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1. Energy gap of topological surface states in proximity to a magnetic insulator

   https://doi.org/10.1038/s42005-023-01327-5  

   Wang, Jiashu ; Wang, Tianyi ; Ozerov, Mykhaylo ; Zhang, Zhan ; Bermejo-Ortiz, Joaquin ; Bac, Seul-Ki ; Trinh, Hoai ; Zhukovskyi, Maksym ; Orlova, Tatyana ; Ambaye, Haile ; et al ( August 2023 , Communications Physics)

   Topological surface-states can acquire an energy gap when time-reversal symmetry is broken by interfacing with a magnetic insulator. This gap has yet to be measured. Such topological-magnetic insulator heterostructures can host a quantized anomalous Hall effect and can allow the control of the magnetic state of the insulator in a spintronic device. In this work, we observe the energy gap of topological surface-states in proximity to a magnetic insulator using magnetooptical Landau level spectroscopy. We measure Pb_1-xSn_xSe–EuSe heterostructures grown by molecular beam epitaxy exhibiting a record mobility and low Fermi energy. Through temperature dependent measurements and theoretical calculations, we show this gap is likely due to quantum confinement and conclude that the magnetic proximity effect is weak in this system. This weakness is disadvantageous for the realization of the quantum anomalous Hall effect, but favorable for spintronic devices which require the preservation of spin-momentum locking at the Fermi level.

    

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2. Decoding chemical information from vibrational spectroscopy data: Local vibrational mode theory

   https://doi.org/10.1002/wcms.1480  

   Kraka, Elfi ; Zou, Wenli ; Tao, Yunwen ( May 2020 , WIREs Computational Molecular Science)

   Modern vibrational spectroscopy is more than just an analytical tool. Information about the electronic structure of a molecule, the strength of its bonds, and its conformational flexibility is encoded in the normal vibrational modes. On the other hand, normal vibrational modes are generally delocalized, which hinders the direct access to this information, attainable only via local vibration modes and associated local properties. Konkoli and Cremer provided an ingenious solution to this problem by deriving local vibrational modes from the fundamental normal modes, obtained in the harmonic approximation of the potential, via mass‐decoupled Euler–Lagrange equations. This review gives a general introduction into the local vibrational mode theory of Konkoli and Cremer, elucidating how this theory unifies earlier attempts to obtain easy to interpret chemical information from vibrational spectroscopy: (a) the local mode theory furnishes bond strength descriptors derived from force constant matrices with a physical basis, (b) provides the highly sought after extension of the Badger rule to polyatomic molecules, (c) and offers a simpler way to derive localized vibrations compared to the complex route via overtone spectroscopy. Successful applications are presented, including a new measure of bond strength, a new detailed analysis of infrared/Raman spectra, and the recent extension to periodic systems, opening a new avenue for the characterization of bonding in crystals. At the end of this review the LMODEA software is introduced, which performs the local mode analysis (with minimal computational costs) after a harmonic vibrational frequency calculation optionally using measured frequencies as additional input.

   This article is categorized under:

   Structure and Mechanism > Molecular Structures

   Theoretical and Physical Chemistry > Spectroscopy

   Software > Quantum Chemistry

   Electronic Structure Theory > Ab Initio Electronic Structure Methods

    

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3. An analytic evaluation of gravitational particle production of fermions via Stokes phenomenon

   Hashiba, Soichiro ; Ling, Siyang ; Long, Andrew J. ( September 2022 , Journal of High Energy Physics)

   A bstract The phenomenon of gravitational particle production can take place for quantum fields in curved spacetime. The abundance and energy spectrum of gravitationally produced particles is typically calculated by solving the field’s mode equations on a time-dependent background metric. For purposes of studying dark matter production in an inflationary cosmology, these mode equations are often solved numerically, which is computationally intensive, especially for the rapidly-oscillating high-momentum modes. However, these same modes are amenable to analytic evaluation via the Exact Wentzel-Kramers-Brillouin (EWKB) method, where gravitational particle production is a manifestation of the Stokes phenomenon. These analytic techniques have been used in the past to study gravitational particle production for spin-0 bosons. We extend the earlier work to study gravitational production of spin-1/2 and spin-3/2 fermions. We derive an analytic expression for the connection matrix (valid to all orders in an adiabatic parameter ħ ) that relates Bogoliubov coefficients across a Stokes line connecting a merged pair of simple turning points. By comparing the analytic approximation with a direct numerical integration of the mode equations, we demonstrate an excellent agreement and highlight the utility of the Stokes phenomenon formalism applied to fermions. We discuss the implications for an analytic understanding of catastrophic particle production due to vanishing sound speed, which can occur for a spin-3/2 Rarita-Schwinger field. 

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4. Evidence of a coupled electron-phonon liquid in NbGe2

   https://doi.org/10.1038/s41467-021-25547-x  

   Yang, Hung-Yu ; Yao, Xiaohan ; Plisson, Vincent ; Mozaffari, Shirin ; Scheifers, Jan P. ; Savvidou, Aikaterini Flessa ; Choi, Eun Sang ; McCandless, Gregory T. ; Padlewski, Mathieu F. ; Putzke, Carsten ; et al ( September 2021 , Nature Communications)

   Whereas electron-phonon scattering relaxes the electron’s momentum in metals, a perpetual exchange of momentum between phonons and electrons may conserve total momentum and lead to a coupled electron-phonon liquid. Such a phase of matter could be a platform for observing electron hydrodynamics. Here we present evidence of an electron-phonon liquid in the transition metal ditetrelide, NbGe₂, from three different experiments. First, quantum oscillations reveal an enhanced quasiparticle mass, which is unexpected in NbGe₂with weak electron-electron correlations, hence pointing at electron-phonon interactions. Second, resistivity measurements exhibit a discrepancy between the experimental data and standard Fermi liquid calculations. Third, Raman scattering shows anomalous temperature dependences of the phonon linewidths that fit an empirical model based on phonon-electron coupling. We discuss structural factors, such as chiral symmetry, short metallic bonds, and a low-symmetry coordination environment as potential design principles for materials with coupled electron-phonon liquid.

    

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5. Visualizing atomic sizes and molecular shapes with the classical turning surface of the Kohn–Sham potential

   https://doi.org/10.1073/pnas.1814300115  

   Ospadov, Egor ; Tao, Jianmin ; Staroverov, Viktor N. ; Perdew, John P. ( November 2018 , Proceedings of the National Academy of Sciences)

   The Kohn–Sham potential$v_{\text{eff}}\left(\mathbf{r}\right)$is the effective multiplicative operator in a noninteracting Schrödinger equation that reproduces the ground-state density of a real (interacting) system. The sizes and shapes of atoms, molecules, and solids can be defined in terms of Kohn–Sham potentials in a nonarbitrary way that accords with chemical intuition and can be implemented efficiently, permitting a natural pictorial representation for chemistry and condensed-matter physics. Let${ϵ}_{\text{max}}$be the maximum occupied orbital energy of the noninteracting electrons. Then the equation$v_{\text{eff}}\left(\mathbf{r}\right)={ϵ}_{\text{max}}$defines the surface at which classical electrons with energy$ϵ\le {ϵ}_{\text{max}}$would be turned back and thus determines the surface of any electronic object. Atomic and ionic radii defined in this manner agree well with empirical estimates, show regular chemical trends, and allow one to identify the type of chemical bonding between two given atoms by comparing the actual internuclear distance to the sum of atomic radii. The molecular surfaces can be fused (for a covalent bond), seamed (ionic bond), necked (hydrogen bond), or divided (van der Waals bond). This contribution extends the pioneering work of Z.-Z. Yang et al. [Yang ZZ, Davidson ER (1997)Int J Quantum Chem62:47–53; Zhao DX, et al. (2018)Mol Phys116:969–977] by our consideration of the Kohn–Sham potential, protomolecules, doubly negative atomic ions, a bond-type parameter, seamed and necked molecular surfaces, and a more extensive table of atomic and ionic radii that are fully consistent with expected periodic trends.

    

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