Physics Colloquium - Fall 2022 Fall 2022

NO LECTURE CAPTURE WAS USED. Video link to be announced.

The Cosmic Microwave Background (CMB) gives a photographic image of the Universe when it was still an “infant,” and its detailed measurements have given us a wealth of information, such as the composition and history of the Universe. The CMB research told us a remarkable story: the structure we see in our Universe, such as galaxies, stars, planets, and eventually ourselves, originated from tiny quantum fluctuations in the period of early Universe called “cosmic inflation.” But is this picture true? In this lecture, I will review the physics of CMB and key results from recent experiments and discuss future prospects for the quest to find out about our origins.

Nora Sherman (U-M Physics)

A Multi-Messenger Search for H0 Using Optical Counterparts to Gravitational Wave Events

Measurements of the Hubble Constant (H0) – a parameter that helps illustrate the expanding behavior of the universe – differ vastly, particularly between those using early- versus late-universe data. To help relieve this tension, the Dark Energy Survey Gravitational Wave group (DESGW) seeks to perform a standard siren measurement of H0 by identifying electromagnetic counterparts to compact binary mergers. Together with the GW data from detections by the LIGO/Virgo/KAGRA Collaboration (LVK), this information allows us to make an H0 measurement independent of traditional methods. In this talk, I will detail DESGW’s pursuit of this measurement, including our tools for observation and analysis, recent key results, ongoing studies critical to the project, and our preparation for the next LVK observing run.

Torben Purz (U-M Physics)

Imaging of Dynamic Exciton Interactions and Coupling in Transition Metal Dichalcogenides

Transition metal dichalcogenides (TMDs) are regarded as a possible material platform for quantum information science, photovoltaics, and related device applications. However, many experimental results on TMDs are only realized at specific spots on the sample, presenting a challenge to the scalability of these applications. Here, we demonstrate multi-dimensional coherent imaging spectroscopy on TMD monolayer and heterostructure samples. This technique enables us to shed light on the spatial dynamics of various material parameters—including dephasing, inhomogeneity, and strain, as well as coherent exciton coupling and charge transfer. We demonstrate that dephasing and inhomogeneity are very sensitive to residual strain in state-of-the-art TMD monolayer samples. At the same time, the coherent coupling strength and charge transfer remain robust across large areas of the heterostructure sample. Our findings strengthen the case for heterostructure TMDs as a next-generation material platform for device applications and introduce a powerful tool in multi-dimensional coherent imaging spectroscopy for material characterization.

Aidan Herderschee (U-M Physics)

The Space of Supersymmetric Theories

I will review how causality non-trivially bounds the space of quantum field theories, focusing in particular on maximally supersymmetric theories in four dimensions. I will then discuss some geometric properties of this space and how to efficiently calculate bounds using linear programming. Finally, I will use some results from string theory to motivate novel conjectures.

Ryan Cardman

Ponderomotive Laser Spectroscopy of Rubidium Rydberg Atoms

Spectroscopy of Rydberg atoms with very large principal quantum numbers, n, has traditionally been performed through coherent absorption and emission of microwaves. In my talk, I describe a newly developed and recently demonstrated method of manipulating an alkali (rubidium) Rydberg atom’s valence electron using phase-modulated laser fields that drive a coherent transition between two Rydberg states. The light-matter interaction for this method originates from the ponderomotive force of the laser field acting on the Rydberg electron, rather than the more commonly observed electric-dipole force (which, along with magnetic-dipole interactions, governs most of modern spectroscopy). Because the ponderomotive interaction fundamentally differs from the electric-dipole force, differences arise in selection rules, which are considerably more relaxed in ponderomotive than in traditional spectroscopy. For instance, high-harmonic transitions can be driven in first-order perturbation theory (i.e. without virtual intermediate states, and without a significant drop in Rabi frequency with increasing order). We also observe and explain a new paradigm for Doppler-free spectroscopy. In my talk, I will describe details of the optical setup, the phase-control of the light, the experimental spectra, our models, and numerical simulations. Applications of ponderomotive Rydberg-atom spectroscopy include site-selective spin manipulations in quantum simulators, high-l-Rydberg-state initialization, and gate operations in Rydberg quantum computers.

Xiaoyu Guo

Ferro-Rotational Domain Walls Revealed by Electric Quadrupole Second Harmonic Generation Microscopy

Domain walls in (multi)ferroics have received tremendous attention recently due to their emergent properties distinct from their domain counterparts. However, in contrast to ferromagnetism and ferroelectricity, it is extremely challenging to study ferro-rotational (FR) domain walls because the FR order is invariant under both spatial-inversion and time-reversal operations and thus hardly couple with conventional probes. Here, we investigate an FR candidate NiTiO3 with electric quadrupole (EQ) second harmonic generation rotational anisotropy (SHG RA) and probe the point symmetries of its two degenerate FR domain states. We then visualize the real-space FR domains and domain walls using scanning EQ SHG microscopy. By taking local EQ SHG RA measurements, we further show the restoration of the mirror symmetry at FR domain walls and prove their nonpolar nature. Our findings not only provide insight into FR domain walls, but also demonstrate a powerful tool for future studies on domain walls of unconventional ferroics.

Robert Saskowski

Towards Unreasonable Effectiveness in AdS5

Gauge/gravity duality has garnered an enormous amount of interest in the last twenty-five years. The prototypical example is the AdS/CFT correspondence, which relates supergravity with a negative cosmological constant to a conformal field theory in one lower dimension. In particular, this relates gravitational and non-gravitational theories. I will focus on the supergravity story, specifically with higher-derivative corrections. In this talk, I will give a brief introduction to the AdS/CFT correspondence and higher derivatives and discuss some recent work with my advisor Jim Liu regarding the universality of supersymmetric four-derivative corrections to minimal supergravity in five spacetime dimensions.

Self-organized complex structures in nature, from hierarchical biopolymers to viral capsids and organisms, offer efficiency, adaptability, robustness, and multifunctionality. How are these structures assembled? Can we understand the fundamental principles behind their formation, and assemble similar structures in the lab using simple inorganic building blocks? What’s the purpose of these complex structures in nature, and can we utilize similar mechanisms to program new functions in metamaterials? In this talk, we will start from the perspective of geometric frustration, to explore answers to these questions. I will discuss our recent work on developing analytic theories based on crystal structures in non-Euclidean space for the self-assembly of nanoparticles into complex structures, mechanical properties of materials in which geometric frustration causes prestress, as well as our ongoing effort in designing topological mechanical metamaterials with and without geometric frustration.

Quantum many-body theories can be used to describe the physics of quantum systems with many strongly interacting particles. In condensed matter physics, these theories are typically applied to effective low-energy lattice models, which are designed to capture only the essential degrees of freedom of a solid. Such models contain phenomenological parameters and are often not predictive.

This talk will summarize recent progress on solving the many-body problem ab-initio, i.e. without adjustable parameters and without the construction of effective low-energy models. We will showcase algorithmic and computational advances that have enabled high-precision calculations of solids with strong quantum effects. A path towards controlled and adaptive many-body simulations is outlined.

LUX-ZEPLIN (LZ) is a dark matter experiment at the 4850’ level of the Sanford Underground Research Facility in Lead, South Dakota. The experiment utilizes a two-phase time projection chamber, containing seven active tonnes of liquefied xenon, to search for weakly interacting massive particles (WIMPs). Auxiliary veto detectors, including a liquid scintillator outer detector, improve the rejection of unwanted background events in the central region of the detector. LZ has been designed to explore much of the parameter space available for WIMP models and is the wordlś most sensitive DM experiment to date. In this talk, I will present the first results of LZ and the contributions of U-M to LZ before giving an outlook on future DM searches with LZ and beyond.

This talk presents an overview of my research on dark energy. Hypothesized as a new form of energy to explain the observed accelerated expansion of the universe, dark energy is one of the most formidable scientific problems of our time. Its discovery, in 1998, was awarded the physics 2011 Nobel prize, yet, its explanation remains elusive. My most well-known work is the DESGW project, which inaugurated the sub-field of multi-messenger cosmology with standard sirens. Rapid growth prospects in this area are fueled by the increased sensitivity of gravitational wave detectors and the discovery capability of cosmic survey instruments. I also pursue precision cosmological measurements using galaxy clusters. These two research thrusts complement each other, as clusters allow us to distinguish between dark energy models that predict the same expansion rate for the universe. Results of this research program will include precision cosmological measurements to enable a breakthrough in our understanding of dark energy.