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DDC: 530 × Subject: Energy ×

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    Evolution of lattice, spin, and charge properties across the phase diagram of FeSe1-x Sx
    (Woodbury, NY : Inst., 2022) Lazarević, N.; Baum, A.; Milosavljević, A.; Peis, L.; Stumberger, R.; Bekaert, J.; Šolajić, A.; Pešić, J.; Wang, Aifeng; Šćepanović, M.; Abeykoon, A. M. Milinda; Milošević, M.V.; Petrovic, C.; Popović, Z.V.; Hackl, R.
    A Raman scattering study covering the entire substitution range of the FeSe1-xSx solid solution is presented. Data were taken as a function of sulfur concentration x for 0≤x≤1, of temperature and of scattering symmetry. All types of excitations including phonons, spins, and charges are analyzed in detail. It is observed that the energy and width of the iron-related B1g phonon mode vary continuously across the entire range of sulfur substitution. The A1g chalcogenide mode disappears above x=0.23 and reappears at a much higher energy for x=0.69. In a similar way the spectral features appearing at finite doping in A1g symmetry vary discontinuously. The magnetic excitation centered at approximately 500 cm-1 disappears above x=0.23 where the A1g lattice excitations exhibit a discontinuous change in energy. The low-energy mode associated with fluctuations displays maximal intensity at the nematostructural transition and thus tracks the phase boundary.
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    Fate of density waves in the presence of a higher-order van Hove singularity
    (College Park, MD : APS, 2023) Zervou, Alkistis; Efremov, Dmitry V.; Betouras, Joseph J.
    Topological transitions in electronic band structures, resulting in van Hove singularities in the density of states, can considerably affect various types of orderings in quantum materials. Regular topological transitions (of neck formation or collapse) lead to a logarithmic divergence of the electronic density of states (DOS) as a function of energy in two dimensions. In addition to the regular van Hove singularities, there are higher-order van Hove singularities (HOVHS) with a power-law divergence in DOS. By employing renormalization group techniques, we study the fate of a spin-density wave phase formed by nested parts of the Fermi surface, when a HOVHS appears in parallel. We find that the phase formation can be boosted by the presence of the singularity, with the critical temperature increasing by orders of magnitude, under certain conditions. We discuss possible applications of our findings to a range of quantum materials such as Sr3Ru2O7, Sr2RuO4, and transition metal dichalcogenides.