# Colloquia

Attend virtually via Zoom:
##### https://web.phys.virginia.edu/Private/Covid-19/colloquium.asp

Friday, April 29, 2022
3:30 PM
Ridley Hall, Room G008

## "Re-inventing fixed-target experiments to probe light dark matter"

Cristina Mantilla , Fermilab
[Host: Dustin Keller]
ABSTRACT:

The search for dark matter is ever-evolving and a number of experiments are underway to search for the constituents of dark matter across a vast range of masses. A largely unexplored regime is that where light dark matter particles, with masses between the electron and proton masses, may interact feebly with ordinary matter. I will discuss how small accelerator experiments can produce these dark matter candidates by scattering energetic particles on a fixed target. I will focus on the DarkQuest experiment at Fermilab that uses a proton beam and builds off of an existing detector and accelerator infrastructure. I will also describe how an electron beam can be complimentary used in the Light Dark Matter experiment (LDMX) at SLAC. I will explain the design and challenges of these experiments and their prospects for characterizing a dark matter signal in the near future.

Hoxton Lecture

Monday, April 25, 2022
7:00 PM
Newcomb Hall, Room Newcomb Hall Theater
Note special date.
Note special time.
Note special room.

## "Probing the Universe with Gravitational Waves"

Barry C. Barish , Professor of Physics, Emeritus, at Caltech, and Distinguished Professor of Physics, at UC Riverside, Nobel Laureate 2017
ABSTRACT:

The discovery of gravitational waves, predicted by Einstein in 1916, is enabling both important tests of the theory of general relativity, and represents the birth of a new astronomy. Modern astronomy, using all types of electromagnetic radiation, has giving us an amazing understanding of the complexities of the universe, and how it has evolved. Now, gravitational waves and neutrinos are beginning to provide the opportunity to pursue some of the same astrophysical phenomena in very different ways, as well as to observe phenomena that cannot be studied with electromagnetic radiation. The detection of gravitational waves and the emergence and prospects for this exciting new science will be explored.

VIDEO:
Joint Physics-Astronomy colloquium

##### Attend virtually via Zoom:   https://web.phys.virginia.edu/Private/Covid-19/colloquium.asp

Friday, April 22, 2022
3:30 PM
Ridley Hall, Room G008

## "The expansion of space, a scaling symmetry, and a mirror-world dark sector"

Professor Lloyd Knox , UC Davis
[Host: Prof. Genya Kolomeisky]
ABSTRACT:

I will introduce, for those unfamiliar with general relativity, the notion of the expansion of space, before going on to discuss a 5 sigma discrepancy between two inferences of the rate of that expansion today. One of those inferences is highly indirect and model dependent, relying on measurements of maps of the cosmic microwave background (CMB), a thermal relic of the hot big bang. I will explain what the CMB is, and show how well our standard cosmological model describes its statistical properties, and how we can use that model to infer the expansion rate today. I will then describe a symmetry under a scaling transformation of all relevant time scales in the problem that can potentially be exploited to reconcile the two inferences. Significant constraints on such a solution come from measurements of the CMB energy density and the abundances of light elements produced in the big bang. The former constraint can be circumvented by use of a ‘mirror world’ dark sector — a copy of the standard model of particle physics with little to no interactions with standard model particles other than via gravity.

Attend virtually via Zoom:
##### https://web.phys.virginia.edu/Private/Covid-19/colloquium.asp

Friday, April 8, 2022
3:30 PM
Ridley Hall, Room G008

## "Electron spin resonance spectroscopy of quantum spin liquids"

Oleg Starykh , The University of Utah, Salt Lake City
[Host: Prof. Dima Pesin]
ABSTRACT:

Much of the current research in quantum magnetism is motivated by the search for an elusive quantum spin liquid (QSL) state of the magnetic matter. A salient feature of this entangled quantum state is the presence of fractionalized elementary excitations such as spin-1/2 spinons, interactions between which are mediated by the emergent gauge field.

In my talk, I provide a historical perspective on quantum spin liquids and shed some light on the origins of  this weird” idea. Following a brief summary of the current state of affairs in QSL research, I describe  how QSLs respond to the magnetic field and demonstrate surprising similarity of the described response to that of neutral Fermi liquids.

I illustrate these theoretical ideas with a recent electron spin resonance measurements on quasi-one-dimensional antiferromagnet K2CuSO4Br2. The experiment leads the first-ever spectroscopic determination of the (backscattering) interaction between spinon excitations of the quantum spin chain.

VIDEO:
Attend virtually via Zoom:
##### https://web.phys.virginia.edu/Private/Covid-19/colloquium.asp

Friday, April 1, 2022
3:30 PM
Ridley Hall, Room G008

## "How to Become a More Effective Mentor/Mentee"

Kirsten Tollefson , Professor and Associate Dean, Graduate School, at Michigan State University
[Host: Prof. Craig Group]
ABSTRACT:

I will discuss what recent research says about the science of effective mentorship and how we can learn to do it better. We will focus on 2 competencies - aligning expectations and maintaining effective communication - through some guided activities. I will give you examples of strategies and resources that can help you become a more effective mentor and mentee.

VIDEO:
Attend virtually via Zoom:
##### https://web.phys.virginia.edu/Private/Covid-19/colloquium.asp

Friday, February 25, 2022
3:30 PM
Online, Room via Zoom
Note special room.

## "A Career Outside the Academia: My Experiences as VP at a small Hi Tech Company"

Dr. Zuyu Zhao , Janis ULT Inc.
[Host: Prof. Bellave Shivaram]
ABSTRACT:

Most of the physics students will work in industry instead of academia after they got the degree (BSc., MS, or Ph.D.)

The speaker will share his personal 30 year experience of working in a small tech company after his post-doc period, including changes and challenges of culture, daily life, responsibilities, etc.

Advice will be discussed with the audience how meet the challenges.

The speaker’s main tasks were to develop ultra-low temperature facilities from 300mK to 10mK. Along with the speaker’s career growth, examples of his achievements are presented.

A brief business summary of the speaker’s company, before and after COVID, is presented at the end of the presentation.

Attend virtually via Zoom:
##### https://web.phys.virginia.edu/Private/Covid-19/colloquium.asp

Friday, February 18, 2022
3:30 PM
Ridley Hall, Room G008

## "Failure to Social Distance: Breaking Gathering Limits in Titan's Lakes"

Alex Rosenthal , University of Virginia - Department of Physics
[Host: Stefan Baessler]
ABSTRACT:

Saturn's moon Titan is a geologically and meteorologically active world seen as a potential location for the development of life. While we cannot immediately answer "Is there life on TItan?" or even "Are there life-forming reactions occurring?" we can take a step back and investigate a more foundational question: "What chemistries are occurring?" The answers to this question are stepping stones to understanding broader processes such as weather cycles, geology, and potentially organic reactions. We attack this question using molecular dynamics simulations. By identifying molecules that cluster or exhibit other interesting behaviors, we hope to identify possible sites for interesting chemical reactions that could produce large or prebiotic molecules, as well as characterize those reactions.

Attend virtually via Zoom:
##### https://web.phys.virginia.edu/Private/Covid-19/colloquium.asp

Friday, February 18, 2022
3:50 PM
Ridley Hall, Room G008
Note special time.

## "Coordinate Space Representation and Average Radius of Quark and Gluon Generalized Parton Distribution Functions"

Zaki Panjsheeri , University of Virginia - Department of Physics
[Host: Stefan Baessler]
ABSTRACT:

The task of the field of nuclear physics called femtography is to image the internal structure of strongly interacting particles, from single protons and neutrons to atomic nuclei. Protons and neutrons are composed of quarks and gluons, but the precise spatial arrangement of the two valence up quarks and one valence down quark, along with the sea quarks and gluons that contribute half of the momentum, remains unknown.  A compelling method for deriving dynamical information about the internal structure of the proton is through the use of generalized parton distributions (GPDs).  Two-dimensional Fourier transforms of GPDs provide insight into matter, charge, and radial distributions of the quarks and gluons inside the nucleon.  We present an explicit calculation of such transforms in a spectator model framework using parametric analytic forms of GPDs, originally constrained using deeply virtual Compton scattering and lattice QCD data.  We compare the valence quarks to the gluon distribution through, i.a., average radii, a notion of distance inside the nucleon, and we present a novel result for the radius of the gluon density.

Attend virtually via Zoom:
##### https://web.phys.virginia.edu/Private/Covid-19/colloquium.asp

Friday, February 18, 2022
4:10 PM
Ridley Hall, Room G008
Note special time.

## "Property Tuning of Layered Materials by Electrochemical Intercalation"

Dawn Ford , University of Virginia - Department of Physics
[Host: Stefan Baessler]
ABSTRACT:

Recent developments in two-dimensional (2D) magnetism have intensified the research on novel van-der Waals magnetic materials to explore new magnetic phenomena in the 2D limit. Among 2D magnetic materials, one model system is metal thiophosphates MPX3 (M = transition metal ions, X = chalcogen ions) in which the antiferromagnetic (AFM) properties are highly dependent on the choice of transition metal M. The van der Waals-type crystal structure allows the mechanical exfoliation of bulk crystals to obtain atomically thin layers. In MPX3, the AFM ordering is found to persist down to the atomically thin limit, making them a promising candidate for future device applications. Furthermore, the layered structure also permits the inter-layer intercalation, which is an effective way to tune the properties. With this motivation, we performed Li and Fe intercalation in NiPS3 by using electrochemical technique. In this method, the electrical potential causes electrons to flow from anode to cathode through the circuit within the battery leading to the intercalation of intercalant ions between the layers of the host sample as shown in figure below. By tuning the amount of charges intercalated during electrochemical intercalation, the number of intercalated ions in the host single crystal can be controlled. NiPS3 exhibits AFM ordering below TN = 155 K and the spin-flop transition above μ0H ≈ 6 T. The goal of this project is to intercalate different Li and Fe content in NiPS3 single crystals and characterize their magnetic properties. Mainly, we will focus on the tuning of the ordering temperature and the spin-flop field of pristine NiPS3. In addition, the transition from AFM state to other states such as ferromagnetism is also one important direction of this project. Li intercalation was found to increase the magnetization value of NiPS3.  Future work will consist of characterizing the changes in the magnetic ordering of Fe intercalated NiPS3 and extending the investigation of Li intercalated NiPS3.

Attend virtually via Zoom:
##### https://web.phys.virginia.edu/Private/Covid-19/colloquium.asp

Friday, January 28, 2022
3:30 PM
Online, Room via Zoom
Note special room.

## "Artificial Intelligence in Spin Physics"

Dustin Keller , University of Virginia - Department of Physics
[Host: Despina Louca]
ABSTRACT:

The landscape of physics research is changing due to the rapid advancement in computing. Traditionally, science is done through observation and experimentation.  While there is no indication yet that this trend will change overnight, there is an increasing likelihood that methods in physics are changing in a way that we must prepare for. New technology, new methods, and new instrumentation must be brought to the forefront to take advantage of the rapid evolution of artificial intelligence and its ubiquitous pervasiveness in all aspects of research and life.  I briefly review some of the advancement in machine learning and how these developments are changing our field using examples in Spin Physics from the recent past, the present, and near the future.

VIDEO:
Attend virtually via Zoom:
##### https://web.phys.virginia.edu/Private/Covid-19/colloquium.asp

Friday, January 21, 2022
3:30 PM
Physics Building, Room 204
Note special room.

## "Studying matter and spacetime with gravitational waves"

Professor David Nichols , University of Virginia - Department of Physics
[Host: Professor Despina Louca]
ABSTRACT:

Gravitational waves have been detected from the mergers of nearly ninety binary black holes during the first three observing runs of the Advanced LIGO and Virgo detectors. In this talk, I will discuss these detections and their implications for understanding fundamental properties of matter and spacetime in two contexts. First, I will review a nonlinear effect in general relativity called the gravitational-wave memory. The effect is characterized by a lasting change in the gravitational-wave strain produced by the energy radiated in gravitational waves. I will describe how this effect is related to the infrared properties of gravity, how the memory effect can be measured with LIGO and Virgo, and how new types of memory effects have been recently predicted. Second, I will discuss how dense distributions of dark matter around a black hole can influence the inspiral of a second compact object and thus the gravitational waves emitted from such a binary. With the planned space-based gravitational-wave detector LISA, the distribution of dark matter on these small scales could be mapped precisely. This would provide a new method to study dark matter: with gravitational waves.

VIDEO:
Attend virtually via Zoom:
##### https://web.phys.virginia.edu/Private/Covid-19/colloquium.asp

Friday, December 3, 2021
3:30 PM
Physics Building, Room 204
Note special room.

## "Floquet-engineering topological Dirac bands in an optical lattice"

Ian Spielman , NIST and The University of Maryland
[Host: Prof. Dima Pesin]
ABSTRACT:

Over the years my group has performed a number of experiments realizing relativistic physics using cold atoms — described by the 1D Dirac Hamiltonian in some degree of approximation.  I will begin by reviewing these results in conjunction with those from the whole of the cold-atom community.

With that backdrop, I describe a spin dependent bipartite Floquet lattice, in which the dispersion relation is linear for all points in the Brillouin zone. The (stroboscopic) Floquet spectrum of our periodically-driven Hamiltonian features perfect spin-momentum locking, and a linear Dirac dispersion.  These bands are protected by a Floquet topological invariant which we directly measure by using quantum state tomography.

VIDEO:
Attend virtually via Zoom:
##### https://web.phys.virginia.edu/Private/Covid-19/colloquium.asp

Friday, November 19, 2021
3:30 PM
Physics Building, Room 204
Note special room.

## "Analogue gravity in cold atom and condensed matter systems "

Professor Daniel Sheehy , Louisiana State University
[Host: Cass Sackett]
ABSTRACT:

In recent years there has been much interest in the field of "analogue gravity", in which cosmological or astrophysical phenomena like Hawking radiation are mimicked in a laboratory experiment.  At LSU, my research group in cold atom/condensed matter theory became interested in this field, motivated by the recent experiment of Eckel and collaborators, [Phys. Rev. X 8, 021021 (2018)] who used a rapidly expanding Bose-Einstein condensate (BEC) to reproduce the inflationary regime of the early universe.  I will present our work on the physics of inflation in expanding BEC's, and discuss other setups to detect analogue gravity phenomena like the Unruh effect.

VIDEO:
Join Zoom Meeting:
##### https://web.phys.virginia.edu/Private/Covid-19/colloquium.asp

Friday, November 12, 2021
3:30 PM
Physics Building, Room 204
Note special room.

## "Physics motivations for future colliders"

Professor Tao Han , University of Pittsburg
[Host: Professor P.Q. Hung]
ABSTRACT:

With the milestone discovery of the Higgs boson at the CERN Large Hadron Collider (LHC), high energy physics has entered a new era. The completion of the “Standard Model” (SM) implies, for the first time ever, that we have a relativistic, quantum-mechanical, self-consistent theoretical framework, conceivably valid up to exponentially high energies, even to the Planck scale. Yet, the SM leaves many unanswered questions both from the theoretical and observational perspectives, including the nature of the electroweak superconductivity and its phase transition, the hierarchy between the particle masses and between the observed scales, the nature of dark matter etc. There are thus compelling reasons to believe that new physics beyond the SM exists. We argue that the collective efforts of future high energy physics programs, in particular the future colliders, hold great promise to uncover the laws of nature to a deeper level.

VIDEO:
Attend virtually via Zoom:
##### https://web.phys.virginia.edu/Private/Covid-19/colloquium.asp

Friday, November 5, 2021
3:30 PM
Physics Building, Room 204
Note special room.

## "Atomtronics for Quantum Sensing"

Professor Malcolm Boshier , Los Alamos National Lab
[Host: Prof. Cass Sackett]
ABSTRACT:

Atomtronics is the emerging technology of building circuits where the current is a flow of ultracold atoms propagating as coherent matter waves inside suitable waveguides.  In this talk I will describe our atomtronic technology in which the waveguides are created with laser light via the optical dipole potential, and then discuss two quantum sensors based on it.  First, we have demonstrated the atomtronic analogue of the dc SQUID and shown that it exhibits the quantum interference that gives the Superconducting Quantum Interference Device its name.   In the conventional SQUID this is seen as a periodic variation of critical current with magnetic flux.  In the atomtronic SQUID it causes a periodic variation of critical current with rotation, enabling the device to function as a gyro.  Second, we are developing an atomtronic version of the Fiber Optic Gyro, in which rotation is measured by the Sagnac effect.  In our device a Bose-Einstein condensate is split, reflected, and recombined inside a waveguide that is translated so that the wavepackets travel around a loop and realize a waveguide Sagnac atom interferometer.

Attend virtually via Zoom:
##### https://web.phys.virginia.edu/Private/Covid-19/colloquium.asp

Friday, October 29, 2021
3:30 PM
Physics Building, Room 204
Note special room.

## "Exploring Gravitational Wave and Dark Matter Physics with the 100-meter-tall MAGIS-100 Atom Interferometer"

Professor Tim Kovachy , Northwestern University
[Host: Prof. Bob Hirosky]
ABSTRACT:

Atom interferometers exploit spatially delocalized quantum states to make a wide variety of highly precise measurements.  Recent technological advances have opened a path for atom interferometers to contribute to multiple areas at the forefront of modern physics, including searches for wave-like dark matter, gravitational wave detection, and fundamental quantum science.  In this colloquium, I will describe MAGIS-100, a 100-meter-tall atom interferometer being built at Fermilab to pursue these directions.  MAGIS-100 will serve as a prototype gravitational wave detector in a new frequency range, between the peak sensitivities of LIGO and LISA, that is promising for pursuing cosmological signals from the early universe and for studying a broad range of astrophysical sources.  In addition, MAGIS-100 will search for wave-like dark matter, probe quantum mechanics in a new regime in which massive particles are delocalized over macroscopic scales in distance and time, and act as a testbed for advanced quantum sensing techniques.  Finally, I will discuss the potential and motivation for follow-on atomic detectors with even longer baselines.

VIDEO:
Attend virtually via Zoom:
##### https://web.phys.virginia.edu/Private/Covid-19/colloquium.asp

Friday, October 22, 2021
3:30 PM
Physics Building, Room 204
Note special room.

## "Quantum Many-Body Physics of Superconducting Qubits"

Professor Leonid Glazman , Yale University
[Host: Dima Pesin]
ABSTRACT:

The ongoing development of superconducting qubits has brought some basic questions of many-body physics to the research forefront, and helped solve several of them. I will address two effects in quantum condensed matter highlighted by the development of a fluxonium qubit. The first one is the so-called cosine-phi problem stemming from the seminal paper of Brian Josephson. It predicted the phase dependence of the dissipative current across the Josephson junction. A fluxonium qubit enabled the observation of the effect, after nearly 50 years of unsuccessful attempts by other techniques. The second one is inelastic scattering ("splitting") of a microwave photon by quantum fluctuations of phase across a Josephson junction. This effect is the elementary mechanism driving the Schmid transition, which predicts a collapse of the Josephson current in a junction influenced by a dissipative environment.

VIDEO:
Attend virtually via Zoom:
##### https://web.phys.virginia.edu/Private/Covid-19/colloquium.asp

Friday, October 8, 2021
3:30 PM
Physics Building, Room 204
Note special room.

## " Muon magnetic anomaly: probing the innermost nature of vacuum"

Professor Dinko Pocanic , University of Virginia - Department of Physics
[Host: Prof. Gordon Cates]
ABSTRACT:

Marking the semi-anniversary of the Fermilab Muon g−2 experiment (E989) Run 1 result, this colloquium will review the current status of the muon magnetic anomaly, the inferred evidence of possible particles outside the Standard Model (SM), and future prospects in this active research field.

The intrinsic magnetic field of a simple object, such as a compass needle, is expressed in terms of its magnetic moment.  The magnetic moment of a point particle, such as the electron, is predicted by relativistic quantum mechanics to be g = 2, in convenient dimensionless units.  For the electron, this prediction fails at the part-per-thousand level; the resulting magnetic anomaly, ae = (g − 2)/2, is due to the electron’s couplings to virtual particles excited in the vacuum.

Muon, the electron’s 200 times heavier cousin, experiences far stronger couplings to massive virtual particles, including possible non-SM exotics. The SM provides a prediction for the muon magnetic anomaly aµ with sub-ppm precision. Hence, a comparably precise measurement of aµ offers a uniquely sensitive test for the presence of non-SM particles in nature. For almost 20 years a tantalizing discrepancy of ∼ 3 – 4σ has persisted between the measurements of aµ, and the SM calculations. The Fermilab Muon g−2 Run 1 result brings a much awaited update to this test, with much more to come.

VIDEO:

Attend virtually via Zoom:
##### https://web.phys.virginia.edu/Private/Covid-19/colloquium.asp

Friday, October 1, 2021
3:30 PM
Physics Building, Room 204
Note special room.

## "Rotation sensing with an atom-interferometer gyroscope"

Professor Cass Sackett , University of Virginia - Department of Physics
[Host: Gordon Cates]
ABSTRACT:

Precision rotation sensing is useful for navigation, geophysics, and tests of fundamental physics. Atom interferometers provide, by some measures, the most sensitive method for rotation sensing achieved to date. However, the best performance requires freely falling atoms in a large experimental apparatus. Many applications, such as navigating a vehicle, will benefit from a more compact geometry. One method to achieve this is by using trapped atoms that are suspended against gravity. We have implemented such an interferometer and used it to measure a rotation rate comparable to that of the Earth. The most recent iteration of the interferometer has demonstrated improvements by a factor of ten in rotation sensitivity and trap stability. A second new apparatus reduces the scale of the vacuum chamber and optical system to roughly the size of a microwave oven.

VIDEO:

Attend virtually via Zoom:
##### https://web.phys.virginia.edu/Private/Covid-19/colloquium.asp

Friday, September 17, 2021
3:30 PM
Physics Building, Room 204
Note special room.

## "Inside the Proton: science fact, speculation, and the stuff of science fiction"

Professor Gordon Cates , University of Virginia - Department of Physics
[Host: Kent Paschke]
ABSTRACT:

Whereas the structure of the atom has been  understood for many years, the internal structure of the proton (and neutron) is the subject of active research.  Understanding the nucleon is difficult because its structure is governed by quantum chromodynamics, or QCD, which has not been solved exactly in the non-perturbative or low-energy regime.  The proton's structure is intriguing, however, for many reasons.  For example, we think of the proton as being made of three quarks, but the mass of those quarks only accounts for about 1% of the proton's mass.  The remaining 99%, and hence 99% of the known mass in the universe, is due to exotic effects associated with the QCD vacuum.  While a great deal of work remains to be done, the way in which we visualize the proton has changed dramatically since the discovery of quarks.  Just as the structure of the atom was unveiled early in the 20th century, the structure of the proton is being unveiled in the first decades of the 21st century.  Another intriguing aspect of the proton arises from the fact that QCD is the only theory in nature that has essentially no free parameters. String theory, that attempts to unify our understanding of gravity and the quantum world, grew out of early efforts to understand the strong interaction.  Since string theory deals with the topology of space and time, it is tempting to believe that a deep understanding of the proton may one day provide a window into even more fundamental questions. The colloquium will cover some recent developments in our understanding of the nucleon, as well as providing a glimpse of where this rich area of research is heading in upcoming years.

VIDEO:

Attend virtually via Zoom:
##### https://web.phys.virginia.edu/Private/Covid-19/colloquium.asp

Friday, September 10, 2021
3:30 PM
Physics Building, Room 204
Note special room.

## "Stirring by staring: Induced non-equilibrium states by measurements in quantum systems"

Israel Klich , University of Virginia - Department of Physics
[Host: Despina Louca]
ABSTRACT:

In quantum mechanics, the role of an observer is fundamentally different from that of a classical observer.  The quantum mechanical observer necessarily plays an active role in the dynamics of the system that it is observing.  This apparent difficulty may be turned into a tool to drive an initially trivial system into a complicated quantum many-body state simply by observing it.  I will present two remarkable examples of states induced by measurement. In the first, we examine the role of a moving density measuring device interacting with a system of fermions, and in particular, show that it would leave behind a wake of purely quantum origin. In the second example, inspired by the recent invention of topological Floquet insulators, we will see how a suitably chosen set of density measurements, repeated periodically, will induce robust chiral edge motion on a lattice of free fermions. Our examples show how quantum mechanical observation can be added as a versatile tool to the arsenal of quantum engineering in condensed matter systems.

VIDEO:
Click on the following link to attend the online colloquium:
##### https://web.phys.virginia.edu/Private/Covid-19/colloquium.asp

Friday, February 12, 2021
2:00 PM
Online, Room via Zoom
Note special time.
Note special room.

## "Complexity of magnetic patterns and self-induced spin-glass state"

Prof. Mikhail Katsnelson , Radboud University of Nijmegen, The Netherlands,
[Host: Dima Pesin]
ABSTRACT:

The origin of complexity remains one of the most important and, at the same time, the most controversial scientific problems. Earlier attempts were based on theory of dynamical systems but did not lead to a satisfactory solution of the problem. I believe that a deeper understanding is possible based on a recent development of statistical physics, combining it with relevant ideas from evolutionary biology and machine learning.

Using patterns in magnetic materials as the main example, I discuss some general problems such as (a) a formal definition of pattern complexity [1]; (b) self-induced spin glassiness due to competing interactions as a way to interpret chaotic patterns [2]; (c) multi-well states intermediate between glasses and ordinary ordered states and their relevance for the problem of long-term memory in complicated systems [3]; and (d) complexity of frustrated quantum spin systems [4]. I will also review a very recent experimental observation of self-induced spin-glass state in elemental neodymium [5].

[1] A. A. Bagrov, I. A. Iakovlev, A. A. Iliasov, M. I. Katsnelson, and V. V. Mazurenko, Multi-scale structural complexity of natural patterns, PNAS 117, 30241 (2020).
[2] A. Principi and M. I. Katsnelson, Spin glasses in ferromagnetic thin films, Phys. Rev. B 93, 054410 (2016); Self-induced glassiness and pattern formation in spin systems due to long-range interactions, Phys. Rev. Lett. 117, 137201 (2016).
[3] A. Kolmus, M. I. Katsnelson, A. A. Khajetoorians, and H. J. Kappen, Atom-by-atom construction of attractors in a tunable finite size spin array, New J. Phys. 22, 023038 (2020).
[4] T. Westerhout, N. Astrakhantsev, K. S. Tikhonov, M. I. Katsnelson, and A. A. Bagrov, Generalization properties of neural network approximations to frustrated magnet ground states, Nature Commun. 11, 1 (2020).
[5] U. Kamber et al, Self-induced spin glass state in elemental and crystalline neodymium, Science 368, eaay6757 (2020).
VIDEO:
Click on the following link to attend the online colloquium:
##### https://web.phys.virginia.edu/Private/Covid-19/colloquium.asp

Friday, February 5, 2021
3:30 PM
Physics Building, Room via Zoom
Note special room.

## "A fermionic triangular-lattice quantum gas microscope "

Peter Schauss , University of Virginia - Physics Dept.
[Host: Bob Jones]
ABSTRACT:

Geometrically frustrated many-body systems show many interesting emerging phenomena, ranging from kinetic frustration to exotic spin ordering and chiral spin liquid phases. Ultracold atom systems offer great tunability and flexibility to realize such systems in a wide parameter range of interactions, densities, and spin-imbalance.

In this talk, I will present our recent results on site-resolved imaging of ultracold fermionic lithium atoms on a triangular optical lattice.

Degenerate Fermi gases with about one tenth of the Fermi temperature have been realized within a crossed dipole trap and successfully loaded into a two-dimensional triangular optical lattice. To characterize this lattice, we observed Kapitza-Dirac scattering using a molecular Bose-Einstein condensate. Collecting the emitted photons during Raman sideband cooling in the triangular lattice using a high-resolution microscope objective enabled the high-fidelity imaging of individual fermionic atoms in the lattice with single-site resolution.

The next step will be the realization of a triangular lattice Hubbard model by implementing an additional optical lattice to increase interactions.

This novel experimental platform will allow us to study spin and density correlations in the triangular Hubbard model to explore signatures of frustration and spin-hole bound states and may lead to a direct observation of non-vanishing chiral correlations.

VIDEO:

To add a speaker, send an email to gdc4k@Virginia.EDU Include the seminar type (e.g. Colloquia), date, name of the speaker, title of talk, and an abstract (if available). [Please send a copy of the email to phys-speakers@Virginia.EDU.]