Physics Colloquium Spring 2024

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

Contact colloquium organizer Dr. Hyunsoo Kim at for the Zoom link.

(Link to main colloquium page)

Compressibility Effects on Turbulence Production



Turbulence in compressible flows plays a critical role in various applications, such as optimizing air/fuel mixing in scramjet combustors. Understanding and controlling turbulence in these scenarios is vital. However, dealing with compressible turbulence poses greater challenges compared to its incompressible counterpart because thermodynamic quantities fluctuate.

In compressible turbulent flows, the production of turbulent kinetic energy depends on both velocity and density fluctuations. The widely used Morkovin's hypothesis in literature relates density fluctuations to velocity and the local Mach number. In this study, we examine the validity of this hypothesis and leverage its expression to redefine turbulence production solely in terms of velocity.

Our results indicate that the planar component of turbulence production remains largely unaffected by compressibility. This finding supports previous research focused on sustaining turbulence production through streamwise vortices. Conversely, the turbulence production terms aligned with the convective velocity are influenced by compressibility, even at relatively low Mach numbers.

This work contributes to refining our understanding of compressible turbulence and its implications for practical applications, particularly in the context of enhancing combustion efficiency in scramjet combustors.


Altermagnetism in 3- and 2-D: simple symmetry constraints and

Igor Mazin
George Mason University, Fairfax, VA

Since many years, the canonical classification of ordered magnets
included noncollinear (with many further subdivisions) and two collinear
types: antiferromagnets (AF), which have net magnetization zero by
symmetry, and ferro/ferrimagnets (FM), which do not have this property.
The two have distinctly different micro- and macroscopic properties. It
was supposed, for instance, that only FM can exhibit spin-splitting of
the electronic bands in absence of spin-orbit coupling AND lack of
inversion symmetry, have anomalous Hall effect (i.e., Hall effect driven
by variation of the Berry phase), magnetooptical effects, suppressed
Andreev scattering in contact with a singlet superconductor etc. A
surprisingly recent development (~2019) is that this classification is
incomplete: there are collinear magnets that would belong to AF by this
classification, but show all characteristics of FM, except the net spin
polarization! They were recently dubbed "altermagnets", AM.
Incidentally, what has also not been fully appreciated was that there
are also materials that have strictly zero net magnetization, but
enforced not by symmetry, but by the Luttinger's theorem, and therefore
truly belonging to the FM class ("Luttinger-compensated ferrimagnets").
In this talk I will present the new classification and explain, in
specific examples, what are the symmetry conditions for AM, why these
are a truly new class deserving a new name, and how their unusual
properties appear. In the second part of the talk I will discuss
single-layer non-AM antiferromagnets, and show how the can be
functionalized to be AM by targeted symmetry lowering, with specific
examples of MnP(S,Se)3 and FeSe, and will discuss novel properties
compared to 3D AM

Synthesis and Characterization of Novel 2D Dirac/Weyl Materials

The discovery of graphene has stimulated enormous interest in two-dimensional (2D) electron gas with linear band structure. 2D Dirac materials possess many intriguing physical properties such as high carrier mobility and zero-energy Landau level for the relativistic dispersion and chiral spin/pseudospin texture. In this talk, we will discuss three new variants of 2D Dirac materials including (1) unpinned 2D Dirac semimetals in α-antimonene1,2, (2) Rashba spin-split 2D Weyl semimetals in α-bismuthene3, and (3) interacting Dirac states in graphene heterostructures4. The results offer new insights into the relativistic behavior of electrons in reduced dimensions. We will review the emergent properties and device applications of relativistic electrons in those 2D Dirac/Weyl semimetals, especially, cloning of Dirac fermions, Moiré flat bands, and spin/valley separators.
[1] Lu et al., Nat. Commun. 13:4603 (2022)
[2] Kowalczyk et al., ACS Nano 14, 1888 (2020)
[3] Lu et al., arXiv:2303.02971(2023)
[4] Lu et al., Advanced Materials 2200625 (2022)

Gravitational wave: messenger of the extreme universe
Neutron stars are the most extreme physical objects known to science. The central densities of neutron stars are orders of magnitude higher than the nuclear density. As a result, they warp the spacetime around them to such an extent that in a binary system, they emanate strong emission of gravitational waves measurable from billions of lightyears away using ground-based gravitational wave detectors like the LIGO. Neutron star collisions are extremely rare events, around one such collision in a million years in a Milky Way-like galaxy, and understanding them is a key to solving some of the most interesting problems in astrophysics: abundance of the periodic table's heavy element, direct measurement of cosmological distances and hence independent measurement of the Hubble constant, understanding how matter behaves in extreme pressure-density environment, etc. In this talk, we will look back into LIGO's quest to make this endeavor a success, which led to the opening of the field of multi-messenger GW-EM astrophysics. I will give a brief introduction to the science of neutron star astrophysics and gravitational waves, and then talk about the new advances in the field in the last seven years. 

Dispersive shock waves in one-dimensional quantum droplets

We explore dispersive hydrodynamic phenomena in quantum droplets of homonuclear cold bosonic mixtures utilizing an extended Gross-Pitaevskii (eGPE) framework. This model supports various self-bound configurations such as droplets, bubbles and kinks. It is shown that the interaction of these structures can lead to the dynamical nucleation of dispersive shock-waves. In this work, we specifically unravel the dispersive regularization of hydrodynamic singularities in the eGPE due to the competition of mean-field repulsion and quantum fluctuation attraction. Interestingly, the controllable generation of dispersive shock-waves with the aid of initial state engineering is demonstrated. This can be understood in terms of families of Riemann problems. We classify and analyze the emergent wave-patterns using the framework of Whitham modulation theory.

Modern tests of Quantum Electrodynamics in the strong-field regime

Quantum electrodynamics (QED) is a well-established physical theory and its predictions have
been confirmed experimentally in various regimes and with extremely high accuracy. However,
there are still areas of QED that deserve theoretical and experimental investigation, especially
when physical processes occur in the presence of intense background electromagnetic fields, i.e., of
the order of the so-called “critical” field of QED [1–3].
After a broad introduction on strong-field QED and I focus on two prominent theoretical exam-
ples of currently open problems in the field: The problems of radiation reaction and that of vacuum
polarization [1–3]. Then, I will show how a newly-developed technology, “flying focus laser beams”
(FFBs), can be employed as a tool to test QED in the strong-field regime and, in particular, its pre-
dictions on radiation reaction and vacuum polarization. In FFBs, in fact, the velocity of the focus
can be “programmed” and it is independent of the group and the phase velocity of the beam itself.
Specifically, by considering either an ultrarelativistic electron beam or a high-energy photon beam
counterpropagating with respect to a FFB, whose focus copropagates with the electrons/photons
at the speed of light, we show that radiation-reaction [4] and vacuum-polarization [5] effects can
be rendered measurable at much lower intensities than conventionally required in a similar setup.
[1] A. Di Piazza, C. M¨uller, K. Z. Hatsagortsyan, and C. H. Keitel, Rev. Mod. Phys. 84, 1177 (2012).
[2] A. Gonoskov, T. G. Blackburn, M. Marklund, and S. S. Bulanov, Rev. Mod. Phys. 94, 045001 (2022).
[3] A. Fedotov, A. Ilderton, F. Karbstein, B. King, D. Seipt, H. Taya, and G. Torgrimsson, Phys. Rep.
1010, 1 (2023).
[4] M. Formanek, D. Ramsey, J. P. Palastro, D. Froula, and A. Di Piazza, Phys. Rev. A 105, L020203
[5] M. Formanek, J. P. Palastro, D. Ramsey, S. Weber, and A. Di Piazza, (2023), arXiv:2307.11734.

Gravitational waves as a probe of the early Universe

Gravitational waves (GWs) from mergers of black holes and neutron stars have been directly detected since 2015. In addition, the detection of stochastic GWs with the pulsar timing arrays was announced in 2023. With these detections, GWs have opened new windows of Cosmology and astrophysics. In particular, GWs are one of the powerful tools for studying the evolution of the early Universe. This is because, thanks to the weakness of the gravitational interaction, GWs are not damped except for their redshift once they are produced, unlike the density perturbations.

In this colloquium, I will focus on one of the cosmological GWs, those induced by density perturbations. Specifically, I will discuss two situations where this kind of GWs becomes an powerful probe. The first case is that the initial amplitudes of small-scale density perturbations are large. This situation is closely related to the production of a sizable amount of primordial black holes (PBHs) as dark matter or the BHs detected by the LIGO-Virgo-KAGRA collaborations. The second case is that the Universe experiences a sudden end of the early matter-dominated era. As concrete examples for this situation, I will mention the PBH and the axion-rotation domination era.

Rydberg Photonics: Utilizing Rydberg excitons for Nonlinear Quantum Optics
Cu2 O, also known as cuprous oxide, has garnered attention as a solid-state material that holds
promise for hosting excitonic Rydberg states. These states are characterized by large principal
quantum numbers (n), resulting in significantly expanded wavefunctions (∝ n 2 ). Consequently,
Cu 2 O exhibits strong dipole-dipole (∝ n 4 ) and van der Waals (∝ n 11 ) interactions, making it an
appealing platform for solid-state quantum technologies. Of particular interest are thin-film Cu 2 O
samples, which can be fabricated with careful control to minimize defects and enable the
observation of extreme single-photon nonlinearities through the Rydberg blockade.
In this study, I present spectroscopic absorption and photoluminescence investigations focused on
Rydberg excitons in synthetic Cu2 O grown on a transparent substrate. Our observations reveal a
series of yellow excitons extending up to the principal quantum number n = 7. Additionally, I
discuss our preliminary findings in synthesizing highly crystalline Cu2 O samples, which have the
potential to enhance the sample quality and enable the investigation of higher Rydberg states.
Furthermore, I present promising preliminary results from coupling Rydberg excitons to silicon
nitride photonic circuitry, marking a significant advancement in the field of Rydberg photonics.
These discoveries open up new avenues for the development of scalable and integrated on-chip
quantum devices based on Rydberg states in Cu2 O.

Rydberg physics: From Ultralong-Range Molecules to Quantum Simulation and Quantum Optimization

A review on the most recent activities in Rydberg physics at the center for optical quantum technologies will be provided. I start out with addressing the exotic properties of ultralong-range Rydberg molecules (ULRM). ULRM possess extreme bond lengths of the order of several micron and huge dipole moments. Their potential energy curves mimic the highly oscillatory structure of the Rydberg wave  function thereby offering new possibilities for engineering molecular properties on vastly different time and length scales. Trilobite and butterfly states can easily be controlled by weak external electric or magnetic fields. I demonstrate that synthetic dimensions based on quantum numbers can be used to design conical intersections and consequently non-adiabatic interaction effects in the spectra of ULRMs. Ultrafast decay processes are a consequence of these intersections. Quenches of external fields then lead to a rich rovibrational quantum dynamics of ULRM.
The second part of this talk focuses on quantum simulation and quantum optimization. I provide evidence for novel quantum phases of strongly interacting many-body Rydberg setups, specifically the so-called bond order density wave is unraveled and the extended control of Luttinger liquid phases is presented. On the quantum optimization side I describe how a local detuning approach can enhance the tweezer array-based control of the famous graph theoretical MIS and Max-Cut problems. The traditional order ~N2 approach is here replaced by a linear system size scaling approach. Finally, I will make a short excursion into our recent work on single atom implementation of integer linear programming. Here, a single Rydberg atom will be used to encode linear and even nonlinear integer problems which are known to be difficult to solve in a  classical manner.