaningfully constrain the very-high-energy component of a gamma-ray burst within the binary neutron star merger horizon of current gravitational wave detectors if it occurs in our field of view. We did not find evidence for emission for any of the events analyzed. The source location of GW170817 was not observable by HAWC at the time of the merger.

We also set flux upper bounds for TXS 0506+056 during the periods when the neutrino flares were identified. For the flare between September 2014 and March 2015 these are the only available limits at very high energy, and are consistent with the low state in high-energy gamma rays reported by the Fermi-LAT Collaboration.

Peizhi Du - June 5, 2019

Dissertation Title: Leptogenesis and phase transition in a warped extra dimension

Date and Time: Wednesday, June 5, 11:00 am

Location: PSC 1136

Dissertation Committee Chair: Prof. Kaustubh Agashe

Committee:

Prof. Zackaria Chacko
Prof. Theodore Jacobson
Prof. Raman Sundrum
Prof. Richard Wentworth

Abstract:

Despite the success of the standard model of particle physics, a few unsolved problems call for new physics beyond the SM. In this thesis, we studied a natural embedding of the well-known high scale type I seesaw mechanism in a warped five dimensional (5D) theory. Such model can address both neutrino mass problem and a profound theoretical problem in the SM, called Planck weak hierarchy problem. In contrast to the usual high scale type I seesaw mechanism in four dimensions, 5D warped seesaw model become TeV scale “inverse” seesaw like model after Kaluza-Klein decomposition into 4D theories. We also showed this model predicts attractive signatures at the Large Hadron Collider. Moreover, the baryon asymmetry of the universe can be naturally generated via leptogenesis mechanism in seesaw models. We demonstrated that the simplified warped seesaw can achieve successful leptogenesis and feature an interesting interplay of high scale generation of the asymmetry and TeV scale washouts. To make this mechanism realistic in the full warped seesaw model, we also studied the phase transition from the high temperature black hole phase to low temperature phase with two branes in 5D theories. According to AdS/CFT duality, such phase transition is dual to the de-confining and confining phase transition in strongly coupled nearly conformal 4D theories. It was previously believed that such phase transition rate is too slow at critical temperature, resulting in a large amount of supercooling and diluting all primordial abundance. We proposed a mechanism to achieve fast phase transition around critical temperature and thus the asymmetry generated from leptogenesis can survive till today.

Neill Warrington - May 22, 2019

Gareth Roberg-Clark - May 15, 2019

Dissertation Title: Suppression of electron thermal conduction in the intracluster medium of galaxy clusters

Date and Time: Wednesday, May 15, 2:00 pm

Location: AVW 2460 (ECE)

Dissertation Committee Chair: Prof. James F. Drake

Committee:

Prof. William Dorland
Prof. Adil Hassam
Prof. Thomas Antonsen
Prof. M. Coleman Miller

Abstract:

Understanding the thermodynamic state of the hot intracluster medium (ICM) in a galaxy cluster requires a knowledge of the plasma transport processes, especially thermal conduction. The basic physics of thermal conduction in plasmas with ICM-like conditions has yet to be elucidated, however. We use particle-in-cell simulations and analytic models to explore the dynamics of an ICM-like plasma (with small gyroradius, large mean-free-path, and strongly sub-dominant magnetic pressure) induced by heat fluxes associated with thermal conduction. In this system $\beta >> 1$ where $\beta = P_{thermal}/P_{mag} = 8\pi nT / B^{2}$ is the ratio of thermal pressure to magnetic field energy density.

Linear theory reveals that whistler waves are driven unstable by electron heat flux, even when the heat flux is weak. The resonant interaction of electrons with these waves then plays a critical role in scattering electrons and suppressing the heat flux. In a 1D model where only whistler modes that are parallel to the magnetic field are captured, the only resonant electrons are moving in the opposite direction to the heat flux and the electron heat flux suppression is small. In 2D or more, oblique whistler modes also resonate with electrons moving in the direction of the heat flux. The overlap of resonances leads to effective symmetrization of the electron distribution function and a strong suppression of heat flux.

In a numerical model with continually supplied heat flux in the system, two thermal reservoirs at different temperatures drive an electron heat flux that destabilizes oblique whistlers. The whistlers grow to large amplitude, $\delta B / B_{0} \simeq 1$, and resonantly scatter the electrons. A surprise is that the resulting steady state heat flux is largely independent of the thermal gradient. The rate of thermal conduction is instead controlled by the finite propagation speed of the whistlers, which act as mobile scattering centers that convect the thermal energy of the hot reservoir.

When the plasma $\beta$ is reduced in the numerical model, we find that a transition takes place between whistler-dominated (high-$\beta$) and double-layer-dominated (low-$\beta$) heat flux suppression. Whistlers saturate at small amplitude in the low $\beta$ limit and are unable to effectively suppress the heat flux. Electrostatic double layers suppress the heat flux to a mostly constant factor of the free streaming value once this transition happens. The double layer physics is an example of ion-electron coupling and occurs on a scale of roughly the electron Debye length. The scaling of ion heating associated with the various heat flux driven instabilities is explored over the full range of $\beta$. The range of plasma-$\beta$s studied in this work makes it relevant to the dynamics of a large variety of astrophysical plasmas, including not just the intracluster medium but hot accretion flows, stellar and accretion disk coronae, and the solar wind.

Joshua Isaacs - May 14, 2019

Dissertation Title: The Physics of High-Intensity Laser-Matter Interactions and Applications

Date and Time: Tuesday, May 14, 3:30 pm

Location: ERF 1207A

Dissertation Committee Chair: Prof. Phillip Sprangle

Committee:

Prof. Thomas Antonsen
Prof. Howard Milchberg
Prof. Joseph Penano
Prof. Antonio Ting
Prof. Rajarshi Roy (Dean's Representative)

Abstract:

This dissertation consists of three separate research topics:

First, the effect of laser noise on the propagation of high-power and high-intensity short pulse lasers in dispersive and nonlinear media is studied. We consider the coupling of laser intensity noise and phase noise to the spatial and temporal evolution of laser radiation. We show that laser noise can have important effects on the propagation of high-power as well as high-intensity lasers in a dispersive and nonlinear medium such as air. We present atmospheric propagation examples of the spatial and temporal evolution of intensity and frequency fluctuations due to noise for laser wavelengths of 0.85 μm, 1 μm, and 10.6 μm.

Next, a concept for all-optical remote detection of radioactive materials is presented and analyzed. The presence of excess radioactivity increases the level of negative ions in the surrounding air region. This can act as a source of seed electrons for a laser-induced avalanche ionization breakdown process. We model irradiated air to estimate the density of negative ions and use a set of coupled rate equations to simulate a subsequent laser-induced avalanche ionization. We find that ion-seeded avalanche breakdown can be a viable signature for the detection of radioactivity, a conclusion which has been experimentally tested and verified.

Finally, we propose and analyze a mechanism to accelerate protons from close to rest in a laser-excited plasma wave. The beating of two counter-propagating laser pulses in a plasma shock-excites a slow forward-propagating wakefield. The trapping and acceleration of protons is accomplished by tapering both the plasma density and the amplitude of the backward-propagating pulse. We present an example in which protons are accelerated from 10 keV to 10 MeV in a distance of approximately 1 cm.

Chen Li - May 10, 2019

Dissertation Title: Novel Approaches to Control of Surface Reactions in Plasma Etching of Electronic Materials

Date and Time: Friday, May 10, 3:00 pm

Location: IREAP conference room

Dissertation Committee Chair: Prof. Gottlieb S. Oehrlein

Committee:

Prof. Steven Anlage
Prof. William Dorland
Prof. Derek Boyd
Prof. Thomas Antonsen

Abstract:

Advanced semiconductor manufacturing requires precise plasma etching control for patterning complex semiconductor device structures. Pattern transfer into dielectric materials is one of the most frequently performed operation and traditionally done using continuous wave (CW) plasma etching processes based on fluorocarbon (FC) chemistries. Such etching methods are facing challenges when critical dimension (CD) approach 10 nm. Issues include low materials etching selectivity, surface damage, roughness and poor etching profile control. In this work, various aspects of low temperature plasma-based etching approaches are tailored for optimal plasma etching performance, including novel gaseous precursors for better control of gas phase and surface processes, tailoring the relative importance of radicals and ion bombardment at surface by sequential processes, and a new way to input energy to surfaces to stimulate etching reactions. We systematically studied the impact of molecular structure parameters of hydrofluorocarbon (HFC) precursors on plasma deposition of fluorocarbon (FC) and material etching performance. The HFC chemical composition and molecular structure such as ring structure, C=C, C≡C, C-O, C-H and degree of unsaturation have dramatic impacts on FC surface polymerization and material etching performance. Further, we report a new atomic layer etching (ALE) technique which temporally separates chemical reactant supply to a surface from ion bombardment induced etching. By this ALE method, the ion bombardment energy can be reduced to ensure low substrate damage and extremely high etching selectivity of two materials. Finally, we developed a hollow cathode electron beam etching system to reduce the energy and momentum input on the material surface by utilizing an electron-radical synergy effect. This present work has unveiled highly promising elements of a new roadmap of next generation semiconductor etching approaches and is expected to impact multiple areas of nanoscience and technology, including plasma etching of post-silicon materials. The use of specially selected gaseous precursor chemistry, temporal separation of radical exposure and energy-induced etching, and finally using electron bombardment for activation of surface etching, challenge our current understanding of semiconductor plasma processing and presents an important step forward in terms of the further industrial development of these approaches.

Andrew Allocca - May 9, 2019

Dissertation Title: Casimir and Optical Phenomena in Two-dimensional Systems

Date and Time: Thursday, May 9, 2:00 pm

Location: PSC 3150

Dissertation Committee Chair: Prof. Victor Galitski

Committee:

Prof. Theodore Einstein
Prof. Mohammad Hafezi
Prof. Frederick Wellstood
Prof. Victor Yakovenko

Abstract:

The nature of the interaction of light with matter is a subject of great interest in condensed matter physics. Here we study the behavior of three electromagnetic effects arising from the coupling of light to two-dimensional electron systems: the Casimir effect, excitons in an insulator, and the formation of polaritons in a cavity.

We begin by examining how the Casimir effect is affected by material properties. First we consider using the Casimir force as a probe of a change in the topology of a material's Fermi surface, called a Lifshitz transition. We study a spin-orbit coupled semiconducting system which can be made to undergo this sort of transition with an external magnetic field, and find that the signature of this transition is a non-analyticity in the Casimir force at the transition point.

We then consider how the phenomenon of weak localization can be used as a test of the role of disorder when describing the Casimir effect between metallic objects. We show how the sensitive dependence of the conductivity of a two-dimensional disordered metal on both temperature and magnetic field should translate into similar sensitivities of the Casimir force, assuming effects of disorder should be included at all.

Next, we examine excitons formed in the bulk of an insulator as the system transitions between topological and trivial insulating phases, finding that the phases have different signatures in the exciton spectrum. This can be understood as an effect of the Berry curvature of the model giving an indirect glimpse of topological properties. We first construct a semiclassical model of the system to develop a qualitative intuition, then move to a numerical calculation in a full quantum model.

Finally, we consider the formation of polaritons inside of a photonic cavity containing a two-dimensional superconducting layer. We show how a coupling can be engineered between cavity photons resonant with a collective mode of the superconductor called the Bardasis-Schrieffer mode, leading to hybridized superconductor polariton states. Motivated by exciton polaritons condensation, we conjecture that a phase-coherent density of these objects could produce an exotic s+id superconducting state.

Andi Tan - May 7, 2019

Dissertation Title: PANDAX-II DARK MATTER DETECTOR AND ITS FIRST RESULTS

Date and Time: Tuesday, May 7, 9:00 am

Location: PSC 2136

Dissertation Committee Chair: Prof. Carter Hall

Committee:

Prof. Xiangdong Ji (Advisor)
Prof. Eun-Suk Seo
Prof. Thomas Cohen
Prof. Da-lin Zhang

Abstract:

The particle physics nature of dark matter (DM) is one of most fundamental scientific questions nowadays. The leading candidates, weakly interacting massive particles (WIMPs), can be directly detected by looking for WIMP-nucleus scattering events in deep underground laboratories. The Particle and astrophysical Xenon (PandaX) project is a series of xenon-based ultra-low background experiments in the China JinPing underground Laboratory (CJPL) targeting the unknown physics of DM and neutrinos. The first and second stage experiments (PandaX-I and II) both utilize dual-phase xenon time projection chamber (TPC) to carry out direct search for the dark matter particles. PandaX-II, a half-ton scale experiment, is currently under operation, and produced leading limits on DM-nucleon spin independent and spin dependent scattering cross sections in 2016 and 2017.

In the dissertation, I focus on the PandaX-II experiment including the development of the detector and data analysis. In 2017, PandaX-II experiment achieved a background level of 0.8×10⁻³ event/kg/day/keV which was the lowest among similar detectors at the time. Compared to other dual-phase xenon detector, we drift electrons by applying bias voltages on the electrodes which producing a stronger uniform electric field at a strength about 400 V/cm. After running for more than three years, more than 97% of 110 3-inch photo multiplier tubes (PMTs) perform stably. In the data analysis, I studied an inefficiency raised from zero length encoding (ZLE), a data suppression firmware of data acquisition system, in run periods with relatively low PMT gain. A data-driven algorithm is developed for the X-Y position reconstruction using the hit pattern of the proportional scintillation on the top PMT array which enlarged the fiducial volume for WIMP search by 20%. The background analysis is important for rare-event search experiments. I introduce the investigation on the intrinsic electron recoil background events from krypton, radon and xenon isotopes.

In this talk, I will present an overview of the project, discuss recent results from PandaX-II, and give an outlook into the future.

Christopher Eckberg - May 7, 2019

Lance Boyer - April 29, 2019

Dissertation Title: Superconductors that Break Time-Reversal Symmetry

Date and Time: Monday, April 29, 4:00 pm

Location: Toll 2202

Dissertation Committee Chair: Dr. Victor Yakovenko

Committee:

Dr. Steven Anlage
Dr. Maissam Barkeshli
Dr. Jeremy Munday
Dr. Victor Galitski

Abstract:

Since 2006 it has been discovered experimentally that the superconducting state spontaneously breaks time-reversal symmetry (TRS) in several materials, such as Sr2RuO4, UPt3, URu2Si2, PrOs4Sb12, and Bi/Ni bilayers. This dissertation studies three physical phenomena related to time-reversal symmetry breaking (TRSB) in these superconductors.

The experimental evidence for TRSB comes from the magneto-optical polar Kerr effect, which is determined by the high frequency ac Hall conductivity. However, these superconductors are also expected to exhibit a spontaneous dc Hall effect in the absence of an applied magnetic field. In the first part of this dissertation we propose a method for measuring the low frequency Hall conductivity in superconductors with TRSB. The method is based on a Corbino disk geometry where an oscillating co-axial magnetic field induces circular electric field, which, in turn, induces radial charge oscillations due to the Hall conductivity.

In the second part, we propose an explanation for the polar Kerr effect observed in the Hidden-Order phase of the heavy-fermion superconductor URu2Si2. Using a Ginzburg-Landau model for a complex order parameter, we show that the system can have a metastable ferromagnetic state, which produces the Kerr signal, even if the Hidden-Order state respects TRS. We predict that applying a reversed magnetic field should reset the system to the non-magnetic ground state, resulting in zero Kerr signal.

In the third part of the dissertation, we investigate the conditions for the existence of a Majorana bound state on a vortex in a 2D d+id superconductor with strong spin-orbit coupling. This TRSB pairing was proposed earlier for the Ni/Bi bilayer. We find that the Majorana bound state can exist for a d+id pairing under conditions similar to those for s-wave pairing.

Josue Morales-Cifuentes - April 24, 2019

Dissertation Title: Submonolayer Adsorbates: Theoretical Studies of Transient Mobility and Symmetry Breaking

Date and Time: Wednesday, April 24, 11:00 am

Location: PSC 2148

Dissertation Committee Chair: Prof. Theodore L. Einstein

Committee:

Prof. Janice E. Ruett-Robey
Prof. John D. Weeks
Prof. Ellen D. Williams
Prof. Alberto Pimpinelli

Abstract:

Weakly bound submonolayer adsorbates provide important insight into fundamental descriptions of physics that would otherwise be masked, or even suppressed, by strong effects such as chemical binding. We focus on two surface effects: transient mobility at the microscopic scale, and symmetry breaking at the atomic one.

We present a novel island nucleation and growth model that explicitly includes, at the microscopic scale, the behavior of transient (ballistic) monomers. At a deposition rate F, monomers are assumed to be in a hot precursor state before thermalizing. In the limiting regimes of fast (diffusive) and slow (ballistic) thermalization, we recover the expected scaling of the island density, N: N ~ F^α. We construct effective exponents α_eff and activation energies to properly characterize the transitional regimes between these limiting regimes. Through these constructs, we describe a rich and complex structure of meta-stable limiting regimes, asymptotic behavior and energetically driven transitions. Application to N(F, T) of recent organic-molecule deposition experiments yields excellent fits. We have also studied, at the atomic scale, an effective potential mechanism that breaks the intrinsic two-fold sublattice (triangular) symmetry of (honeycomb) graphene using DFT calculations (VASP ver 5.3.3).

We choose the specific system CF₃Cl substrates on graphene, to benefit from experimental results obtained locally. Using ab initio van der Waals density functionals, we discover a physisorbed phase with binding energies of about 280meV. For low coverages, sublattice symmetry breaking effects are responsible for gap openings of 4meV; contrastingly, in large coverages it is the formation of ordered overlayers that opens gaps nearly 5 times as large, of roughly 15meV. We discover that in both cases, it is the breaking of the symmetry between graphene's two sublattices that induces symmetry breaking by means of adsorbate interactions that favor large ordered regions on a single sublattice, coverage itself is insignificant.

Zachary Raines - April 22, 2019

Dissertation Title: Hybridization and enhancement processes in quasi-two dimensional superconductors

Date and Time: Monday, April 22, 2:00 pm

Location: PSC 2136

Dissertation Committee Chair: Prof. Victor M. Galitski

Committee:

Prof. Alexey Gorshkov
Prof. Richard Greene
Prof. Jay Deep Sau
Prof. John Weeks

Abstract:

Superconductivity is a field with a great many branches and applications. In this dissertation, we focus on two specific processes in superconductors – light-induced enhancement and hybridization of collective modes – in two types of quasi-two dimensional materials – either the loosely coupled planes of a layered superconductor or a superconducting thin film.

Motivated by experiments in the cuprates that have seen evidence of a transient superconducting state upon optical excitation we study the effects of inter-plane tunneling on the competition between superconductivity and charge order. We find that an optical pump can suppress the charge order and simultaneously enhance superconductivity, due to the inherent competition between the two. Taking into account that the charge order empirically shows a broad peak in c-axis momentum, we consider a model of randomly oriented charge ordering domains and study how interlayer coupling affects the competition of this order with superconductivity.

Also in the cuprates, several groups have reported observations of collective modes of the charge order present in underdoped cuprates. Motivated by these experiments, we study theoretically the oscillations of the order parameters, both in the case of pure charge order, and for charge order coexisting with superconductivity. Using a hot-spot approximation we find in the coexistence regime two Higgs modes arising from hybridization of the amplitude oscillations of the different order parameters. We explore the damping channels of these hybrid modes.

As another means of enhancing superconductivity we consider coupling a two-dimensional superconducting film to the quantized electromagnetic modes of a microwave resonator cavity. We find that when the photon and quasiparticle systems are out of thermal equilibrium, a redistribution of quasiparticles into a more favorable non-equilibrium steady-state occurs, thereby enhancing superconductivity in the sample, a fluctuation analog of a phenomenon known as the Eliashberg effect.

Finally, following the recent success of realizing exciton-polariton condensates in cavities, we examine the hybridization of cavity photons with two types of collective modes in superconductors. Enabled by the recently predicted and observed supercurrent-induced linear coupling between these excitations and light, we find that significant hybridization between the superconductor's collective modes and resonant cavity photons can occur.

Wan-Ting Liao - April 4, 2019

Dissertation Title: Investigation of Tunneling in Superconductors in a Millikelvin Scanning Tunneling Microscope

Date and Time: Thursday, April 4, 1:00 pm

Location: PSC 1136

Dissertation Committee Chair: Dr. Christopher Lobb

Committee:

Dr. Frederick Wellstood
Dr. Michael Dreyer
Dr. Kevin Osborn
Dr. John Cummings

Abstract:

I describe my use of a dual-point millikelvin scanning tunneling microscope (STM) to observe tunneling into superconducting TiN and Nb surfaces. After describing the STM, I present tip-sample conductance-voltage characteristics measured on 25 nm and 50 nm thick films of superconducting TiN that show large qualitative variations at 0.5 K. At some locations the characteristics show a clear superconducting gap, as expected for conventional superconductor-normal (S-N) tunneling through a low transparency behavior, while at other locations I see a distinct zero-voltage conductance peak, as expected for S/N Andreev tunneling through high transparency barrier. I fit the Blonder-Tinkham-Klapwijk (BTK) model of Andreev tunneling to the data and use this to construct spatially resolved maps of the superconducting gap Δ, the local temperature T and the barrier transparency Z. I observe an unexpected correlation between variations in Δ, T and Z and discuss some possible causes, including local heating and surface contamination. I next describe I(V) (current-voltage) characteristics obtained as a function of the tunneling resistance R_n by varying the distance between a Nb STM tip and a Nb sample at 50 mK and 1.5 K. Depending on R_n, the junction can be in the phase-diffusion regime, the underdamped small junction limit or the point contact regime. The characteristics show sub-gap current steps that depend strongly on R_n, as expected from multiple Andreev reflection (MAR) effects. To better understand this behavior, I generalized the multiple Andreev reflection (MAR) theory of Averin and Bardas to the case where the junction electrodes can have different Δ. Fitting this MAR theory to the I(V) data, I extract the gap Δ_sampleof the sample, the gap Δ_tipof the tip, and the barrier transparency Z, all as a function of R_n. I find that Δ_sample = 1.5 meV, nearly the full gap of bulk Nb, but Δ_tip = 0.67 meV for R_n> 10kΩ and it decreases for R_n≤ 10 kΩ. I conclude with a discussion of some of the implications for STM of superconductors.

Zachary Eldredge - April 4, 2019

Zachary Epstein - April 3, 2019

Prasoon Gupta - April 1, 2019

Dina Genkina - February 13, 2019

Neal Pisenti - January 14, 2019

PhD Defenses 2016

Richard Knoche - December 9, 2016

Jeffery Demers - December 9, 2016

Xu Jiang - December 2, 2016

Ismail Volkan Inlek - November 7, 2016

David Green - November 2, 2016

Alex Jeffers - October 31, 2016

Shantanu Debnath - September 28, 2016

Xunnong Xu - September 26, 2016

Kale Johnson - September 19, 2016

Wrick Sengupta - August 30, 2016

Joshua Wood - August 12, 2016

Jacob Smith - August 11, 2016

Rachel Lee - July 27, 2016

Daniel Barker - July 25, 2016

Aaron Lee - July 19, 2016

Ryan Maunu - July 14, 2016

Anton de la Fuente - July 12, 2016

Matthew Adams - July 11, 2016

Christopher Verhaaren - June 8, 2016

PhD Defenses 2017

Jack Wimberley - December 13, 2017

Stephen Ragole - December 13, 2017

Erin Sohr - December 11, 2017

Jonathan Larson - December 8, 2017

Hilary Hurst - December 7, 2017

Matthew Reed - December 6, 2017

Benjamin Reschovsky - November 20, 2017

Andika Putra - November 17, 2017

Nightvid Cole - November 15, 2017

Eric Rosenthal - November 3, 2017

Renxiong Wang - October 25, 2017

David Wong-Campos - October 16, 2017

Ayoti Patra - September 15, 2017

Joseph L. Garrett - August 18, 2017

Tian Li - August 10, 2017

Ranchu Mathew - August 10, 2017

Joyce Coppock - August 1, 2017

Chang-Hun Lee - July 26, 2017

Sungwoo Hong - July 13, 2017

Nihal Jhajj - July 7, 2017

George Hine - May 25, 2017

Jacob Tosado - May 11, 2017

Gina Quan - April 27, 2017

Kenneth Wright - April 24, 2017

Nathaniel Steinsultz - April 10, 2017

Setiawan - April 7, 2017

Pablo Andrés Solano Palma - March 31, 2017

Thomas Rensink - January 20, 2017

PhD Defenses 2018

Clayton Crocker - December 13, 2018

Min-A Cho - December 11, 2018

Safa Motesharrei - October 29, 2018

David Meyer - October 26, 2018

Jeffrey Magill - September 21, 2018

Amit Nag - August 30, 2018

Cody Ballard - August 22, 2018

Simón Riquelme Muñoz - August 9, 2018

Shavindra Premaratne - July 12, 2018

Chun-Xiao Liu - July 6, 2018

David Somers - July 5, 2018

Chiao-Hsuan Wang - June 11, 2018

Mark Eichenlaub - May 30, 2018

Yee Lam Elim Cheung - May 2, 2018

Antony Speranza - April 27, 2018

Caroline Figgatt - March 22, 2018

Qin Liu - March 14, 2018

Jason Andrews - February 16, 2018

Kiersten Ruisard - February 14, 2018

Avinash Kumar - January 26, 2018

Jon Balajthy - January 19, 2018

PhD Defenses 2019

Noah Sennett - December 13, 2019

Andrew Glaudell - December 9, 2019

Andrew Smith - December 5, 2019

AnaValdés-Curiel - December 2, 2019

Ali Hamed Moosavian - November 19, 2019

Jeremy Young - November 6, 2019

Sanjukta Krishnagopal - November 4, 2019

Jaideep Pathak - October 18, 2019