Theoretical Physics Institute Seminar

University of Alberta

Organizer: Igor Boettcher

Seminar

  • Thursday 12:40-14:00
  • Room L1-029
  • Talk starts at 12:50, pizza will be served at 12:40.

Schedule Fall 2024

  • Nov 28, 2024: Matthias Kaminski (University of Alabama)
     

Schedule Winter 2024

  • May 30, 2024: Christoph Ternes (Gran Sasso)
     
  • May 16, 2024: Shouryya Ray (University of Southern Denmark)
     
  • April 26, 2024: Manu Paranjape (Universite de Montreal)
     
    Extra seminar: Friday
     
    What is the gravitational field of a non-local quantum superposition?
     
    We probe the gravitational field of a massive particle in a non-local quantum superposition through gravitational scattering with a massless particle. We find that the scattering is essentially insensitive to the quantum non-locality. We comment on the sensibility of the Newton-Schroedinger approach and analysis of the problem.
     
  • April 25, 2024: Mireia Tolosa-Simeon (Ruhr University Bochum)
     
    Analog gravity in Dirac materials
     
    Condensed matter systems can be used in various scenarios to emulate and study phenomena from a completely different field of physics, for example, elementary particle physics or gravity. Such analog condensed matter models provide a novel perspective to approach questions that are not directly accessible in the original systems as they can potentially be realized experimentally in a well-controlled setup.
    In this project [1], we address the problem of cosmological fermion production in expanding universes using Dirac materials as analog models. Recently, two-dimensional moire Dirac materials, such as twisted bilayer graphene (TBG), have been established as highly tunable condensed matter platforms allowing us to manipulate electronic band structures and interaction effects in a controlled manner. A remarkable feature of Dirac materials is the presence of fermionic low-energy excitations, described by a quasirelativistic Dirac equation where the velocity of light is replaced by the Fermi velocity. The Fermi velocity can be tuned dynamically over several orders of magnitude leading to a time-dependent metric for the Dirac fermions. In addition, we consider the presence of time-dependent Dirac masses that may originate from symmetry breaking and lead to a finite band gap in the energy dispersion. These ingredients allow us to construct an analog model for the phenomenon of cosmological fermion production in expanding universes, arising due to a time-dependent metric and conformal symmetry breaking.
    [1] M. Tolosa-Simeon, M. M. Scherer and S. Floerchinger, Analog of cosmological particle production in moire Dirac materials, arXiv:2307.09299 (2023)
     
  • April 18, 2024: Eric Poisson (University of Guelph)
     
    Twist and shout: gravitomagnetic tidal resonances in binary inspirals
     
    I know, the second part of the title sounds intimidating. Perhaps you're already thinking: this talk is not for me. But resist and come anyway! I promise that I will make the subject accessible to all, and that the physics is actually pretty exciting. What is it about? The context for the talk comes from the ongoing effort to measure gravitational waves from coalescing compact binaries, and to use these observations to learn something about the intimate nature of black holes and neutron stars. Black holes have no internal structure and are therefore boring, but neutron stars are deeply mysterious, featuring matter at densities that far exceed what can be found in ordinary nuclear matter in laboratories. A key to an understanding of their interior comes from the tidal deformation of each neutron star when the binary system is approaching merger, and its imprint on the emitted gravitational waves. I shall describe a less familiar type of tidal field predicted by general relativity, associated with the gravity produced by mass currents (gravitomagnetism). I will show how this tidal field can excite modes of vibration of a rotating neutron star (inertial modes) and produce resonances. The resonances have a large impact on the binary's orbital motion, and this can be measured in the emitted gravitational waves, giving us a new handle on the intimate nature of neutron stars.
     
  • April 11, 2024: Christoph Simon (University of Calgary)
     
    Could quantum entanglement play a role in the brain?
     
    I will begin by discussing two principal motivations for asking the question whether quantum entanglement might play a role in the brain. One is related to the binding problem of consciousness, the other to quantum technology and evolution. I will then describe our work on how entanglement could potentially be implemented in the brain. I will focus on two physical systems that a priori seem well suited for this purpose, namely photons and spins. I will describe how photons are emitted by cells and seem to be guided by axons in the brain, potentially serving as biological signals and maybe flying qubits. I will then turn to spins, describing how the radical pair mechanism, which involves electron and nuclear spins, can explain magnetic field sensing by birds and other animals, but maybe also many other magneto-sensitive biological phenomena. Nuclear spins appear particularly promising as potential stationary qubits. I will conclude by a short discussion of neuromorphic quantum computing, which might give a flavor of how entanglement could be useful to the brain.
     
  • April 9, 2024: Pablo Basteiro (Wuerzburg University)
     
    Extra: Joint with CMP seminar
    Special date and time: Tuesday 12:30-13:30
     
    Entanglement in interacting Majorana chains and transitions of von Neumann algebras
     
    Analytical insights into interacting quantum many-body systems are hard to come by. A particularly difficult aspect to study is the the precise characterization of the phase diagram of a system based on its entanglement properties. Recently, a version of this problem has been tackled in the context of holography via a novel take on an old paradigm, namely the theory of von Neumann algebras. Different types of algebras are known to encode distinct entanglement properties, and identifying their occurrence provides new perspectives into the different phases of a system.
    In this talk, I introduce a model of Majorana fermions with two-site interactions consisting of a general function of the fermion bilinear. The models are exactly solvable in the limit of a large number of on-site fermions. In particular, I study a four-site chain, which exhibits a quantum phase transition controlled by the hopping parameters and manifests itself in a discontinuous entanglement entropy. Based on these results, I identify transitions between types of von Neumann operator algebras throughout the phase diagram. In the strongly interacting limit, this transition occurs in correspondence with the quantum phase transition. This study provides a novel application of the theory of von Neumann algebras in the context of quantum many-body systems.
     
  • April 4, 2024: Lukas Muechler (Pennsylvania State University)
     
    Bridging Topological Band Theory and Molecular Chemistry: A Novel Approach to Understanding Reaction Dynamics
     
    The integration of topology into the analysis of electronic states in crystalline materials has had a revolutionary impact on the field of condensed matter physics. Topological band theory has delivered new approaches and tools to characterize the electronic structure of materials, resulting in the discovery of new phases of matter with exotic properties. In the framework of topological band theory, the crossings between energy levels of electrons are characterized by topological invariants, which predict the presence of topological boundary states. Given the common occurrence of energy level crossings on molecular potential energy surfaces, extending topological concepts to molecular systems holds potential for significantly enhancing our comprehension of reaction dynamics. However, the disparate quantum mechanical frameworks used to describe solids and molecules present substantial challenges. This talk will delve into recent efforts of our group to reconcile these two domains, focusing on the characterization of features of the potential energy surface such as conical intersections and second order saddle points using topological invariants, and exploring their implications on reaction dynamics. We demonstrate our results by studying 4 pi electrocyclization reactions.
     
  • March 28, 2024: Frank Marsiglio (University of Alberta)
     
    The Theory of Superconductivity: What we know, and (more importantly) what we don't know
     
    This (hydride-free) talk will try to accomplish two slightly conflicting tasks. First, it is to serve as an introduction to the theory of superconductivity (Bardeen-Cooper-Schrieffer (BCS) and its more prominent "spin-offs") that would be suitable, for example, in an introductory graduate condensed matter course. Second, it will illustrate some present-day applications of some of these theories to current questions being posed for superconductors in general. Hopefully this still leaves a little time for the topic of the last part of the title.
     
  • March 21, 2024: John Bechhoefer (Simon Fraser University)
     
    What can Maxwell's demon do? - Experimental performance limits for information-fueled engines
     
    Information engines are a modern realization of the Maxwell-demon thought experiment. They exploit "favourable fluctuations" of a heat bath to generate work, at the cost of dissipation in a measuring device. We designed a simple information engine using optical tweezers and feedback to raise a "heavy" micron-sized trapped bead diffusing in water against gravity, without doing any external work. We first explore the conditions that maximize engine performance and achieve powers (~1000 kT/s) and speeds (~190 mu m/s) that compare to similarly sized bacteria. We then show that naively implemented information engines have a phase transition where they fail to function when measurements are too noisy. Adopting a more sophisticated measurement "filter" can eliminate the phase transition. Finally, placing the bead in an environment with "extra" nonequilibrium fluctuations can dramatically increase power output. These experiments suggest that what was once "just a thought experiment" may have practical applications.
     
  • March 14, 2024: Priya Sharma (University of Surrey)
     
    Inverse Faraday Effect in Fermi Liquids
     
    In the Fermi liquid metallic state, a static local magnetic moment is induced on the application of a circularly polarized electromagnetic wave, via the Inverse Faraday Effect (IFE). The direction of this moment is along the direction of propagation of light, and the magnitude of the moment depends on the frequency of light, the temperature and various material parameters characteristic of the metal. I will present a microscopic formulation that describes this effect and predicts the magnitude of induced moments with examples. I propose an analogous effect in the Fermi liquid state of He-3. A static circulating current is induced when liquid He-3 is driven by a circularly polarized transverse acoustic wave. The propagation of transverse sound in He-3 has been predicted but not observed thus far. I propose an analogue of the inverse Faraday effect as a scheme to experimentally demonstrate the propagation of transverse sound in the coupled system of 3He-aerogel. I estimate the magnitude of induced circulating currents for this system and find that these are within the range of experimental measurement.
     
  • March 7, 2024: Bastian Hess (Wuerzburg University)
     
    Geometry with curvature, torsion and non-metricity: Boundary terms in the geometrical trinity of gravity
     
    Motivated by applying the AdS/CFT correspondence to spin transport, we will briefly introduce manifolds and the concept of tangent spaces. In these spaces we introduce frame, connection and metric as dynamical fields. Their corresponding field strengths will be curvature, torsion and non-metricity. Considering manifolds with boundary requires to understand the Gibbons-Hawking-York boundary terms in the presence of torsion, curvature and non-metricity. We will derive a universal equation for calculating them for arbitrary Lagrangians. Subsequently, we apply this universal equation to the geometrical trinity of gravity to give a correct interpretation of the boundary terms in this trinity.
     
  • February 29, 2024: Tyler Cocker (Michigan State University)
     
    Theoretical tools and open theoretical challenges for ultrafast terahertz microscopy on the atomic scale
     
    Terahertz scanning tunneling microscopy (THz-STM) has emerged as a leading experimental technique over the last ten years, bringing new opportunities in the previously unexplored domain of ultrafast dynamics on the atomic scale. As the scope of THz-STM experiments broadens, new theoretical tools are needed to address open questions in materials science. In this talk, I will describe a recent approach that connects THz-STM measurements with the time-dependent local density of electronic states. I will further discuss how conventional theoretical tools like density functional theory have been used to understand THz-STM images, and outline some objectives for future theoretical treatments of atomic-scale nonequilibrium dynamics.
     
  • February 22, 2024: No seminar (reading week)
     
  • February 15, 2024: Pavel Kovtun (University of Victoria)
     
    Relativistic dissipation: from hydrodynamics to effective field theory
     
    I will discuss three related questions: how to unify the Navier-Stokes equations of fluid mechanics with relativity; what useful things one can learn from dispersion relations ω(k); which framework can implement the fluctuation-dissipation relations for relativistic hydrodynamic fluctuations.
     
  • February 8, 2024: Charles Cao (Virginia Tech)
     
    online
     
    Holographic Magic
     
    Entanglement has been a pivotal concept in quantum manybody physics and quantum gravity. However, it alone does not capture the full quantum landscape. While entanglement gives the power beyond classical correlations, quantum advantage implies that there are systems that are easy to simulate on a quantum computer but hard classically. Empirically, this notion of classical hardness is connected to non-stabilizerness, which is colloquially known as magic. Magic provides a new window for understanding complex quantum systems in a way that is distinct from entanglement. In this talk, we will examine the role of magic in quantum manybody systems and discuss how the interplay between entanglement and magic is important for the spectral properties of a state and the emergence of gravity in AdS/CFT.
     
  • February 1, 2024: David T. Stephen (University of Colorado Boulder)
     
    online
     
    Quantum circuits for constructing topological phases of matter
     
    The modern perspective on the classification of phases of matter is given in terms of circuit complexity: two many-body quantum states belong to the same phase of matter if and only if they can be related by a finite-depth quantum circuit. In this talk, I will discuss how quantum circuits can also be used to transform between different phases of matter. First, I will introduce a special class of linear-depth quantum circuits called sequential circuits which suffice to transform between a majority of known gapped phases. Second, I will show how certain phases can be related using finite-depth circuits if one is allowed to employ long-range interactions. Finally, I will show how certain quantum states can serve as many-body catalysts which facilitate transformations between certain phases of matter. The circuits constructed in this talk provide new insights into the entanglement structure of topological phases of matter and also new routes to prepare these phases in quantum devices.
     
  • January 25, 2024: Sergey Sibiryakov (McMaster University, Perimeter Institute)
     
    False vacuum decay out of equilibrium
     
    False vacuum decay in field theory is traditionally described using Euclidean methods adapted to systems in thermal equilibrium. I will discuss how to go beyond this restriction and describe the false vacuum decay from a general non-equilibrium state. As specific examples I will consider the catalysis of false vacuum decay by an evaporating black hole and a toy model of field theory with time-dependent boundary conditions. I will also mention new observables pertaining to the real-time picture of false vacuum activation.
     
  • January 18, 2024: Duncan O'Dell (McMaster University)
     
    The Abraham-Minkowski controversy: an ultracold atom perspective
     
    Over 100 years ago Max Abraham and Hermann Minkowski proposed rival theoretical expressions for the momentum of light in a medium. Since then a large number of theoretical and experimental studies have been conducted which have variously supported one or the other theory. The subject remains controversial to this day, but the advent of ultracold atoms allows for simple and clean measurements that may lead to greater clarity. I will discuss this subject from the perspective of ultracold atoms and point out some unexpected connections to seemingly different physics such as the He-McKellar-Wilkens phase (the geometric phase acquired by an electric dipole moving in a magnetic field).
     

Schedule Fall 2023

  • December 18, 2023: Joel Hutchinson (University of Basel)
     
    Special date and time. Monday, 1-2pm
     
    New phenomena in strongly correlated van der Waals heterostructures
     
    Recent advances in the characterization and manipulation of van der Waals materials have shown that few-layer heterostructures offer a unique platform for studying strongly correlated physics in two-dimensions (2D). In this talk I will present two theoretical case studies where electron interactions result in new physics in few-layer systems. In the first study, we find that the charge susceptibility in bilayers has peaks arising from scattering across the Fermi surfaces, not seen in the usual Lindhard function. In a conventional 2D electron gas, such peaks are suppressed by screening. In a bilayer system, however, we find that these peaks give rise to an enhanced response of out-of-plane dipoles to a local potential difference across the layers. In the second study, we show that when an antiferromagnetic insulator (AFMI) is sandwiched between two transition metal dichalcogenide (TMD) monolayers, magnons in the AFMI can mediate an interaction between electrons in the TMDs which gives rise to interlayer Cooper pairing. This results in a chiral p-wave superconducting gap, which gives rise to topological Majorana edge modes.
     
  • December 7, 2023: Javier Reynoso Cordova (University of Alberta)
     
    Astrophysical signals of particle Dark Matter
     
    Dark Matter is an important constituent of the standard cosmological model, however there is not a non-gravitational detection so far. There are multiple models suggesting Dark Matter is a particle and an extension of the Standard Model. In this talk I will briefly discuss the prediction and detection of some astrophysical signatures a particle dark matter can produce, specially Gamma-rays, cosmic-rays and synchrotron radiation. I will also briefly discuss the current and future experimental settings and efforts people are conducting to detect such non standard interactions.
     
  • November 30, 2023: Alexander Penin (University of Alberta)
     
    Oscillating Fields, Emergent Gravity and Particle Traps
     
    Rapidly oscillating fields appear in a variety of nontrivial physical systems from Kapitza pendulum to Paul traps and Floquet materials. Surprisingly, a systematic high precision theoretical description of such systems is not yet available. In this talk I discuss recent progress in this field and present a number of results connecting dynamical systems, general relativity and quantum field theory. The high-order perturbative expression for the classical and quantum effective action for the large-scale dynamics of charged particles in a rapidly oscillating field are presented. Remarkably, the action models the effects of general relativity on the motion of nonrelativistic particles, with the values of the emergent curvature and speed of light determined by the field spatial distribution and frequency.
     
  • November 23, 2023: Steven Rayan (University of Saskatchewan)
     
    Joint TPI/Math colloquium. Room CAB 239, 4-5 pm
     
    Quantum Matter, Moduli Spaces, and a Higher Spectral Correspondence
     
    The moduli space of Higgs bundles on a complex algebraic curve - an object that originates from Yang-Mills theory in theoretical physics, but which now lies at the interface of algebraic geometry and geometric representation theory - is perhaps best understood structurally through the spectral correspondence, which abelianizes each Higgs bundle at the cost introducing an additional curve typically of higher genus. This curve is called the spectral curve of the Higgs bundle as it encodes the spectrum of the Higgs field. This abelianization leads to the now famous Hitchin fibration. At the same time, algebraic curves of arbitrary genus and moduli spaces associated with them have recently become key objects in a different part of physics, namely condensed matter physics, due to the advent of 2-dimensional hyperbolic quantum matter, whose electronic band theory was developed by J. Maciejko and myself and which has been further explored both theoretically and experimentally by various groups internationally, including that of I. Boettcher. These two pictures overlap when the position curve of the band theory coincides with the spectral curve of a Higgs bundle, as elicited in my joint work with E. Kienzle. This talk will attempt to introduce the two pictures (without any assumptions of knowledge in either algebraic geometry or theoretical physics) and I will point out a few intriguing things that happen in the overlap. One of these observations is an expansion of the Hitchin fibration that anticipates a higher spectral correspondence that I have been exploring in joint work with K. Banerjee. There will be lots of visual illustrations to support the talk.
     
  • November 9, 2023: Shinji Mukohyama (Yukawa Institute, Kyoto)
     
    Effective field theory of black hole perturbations with timelike scalar profile
     
    Many dark energy (DE) models are based on a scalar field with timelike gradient. In this talk we begin with a review of the systematic construction of the effective field theory (EFT) describing perturbations around the Minkowski background with a timelike scalar profile and its extension to cosmological backgrounds, i.e. the ghost condensation and the EFT of inflation/DE. If one hopes to learn something about the EFT of DE from black holes (BHs) then one needs to consider BH solutions with timelike scalar profiles. We thus extend the EFT to arbitrary backgrounds. Finally, as an application of the general EFT, we study odd-parity perturbations around a spherically symmetric, static black hole background with a timelike scalar field responsible for DE and compute quasi-normal mode frequencies.
     
  • October 19, 2023: Shiwei Zhang (Flatiron Institute)
     
    Magnetism and superconductivity - insights from computations on the Hubbard model
     
    The Hubbard model is fundamental to quantum many-body physics. Since the discovery of high-temperature superconductivity, it has been a focal point in condensed matter and more recently also in the field of ultracold atoms. The properties of the two-dimensional Hubbard model are often the outcome of a delicate balancing act between multiple competing or co-existing tendencies. This makes accurate computations essential, but also extremely challenging. Through advances in methodologies and the combined use of complementary methods, we are seeing a new era of rapid progress. I will discuss some of these developments, and what they have revealed about the physics of the Hubbard model.
     
  • October 5, 2023: Nikolay Prokof'ev (University of Massachusetts, Amherst)
     
    Bi-polaron superconductivity in the low density limit
     
    It has been assumed for decades that high values of Tc from the electron-phonon coupling are impossible. At weak-to-intermediate coupling strength this result follows from the Migdal-Eliashberg theory, while at strong coupling, when bipolarons form, the transition temperatures are low because of the large effective mass enhancement. However, the latter conclusion was based on numerical solutions of the Holstein model. I will discuss a different model with electron-coupling based on the displacement modulated hopping of electrons and argue that much larger values of the bipolaron Tc can be achieved in this setup. Non-locality of the problem gives rise to small-size, yet relatively light bipolarons, which can be studied by an exact sign-problem-free quantum Monte Carlo approach even in the presence of strong Hubbard and Coulomb potentials. We find that Tc in this model generically and significantly exceeds typical upper bounds based on Migdal-Eliashberg theory or superfluidity of Holstein bipolarons, and, thus, offers a route towards the design of high-Tc superconductors via functional material engineering. Finally, there are indications for even better prospects in systems with non-linear electron-phonon coupling.