# Physics Colloquium Spring 2022

Thursdays 4:00 p.m. 104 Physics or online via Zoom.

Contact colloquium organizer Dr. Shun Saito at saitos@mst.edu for the Zoom link.



Title: Pairing at Zero Density

Abstract: As far as we know all superconductors are condensates of paired electrons. Thus, pairing is an essential stage in the formation of a superconductor. According to standard lore, the key to overcoming the strong electron-electron repulsion lies in their ability to exchange phonons, which vibrate at low frequency (the Debye frequency), orders of magnitude smaller than the electronic frequency (the Fermi energy). Moreover, the superconducting transition temperature is an emergent scale that is exponentially sensitive to the density of free charge carriers, thus bounding the density from below. The discovery of superconductivity in doped materials (semimetals, semiconductors, doped Mott insulators, twisted bilayer graphene etc.) challenges this understanding by defying these basic requirements. In this talk I will discuss these issues in detail. I will then present a concrete example exhibiting superconductivity at any charge density including zero; A Dirac semimetal in a crystal that is undergoing a structural quantum phase transition. Thus, providing a new mechanism for superconductivity that does not suffer from a vanishing density and possibly opens a path to understand the low-density superconductors.

Title: Cosmology in the machine learning era

Abstract: Advances in machine learning are triggering a revolution across fields. In this talk I will show how machine learning is helping cosmologists to tackle decades-old problems. I will first describe some of the most important problems in cosmology. I will then illustrate the need for a large collection of numerical simulations that describe the Universe and its components within the theoretical uncertainties. Next, I will show how machine learning can be used as the glue between cosmology, astrophysics, and numerical simulations to maximize the scientific return of cosmological surveys and astronomical data in general.
Title: Ultrafast population inversion in nitrogen molecular ions by intense laser fields

Abstract: When an intense, femtosecond laser pulse is focused in air, coherent emission referred to as “air lasing” is generated. The lasing emission at 391 nm, corresponding to the transition from the electronically excited B state to the electronic ground X state of N2+ is particularly interesting, since strong-field ionization of N2 is not expected to produce B-X population inversion.
By observing air lasing using few-cycle laser pulses, we showed in 2015 [1] that the B-X population inversion is formed on a few-fs time scale through the post-ionization excitation of N2+. In recent measurements, we showed that the 391 nm lasing signal can be enhanced by 5 orders of magnitude by combining a polarization-gated 800 nm driving pulse with a 1600 nm IR pulse [2], and that the lasing signal contains signatures of the coherent rotational, vibrational, and electronic motion of N2+ [3].
In the talk, I will give an overview of air lasing, explain the mechanism behind the ultrafast B-X population inversion in N2+, and discuss the theoretical simulation of the excitation process.
[1] H. Xu, E. Lötstedt, A. Iwasaki, and K. Yamanouchi, Nat. Commun. 6, 8347 (2015).
[2] H. Li et al., Phys. Rev. Lett. 125, 053201 (2020).
[3] T. Ando et al., Phys. Rev. Lett. 123, 203201 (2019).

Search for Light Bosons with King and Super-King Plots

There are many defects in the standard model of particle physics, such as the evident need
for some form of dark matter. The main motivation for the present talk is the potential to
complement or even extend what can be learned from high-energy particle experiments with
high accuracy atomic-physics measurements at low energy in the search for new physics be-
yond the standard model. One such search is the use of high precision measurements of the
atomic isotope shift in a sequence of isotopes that would be sensitive to an electron-neutron
interaction mediated by light bosons. If such an interaction existed, it would presumably
vary linearly with the number of neutrons in the nucleus, and so would produce a systematic
deviation in the isotope shift measurements. The so-called King plot provides a sensitive
method to search for such deviations. By taking second differences, we have recently devel-
oped a super-King plot [1] that can be applied to light heliumlike ions where high-precision
calculations are possible to assist the interpretation of laboratory measurements of isotope
shifts. Results of calculations and prospects for future experiments will be presented.
[1] G.W.F. Drake, H.S. Dhindsar and V.J. Martin, Phys. Rev. A 105, L060801 (2021).

Title: Ultrafast chirality: twisting light to twist electrons
Abstract:

I will describe several new, extremely efficient approaches to chiral discrimination and enantio-sensitive molecular manipulation, which take advantage of ultrafast electronic response [1,2].  One of them is based on the   new concept of synthetic chiral light [3,4], which can be used to trigger bright nonlinear optical response in the molecule of a desired handedness while keeping its mirror twin dark in the same frequency range. The other is based on the new concept of  geometric magnetism in photoionization of chiral molecules and leads to a new class of enantiosensitive observables in photoionization [5]. Crucially, the emergence of these new observables is associated with ultrafast excitation of chiral electronic or vibronic currents prior to ionization and can be viewed as their unique signature.

[1] S Beaulieu et al, Photoexcitation circular dichroism in chiral molecules, Nature Physics 14 (5), 484 (2018)

[2] AF Ordonez, O Smirnova, Generalized perspective on chiral measurements without magnetic interactions, Physical Review A 98 (6), 063428, 2018

[3] D Ayuso et al, Synthetic chiral light for efficient control of chiral light–matter interaction, Nature Photonics 13 (12), 866-871, (2019)

[4] D. Ayuso et al "Enantio-sensitive unidirectional light bending”,  Nat Commun 12, 3951 (2021)

[5] AF Ordonez, D Ayuso, P. Decleva, O Smirnova “Geometric magnetism and new enantio-sensitive observables in photoionization of chiral molecules” http://arxiv.org/abs/2106.14264

Title: Universal Physics of 2 or 3 or 4 Strongly Interacting Particle
By Chris Greene, Purdue University

Recent developments in the field of a few interacting particles with
nonperturbative interactions will be reviewed, focusing on ultracold
atomic physics, but with one recent application to the few-nucleon
problem as well.  Some of these studies are intimately connected with
the Efimov effect, while others go beyond the standard Efimov effect
with its remarkable infinity of long-range energy levels.  Some of our
relevant references addressing those topics are listed below.

[1] Nonadiabatic Molecular Association in Thermal Gases Driven by
Radio-Frequency Pulses, Phys. Rev. Lett. 123, 043204 (2019), with Panos
Giannakeas, Lev Khaykovich, and Jan-Michael Rost.
[2]  Nonresonant Density of States Enhancement at Low Energies for Three
or Four Neutrons, Phys. Rev. Lett. 125, 052501 (2020), with Michael
Higgins, Alejandro Kievsky, and Michele Viviani
[3] Efimov physics implications at p-wave fermionic unitarity, Phys Rev
A 105, 013308 (2022), with Yu-Hsin Chen.
[4] Ultracold Heteronuclear Three-Body Systems: How Diabaticity Limits
the Universality of Recombination into Shallow Dimers, Phys. Rev. Lett.
120, 023401 (2018), with Panos Giannakeas.



Simulating the Formation of our Milky Way Galaxy

Within the cosmic web, galaxies like our own Milky Way form as gas flows along cosmic filaments into dark-matter halos, fueling the formation of stars, while the resultant feedback from stars drives strong outflows of gas. Understanding this nonlinear interplay between cosmic inflows and feedback-driven outflows is one of the most significant questions in astrophysics and cosmology, and it requires a new generation of supercomputer simulations that can achieve high dynamic range to resolve the scales of stars within a cosmological environment. I will describe how we use massively parallelized cosmological zoom-in simulations to model the physics of galaxy formation at unprecedented resolution. I will discuss new insight into the formation of our Milky Way galaxy, including the faintest-known galaxies that orbit around it. These low-mass galaxies trace structure formation on the smallest cosmological scales and have presented the most significant challenges to the cold dark matter paradigm. I will describe how these new generations of simulations are allowing us to shed light on dark matter.

TITLE: Working with Giorgio Parisi: Quantum Field Theory in Action

ABSTRACT:
Quantum field theories such as quantum electrodynamics belong to the jewels of
theoretical physics: They have allowed us to understand energy levels of simple
atomic systems to an accuracy of 13 or 14 decimals, and to calculate the
anomalous magnetic moment of the electron to 10 digits. On a completely
different footing, the renormalization-group analysis of the critical point of
the so-called N-vector model has allowed us to calculate critical exponents of
phase transitions to unprecendented accuracy. Yet, all these methods rely on
perturbative methods, which are encapsulated in so-called Feynman diagrams.
These are diagrams which illustrate the perturbative corrections to a physical
quantity in perturbation theory. In higher orders, the number of diagrams grows
factorially, and all perturbation series eventually diverge, in the form of
asymptotic series. While this fact does not significantly diminish the
predictive power of quantum electrodynamics, in view of the smallness of the
fine-structure constant alpha ~ 1/137, which is the quantum
electrodynamic coupling parameter, the problem manifests itself prominently in
the N-vector model, where the critical point (zero of the so-called $\beta$
function) is reached for coupling parameters of order unity. Decades of
efforts into the calculation of higher-order Feynman diagrams typically end at
the five-loop level (quantum electrodynamics) or seven-loop level (N-vector
model). With Giorgio Parisi (Nobel Laureate, 2021, Rome) and Jean Zinn-Justin
(CEA Saclay), we have been finding ways to overcome the predictive limits of
perturbation theory, paving the way for diagrammatic expansions about infinite
loop order, and showing that, in infinite loop order, the evaluation of Feynman
diagrams becomes, again, a manageable task.

Unraveling the superconducting state of UTe2

Priscila F. S. Rosa
Los Alamos National Laboratory, Los Alamos, NM 87545 USA

Spin-triplet superconductors are a promising route in the search for topological superconductivity, and UTe2 is a recently discovered contender. In this talk, I will present a brief overview of key experimental results on the superconducting state of UTe2 as well as some of its outstanding puzzles. I will then focus on recent developments in sample synthesis combined with thermodynamic measurements that shed light on the role of disorder and magnetic fluctuations in UTe2. At the end of the talk, I will highlight some of the pressing outstanding open questions regarding the superconducting order parameter of UTe2.

Nonlinear gravity in the late Universe

In this talk I will describe a number of novel ideas that arise in a nonlinear general relativistic treatment of cosmology, and recent progress modeling cosmological observables in this context. These advances provide us with insight into subtle gravitational effects that allow us to test general relativity in new regimes; yet if unaccounted for, these same effects may mislead us in our search for new physics. I will describe how these models not only allow us to study the behavior of general relativity at a nonlinear level, but provide us with a way to test the fundamental assumptions our cosmological models are built upon.

Imaging Domain switching and Revealing Emergent Orders in 2D Antiferromagnets

The family of monolayer two-dimensional (2D) materials hosts a wide range of interesting phenomena, including superconductivity, charge density waves, topological states and ferromagnetism, but direct evidence for antiferromagnetism in the monolayer has been lacking. Antiferromagnets have attracted enormous interest recently in spintronics due to the absence of stray fields and their terahertz resonant frequency. Despite the great advantages of antiferromagnetic spintronics,  controlling and directly detecting  Neel vectors in 2D materials have been challenging. In my talk, I will show that we have developed a sensitive second harmonic generation  (SHG) microscope and detected long-range Neel antiferromagnetic (AFM) order down to the  monolayer in MnPSe3[1].    In MnPSe3, we observed the switching of an Ising-type  Neel vector  reversed by the time-reversal operation.  We rotated them by an arbitrary angle irrespective of the lattice by applying uniaxial strain. The phase transition in the presence of strain in MnPSe3 falls into the Ising universality class instead of the XY type, and the Ising Neel vector is locked to the strain. I will also talk about using the newly developed optical confocal microscopy to image other emergent orders in 2D antiferromagnets[2, 3,4].

References:
1. Ni et al. Nature Nanotechnology 16, 782-787 (2021)
2. Ni et al.  Phys. Rev. Lett. 127, 187201 (2021)
3. Zhang et al. Nature Photonics (2022)
4. Ni, et al. Nano Letters (2022)

Fuller Prize Finalists 2022

Reece Beattie-Hauser: Scalar susceptibility of a diluted classical XY model

Charlie Kropp: Investigation of Time-Delay, Transmission, and Deposition Eigenchannels

Anthony Lonsdale: Application of Molecular Spin Dynamics to Thermal Transport Problems

Ethan Pham: Electric Field Exfoliation for Two-Dimensional Nanolayered Materials

Jordan Stevens: Early Dark Energy in light of precise cosmological observations

Title: Superconducting Energy Gap Probed by Particle Irradiations

Abstract:

Recent discovery of the room temperature superconductor [1] has attracted renewed attention in the field of superconductivity. The superconductor is a material that shows zero resistivity and Meissner effect below its critical temperature. These unique properties are originated from the pairing of electrons below its critical temperature by opening a superconducting energy gap. Thus, investigating the symmetry of the superconducting energy gap is the first and most important step to understand its properties.

High-energy particle irradiations (electron, proton, heavy ion, etc.) have been used to study various superconductors. Especially, the electron irradiation was effectively used to identify the symmetry of superconducting energy gaps. In this talk, I will describe how the particle irradiations help understand the superconducting energy gaps of (Ba1-xKx)Fe2As2 iron-based superconductor [2], NbSe2 [3], and YBa2Cu3O7-δ [4].

[1] E. Snider et al., Nature 586, 373 (2020).

[2] K. Cho, et al., SCIENCE ADVANCES 2, e1600807 (2016).

[3] K. Cho, et al., Nature Communications 9, 2796 (2018).

[4] K. Cho, et al., Phys. Rev. B 105, 014514 (2022).