Thursdays 4:00 p.m. 104 Physics or online via Zoom.
Contact colloquium organizer Dr. Hyunsoo Kim at email@example.com 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
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.
 Lu et al., Nat. Commun. 13:4603 (2022)
 Kowalczyk et al., ACS Nano 14, 1888 (2020)
 Lu et al., arXiv:2303.02971(2023)
 Lu et al., Advanced Materials 2200625 (2022)