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Register a new userCoexisting Fermi Liquid and Strange Metal Phenomena in Sr$_2$RuO$_4$
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The strange metal is an enigmatic phase whose properties are irreconcilable with the established Fermi liquid theory of conductors. A fundamental question is whether a strange metal and a Fermi liquid are distinct phases of matter, or whether a material can be intermediate between or in a superposition of the two. We studied the collective density response of the correlated metal Sr$_2$RuO$_4$ by momentum-resolved electron energy-loss spectroscopy (M-EELS). We discovered that a broad continuum of non-propagating charge fluctuations (a characteristic of strange metals) and also a dispersing Fermi liquid-like collective mode at low energies and long wavelengths coexist in the same material at the same temperature. These features exhibit a spectral weight redistribution and velocity renormalization when we cool the material through the quasiparticle coherence temperature. Our results show not only that strange metal and Fermi liquid phenomena can coexist but also that Sr$_2$RuO$_4$ serves as an ideal test case for studying the interaction between the two.
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We report a polarization-resolved Raman spectroscopy study of the orbital dependence of the quasiparticles properties in the prototypical multi-band Fermi liquid Srtextsubscript{2}RuOtextsubscript{4}. We show that the quasiparticle scattering rate displays $omega^{2}$ dependence as expected for a Fermi liquid. Besides, we observe a clear polarization-dependence in the energy and temperature dependence of the quasiparticle scattering rate and mass, with the $d_{xz/yz}$ orbital derived quasiparticles showing significantly more robust Fermi liquid properties than the $d_{xy}$ orbital derived ones. The observed orbital dichotomy of the quasiparticles is consistent with the picture of Srtextsubscript{2}RuOtextsubscript{4} as a Hunds metal. Our study establishes Raman scattering as a powerful probe of Fermi liquid properties in correlated metals.
We report optical measurements demonstrating that the low-energy relaxation rate ($1/tau$) of the conduction electrons in Sr$_2$RuO$_4$ obeys scaling relations for its frequency ($omega$) and temperature ($T$) dependence in accordance with Fermi-liquid theory. In the thermal relaxation regime, $1/taupropto (hbaromega)^2 + (ppikB T)^2$ with $p=2$, and $omega/T$ scaling applies. Many-body electronic structure calculations using dynamical mean-field theory confirm the low-energy Fermi-liquid scaling, and provide quantitative understanding of the deviations from Fermi-liquid behavior at higher energy and temperature. The excess optical spectral weight in this regime provides evidence for strongly dispersing resilient quasiparticle excitations above the Fermi energy.
The Hall coefficient $R_H$ of Sr$_2$RuO$_4$ exhibits a non-monotonic temperature dependence with two sign reversals. We show that this puzzling behavior is the signature of two crossovers which are key to the physics of this material. The increase of $R_H$ and the first sign change upon cooling are associated with a crossover into a regime of coherent quasiparticles with strong orbital differentiation of the inelastic scattering rates. The eventual decrease and the second sign change at lower temperature is driven by the crossover from inelastic to impurity-dominated scattering. This qualitative picture is supported by quantitative calculations of $R_H(T)$ using Boltzmann transport theory in combination with dynamical mean-field theory, taking into account the effect of spin-orbit coupling. Our insights shed new light on the temperature dependence of the Hall coefficient in materials with strong orbital differentiation, as observed in Hunds metals.
We present a comprehensive angle-resolved photoemission spectroscopy study of Ca$_{1.8}$Sr$_{0.2}$RuO$_4$. Four distinct bands are revealed and along the Ru-O bond direction their orbital characters are identified through a light polarization analysis and comparison to dynamical mean-field theory calculations. Bands assigned to $d_{xz}, d_{yz}$ orbitals display Fermi liquid behavior with fourfold quasiparticle mass renormalization. Extremely heavy fermions - associated with a predominantly $d_{xy}$ band character - are shown to display non-Fermi-liquid behavior. We thus demonstrate that Ca$_{1.8}$Sr$_{0.2}$RuO$_4$ is a hybrid metal with an orbitally selective Fermi liquid quasiparticle breakdown.
Sr$_2$RuO$_4$, an unconventional superconductor, is known to possess an incommensurate spin density wave instability driven by Fermi surface nesting. Here we report a static spin density wave ordering with a commensurate propagation vector $q_c$ = (0.25 0.25 0) in Fe-doped Sr$_2$RuO$_4$, despite the magnetic fluctuations persisting at the incommensurate wave vectors $q_{ic}$ = (0.3 0.3 L) as in the parent compound. The latter feature is corroborated by the first principles calculations, which show that Fe substitution barely changes the nesting vector of the Fermi surface. These results suggest that in addition to the known incommensurate magnetic instability, Sr$_2$RuO$_4$ is also in proximity to a commensurate magnetic tendency that can be stabilized via Fe doping.
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