Exotic Surface Stripe Orders in Correlated Kagome Metal CsCr3Sb5

TL;DR

This study investigates CsCr3Sb5, a correlated kagome metal, to understand how surface stripe orders relate to bulk electronic states and potential superconductivity. By combining STM, ARPES (including nano- and laser-ARPES), and DFT, the authors discover distinct surface stripe orders on two terminations: a mixed $2a_0\times a_0$/$3a_0\times a_0$ pattern on Cs-terminated surfaces and a $4a_0\times\sqrt{3}a_0$ pattern on Sb-terminated surfaces, with the kagome flat band residing about $\sim$330 meV above the Fermi level. The data show that the strong electron correlations originate from Coulomb interactions and Hund's coupling rather than from the flat bands near $E_F$, and reveal a ~100 meV electron-boson coupling mode; the Sb $p$-band remains largely uncoupled to the bulk density-wave order across temperature. The results highlight a pronounced surface–bulk interplay in CsCr3Sb5, challenging the notion that kagome flat bands drive its correlated physics and suggesting new avenues to explore surface-induced states and their relation to superconductivity in this material.

Abstract

The newly discovered kagome superconductor CsCr3Sb5 exhibits distinct features with flat bands and unique magnetism, providing a compelling platform for exploring novel quantum states of correlated electron systems. Emergent charge order in this material is a key for understanding unconventional superconductivity, but it remains unexplored at the atomic scale and the underlying physics is elusive. Here, we identify and unreported stripe orders on the surface which are distinct from the bulk and investigate the underlying bulk electronic properties using a combination of scanning tunneling microscopy (STM), angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations. Specifically, a mixture of 2a0 * a0 and 3a0 * a0 stripe order is found on Cs-terminated surface while 4a0 * root3a0 stripe order is found on the Sb-terminated surface. The electronic spectra exhibit strongly correlated features resembling that of high temperature superconductors, with kagome flat bands lying about 330 meV above EF, suggesting that the electron correlations arise from Coulomb interactions and Hund's coupling. Moreover, a distinct electron-boson coupling mode is observed at approximately 100 meV. These findings provide new insights into the interplay between surface and bulk charge orders in this strongly correlated kagome system.

Figures (5)

• Figure 1: Crystal structure, electronic band structure and temperature-driven phase transition of CsCr$_3$Sb$_5$ single crystals.a, Hexagonal crystal structure of CsCr$_3$Sb$_5$ from both the axonometric and top view. b, Electronic band structure and the corresponding electronic density of states (DOS) of CsCr$_3$Sb$_5$ from the first-principles calculations. The van Hove singularities (vHS) are indicated by the black arrows and the flat bands are highlighted by the grey box. c, Temperature-dependent in-plane resistivity of CsCr$_3$Sb$_5$ single crystal. The bad metallic behavior at high temperatures is interrupted by a pronounced peak at $T_{\rho}$ = 54.6 K, which relates to an antiferromagnetic transition and structural modulation. d, Specific heat ($C$) of CsCr$_3$Sb$_5$, showing a sharp peak at 54.2 K. Inset is a plot and corresponding linear fit of $C/T$ versus $T^2$. e and f, STM topographies taken on the Cs- and Sb-terminated surfaces, respectively. Dark dots on the Cs termination indicate Cs vacancies while bright dots on the Sb termination correspond to Cs adatoms. g, Typical dI/dV spectrum measured on the Cs-terminated surfaces. The peaks at –140 and 330 meV can be attributed to the vHS and flat band, respectively, in alignment with the theoretical calculations. STM setup condition: $V_{s}$ = 2.5 V, $I_{t}$ = 100 pA (e); $V_{s}$ = 1 V, $I_{t}$ = 500 pA (f).
• Figure 2: Distinct surface stripe orders of CsCr$_3$Sb$_5$.a, High-resolution STM topography taken on the Cs-terminated surface with a mixture of 2$a_0$×$a_0$ and 3$a_0$×$a_0$ stripe superstructures. Every three 2a$_0$ stripes are separated by a 3a$_0$ stripe. b, Fast Fourier transform (FFT) of a. Superstructure modulation peak shows at 4/9 $Q_{Bragg}$. c, High-resolution STM topography taken on the Sb-terminated surface with a 4$a_0$×$\sqrt{3}$$a_0$ stripe superstructure. The illustration shows the honeycomb structure with a lattice constant of 5.39 Å. d, FFT pattern of c. The $q_{2a_0}$ and $q_{4a_0}$ are rotated by 30° relative to $Q_{Bragg}$. e, The STM topography of the Sb-terminated surface with a 4$a_0$ stripe superstructure. f, The dI/dV spectra were acquired from the bright and dark regions at the marked cross in (e). g, A series of dI/dV spectra along the green arrow in (e) encompasses two 4$a_0$ periods. h, The spatial evolution of the dI/dV intensity, extracted from the panel (g) at ±80 mV, shows a clear 4$a_0$ periodicity, with the peaks at −80 mV and the troughs at 80 mV aligning in phase. i and j, dI/dV mapping at 30 mV (i) and −30 mV (j), a defect is marked with a red dotted circle. k, Line profiles along the same region, marked by arrows in (i) and (j), showing the peak-to-dip intensity contrast with a 4$a_0$ period. 4$a_0$$\times$$\sqrt{3}a_0$ stripe superstructure is marked as a black rectangle in (e), (i) and (j). STM setup condition: (a) $V_s$ = 130 mV, $I_t$ = 1.0 nA; (c) $V_s$ = −300 mV, $I_t$ = 100 pA; (e) $V_s$ = −30 mV, $I_t$ = 1.0 nA; (i) $V_s$ = 30 mV, $I_t$ = 1.0 nA; (j) $V_s$ = −30 mV, $I_t$ = 1.0 nA.
• Figure 3: Momentum-resolved electronic structure and the energy position of kagome flat bands of CsCr$_3$Sb$_5$.a, Photoemission spectra of CsCr$_3$Sb$_5$ measured with 92.8 eV photons, showing the Sb 4$d$ core level, which indicates different surface terminations. b, Constant energy contour mapping at -0.4 eV on the Cs-terminated surface obtained using circularly polarized light. c, Spectral images along different momentum paths indicated by the yellow lines in b. The overlaid dashed lines represent the corresponding calculated bands and the blue dashed lines highlight the kagome flat bands. d and e, Same as b,c but for the Sb-terminated surface. f, Fermi surface mapping measured with a 7 eV laser light source. A sample bias of –50 eV was applied to achieve a large momentum detection range. The light polarization was linear horizontal. g, $E-k$ spectral image and the corresponding energy distribution curve (EDC) stacks along the horizontal cut (cut4) across $\Gamma$, as indicated in f. The red dashed boxes highlight the clean Fermi cutoff without any flat-band spectral feature. h, Same as g but along the vertical cut (cut5) indicated in f. i, ARPES spectral image overlaid with DFT vHS. j, dI/dV spectra obtained by combining data from the Cs- and Sb-terminated surfaces. Flat bands and vHS are labeled with the blue and yellow arrows, respectively.
• Figure 4: Electron correlation and distinct electron-boson coupling in CsCr$_3$Sb$_5$.a, $E-k$ spectral image of CsCr$_3$Sb$_5$ along the cut indicated in Fig. \ref{['fig:Fig2']}f (cut4). b, Same as a but for CsV$_3$Sb$_5$. c and d, Energy distribution curve stacks corresponding to the spectral images in a and b, respectively. The blue dashed lines indicate the quasiparticle peaks. The solid blue line denotes incoherent spectra at high binding energies. The red shaded line indicates the waterfall-like spectral feature. e, Fitted band dispersions of CsCr$_3$Sb$_5$ (black circles) and CsV$_3$Sb$_5$ (red circles) from a and b. Kink positions are denoted by the arrows. f, Real part of the self-energy of CsCr$_3$Sb$_5$ extracted from e, assuming a linear bare-band dispersion.
• Figure 5: Temperature evolution of the electronic band originating from the Sb $p$ orbital.a, ARPES spectral images of the band along the cut indicated in Fig. \ref{['fig:Fig2']}f (cut4) measured at different temperatures. Warm-up and cool-down cycles were performed to rule out extrinsic effects introduced by sample degradation. b, Corresponding momentum distribution curves at the Fermi level of the $E-k$ spectra shown in a for the cool-down process. c, Fitted peak width of b, showing no significant change across the transition temperature for either the warm-up or cool-down processes.