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1. Physical, numerical, and computational challenges of modeling neutrino transport in core-collapse supernovae

   https://doi.org/10.1007/s41115-020-00010-8  

   Mezzacappa, Anthony ; Endeve, Eirik ; Messer, O. E. ; Bruenn, Stephen W. ( December 2020 , Living Reviews in Computational Astrophysics)

   Abstract The proposal that core collapse supernovae are neutrino driven is still the subject of active investigation more than 50 years after the seminal paper by Colgate and White. The modern version of this paradigm, which we owe to Wilson, proposes that the supernova shock wave is powered by neutrino heating, mediated by the absorption of electron-flavor neutrinos and antineutrinos emanating from the proto-neutron star surface, or neutrinosphere. Neutrino weak interactions with the stellar core fluid, the theory of which is still evolving, are flavor and energy dependent. The associated neutrino mean free paths extend over many orders of magnitude and are never always small relative to the stellar core radius. Thus, neutrinos are never always fluid like. Instead, a kinetic description of them in terms of distribution functions that determine the number density of neutrinos in the six-dimensional phase space of position, direction, and energy, for both neutrinos and antineutrinos of each flavor, or in terms of angular moments of these neutrino distributions that instead provide neutrino number densities in the four-dimensional phase-space subspace of position and energy, is needed. In turn, the computational challenge is twofold: (i) to map the kinetic equations governing the evolution of these distributionsmore »or moments onto discrete representations that are stable, accurate, and, perhaps most important, respect physical laws such as conservation of lepton number and energy and the Fermi–Dirac nature of neutrinos and (ii) to develop efficient, supercomputer-architecture-aware solution methods for the resultant nonlinear algebraic equations. In this review, we present the current state of the art in attempts to meet this challenge.« less
2. Dirac Fermion Cloning, Moiré Flat Bands and Magic Lattice Constants in Epitaxial Monolayer Graphene

   https://doi.org/10.1002/adma.202200625  

   Lu, Qiangsheng ; Le, Congcong ; Zhang, Xiaoqian ; Cook, Jacob ; He, Xiaoqing ; Zarenia, Mohammad ; Vaninger, Mitchel ; Miceli, Paul F. ; Singh, David J. ; Liu, Chang ; et al ( April 2022 , Advanced Materials)

   Tuning interactions between Dirac states in graphene has attracted enormous interest because it can modify the electronic spectrum of the two-dimensional material, enhance electron correlations, and give rise to novel condensed-matter phases such as superconductors, Mott insulators, Wigner crystals and quantum anomalous Hall insulators. Previous works predominantly focus on the flat band dispersion of coupled Dirac states from different twisted graphene layers. In this work, we propose a new route to realizing flat band physics in monolayer graphene under a periodic modulation from substrates. We take graphene/SiC heterostructure as a prototypical example and demonstrate experimentally that the substrate modulation leads to Dirac fermion cloning and consequently, the proximity of the two Dirac cones of monolayer graphene in momentum space. Our theoretical modeling captures the cloning mechanism of Dirac states and indicates that Moiré flat bands can emerge at certain magic lattice constants of the substrate, specifically when the period of modulation becomes nearly commensurate with the (√3×√3)𝑅30∘ supercell of graphene. The results show that epitaxial single monolayer graphene on suitable substrates is a promising platform for exploring exotic many-body quantum phases arising from interactions between Dirac electrons.
3. Graphene quantum dots, graphene non-circular n{p{n-junctions: Quasi-relativistic pseudopotential approach

   H. V. Grushevskaya, G. G. ( January 2018 , NATO science for peace and security series. A, Chemistry and biology)

   We have constructed a continuous model of graphene quantum dot (GQD) as the hydrodynamic limit of the discrete model of n{p{n graphene junction in the form of a rhombic supercell on the graphene plane. The topological type of the proposed GQD-model corresponds bijectively to the GQD-edge topology and can be similar to a sphere or torus. The Hamiltonian of the discrete model of n{p{n graphene junction is chosen to be the Dirac{Weyl type with one Dirac point and 6 pairs of Weyl nodes{antinodes in the folding-zone approximation. The bending-band structure of the proposed GQD-model is ensured by a GQD pseudopotential barrier, which is given by a set of well pseudopotentials for individual carbon atoms of the GQD. The main speci c feature of the structure of electron levels of both spherical and toroidal GQDs is the self-similar energy bands located subsequently one behind another on the energy scale. The atom-like distribution of the electron density is realized from the geometric viewpoint only for toroidal GQDs due to the absence of the curvature for a torus. Though the quasi-zero-energy band exists for spherical and toroidal GQDs, no electron density is present on this band for toroidal GQDs. This causes the formationmore »of a pseudogap between the hole and electron bands, because of the absence of the electron density at the quantum dot center like the case of an ordinary atom. However, the confinement of the electron density is observed for both spherical and toroidal GQDs.« less
4. Long-timescale stability in CMB observations at multiple frequencies using front-end polarization modulation

   https://doi.org/10.1117/12.2629723  

   Cleary, Joseph ; Datta, Rahul ; Appel, John ; Bennet, Charles ; Chuss, David ; Denes Couto, Julliana ; Dahal, Sumit ; Espinoza, Francisco ; Essinger-Hileman, Thomas ; Harrington, Kathleen ; et al ( August 2022 , Proceedings Volume 12190, Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy XI)

   Zmuidzinas, Jonas ; Gao, Jian-Rong (Ed.)

   The Cosmology Large Angular Scale Surveyor (CLASS) is a telescope array observing the Cosmic Microwave Background (CMB) at frequency bands centered near 40, 90, 150, and 220 GHz. CLASS measures the CMB polarization on the largest angular scales to constrain the inflationary tensor-to-scalar ratio and the optical depth due to reionization. To achieve the long time-scale stability necessary for this measurement from the ground, CLASS utilizes a front-end, variable-delay polarization modulator on each telescope. Here we report on the improvements in stability afforded by front-end modulation using data across all four CLASS frequencies. Across one month of modulated linear polarization data in 2021, CLASS achieved median knee frequencies of 9.1, 29.1, 20.4, and 36.4 mHz for the 40, 90, 150, and 220 GHz observing bands. The knee frequencies are approximately an order of magnitude lower than achieved via CLASS pair-differencing orthogonal detector pairs without modulation.
5. Shadow surface states in topological Kondo insulators

   https://doi.org/10.1088/1367-2630/ac4124  

   Ghazaryan, Areg ; Nica, Emilian M. ; Erten, Onur ; Ghaemi, Pouyan ( December 2021 , New Journal of Physics)

   The surface states of 3D topological insulators in general have negligible quantum oscillations (QOs) when the chemical potential is tuned to the Dirac points. In contrast, we find that topological Kondo insulators (TKIs) can support surface states with an arbitrarily large Fermi surface (FS) when the chemical potential is pinned to the Dirac point. We illustrate that these FSs give rise to finite-frequency QOs, which can become comparable to the extremal area of the unhybridized bulk bands. We show that this occurs when the crystal symmetry is lowered from cubic to tetragonal in a minimal two-orbital model. We label such surface modes as ‘shadow surface states’. Moreover, we show that the sufficient next-nearest neighbor out-of-plane hybridization leading to shadow surface states can be self-consistently stabilized for tetragonal TKIs. Consequently, shadow surface states provide an important example of high-frequency QOs beyond the context of cubic TKIs.