Mott-Derived Local Moments and Kondo Hybridization in a d-electron Kagome lattice

Abstract

Unlike canonical Kondo lattices in f-electron systems, where localized f orbitalsnaturally provide local moments, d-electron Kondo lattices require a distinct mechanism for local-moment formation. However, the study of d-electron Kondo lattices in bulk materials remains far from settled, particularly with regard to the microscopic origin of the local moments. Here, we report a microscopic mechanism for this process in the bilayer kagome metal CsCr6Sb6, where strong correlations drive a Mott splitting of the kagome flat band to supply the requisite local moments. By combining STM/STS and ARPES, we resolve a spectroscopic hierarchy between high-energy correlation effects and low temperature hybridization. Low-temperature STS reveals a robust asymmetric suppression of the density of states near EF that is well captured phenomenologically by a Fano-type lineshape, while ARPES detects a sharp quasiparticlepeak near EF. These low-energy signatures evolveon the same temperature scale and disappear upon warming, consistent with the onset of Kondo hybridization. At the same time, STS resolves symmetric humps at approximately +-50 mV and ARPES identifies a weakly dispersive feature around 50 meV below EF; unlike the near-EF hybridization signatures, these features persist to substantially higher temperatures. This separation of energy and temperature scales supports a two-stage picture in which a kagome flat band first undergoes correlation-driven splitting into lower and upper Hubbard bands, and the occupied lower Hubbard band supplies the local moments that later hybridize with itinerant electrons at lower temperature. Our results therefore move beyond the phenomenology of a kagome Kondo lattice candidate and instead provide a microscopic spectroscopic picture linking Mottness to Kondo hybridization in a frustrated d-electron system.

Figures (13)

• Figure 1: Crystal structure and transport properties of CsCr$_6$Sb$_6$. (a) Side view of the crystal structure. The green and gray planes represent the Cs-terminated and Sb2-terminated surfaces, respectively. The double kagome (DK) planes are located between the Sb2 layers. (b) Atomic configuration of the Cs-terminated surface. (c) Atomic model of the Sb2-terminated surface. (d) Temperature dependence of resistance for bulk CsCr$_6$Sb$_6$. Inset: first derivative d$R$/d$T$ showing a kink at $T_{\mathrm{F}} \sim 75$ K, indicative of frustrated magnetism. The characteristic temperature $T_{\mathrm{K\_onset}}$ is extracted from the experimental results of STM and ARPES.
• Figure 2: STM/STS probes Kondo and Mott physics. (a) Atomically resolved STM image of the Sb2-terminated surface (200 mV, 60 pA). The inset shows a magnified view of the black-boxed region, with gray circles indicating the positions of Sb2 atoms. (b) Atomically resolved STM topographic image of the Cs-terminated surface (200 mV, 10 pA) and the corresponding Fourier transform (FT), revealing $\sqrt{3} \times \sqrt{3}$ and $\sqrt{3} \times 1$ reconstructions, highlighted by green and blue ellipses, respectively. (c) Spatially averaged d$I$/d$V$ spectrum on the Sb2-terminated surface at 5 K. The purple shading in panels c-f highlights the $\pm 50$ mV humps. Dashed lines indicate the energy positions of peaks. (d) Spatially averaged d$I$/d$V$ spectrum acquired on the Cs surface at 5 K (black curve). Spectra measured on two distinct Cs reconstructions are shown in green and blue. Dashed lines mark the energy positions of peaks around the Fermi level. (e) Temperature-dependent spatially averaged d$I$/d$V$ spectra on the Sb2-terminated surface. Dotted curves are obtained by thermally broadening the 5 K spectrum using the derivative of the Fermi–Dirac distribution. The curves are offset for clarity. (f) Temperature-dependent spatially averaged d$I$/d$V$ spectra on the Cs-terminated surface. (g)-(h) Spatially averaged d$I$/d$V$ spectrum at 5 K (black circles) fitted with a Fano lineshape (orange) superimposed on a Gaussian peak (blue) for the Sb2-terminated surface and Cs-terminated surface, respectively.
• Figure 3: Kondo hybridization observed by ARPES in a kagome lattice. (a) Fermi surface measured at 16 K using laser source (h$\nu$ = 6.994 eV with −69 V Bias, see Methods), obtained by integrating spectral intensity within $\pm 10$ meV energy window with respect to the $E_{\mathrm{F}}$ and symmetrized assuming three-fold rotational symmetry. Blue curves schematically depict the shape of Fermi surface. (b)-(d) Band structures along the $\Gamma$–K, $\Gamma$–M, and M–K directions at 16 K, respectively. Four distinct bands can be distinguished, labeled $\alpha, \beta, \varepsilon, \gamma$ (highlighted by black dotted lines and triangular markers). The purple rectangular shaded areas around 50 meV below the $E_{\mathrm{F}}$ mark the location of the flat band ($\beta$ band). (e) Temperature-dependent band structures measured at the M point along the K–M–K direction without bias. The purple shading in panel e-f marks the position of the flat band near 50 meV below $E_{\mathrm{F}}$. (f) Low-temperature band structure measured at 16 K along the $\Gamma$-K direction with -49 V bias. (g)-(i) Temperature dependent energy distribution curves (EDCs) taken at the momentum positions indicated by the green arrow in panel e and the black and red arrows in panel f, corresponding to the Fermi momenta at M, near K and $\Gamma$ point, respectively. the EDCs were fitted after subtracting a Shirley background using two Lorentzian components multiplied by the Fermi–Dirac distribution at the corresponding temperatures. The two components represent the hump structure located at approximately $-50$ meV and the quasiparticle peak near the Fermi level, respectively. The fitted components at 16 K are shown as gray (Shirley background), purple ($-50$ meV hump), and red (quasiparticle peak) shaded areas. (j) Temperature dependence of the normalized Lorentzian spectral weight of the quasiparticle peak extracted from the EDC fittings in panels g-i. All EDCs are normalized at 150 meV below $E_{\mathrm{F}}$ and vertically offset for clarity. Solid lines serve as guides to the eye.
• Figure 4: Schematic illustration of correlation-driven Kondo hybridization. (a) Evolution of the density of states (DOS). In the absence of strong correlations, the half-filled kagome flat band lies at the $E_{\mathrm{F}}$. (b) Upon introducing strong correlations, the kagome flat band undergoes a Mott splitting into lower Hubbard band (LHB) and upper Hubbard band (UHB), opening a Mott gap. (c) At temperature below $T_{\mathrm{K\_onset}}$, Kondo hybridization sets in, manifested by the emergence of a Kondo resonance near the $E_{\mathrm{F}}$. (d)-(e) Schematic band structures along high-symmetry directions based on ARPES results, illustrating momentum-dependent Kondo hybridization below $T_{\mathrm{K\_onset}}$. The conduction bands ($\varepsilon, \gamma$) hybridize with renormalized flat band (green line), forming upper and lower hybridized bands (gradient-colored lines) described within the periodic Anderson model. The hybridization strength is $V_k = 6$ meV and the renormalized flat-band level is $\epsilon_0 = 3$ meV. The $\alpha$ band remains non-hybridized.
• Figure S1: Reproducibility of STM topographies and spatially averaged d$I$/d$V$ spectra on the Sb2 surface. (a) STM topographies acquired from three different samples, with occasional bright Cs clusters. (b) Corresponding spatially averaged d$I$/d$V$ spectra measured under different experimental conditions on the clean Sb2 surface.