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Register a new userElectronic instabilities of kagome metals: saddle points and Landau theory
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We study electronic instabilities of a kagome metal with a Fermi energy close to saddle points at the hexagonal Brillouin zone face centers. Using parquet renormalization group, we determine the leading and subleading instabilities, finding superconducting, charge, orbital moment, and spin density waves. We then derive and use Landau theory to discuss how different primary density wave orders give rise to charge density wave modulations, as seen in the AV$_3$Sb$_5$ family, with A=K,Rb,Cs. The results provide strong constraints on the mechanism of charge ordering and how it can be further refined from existing and future experiments.
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By means of the density functional theory in combination with the dynamical mean-field theory, we tried to examine the electronic structure of hexagonal FeGe, in which the Fe atoms form a quasi-2D layered Kagome lattice. We predict that it is a representative Kagome metal characterized by orbital selective Dirac fermions and extremely flat bands. Furthermore, Fes 3$d$ electrons are strongly correlated. They exhibit quite apparent signatures of electronic correlation induced by Hunds rule coupling, such as sizable differentiation in band renormalization, non-Fermi-liquid behavior, spin-freezing state, and spin-orbital separation. Thus, FeGe can be regarded as an ideal platform to study the interplay of Kagome physics and Hundness. 5
The synthesis and characterization of vanadium-based kagome metals YV$_6$Sn$_6$ and GdV$_6$Sn$_6$ are presented. X-ray diffraction, magnetization, magnetotransport, and heat capacity measurements reveal an ideal kagome network of V-ions coordinated by Sn and separated by triangular lattice planes of rare-earth ions. The onset of low-temperature, likely noncollinear, magnetic order of Gd spins is detected in GdV$_6$Sn$_6$, while V-ions in both compounds remain nonmagnetic. Density functional theory calculations are presented modeling the band structures of both compounds, which can be classified as $mathbb{Z}_2$ topological metals in the paramagnetic state. Both compounds exhibit high mobility, multiband transport and present an interesting platform for controlling the interplay between magnetic order associated with the $R$-site sublattice and nontrivial band topology associated with the V-based kagome network. Our results invite future exploration of other $R$V$_6$Sn$_6$ ($R$=rare earth) variants where this interplay can be tuned via $R$-site substitution.
Understanding the link between a charge density wave (CDW) instability and superconductivity is a central theme of the 2D metallic kagome compounds $A$V$_3$Sb$_5$ ($A$=K, Rb, and Cs). Using polarization-resolved electronic Raman spectroscopy, we shed light on Fermi surface fluctuations and electronic instabilities. We observe a quasielastic peak (QEP) whose spectral weight is progressively enhanced towards the superconducting transition. The QEP temperature-dependence reveals a steep increase in coherent in-plane charge correlations within the charge-density phase. In contrast, out-of-plane charge fluctuations remain strongly incoherent across the investigated temperature range. In-plane phonon anomalies appear at $T^*sim 50$~K in addition to right below $T_{mathrm{CDW}}sim 95$~K, while showing no apparent evidence of reduced symmetry at low temperatures. In conjunction with the consecutive phonon anomalies within the CDW state, our electronic Raman data unveil additional electronic instabilities that persist down to the superconducting phase, thereby offering a superconducting mechanism.
A current challenge in condensed matter physics is the realization of strongly correlated, viscous electron fluids. These fluids are not amenable to the perturbative methods of Fermi liquid theory, but can be described by holography, that is, by mapping them onto a weakly curved gravitational theory via gauge/gravity duality. The canonical system considered for realizations has been graphene, which possesses Dirac dispersions at low energies as well as significant Coulomb interactions between the electrons. In this work, we show that Kagome systems with electron fillings adjusted to the Dirac nodes of their band structure provide a much more compelling platform for realizations of viscous electron fluids, including non-linear effects such as turbulence. In particular, we find that in stoichiometric Scandium (Sc) Herbertsmithite, the fine-structure constant, which measures the effective Coulomb interaction and hence reflects the strength of the correlations, is enhanced by a factor of about 3.2 as compared to graphene, due to orbital hybridization. We employ holography to estimate the ratio of the shear viscosity over the entropy density in Sc-Herbertsmithite, and find it about three times smaller than in graphene. These findings put, for the first time, the turbulent flow regime described by holography within the reach of experiments.
Following the discovery of a new family of kagome prototypical materials with structure AV$_3$Sb$_5$ (A = K, Rb, Cs), there has been heightened interest in studying correlation-driven electronic phenomena in these kagome lattice systems. The study of these materials has gone beyond magneto-transport measurements to reveal exciting features such as Dirac bands, anomalous Hall effect, bulk superconductivity with $T_c$ $sim$ 0.9 K-2.5 K, and the observation of charge density wave instabilities which suggests an intertwining of topological physics and new quantum orders. Moreover, very recent works on numerous types of experiments have appeared further examining the unconventional superconductivity and the exotic electronic states found within these kagome materials. Theories on the strong interactions that play a role in these systems have been proposed to shed light on the nature of these topological charge density waves. In this brief review, we summarize these recent experimental findings and theoretical proposals to connect them with the concepts of topological physics and strongly-correlated electron systems.
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