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1. Dirac lines and loop at the Fermi level in the time-reversal symmetry breaking superconductor LaNiGa2

   https://doi.org/10.1038/s42005-021-00771-5  

   Badger, Jackson R.; Quan, Yundi; Staab, Matthew C.; Sumita, Shuntaro; Rossi, Antonio; Devlin, Kasey P.; Neubauer, Kelly; Shulman, Daniel S.; Fettinger, James C.; Klavins, Peter; et al (December 2022, Communications Physics)

   Abstract Unconventional superconductors have Cooper pairs with lower symmetries than in conventional superconductors. In most unconventional superconductors, the additional symmetry breaking occurs in relation to typical ingredients such as strongly correlated Fermi liquid phases, magnetic fluctuations, or strong spin-orbit coupling in noncentrosymmetric structures. In this article, we show that the time-reversal symmetry breaking in the superconductor LaNiGa 2 is enabled by its previously unknown topological electronic band structure, with Dirac lines and a Dirac loop at the Fermi level. Two symmetry related Dirac points even remain degenerate under spin-orbit coupling. These unique topological features enable an unconventional superconducting gap in which time-reversal symmetry can be broken in the absence of other typical ingredients. Our findings provide a route to identify a new type of unconventional superconductors based on nonsymmorphic symmetries and will enable future discoveries of topological crystalline superconductors. 

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2. Monte Carlo study of the pseudogap and superconductivity emerging from quantum magnetic fluctuations

   https://doi.org/10.1038/s41467-022-30302-x  

   Jiang, Weilun; Liu, Yuzhi; Klein, Avraham; Wang, Yuxuan; Sun, Kai; Chubukov, Andrey V.; Meng, Zi Yang (May 2022, Nature Communications)

   Abstract The origin of the pseudogap behavior, found in many high-T_csuperconductors, remains one of the greatest puzzles in condensed matter physics. One possible mechanism is fermionic incoherence, which near a quantum critical point allows pair formation but suppresses superconductivity. Employing quantum Monte Carlo simulations of a model of itinerant fermions coupled to ferromagnetic spin fluctuations, represented by a quantum rotor, we report numerical evidence of pseudogap behavior, emerging from pairing fluctuations in a quantum-critical non-Fermi liquid. Specifically, we observe enhanced pairing fluctuations and a partial gap opening in the fermionic spectrum. However, the system remains non-superconducting until reaching a much lower temperature. In the pseudogap regime the system displays a “gap-filling rather than “gap-closing behavior, similar to the one observed in cuprate superconductors. Our results present direct evidence of the pseudogap state, driven by superconducting fluctuations. 

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3. Variation of the elliptical Fermi surface for a two-dimensional electron gas with anisotropic mass

   https://doi.org/10.1088/1742-6596/2164/1/012023  

   Ciftja, Orion (March 2022, Journal of Physics: Conference Series)

   Abstract We consider a two-dimensional electron gas in the thermodynamic (bulk) limit. It is assumed that the system consists of fully spin-polarized (spinless) electrons with anisotropic mass. We study the variation of the shape of the expected elliptical Fermi surface as a function of the density of the system in presence of such form of internal anisotropy. To this effect, we calculate the energy of the system as well as the optimum ellipticity of the Fermi surface for two possible liquid states. One corresponds to the standard system with circular Fermi surface while the second one represents a liquid anisotropic phase with a tunable elliptical deformation of the Fermi surface that includes the state that minimizes the kinetic energy. The results obtained shed light on several possible scenarios that may arise in such a system. The competition between opposing tendencies of the kinetic energy and potential energy may lead to the stabilization of liquid phases where the optimal elliptical deformation of the Fermi surface is non-obvious and depends on the density as well as an array of other factors related to the specific values of various parameters that characterize the system. 

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4. Rise and fall of Landau’s quasiparticles while approaching the Mott transition

   https://doi.org/10.1038/s41467-021-21741-z  

   Pustogow, Andrej; Saito, Yohei; Löhle, Anja; Sanz Alonso, Miriam; Kawamoto, Atsushi; Dobrosavljević, Vladimir; Dressel, Martin; Fratini, Simone (December 2021, Nature Communications)

   null (Ed.)

   Abstract Landau suggested that the low-temperature properties of metals can be understood in terms of long-lived quasiparticles with all complex interactions included in Fermi-liquid parameters, such as the effective mass m ⋆ . Despite its wide applicability, electronic transport in bad or strange metals and unconventional superconductors is controversially discussed towards a possible collapse of the quasiparticle concept. Here we explore the electrodynamic response of correlated metals at half filling for varying correlation strength upon approaching a Mott insulator. We reveal persistent Fermi-liquid behavior with pronounced quadratic dependences of the optical scattering rate on temperature and frequency, along with a puzzling elastic contribution to relaxation. The strong increase of the resistivity beyond the Ioffe–Regel–Mott limit is accompanied by a ‘displaced Drude peak’ in the optical conductivity. Our results, supported by a theoretical model for the optical response, demonstrate the emergence of a bad metal from resilient quasiparticles that are subject to dynamical localization and dissolve near the Mott transition. 

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5. Hidden and mirage collective modes in two dimensional Fermi liquids

   https://doi.org/10.1038/s41535-020-0250-4  

   Klein, Avraham; Maslov, Dmitrii L.; Chubukov, Andrey V. (December 2020, npj Quantum Materials)

   null (Ed.)

   Abstract The longstanding view of the zero sound mode in a Fermi liquid is that for repulsive interaction it resides outside the particle-hole continuum and gives rise to a sharp peak in the corresponding susceptibility, while for attractive interaction it is a resonance inside the particle-hole continuum. We argue that in a two-dimensional Fermi liquid there exist two additional types of zero sound: “hidden” and “mirage” modes. A hidden mode resides outside the particle-hole continuum already for attractive interaction. It does not appear as a sharp peak in the susceptibility, but determines the long-time transient response of a Fermi liquid and can be identified in pump-probe experiments. A mirage mode emerges for strong enough repulsion. Unlike the conventional zero sound, it does not correspond to a true pole, yet it gives rise to a peak in the particle-hole susceptibility. It can be detected by measuring the width of the peak, which for a mirage mode is larger than the single-particle scattering rate. 

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