## Wen-Chen Lin - June 12, 2023

Dissertation Title: **High Magnetic Field Studies of Unconventional Superconductor UTe2: Unveiling the Phase Diagram and Unconventional Hall Effect**

Date and Time: **Monday, June 12, 2:00 pm **

Location: ** Toll 2219**

Dissertation Committee Chair:** Prof. Johnpierre Paglione**

Committee:

Prof. Nicholas Butch

Prof. Steven Anlage

Prof. Min Ouyang

Prof. Efrain Rodriguez

Abstract:

The recent discovery of superconductivity in UTe2 has drawn strong attention owing to a fascinating list of properties -- including the absence of magnetic order at ambient pressure, extremely high upper critical fields – that have led to proposals of spin-triplet pairing, and multiple superconducting phases.

The first part of this dissertation unveils the comprehensive (H, T, P) phase diagram of UTe2 under magnetic fields reaching 41 T along the crystallographic b-axis, combined with applied pressures of up to 18 kbar. Utilizing magnetoresistance and tunnel diode oscillator measurements, we investigated the pressure-induced evolutions of multiple phases. Our findings indicate that the superconductivity in UTe2 is encapsulated within two orthogonal magnetic phases, specifically, a field-induced polarized phase and a pressure-induced magnetic ground state. This establishes the boundaries of triplet superconductivity in this system.

In the latter part of the thesis, we delve into an unconventional Hall effect in UTe2 that is induced by an in-plane magnetic field. An extensive study was conducted on the field and angle dependence of the Hall response under in-plane fields, confirming its field-odd symmetry nature, which aligns with the recently identified in-plane Hall effect. We examined the field dependence of the Hall response up to 33 T, and observed a remarkable enhancement, indicating a significant linkage to the well-identified metamagnetic transition. In addition, we will discuss the symmetry restrictions on the in-plane Hall response, and evidence of symmetry lowering in our system.

## Joseph Murray - June 6, 2023

Dissertation Title: **Low Temperature Scanning Tunneling Microscopy of Topological Materials and Magnetic Structures**

Date and Time: **Tuesday, June 6, 10:30 am**

Location: **PSC 2148**

Dissertation Committee Chair:** Prof. Chris Lobb**

Committee:

Dr. Robert Butera

Dr. Michael Dreyer

Prof. Johnpierre Paglione

Prof. John Cumings (Dean’s representative)

Abstract:

Scanning tunneling microscopy (STM) provides an opportunity to study the physical and electromagnetic properties of surfaces at the atomic scale. When performed at low temperatures, in high magnetic fields, and with a variety of different probes, it offers a wide range of methods by which novel materials of great practical and theoretical interest can be evaluated, characterized, and even fabricated with atomic precision.

This thesis describes three independent STM studies performed at cryogenic temperatures. In the first, we examined oxygen-doped aluminum films with anomalously high kinetic inductance. A suggested explanation was the migration of oxygen to the grain boundaries, forming a percolation network separated by Josephson links. To determine the coupling between grains, the films were studied using milliKelvin STM performed with a superconducting tip.

In the second study, transport measurements indicated the possible presence of a topological Hall effect in thin films of Cr2Te3, induced by the presence of topologically non-trivial magnetic textures called magnetic skyrmions. In order to provide more decisive evidence, we studied the films using spin-polarized STM at 4K.

Finally, we present an in-situ modification to our 4K STM which permits us to current-bias our samples during STM operation. The modification can be used to study non-equilibrium effects such as spin accumulation induced by a current through a spin Hall material and the spin-momentum locking which is present at the surface of topological insulators.

## Tamoghna Barik - May 25, 2023

Dissertation Title:** Tunneling properties in Fe-based superconductor, FeTe _{.55}Se_{.45} (FTS)**

Date and Time:

**Thursday, May 25, 10:30 AM**

Location:

**ATL 4402**

Dissertation Committee Chair:

**Prof. Jay D Sau**Committee:

Prof. Sankar Das Sarma

Prof. Victor Yakovenko

Prof. Maissam Barkeshli

Prof. Ichiro Takeuchi (Dean’s representative)

Abstract:

The Fe-based chalcogenide compound, FeTe_{1-x}Se_{x} (FTS), has recently attracted attention as a potential candidate for a readily available platform for topological superconducting (TSC) phase on its surface. The co-existence of the strong topological insulating (TI) phase and cylindrical Fermi sheets provide the two necessary ingredients for the TSC phase on its surface.

The strong TI phase in FTS depends on the relative Te/Se composition. The widely studied FTS sample, FeTe_{.55}Se_{.45}, interestingly, lies close to the topological-trivial phase boundary which is estimated to be close to x = .5 by a recent experiment. Additionally, a typical sample of FTS suffers from fluctuations in Te/Se composition at multiple length scales due to its alloy nature. Thus, in a topological FTS sample such fluctuations can give rise to regions where the phase is driven out of its topological nature. Such trivial domains would be scattered throughout the sample. After carefully exploring the effects of such topological domain disordered phase in an effective model of FTS, we conclude that its resultant signatures can be distinguished in a scanning tunneling spectroscopy (STS) measurement of the topological surface states (TSS) density.

Another possibility of an exotic phase arises on the TSC surface when a linear defect forms a Josephson junction (JJ) characterized by a phase shift of 𝜋 for the SC order parameter (OP). Such a 𝜋-JJ induces exotic helical Majorana modes - signatures of which has been observed by a recent tunneling experiment on the surface of FTS in the density of states (DOS) within the SC gap at a crystalline domain wall (DW) associated with half-unit-cell-shift (HUCS). Observation of such signatures naturally poses question about the origin of such 𝜋-JJ which is yet to be fully understood. We propose a mechanism that stabilizes a 𝜋-junction at the HUCS DW when the intrinsic superconducting pairing is of s_{±} character which is the case for bulk FTS. We argue that if the DW induced inter-pocket transmission between the Γ and M pockets in FTS is strong enough, the resultant enhancement of the two OPs of opposite sign stabilizes the 𝜋-junction.

## Naren Manjunath - May 24, 2023

Dissertation Title: **Classification and characterization of crystalline topological invariants in quantum many-body states**

Date and Time: **Wednesday, May 24, 10:30 am**

Location: ** ATL 4402 (CMTC conference room)**

Dissertation Committee Chair:** Professor Maissam Barkeshli (advisor)**

Committee:

Professor Jay Deep Sau

Professor Sankar Das Sarma

Professor Johnpierre Paglione

Professor Mohammad Hafezi (Dean’s Representative)

Abstract:

The theory of topological phases of matter, now a major direction in condensed matter physics, is framed around two complementary problems. The first is to mathematically classify topological states of matter given various symmetries, in terms of suitable topological invariants. The second is to characterize a given state by numerically or experimentally extracting the topological invariants associated with it. Although remarkable progress has been made for topological states with internal symmetries, major open questions remain in the case of many-body systems with crystalline symmetries, especially those with a nonzero Chern number and a magnetic field.

The first part of this thesis develops a theory of crystalline topological response in (2+1) dimensions based on the idea of crystalline gauge fields and their effective actions, which we derive using topological quantum field theory. We use this to obtain a complete classification of topological states with U(1) charge conservation, discrete magnetic translation, and point group rotation symmetries, finding several new invariants. We separately consider symmetry-enriched topological states of bosons, which admit anyonic excitations with fractional statistics, and invertible fermionic states, which do not.

The second part of this thesis focuses on numerically extracting these invariants from many-body invertible states. First, we study two quantized invariants, the discrete shift, and a charge polarization which is quantized by rotational symmetries. We show how to extract these invariants in multiple different ways, which include the fractional charge bound to lattice defects, as well as the angular and linear momentum of magnetic flux. Thereafter, we obtain a complete characterization of the theoretically predicted invariants, by studying the expectation value of the ground state under partial rotation operators.

An immediate application of these ideas is to fully characterize the celebrated Hofstadter model of spinless free fermions on a square lattice. Although the Chern number and filling were first computed in this model in 1982, our theory predicts seven nontrivial invariants, including four new invariants which depend on the crystalline symmetry. We compute these numerically and obtain several additional colorings of Hofstadter's butterfly.

## Saurabh Vasant Kadam - May 23, 2023

Dissertation Title: **Theoretical developments in lattice gauge theory for applications in double-beta decay processes and quantum simulation**

Date and Time: **Tuesday, May 23, 12:00 pm**

Location: ** PSC 2136**

Dissertation Committee Chair:** Zohreh Davoudi**

Committee:

Paulo Bedaque

Zackaria Chacko

Manuel Franco Sevilla

Konstantina Trivisa, Dean’s Representative

Abstract:

Nuclear processes have played, and continue to play, a crucial role in unraveling the fundamental laws of nature. They are governed by the interactions between hadrons, and in order to draw reliable conclusions from their observations, it is necessary to have accurate theoretical predictions of hadronic systems. The strong interactions between hadrons are described by quantum chromodynamics (QCD), a non-Abelian gauge theory with symmetry group SU(3). QCD predictions require non-perturbative methods for calculating observables, and as of now, lattice QCD (LQCD) is the only reliable and systematically improvable first-principles technique for obtaining quantitative results. LQCD numerically evaluates QCD by formulating it on a Euclidean space-time grid with a finite volume, and requires formal prescriptions to match numerical results with physical observables.

This thesis provides such prescriptions for a class of rare nuclear processes called double beta decays, using the finite volume effects in LQCD framework. Double beta decay can occur via two different modes: two-neutrino double beta decay or neutrinoless double beta decay. The former is a rare Standard Model transition that has been observed, while the latter is a hypothetical process whose observation can profoundly impact our understating of Particle Physics. The significance and challenges associated with accurately predicting decay rates for both modes are emphasized in this thesis, and matching relations are provided to obtain the decay rate in the two-nucleon sector. These relations map the hadronic decay amplitudes to quantities that are accessible via LQCD calculations, namely the nuclear matrix elements and two-nucleon energy spectra in a finite volume. Finally, the matching relations are employed to examine the impact of uncertainties in the future LQCD calculations. In particular, the precision of LQCD results that allow constraining the low energy constants that parameterize the hadronic amplitudes of two-nucleon double beta decays is determined.

Lattice QCD, albeit being a very successful framework, has several limitations when general finite-density and real-time quantities are concerned. Hamiltonian simulation of QCD is another non-perturbative method of solving QCD that, by its nature, does not suffer from those limitations. With the advent of novel computational tools, like tensor network methods and quantum simulation, Hamiltonian simulation of lattice gauge theories (LGTs) has become a reality. However, different Hamiltonian formulations of the same LGT can lead to different computational-resource requirements with their respective system sizes. Thus, a search for efficient formulations of Hamiltonian LGT is a necessary step towards employing this method to calculate a range of QCD observables. Toward that goal, a loop-string-hadron (LSH) formulation of an SU(3) LGT coupled to dynamical matter in 1+1 dimensions is developed in this thesis. Development of this framework is motivated by recent studies of the LSH formulation of an SU(2) LGT that is shown to be advantageous over other formulations, and can be extended to higher-dimensional theories and ultimately QCD.

## En-Jui Kuo - May 16, 2023

Dissertation Title: **Quantum Simulation of Bosonic Systems and Applications of Machine Learning**

Date and Time: **Tuesday, May 16, 11:00 am**

Location: **PSC 3150 **

Dissertation Committee Chair:** Mohammad Hafezi **

Committee:

Alexey Gorshkov

Maissam Barkeshli

Victor Albert

Amin Gholampour

Abstract:

First, we introduce the notion of "generalized bosons," whose exchange statistics resemble those of bosons, but the local bosonic commutator $[a_i,a_i^{\dagger}]=1$ is replaced by an arbitrary single-mode operator that is diagonal in the generalized Fock basis. Examples of generalized bosons include boson pairs and spins. We consider the analogue of the boson sampling task for these particles and observe that its output probabilities are still given by permanents, so the results regarding the difficulty of sampling carry over directly. Finally, we propose implementations of generalized boson sampling in circuit-QED and ion-trap platforms.

In the rest of the thesis, we move on to different topics. Firstly, we incorporate machine learning techniques in quantum information. We use machine learning to classify rational two-dimensional conformal field theories (CFTs). We first use the energy spectra of these minimal models to train a supervised learning algorithm. In contrast to conventional methods that are typically qualitative and involve system size scaling, our method quantifies the similarity of the spectrum of a system at a fixed size to candidate CFTs. Such an approach allows us to correctly predict the nature and value of critical points of several strongly correlated spin models using only their energy spectra. Our results are also relevant for the ground-state entanglement Hamiltonian of certain topological phases of matter described by CFTs. Remarkably, we achieve high prediction accuracy by only using the lowest few Rényi entropies as the input. Finally, using autoencoders, an unsupervised learning algorithm, we find a hidden variable that has a direct correlation with the central charge and discuss prospects for using machine learning to investigate other conformal field theories, including higher-dimensional ones.

Next, we demonstrate how machine learning techniques, especially unsupervised learning algorithms, can be used to study Symmetry-Protected Topological (SPT) phases of matter. SPT phases are short-range entangled phases of matter with a non-local order parameter that are preserved under a local symmetry group. Here, we use an unsupervised learning algorithm, namely diffusion maps, to differentiate between symmetry-broken phases and topologically ordered phases and between non-trivial topological phases in different classes. Specifically, we show that phase transitions associated with these phases can be detected in various bosonic and fermionic models in one dimension, including the interacting SSH model, the AKLT model and its variants, and weakly interacting fermionic models. Our approach provides a cost-effective computational method for detecting topological phase transitions associated with SPT systems, which can also be applied to experimental data obtained from quantum simulators.

## Batoul Banihashemi - May 16, 2023

Dissertation Title: **Thermodynamics of quantum gravitational ensembles**

Date and Time: **Tuesday, May 16, 10:30 am**

Location: **PSC 1136 **

Dissertation Committee Chair:** **Theodore Jacobson

Committee:

Alessandra Buonanno

Christopher Jarzynski, Dean’s Representative

Raman Sundrum

Brian Swingle

Abstract:

The discovery of black hole thermodynamics and its extension to cosmological horizons demonstrated a deep connection between thermodynamics and the nature of spacetime as a quantum system. It is then of great importance to properly understand the statistical mechanics of gravitational systems with horizon from first principles. While employing a partition function and the gravitational “Euclidean path integral” produces the expected physical result for entropy, a number of fundamental questions about the underlying analysis persist. This dissertation sharpens and resolves some puzzles regarding statistical mechanics of gravitational ensembles and the gravitational path integral, with a focus on cosmological horizon and de Sitter space.

The main questions addressed in this dissertation are: how is the entropy of de Sitter space derived in absence of any boundary on which the statistical ensemble can be properly defined? What is the correct interpretation of the first law of de Sitter horizon, according to which the horizon area shrinks upon adding matter in de Sitter static patch? And finally, how can entropy of horizon-bounded systems be derived from a Hamiltonian approach and phase space path integral, without the trickery of the gravitational Euclidean path integral? The first two questions are answered by introducing an artificial boundary in the system on which a gravitational ensemble can be properly defined. Once the ensemble is defined, the semiclassical approximation of the statistical partition function yields the entropy, and the interpretation of the de Sitter first law becomes clear by identifying the system energy as the quasilocal energy defined on the boundary. To tackle the last question, the real-time phase space path integral is utilised in the Hamiltonian formulation which maintains connection to the Hilbert space of the system, and it is found that the horizon entropy is derived from a nearly Lorentzian stationary point.

## Gong Cheng - May 16, 2023

Dissertation Title: **Quantum information scrambling and protection in many-body systems**

Date and Time: **Tuesday, May 16, 2:00 pm**

Location: **PSC 3150**

Dissertation Committee Chair:** Prof. Maissam Barkeshli **

Committee:

Prof. Brian Swingle (Co-Chair)

Prof. Paulo Bedaque

Prof. Victor Albert

Prof. Xiaodi Wu (Dean’s representative)

Abstract:

This work focuses on two topics in quantum information theory: the scrambling of quantum information and the preservation of quantum information in large degrees of freedom. The primary object I investigate in this topic is the Out-of-time-order correlator (OTOC), which probes the dynamics of quantum information as it spreads from localized degrees of freedom to those that are distributed throughout the system. Meanwhile, the goal of studying quantum information protection is to construct a system that can preserve quantum information for a sufficiently long time when coupled to a finite-temperature environment.

The many-body systems analyzed in this work belong or are related to a class of strongly interacting systems known as holographic quantum models. The standard examples in this class are believed to be equivalent to gravitational theory in spacetime that is one-dimensional higher than that the quantum model lives in. Therefore, the results may also provide insights into topics in quantum gravity.

The first part of the thesis explores the scrambling dynamics close to a critical point where conformal symmetry emerges. The second case deals with the scrambling dynamics with conservation law constraints in holographic quantum field theory. The result also clarifies how conserved charges influence the dynamics in the bulk dual.

The third part of the thesis presents a matrix model with a large matrix rank N that belongs to the class of approximate quantum error correction codes. We investigate its thermal stability by coupling it to a thermal bath and demonstrate that it behaves as a self-correcting quantum memory at finite temperature. The coherent memory time scales polynomially with the system size N.

## Troy Sewell - May 15, 2023

Dissertation Title: **Variational Algorithms and Resources for Near-Term Quantum Simulation**

Date and Time: **Monday, May 15, 3:00 pm**

Location: **PSC 3204 **

Dissertation Committee Chair:** Jay Sau **

Committee:

Stephen Jordan, Advisor / Co-chair

Brian Swingle

Maissam Barkeshli

Andrew Childs, Dean’s Representative

Abstract:

The difficulty of efficiently simulating quantum many-body systems was one of the first motivations for developing quantum computers and may also be one of the first applications to find practical computational advantage on real quantum hardware. With the relatively recent advent of publicly available quantum technologies, we have now entered the era of noisy intermediate-scale quantum (NISQ) computing. The capabilities of these technologies are evolving rapidly, and with them the computational affordances to which we have access. The time is now ripe to test the capabilities of existing quantum hardware and leverage them to the best of our ability toward achieving a practical quantum computational advantage.

In this dissertation, we address these aims by benchmarking a class of variational multi-scale quantum circuits for state preparation which are locally robust against noise. We demonstrate the advantages of these multi-scale circuits compared to a purely local circuit ansatz using the critical transverse-field Ising model to optimize circuit parameters, numerically test the noise resilience of observables and customized error mitigation techniques using a local gate noise model, and demonstrate the robustness of local subregion preparation on an existing ion-trap quantum computer. We then show how the ground state optimized circuit can be simply extended to an ansatz for thermal state preparation using the separation of energy scales afforded by the multi-scale circuit structure.

Additionally, we evaluate the quantum resources needed for some quantum simulation tasks. We estimate the gate complexity of the site-by-site algorithm for fault-tolerant ground state preparation, which we extend to the case of degenerate Hamiltonians. Using matrix product states we evaluate the non-stabilizer quantum resources needed to represent thermalized subregions of a chaotic Potts model, which we use to address the feasibility for classical simulation of quantum hydrodynamics.

## Stefano Antonini - May 15, 2023

Dissertation Title: **Holographic Cosmological Models and the AdS/CFT Correspondence**

Date and Time: **Monday, May 15, 10:00 AM**

Location: **PSC 3150**

Dissertation Committee Chair:** Theodore Jacobson **

Committee:

Zackaria Chacko

Christopher Jarzynski (Dean’s Representative)

Raman Sundrum

Brian Swingle (Academic Advisor)

Abstract:

The formulation of a quantum theory of gravity is a central open problem in theoretical physics. In recent years, the development of holography---and in particular the Anti-de Sitter/Conformal Field Theory (AdS/CFT) correspondence---provided a new framework to investigate quantum gravity and led to consistent advancement. However, how to describe cosmology within holography remains an unanswered question whose solution could determine whether holography is able to capture physics in our universe.

This dissertation describes a new proposal for embedding cosmological physics in the holographic paradigm. This is articulated in two different but related approaches, both involving time-symmetric Big Bang-Big Crunch cosmologies with negative cosmological constant Λ .

In the first approach, the cosmological universe is given by a four-dimensional end-of-the-world brane moving in a five-dimensional AdS black hole spacetime. The proposed holographic dual description is given by a boundary conformal field theory. Under specific conditions, gravity is localized on the brane and effectively four-dimensional: an observer living on the brane is unaware of the existence of the extra dimension. In this dissertation, I show how these conditions can be met in an AdS-Reissner-Nordström background while retaining a holographic dual description.

The second approach focuses on spatially flat Λ<0 cosmologies which analytically continue to Euclidean wormholes connecting two asymptotic AdS boundaries. The proposed dual theory is given by two holographic 3D CFTs coupled by non-holographic 4D degrees of freedom on a strip. A different analytic continuation of the Euclidean wormhole leads to a Lorentzian traversable wormhole. After discussing the general features of these holographic cosmologies, I describe how the traversable wormhole can be reconstructed from the dual theory and how the existence of the former constrains the latter. Finally, I show that these Λ<0 cosmologies can undergo phases of accelerated expansion and match observational data for the scale factor evolution.

## Sanket Doshi - May 10, 2023

Dissertation Title: ** Dark Matter and Neutrino Masses from a Composite Hidden Sector**

Date and Time: **Wednesday, May 10, 1:30pm**

Location: **PSC 2204 **

Dissertation Committee Chair:** Dr. Zackaria Chacko **

Committee:

Dr. Raman Sundrum

Dr. Kaustubh Agashe

Dr. Massimo Ricotti (Dean’s Representative)

Dr. Christopher Palmer

Abstract:

Despite the remarkable success of the Standard Model in explaining the interactions of the elementary particles, there is now indisputable evidence that it is incomplete. Although the Standard Model predicts that neutrinos are massless, over the last few decades experiments have established that the masses of the neutrinos, although very small, are nonvanishing. Furthermore, cosmological and astrophysical observations have established that about 80% of the matter in the universe is composed of some form of non-luminous dark matter, but there is no particle in the Standard Model that can play this role. Any explanation of the origin of neutrino masses and the nature of dark matter therefore requires physics beyond the SM. In this thesis, we present a novel class of models that can explain both the origin of neutrino masses and the observed abundance of dark matter.

In these models, the particle that constitutes dark matter arises as the composite state of a strongly coupled hidden sector that couples to the Standard Model through the neutrino portal. A discrete symmetry ensures that the dark matter particle is stable and does not decay. The hidden sector is in thermal equilibrium with the Standard Model in the early universe. The abundance of dark matter is set by its annihilation into final states containing neutrinos. The neutrino portal coupling also gives rise to small Majorana masses for the neutrinos through the inverse seesaw mechanism, with the role of the singlet neutrinos being played by composite states. The Standard Model neutrinos mix with the singlet neutrinos, and so the Standard Model neutrinos are partially composite in this framework. The dynamics of the hidden sector is taken to be approximately conformal in the ultraviolet, and a relevant deformation leads to breaking of the conformal symmetry in the infrared. Since the hidden sector is uncharged under the Standard Model gauge groups, the compositeness scale can lie below the weak scale, leading to striking experimental signals.

We employ the AdS/CFT correspondence to construct a holographic dual of this scenario. This takes the form of a Randall Sundrum model with two branes. Within this framework we explore the signals of these models at various current and future experiments. These include searches for lepton flavor violation in and conversion experiments, direct and indirect detection of dark matter and searches at colliders and beam dumps. We determine the current bounds on this scenario and show that future experiments can significantly expand the reach.

## John Collini - April 28, 2023

Dissertation Title: **HIGH PRESSURE DRIVEN EVOLUTION OF CHARGE ANDSTRUCTURAL ORDER IN NEMATIC SUPERCONDUCTOR, Ba1−xSrxNi2As2**

Date and Time:

**Friday, April 28, 1:30 pm**

Location:

**Toll 0360. QMC Conference Room**

Dissertation Committee Chair:

**Johnpierre Paglione**

Committee:

Richard Green

Nicholas Butch

Ichiro Takeuchi

Jeffrey Lynn

Abstract:

The desire for a complete understanding of high temperature unconventional superconductivity

has illustrated a necessity for the study of non-magnetic sources of superconducting

enchantment, such as nematically driven fluctuations and charge order fluctuations. BaNi2As2,

a non-magnetic counterpart to high Tc superconductor BaFe2As2, shows a six-fold superconducting

enhancement neighboring charge and nematic orders, positioning it as an excellent candidate

for studying the interactions between charge order, nematic order, and enhanced superconductivity.

In this thesis, I will present X-ray diffraction and electrical transport evidence for the

development of complex charge order within the system as functions of isovalent chemical

substitution via Ba1−xSrxNi2As2 and applied hydrostatic pressure. The discovery of three

separate charge order will be detailed: an incommensurate charge order at Q = 0.28

and two commensurate charge orders at Q = 0.33 and Q = 0.5. X-ray diffraction

measurements of the Q = 0.28 charge order will be used to show a strong correlation

between it and a previously established nematic order for Ba1−xSrxNi2As2. Applied

pressure of BaNi2As2 up to 10.4 GPa will detail the development of all three charge orders and be used to show a correlation between pressure and isovalvent substitution in BaNi2As2. The critical substitution of 71% Sr and the critical pressure of 9 ± 0.5 GPa will be directly compared by X-ray measurements of their lattice parameters, revealing a collapsed tetragonal phase. This phase is shown to be analogous to the collapsed

tetragonal phase of the Fe-pnictide superconductors, likely playing a key role seen at

the critical substitution and pressure of BaNi2As2

## Rahul Gaur - April 14, 2023

Dissertation Title: **Optimization of high-beta fusion devices against linear instabilities**

Date and Time: **Friday, April 14, 9:45 AM **

Location: **ERF 1207 Large conference room, Energy Research Facility**

Dissertation Committee Chair:** Prof. William Dorland **

Committee:

Dr. Ian Abel

Dr. Matt Landreman

Prof. Alexander Philippov

Prof. Thomas Antonsen (Dean’s representative)

Abstract:

Magnetic confinement fusion is a technique in which a strong magnetic field is used to contain a hot plasma, which enables nuclear fusion. Regarding overall energy efficiency, the two most promising magnetic confinement concepts are tokamaks (axisymmetric devices) and stellarators (nonaxisymmetric devices). The power P produced by a magnetically confined nuclear fusion device is proportional to , where V is the device's volume, β is the plasma pressure - magnetic pressure ratio, and B is the magnetic field strength. Most tokamaks and stellarators currently in operation are low-β devices. Broadly speaking, there are three ways to increase P, one may increase the operating β (as in Spherical Tokamaks), the magnetic field (as in the SPARC and EAST), or the volume of the device (as in ITER and W7X). The cost of these devices is proportional to V, making large devices extremely expensive. Similarly, a large magnetic field (>10T) requires superconducting magnets that, even after the recent innovations in HTS (High-Temperature Superconductors), are expensive to operate.

In addition, more research needs to be done on the effect of neutron flux on the lifecycle of HTS magnets. High-β devices are an attractive idea for producing fusion energy efficiently. However, a high β generally also implies a large gradient in plasma pressure that can be a source of numerous magnetohydrodynamic (MHD) and kinetic instabilities. If fusion devices could be optimized against such instabilities, high-β operation would become attractive compared to high-field or large-volume reactors. Therefore, this thesis examines the stability of the high-β tokamak and stellarator equilibrium equilibria.

We will start by investigating the stability of high-β tokamaks and stellarator equilibria

against the infinite-n, ideal ballooning mode, an important pressure-driven magnetohydrodynamic (MHD) instability. We optimize these equilibria for stability against the ideal-ballooning mode. To achieve this, we formulate a gradient-based adjoint technique and demonstrate its speed and effectiveness by stabilizing these equilibria. We also formulate how this technique can be easily extended to low-n ideal-MHD modes in both tokamaks and stellarators.

After demonstrating the process of stabilizing against ideal MHD unstable modes, we analyze the kinetic stability of high-β tokamak and stellarator equilibria by numerically solving the δf gyrokinetic model – a simplified version of the Vlasov-Maxwell model. These kinetic instabilities are driven by temperature and density gradients. To better understand these instabilities, we scan multiple values of the plasma β, temperature and density gradients, and plasma boundary shapes, discovering useful relationships between equilibrium-dependent quantities and growth rates of these instabilities.

From the microstability study, we find that electromagnetic effects are important for high-β devices. Hence, we use the numerical tools and knowledge derived from the previous chapters to write an optimization framework that searches for axisymmetric equilibria that are stable to electromagnetic kinetic instabilities. Due to the similarity between axisymmetry and quasisymmetry – a hidden symmetry in stellarators – we extend the microstability optimizer to search for high-β quasisymmetric stellarator equilibria.

## Rui Zhang - April 11, 2023

Dissertation Title: **Lattice Quantum Chromodynamics (QCD) Calculations of Parton Physics with Leading Power Accuracy in Large Momentum Expansion**

Date and Time: **Tuesday, April 11, 2:00 pm**

Location: **PSC 3150**

Dissertation Committee Chair:** Xiangdong Ji **

Committee:

Thomas Cohen

Drew Baden

Zohreh Davoudi

Alice Migenery

Abstract:

Parton distributions describing how momenta of quarks and gluons are distributed inside a hadron moving at the speed of light, are important inputs to the standard model prediction of collider physics. Their non-perturbative nature does not allow traditional perturbative calculations from quantum field theory. Besides a global fitting to experimental data, it is also possible to calculate parton physics from lattice-QCD, a first principle non-perturbative Monte Carlo simulation of the strong interaction on super computers. Among the different strategies to extract information for parton physics, the large momentum effective theory, based on a large momentum expansion of non-local Euclidean correlation functions, allows us to directly calculate the momentum fraction x-dependence. When matching the lattice-QCD calculations to the physical parton physics in the large momentum expansion, there are unavoidable power corrections in the expansion parameter Lambda/Pz, which is determined by the QCD characteristic non-perturbative scale Lambda~300 MeV and the hadron momentum Pz, and the leading term appears as order Lambda/2xPz due to the linear divergent self-energy of Wilson line in the Euclidean lattice correlators. For current lattice calculations of Pz~2 to 3 GeV, this correction can be as large as 30% at small x, dominating the uncertainties in the calculation. Achieving power accuracy in linear order of Lambda/Pz is thus crucial for a high precision calculation of the parton physics from lattice.

In this dissertation, I summarize our work to eliminate this linear correction by consistently define the renormalization for the linear divergence in lattice data and the resummation scheme of the factorially growing infrared-renormalon series in the perturbative matching. We show that the method significantly reduces the linear uncertainty by a factor of 3 to 5 and improves the convergence of the perturbation theory. We then apply the strategy to the calculation of pion distribution amplitude, which describes the pion light-cone wave function in a quark-antiquark pair. The method improves the short distance behavior of the renormalized lattice correlations, which is now consistent with the prediction of the short distance operator product expansion, showing a reasonable value for the moments of pion distribution amplitude. We also develop the first strategy to resum the large logarithms in the matching to physical pion distribution amplitude when the momentum of quark or antiquark in the pion are small, that could improve the accuracy of the prediction near the endpoint regions. After extracting the x-dependence from the large momentum expansion in mid-x region, we complete the endpoint regions by fitting to the short distance correlations. Then a complete x-dependence is obtained for the pion distribution amplitude, which suggests a broad distribution compared to previous lattice calculations or model predictions.

## Yuxun Guo - April 6, 2023

Dissertation Title: **UNRAVELING THE NUCLEON 3D STRUCTURE FROM EXPERIMENT, LATTICE AND GLOBAL ANALYSIS**

Date and Time: **Thursday, April 6, 11:00 am**

Location: **PSC 2136**

Dissertation Committee Chair:** Prof. Xiangdong Ji **

Committee:

Professor Kaustubh Agashe

Professor Zackaria Chacko

Professor Thomas Cohen

Professor Da-Lin Zhang

Abstract:

Nucleon 3D structure has been one of the most important goals in modern nuclear physics, which will provide, among other insights, an intuitive understanding of how the fundamental properties of the nucleon, such as its mass and spin, arise from the underlying quark and gluon degrees of freedom. I will present studies the 3D structure of nucleon via generalized parton distributions (GPDs) with inputs from experiments, lattice and global fits. I will discuss various exclusive measurements at HERA, JLab that can be used to probe the nucleon 3D structures as well as progresses in lattice QCD calculation related to nucleon 3D structures. I will also introduce a global analysis program that combines both experiment and lattice inputs and generate the state-of-art GPDs, which will shed a refreshing new light on the problem of proton structure and confinement.

## DinhDuy Vu - March 29, 2023

Dissertation Title: **Topology, localization, and spontaneous symmetry breaking in nonequilibrium many-body systems**

Date and Time: **Wednesday,March 29, 10:30 am**

Location: **ATL 4402 (CMTC conference room)**

Dissertation Committee Chair: **Prof. Sankar Das Sarma**

Committee:

Prof. Jay Deep Sau

Prof. Maissam Barkeshli

Prof. Alicia Kollar

Prof. Christopher Jarzynski (Dean's Representative)

Abstract:

Exotic many-body phenomena are usually associated with the ground state of a time-independent Hamiltonian. It is natural to ask whether these physics can survive in a dynamic setting. Under a generic drive, the steady equilibrium state is most likely an infinite-temperature featureless thermal state. However, there exist exceptional cases where thermalization either does not happen or is delayed for a sufficiently long time, called nonequilibrium many-body systems. In this thesis, we study mechanisms that can generate nonequilibrium dynamics: many-body localization, prethermalization, and projective measurements. We then demonstrate that the resulting quantum states can host a wide variety of many-body phenomena similar to the groundstate, focusing on three aspects: topology, localization, and spontaneous symmetry breaking.

## Kaustubh Wagh- March 27, 2023

Dissertation Title: **REGULATING GENE EXPRESSION: THE ROLE OF TRANSCRIPTION FACTOR DYNAMICS**

Date and Time: **Monday, March 27, 12:30 PM**

Location: **Conference Room, 1116, Institute for Physical Science and Technology (IPST) Building**

Dissertation Committee Chair: **Prof. Arpita Upadhyaya **

Committee:

Dr. Gordon L. Hager

Dr. Michelle Girvan

Dr. Wolfgang Losert

Dr. Helim Aranda-Espinoza

Abstract:

The genetic information encoded within our DNA is converted into RNA in a process called transcription. This is a tightly regulated process where multiple proteins act in concert to activate appropriate gene expression programs. Transcription factors (TFs) are key players in this process, with TF binding being the first step in the assembly of the transcriptional machinery. TFs are sequence-specific DNA binding proteins that bind specific motifs within chromatin. How TFs navigate the complex nuclear microenvironment to rapidly find their target sites remains poorly understood. Technological advances over the past 20 years have enabled us to follow single TF molecules within live cells as they interact with chromatin. Most TFs have been shown to exhibit power law distributed residence times, which arise from the broad distribution of binding affinities within the nucleus. This blurs the line between specific and non-specific binding and renders it impossible to distinguish between different binding modes based on residence times alone.

In this dissertation, I combine single molecule tracking (SMT) with statistical algorithms to identify two distinct low-mobility states for chromatin (histone H2B) and bound transcriptional regulators within the nucleus. On our timescales, the TF mobility states represent the mobility of the piece of chromatin that they are bound to. Ligand activation results in a dramatic increase in the proportion of steroid receptors in the lowest mobility state. Mutational analysis revealed that only chromatin interactions in the lowest mobility state require an intact DNA-binding domain as well as oligomerization domains. Importantly, these states are not spatially separated as previously believed but in fact, individual H2B and chromatin-bound TF molecules can dynamically switch between them. Single molecules presenting different mobilities exhibit different residence time distributions, suggesting that the mobility of a TF is intimately coupled with their temporal dynamics. This provides a way to identify different binding modes that cannot be detected by measuring residence times alone. Together, these results identify two unique and distinct low-mobility states of chromatin that appear to represent common pathways for transcription activation in mammalian cells.

Next, I demonstrate how SMT can complement genome wide assays to paint a complete picture of gene regulation by TFs using two case studies: corticosteroid signaling and endocrine therapy resistance in breast cancer. Finally, I conclude with a roadmap for future work on examining the role of mechanical cues within the cellular microenvironment (such as stiffness and topography) in regulating TF dynamics and gene expression.

## Yipeng Sun- March 1, 2023

Dissertation Title: **MEASUREMENT OF R(D(∗)) IN SEMILEPTONIC B DECAYS AND UPGRADE OF THE LHCB UPSTREAM TRACKER**

Date and Time: **Wednesday,March 1, 2023, 1:00 pm**

Location: **PSC3204**

Dissertation Committee Chair: **Prof. Manuel Franco Sevilla **

Committee:

Dr. Abohassan Jawahery

Dr. Zakaria Chacko

Dr. Christopher Palmer

Dr. Alice Mignerey

Abstract:

The LHCb experiment at the Large Hadron Collider provides a unique opportunity to study flavor physics with high luminosity. One topic in flavor physics is lepton flavor universality (LFU), a property of the standard model (SM) which requires the three generations of leptons (e,µ, τ) couple to gauge bosons of the eletroweak interactions with) couple to gauge bosons of the eletroweak interactions with the same strength. It is an important probe for testing the validity of the SM and possibly providing hints to new physics beyond the SM. This thesis presents a preliminary framework for the measurement of R(D(∗)), a proxy to test LFU, defined as the ratio of branching fractions B(B → D(∗)τ −ντ)/B(B → D(∗)µ−νµ), with LHCb 2016 data. Another topic of the thesis is the upgrade of the LHCb Upstream Tracker (UT) which greatly increases the readout rate of the detector and removes limitations due to hardware trigger, paving the way for future precision measurements with even higher luminosity.

## Zishuo Yang - February 20, 2023

Dissertation Title: **Experimental study of semitauonic B_c decays and development of the Upstream Tracker electronics for the LHCb upgrade**

Date and Time: **Monday, February 20, 9:00 am**

Location: **PSC 3150**

Dissertation Committee Chair: **Prof. Hassan Jawahery **

Committee:

Dr. Thomas Cohen

Dr. Sarah Eno

Dr. Manuel Franco Sevilla

Dr. Richard Mushotzky

Abstract:

The LHCb experiment at the Large Hadron Collider is designed for studying the properties of heavy quarks and CP violation to indirectly search for new physics beyond the Standard Model. The first topic of this dissertation is a study of semitauonic B_c meson decays at LHCb to test the universality of the couplings of charged leptons in electroweak interactions, which is known as lepton flavor universality in the Standard Model. The second topic of this dissertation is the development of readout electronics for a new silicon-strip tracking detector, the Upstream Tracker, to upgrade the LHCb detector. The upgraded LHCb detector will collect much more data in the upcoming runs of the Large Hadron Collider.