Colloquium
Spring 2024 Colloquia will be held in Room 307 of the Science and Engineering Research Facility on Mondays at 3:30 PM, EST.
Neuromorphic Computing from the Computer Science Perspective: Algorithms and Applications
Neuromorphic computing is a popular technology for the future of computing. Much of the focus in neuromorphic computing research and development has focused on new architectures, devices, and materials, rather than in the software, algorithms, and applications of these systems. In this talk, I will overview the field of neuromorphic from the computer science perspective. I will give an introduction to spiking neural networks, as well as some of the most common algorithms used in the field. Finally, I will discuss the potential for using neuromorphic systems in real-world applications from scientific data analysis to autonomous vehicles.
The Ties That Bind: Understanding Nuclear Forces from Lattice QCD
There are many open questions in nuclear physics which only lattice QCD may be able to answer. One example is understanding the nature and origin of the fine-tuning of interactions between nucleons and nuclei observed in nature. The first step toward building a bridge between the underlying theory, QCD, and nuclear observables is full control over one- and two-nucleon systems. While enormous strides have been made in recent years in precision calculations of single-nucleon observables, the history of two-nucleon calculations has generated more questions than answers. In particular, there is a controversy in the literature between calculations performed using different theoretical techniques, even for calculations far from the physical point, chosen due to the exponentially simpler computational properties. In this talk, I will present the history and challenges behind one- and two-nucleon calculations in lattice QCD, as well as advances in understanding and controlling the associated systematics.
Flat-bands as A Pathway from Theorists' Fantasy Land to Reality
Over recent decades, the study of strongly correlated quantum materials, in which strong interactions between particles push the system into the non-perturbative regime, has revealed a plethora of new quantum states, each with unique physical properties beyond the reach of perturbation theory. A key hurdle in this arena is the non-perturbative nature of these states, making theoretical description and prediction of them a significant challenge. This talk aims to shed light on how flat band systems provide a distinctive platform for various nontrivial correlated phenomena to emerge as exact solutions in theoretical analysis. This facilitates reliable prediction and robust guidance to identify novel quantum states of matter. Examples such as non-Fermi liquids and the fractional quantum anomalous Hall effect will be illuminated, along with a discussion on yet-to-be-observed quantum states that might emerge in flat band systems, such as fractional quantum anomalous Hall smectic states.
Neutron Decay Probes of the Standard Model
A free neutron provides the simplest example of nuclear $\beta$-decay, leading to a unique suite of tests for fundamental parameters of electroweak theory and the Standard Model of particle physics. A free neutron decays into a proton, electron, and antineutrino. This decay can be used to extract the CKM quark-mixing matrix element $V_{ud}$ without the need for nuclear structure corrections, which could resolve present tensions or hunt for new physics. This extraction requires two measurements: the neutron lifetime, $\tau_n$; and the relative coupling strength of the Vector and Axial-Vector currents in the weak interaction, $\lambda$. This talk will provide an overview of this decay process, beginning with measurements of the neutron lifetime. Then, this talk will focus on measuring $\lambda$, presenting an early look at results from the Nab experiment presently commissioning at Oak Ridge National Laboratory.
Lattice Simulations of Nuclear Many-Body Systems
This colloquium introduces the underlying theory and computational algorithms used to simulate the low-energy interactions of protons and neutrons using a three-dimensional lattice grid. Some of the topics to be discussed are nuclear clustering, intrinsic shapes, nuclear binding energies and charge radii, the nuclear equation of state, the liquid-vapor transition in nuclear matter, and superfluidity.
Emergent Photons and Fractionalized Excitations in a Quantum Spin Liquid
A quantum spin liquid (QSL) arises from a highly entangled superposition of many degenerate classical ground states in a frustrated magnet, and is characterized by emergent gauge fields and deconfined fractionalized excitations (spinons). Because such a novel phase of matter is relevant to high-transition-temperature superconductivity and quantum computation, the microscopic understanding of QSL states is a long-sought goal in condensed matter physics. Although Kitaev QSL exists in an exactly solvable spin-1/2 (S=1/2) model on a two-dimensional (2D) honeycomb lattice, there is currently no conclusive identification of a Kitaev QSL material. The 3D pyrochlore lattice of corner-sharing tetrahedra, on the other hand, can host a QSL with U(1) gauge fields called quantum spin ice (QSI), which is a quantum (with effective S=1/2) analog of the classical (with large effective moment) spin ice. The key difference between a QSI and classical spin ice is the predicted presence of the linearly dispersing collective excitations near zero energy, dubbed the "photons" arising from emergent quantum electrodynamics, in addition to the spinons at higher energies. Recently, 3D pyrochlore systems Ce2M2O7 (M = Sn, Zr, Hf) have been suggested as effective S=1/2 QSI candidates, but there has been no evidence of quasielastic magnetic scattering signals from photons, a key signature for a QSI. Here, we use polarized neutron scattering experiments on single crystals of Ce2Zr2O7 to conclusively demonstrate the presence of magnetic excitations near zero energy at 50 mK in addition to the signatures of spinons at higher energies. By comparing the energy (E), wave vector (Q), and polarization dependence of the magnetic excitations with theoretical calculations, we conclude that Ce2Zr2O7 is the first example of a dipolar-octupolar π-flux QSI with dominant dipolar Ising interactions, therefore identifying a microscopic Hamiltonian responsible for a QSL.
Neutrinoless Double-Beta Decay and the Neutrino
Understanding the origin of life on Earth motivates many of the questions that drive inquiry across all scientific subfields. Certainly, such questions influence nuclear and particle physics research. For example, the matter-antimatter asymmetry observed in today's Universe is necessary for our existence, but its origin in not well understood. The neutrino may play a significant role in understanding this asymmetry. Specifically, a promising class of theories that explains the asymmetry requires that the neutrino be its own anti-particle. The nuclear process of neutrinoless double-beta decay (0νΒΒ) can only occur if neutrinos have mass and are their own antiparticle. Although it is known that neutrinos have a small mass, we do not know the value or their particle-antiparticle nature. If a rate for 0νΒΒ is measured it will help elucidate the mass, but critically, 0νΒΒ is the only feasible experimental technique to determine if light neutrinos are their own antiparticle. This situation has resulted in a great deal of excitement for 0νΒΒ research.
This Colloquium will discuss the motivations for the search for 0νΒΒ, the experimental issues, and the use of the radiation-detection technology of germanium detectors to search for this process; the Majorana and LEGEND experiments.
Exploring Many Body Excitons with Resonant Inelastic X-ray Scattering
Excitons are quasiparticles that emerge when a valence electron is promoted in energy to the conduction states, leaving behind a hole that interacts with the electron. Many aspects of exciton physics in traditional insulators are well understood. However, in correlated quantum materials, the situation becomes richer and more complex due to the emergence of many-body excitons, which involve strong electron-electron and electron-spin interactions. In this talk I will explain the technique of resonant inelastic x-ray scattering [1], which we have recently been applying to several aspects of exciton physics. This includes the identification of an antiferromagnetic excitonic insulator state in Sr3Ir2O7 [2], determining the nature of propagating magnetically propagating Hund's excitons in NiPS3 [3], and the Floquet renormalization of the charge-transfer exciton in La2-xSrxCuO4.
[1] Exploring Quantum Materials with Resonant Inelastic X-Ray Scattering, M. Mitrano, S. Johnston, Young-June Kim, and M. P. M. Dean, submitted (2024)
[2] Antiferromagnetic excitonic insulator state in Sr3Ir2O7, D. G. Mazzone et al., Nature Communications 13, 913 (2022)
[3] Magnetically propagating Hund's exciton in van der Waals antiferromagnet NiPS3, W. He et al., in press at Nature Communications (2024)
There's Plenty of Interaction at the Bottom
The formulation of quantum mechanics in the late 1920s forever changed physics. More recently, quantum materials have emerged, offering fascinating opportunities in condensed matter physics. Elementary interactions among elements such as electrons, phonons, and other quasiparticles in quantum materials give rise to the emergence of intriguing phases and offer enormous opportunities for the development of quantum technologies. But investigating these interactions at the relevant length scale requires high-resolution methods. Traditional far-field optical imaging and spectroscopy techniques are constrained by the diffraction limit of light. Interestingly, during the same period in the late 1920s, a visionary scientist named Synge introduced a groundbreaking concept that could circumvent the diffraction limit. Synge shared his idea with Einstein, who encouraged him to publish it. After many years of various pioneering works by different groups, a powerful modern nano-optical technique, a variant of Synge's original idea, was born. In this talk, I will introduce this technique and give examples of high-resolution probing of nanoscale physical phenomena and interactions in two classes of quantum materials: correlated oxides and van der Waals (vdW) crystals. Our recent results reveal how an applied field perturbs dopant distribution at the nanoscale in correlated oxides such as rare-earth nickelates (RNiO3 where R = rare-earth element), leading to ordered reconfigurable phases. This reconfigurability enables the design of robust artificial synapses and opens new frontiers for fundamental understanding of memory, learning, and information retention for brain-inspired information processing. Correlated oxides also provide exciting opportunities to reconfigure polaritons, hybrid light-matter modes, in vdW crystals at the nanoscale, due to their highly tunable local optical and electronic properties. I will introduce a hybrid polaritonic-oxide heterostructure platform consisting of vdW crystals, such as hexagonal boron nitride or alpha-phase molybdenum trioxide, transferred on nanoscale oxygen vacancy patterns on the surface of correlated perovskite oxides. Hydrogenation and temperature modulation allow spatially localized conductivity modulation of the oxide nanoscale patterns, enabling robust real-time modulation and nanoscale reconfiguration of hyperbolic polaritons.
Artificial Intelligence for Quantum Science and Materials for Extremes – the new NSF MRSEC at University of Tennessee
The University of Tennessee has recently been awarded a prestigious Materials Research Science and Engineering Center by the National Science Foundation. The Center for Advanced Materials and Manufacturing is hosted at IAMM HQ, Knoxville as well as the Shull Wollan Center on the ORNL Campus. This Center builds on longstanding partnerships with ORNL in advanced computing, neutron science, and materials. The MRSEC involves more than 80 faculty, postdocs, and students and joins the network of MRSECs nationwide in offering unique capabilities to the national science community. This talk will present the center, its research, education, and outreach activities as well as its connections with the facilities and ways to get involved.
The Secret Life of Quarks: from the Lab to the Cosmos
The strongest fundamental force of nature generates ~96% of the mass of the visible universe and binds together the building blocks of quantum chromodynamics, quarks and gluons, within the proton. At temperatures of a few trillion Kelvin or densities tens of trillions times more dense than iron, these quarks are no longer bound within protons and can form entirely new states of matter. It is possible to unlock these quark phases in heavy-ion collisions that reach the hottest temperatures on Earth or potentially in the core of neutron stars that have densities many times larger than that of a nucleus. In this talk, we will explore different ways to study the phases of matter of quarks from the lab using heavy-ion collision and in the cosmos using X-Ray measurements of neutron stars or gravitational waves from binary neutron star mergers.
QCD for New Physics Searches at the Sensitivity Frontier
Questions that drive searches for physics beyond the Standard Model include the physical origin of the cosmic baryon asymmetry and of dark matter. Quark dynamics, as realized through the theory of quantum chromodynamics (QCD), can appear in these studies in very different ways. In this talk, I develop these possibilities explicitly, first describing the role of QCD in ultra-sensitive searches for new physics, particularly at low energies, and then turning to how its features could be exploited in describing the undiscovered universe, along with the essential observational and experimental tests that could confirm them.