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Colloquium

The Spring 2021 Colloquia will be held over Zoom on Mondays at 3:30 PM, EST.

To register, please contact Chrisanne Romeo (cromeo@utk.edu)

Spring Colloquium Chair, Haidong Zhou (hzhou10@utk.edu)


Colloquium Archives
Spring 2021 Schedule
Date
Speaker
Title
Host

January 25

No Colloquium

 

 

February 1

Weiwei Xie
Rutgers University

Chemistry Perspectives to Novel Quantum Materials: How to Count Electrons and Predict New Materials?

Haidong Zhou

February 8

Susan Gardner
University of Kentucky

Hunting Baryon Number Violation: Connections and Implications

Yuri Kamyshkov

February 15

Nikolay Prokof’ev
UMass Amherst

Fermionic Sign Problem: an Exaggerated Myth

Adrian Del Maestro

February 22

Michael Snow
Indiana University Bloomington

Laboratory Cosmology with Slow Neutrons

Yuri Kamyshkov

March 1

Huibo Cao
ORNL

Field-Tunable Toroidal Moment in a Chiral-Lattice Magnet

Haidong Zhou

March 8

William Wester
Fermilab

Search for Axion Dark Matter

Josh Barrow/Yuri Kamyshkov

March 15

Bob Dubois
UT Psychology

TBD

Graduate Physics Society/Igor Bernardi

March 22

Emil Bozin
Brookhaven National Lab

Unmasking of the High Temperature Orbital Precursors in Quantum Materials

Haidong Zhou

March 29

Jie Xiao
Johns Hopkins School of Medicine

TBD

Jaan Mannik

April 5

Nitin Samarth
Penn State

TBD

Joon Sue Lee

April 12

Antia Botana
Arizona State University

TBD

Haidong Zhou

April 19

Laura Jeanty
University of Oregon

TBD

Tova Holmes

April 26

Honors Day

 

 


Abstracts
February 1

Weiwei Xie, Rutgers University

Chemistry Perspectives to Novel Quantum Materials: How to Count Electrons and Predict New Materials?

Design and discovery of new quantum materials will accelerate the development of new technologies in the future. I will report my group research progress in the past 4 years, mainly focusing on the new superconductors and new magnetic topological quantum materials. My group recently discovered several new superconductors. I will explain our interpretation of this work. More importantly, we are trying to use chemical bonding concept to predict the existence of superconductivity in the materials. Magnetic topological quantum materials (MTQMs) can give rise to forefront electronic properties such as the quantum anomalous Hall effect, axion electrodynamics and Majorana fermions. In our group, we used chemistry electron count rules and structure-property relationship to design new MTQMs. I will describe how to design and prove the material candidate as a new MTQM from both experimental and theoretical aspects and show how topological electronic states and magnetism interplay in the new material.


February 8

Susan Gardner, University of Kentucky

Hunting Baryon Number Violation: Connections and Implications

The established cosmic preponderance of baryons over antibaryons is usually regarded as evidence for baryon number violation, but we have as yet to discover anything of its nature. For example, baryon number B or the difference of baryon number and lepton number L, B-L, could be a gauge symmetry, and this symmetry may be explicitly or spontaneously broken or both. These ideas also thread through theoretical models of the neutrino mass and/or of dark matter. I will survey the possibilities, their connections and implications, emphasizing the suite of experiments that test the various scenarios of B violation and noting the constraints that also follow from the observed properties of neutron stars.


February 15

Nikolay Prokof’ev, UMass Amherst

Fermionic Sign Problem: an Exaggerated Myth

Feynman diagrams are the most celebrated and powerful tool of theoretical physics usually associated with an analytic approach. I will argue that diagrammatic expansions are also an ideal numerical tool with enormous and yet to be explored potential for solving interacting fermionic systems by direct simulation of connected Feynman diagrams. Though the original series based on bare propagators and interactions are sign-alternating and often divergent one can still compute the correct answer behind them by using appropriate series re-summation techniques, conformal mappings, asymptotic series analysis, and homotopic action tools. Ultimately, the diagrammatic expansion can always be made convergent! When dealing with Feynman diagrams, the conventional fermionic sign problem is simply absent for regular systems because the entire setup is valid in the thermodynamic limit. Instead, fermionic sign is a “blessing” because it leads to massive cancellation of high-order diagrams and ultimate convergence of the re-summed series. For illustration, I will discuss results for the unitary Fermi gas, the Fermi-Hubbard model, and jellium (homogeneous electron gas).


February 22

Michael Snow, Indiana University Bloomington

Laboratory Cosmology with Slow Neutrons

Experiments using slow neutrons can address interesting scientific questions in particle physics/astrophysics/cosmology. In this talk I will concentrate specifically on slow neutron experiments which offer input into Big Bang Cosmology or which can search for some of the ingredients that may have led to the matter-antimatter asymmetry of the universe.


March 1

Huibo Cao, Oak Ridge National Laboratory

Field-Tunable Toroidal Moment in a Chiral-Lattice Magnet

As one of very few macroscopic manifestations of quantum coherence, magnetic order is at the crux of condensed matter physics. Assisted with neutrons, magnetism beyond ferromagnetism such as antiferro- ferri- and non-collinear magnetic order was integrated in our knowledge only in the last decades. Nowadays, our interest has been moved to topologically nontrivial magnetism and magnetically coupled multiferroic systems. Here I will introduce our recent observation using neutrons of a field-tunable toroidal moment in a chiral-lattice magnet. A toroidal dipole moment appears independent of the electric and magnetic dipole moment in the multipole expansion of electrodynamics. It arises naturally from vortex-like arrangements of spins. Observing and controlling spontaneous long-range orders of toroidal moments are highly promising for spintronics but remain challenging. Here we demonstrate that a vortex-like spin configuration with a staggered arrangement of toroidal moments, a ferritoroidal state, is realized in a chiral triangular-lattice magnet BaCoSiO4. Upon applying a magnetic field, we observe multi-stair toroidal transitions correlating directly with metamagnetic transitions. We establish a first-principles microscopic Hamiltonian that explains both the formation of toroidal states and the metamagnetic toroidal transition as a combined effect of the magnetic frustration and the Dzyaloshinskii-Moriya interactions allowed by the crystallographic chirality in BaCoSiO4.


March 8

William Wester, Fermilab

Search for Axion Dark Matter

The axion is a proposed particle whose existence might account for much of the dark matter of the universe. This same particle also arises as the solution to the strong-CP problem of particle physics. There have several decades of experiments that have attempted to detect new particles with many of the properties of the axion - but without being sensitive to the preferred region of parameter space. I will report on the Axion Dark Matter Experiment, ADMX, which is currently running and achieving the required sensitivity towards potential discovery.


March 22

Emil Bozin, Brookhaven National Lab

Unmasking of the High Temperature Orbital Precursors in Quantum Materials

The rich physics associated with the emergence of technologically relevant quantum orders in materials stems from complex interaction of electronic charge, spin, and orbitals, and their coupling to the host lattice. In transition metal systems with partial filling of d-manifolds novel properties often engage the orbital sector. Systems exhibiting orbital degeneracy and/or electronic frustration imposed by their lattice topology are particularly interesting, as orbitals couple both to the spin, via electronic interactions, and to the lattice, via Jahn-Teller mechanisms. The removal of this orbital degeneracy and the subsequent relief of frustration then impact symmetry lowering and material properties. The electronic complexity of the low temperature ordered symmetry-broken states has been thoroughly studied in systems displaying diverse emergent behaviors such as frustrated magnetism, colossal magnetoresistivity, charge and orbital order, metal-insulator transition, pseudogap, and high temperature superconductivity. Their understanding employs Fermi surface nesting, Peierls, and band Jahn-Teller mechanisms, among others.

In systems where orbital degeneracies are anticipated, crystallographic symmetry lowering at the temperature driven structural phase transitions is often assumed to imply simultaneous orbital degeneracy lifting (ODL) by engaging some cooperative mechanism. Consequently, seemingly mundane high temperature regimes possessing high crystallographic symmetry remain much less explored. In contrast to this concept, recent utilization of probes sensitive to local symmetry qualified the ODL as a local electronic effect existing at temperature well above the global symmetry breaking transitions.

This presentation will showcase a sensitive local structural technique, x-ray atomic pair distribution function analysis, as it reveals the presence of fluctuating local-structural distortions at high temperature of several transition metal-based quantum materials exhibiting orbital-selective ground states. We argue that this hitherto overlooked fluctuating symmetry-lowering is electronic in nature, thereby modifying the energy-level spectrum and electronic and magnetic properties. The origin is a local, spatio-temporally fluctuating, orbital degeneracy lifted state, that acts as a precursor to electronic phenomena observed at low temperature. It will be demonstrated that such local orbital states come in many flavors (e.g. engaging single [1,2] or multiple transition metal d orbitals [3]), and that they are likely to exist both in the proximity to itinerant-to-localized crossover [1,3] and deep in the Mott insulating regime where charge fluctuations are suppressed [4]. These observations suggest that such precursor states are likely to be widespread amongst diverse classes of partially filled nominally degenerate d-electron systems, with potentially broad implications for our understanding of their properties.

[1] E.S. Bozin et al., Nature Comms. 10, 3638 (2019).
[2] R.J. Koch et al., Phys. Rev. B 100, 020501(R) (2019).
[3] L. Yang et al., Phys. Rev. B 102, 235128 (2020).
[4] R.J. Koch et al., arXiv:2009.14288 (2021).



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