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1. 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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2. Electronic anisotropy and rotational symmetry breaking at a Weyl semimetal/spin ice interface

   https://doi.org/10.1126/sciadv.adr6202  

   Wu, Tsung-Chi; Chang, Yueqing; Wu, Ang-Kun; Terilli, Michael; Wen, Fangdi; Kareev, Mikhail; Choi, Eun Sang; Graf, David; Zhang, Qinghua; Gu, Lin; et al (June 2025, Science Advances)

   In magnetic pyrochlore materials, the interplay of spin-orbit coupling, electronic correlations, and geometrical frustration gives rise to exotic quantum phases, including topological semimetals and spin ice. While these phases have been observed in isolation, the interface-driven phenomena emerging from their interaction have never been realized previously. Here, we report on the discovery of interfacial electronic anisotropy and rotational symmetry breaking at a heterostructure consisting of the Weyl semimetal Eu₂Ir₂O₇and spin ice Dy₂Ti₂O₇. Subjected to magnetic fields, we unveil a sixfold anisotropic transport response that is theoretically accounted by a Kondo-coupled heterointerface, where the spin ice’s field-tuned magnetism induces electron scattering in the Weyl semimetal’s topological Fermi-arc states. Furthermore, at elevated magnetic fields, we reveal a twofold anisotropic response indicative of the emergence of a symmetry-broken many-body state. This discovery showcases the potential of pyrochlore frustrated magnet/topological semimetal heterostructures in search of emergent interfacial phenomena. 

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3. The reverse quantum limit and its implications for unconventional quantum oscillations in YbB12

   https://doi.org/10.1038/s41467-024-45801-2  

   Mizzi, Christopher_A; Kushwaha, Satya_K; Rosa, Priscila_F_S; Phelan, W_Adam; Arellano, David_C; Pressley, Lucas_A; McQueen, Tyrel_M; Chan, Mun_K; Harrison, Neil (February 2024, Nature Communications)

   Abstract The quantum limit in a Fermi liquid, realized when a single Landau level is occupied in strong magnetic fields, gives rise to unconventional states, including the fractional quantum Hall effect and excitonic insulators. Stronger interactions in metals with nearly localizedf-electron degrees of freedom increase the likelihood of these unconventional states. However, access to the quantum limit is typically impeded by the tendency off-electrons to polarize in a strong magnetic field, consequently weakening the interactions. In this study, we propose that the quantum limit in such systems must be approached in reverse, starting from an insulating state at zero magnetic field. In this scenario, Landau levels fill in the reverse order compared to regular metals and are closely linked to a field-induced insulator-to-metal transition. We identify YbB₁₂as a prime candidate for observing this effect and propose the presence of an excitonic insulator state near this transition. 

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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)

   Abstract 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. Electronic Transport and Interaction of Lattice Dynamics in Topological Nodalline Semimetal HfAs ₂ Single Crystals

   https://doi.org/10.1002/adfm.202316775  

   Muhammad, Zahir; Hussain, Ghulam; Islam, Rajbul; Zawadzka, Natalia; Hossain, Md_Shafayat; Iqbal, Obaid; Babiński, Adam; Molas, Maciej_R; Xue, Fei; Zhang, Yue; et al (June 2024, Advanced Functional Materials)

   Abstract Topological semimetals represent a novel class of quantum materials displaying non‐trivial topological states that host Dirac/Weyl fermions. The intersection of Dirac/Weyl points gives rise to essential properties in a wide range of innovative transport phenomena, including extreme magnetoresistance, high mobilities, weak antilocalization, electron hydrodynamics, and various electro‐optical phenomena. In this study, the electronic, transport, phonon scattering, and interrelationships are explored in single crystals of the topological semimetal HfAs₂. It reveals a weak antilocalization effect at low temperatures with high carrier density, which is attributed to perfectly compensated topological bulk and surface states. The angle‐resolved photoemission spectroscopy (ARPES) results show anisotropic Fermi surfaces and surface states indicative of the topological semimetal, further confirmed by first‐principle density functional theory (DFT) calculations. Moreover, the lattice dynamics in HfAs₂are investigated both with the Raman scattering and density functional theory. The phonon dispersion, density of states, lattice thermal conductivity, and the phonon lifetimes are computed to support the experimental findings. The softening of phonons, the broadening of Raman modes, and the reduction of phonon lifetimes with temperature suggest the enhancement of phonon anharmonicity in this new topological material, which is crucial for boosting the thermoelectric performance of topological semimetals. 

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