$\mathrm{Cs}{\mathrm{V}}_3{\mathrm{Sb}}_5$: A ${\mathbb{Z}}_2$ Topological Kagome Metal with a Superconducting Ground State

Recently discovered alongside its sister compounds ${\mathrm{KV}}_3{\mathrm{Sb}}_5$ and ${\mathrm{RbV}}_3{\mathrm{Sb}}_5$, ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ crystallizes with an ideal kagome network of vanadium and antimonene layers separated by alkali metal ions. This work presents the electronic properties of ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$, demonstrating bulk superconductivity in single crystals with a $T_c=2.5\mathrm{K}$. The normal state electronic structure is studied via angle-resolved photoemission spectroscopy and density-functional theory, which categorize ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ as a ${\mathbb{Z}}_2$ topological metal. Multiple protected Dirac crossings are predicted in close proximity to the Fermi level ($E_F$), and signatures of normal state correlation effects are also suggested by a high-temperature charge density wavelike instability. The implications for the formation of unconventional superconductivity in this material are discussed.

Figure 1

$\mathrm{Cs}{\mathrm{V}}_3{\mathrm{Sb}}_5$ is a layered compound with a structurally perfect kagome network of vanadium. There are two distinct Sb sites in the structure: (1) a simple hexagonal net woven into the kagome layer and (2) graphitelike layers of antimony (antimonene) above and below the kagome layer. All bonds $\le 3.2\AA$ have been drawn in the isometric perspective.

Figure 2

(a),(c),(e) Full temperature ranges for the magnetization, electrical resistivity, and heat capacity, respectively, shown for single crystals of ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$. All measurements indicate the presence of an anomaly $T^{*}$ at 94 K, suspected to be an electronic instability (e.g., charge ordering). The inset in (a) shows line cuts through x-ray diffraction data below and above $T^{*}$. Dashed lines denote the appearance of half-integer reflections. (b),(d),(f) Field-dependent measurements at low temperatures, showing the onset of superconductivity in magnetization, resistivity, and heat capacity, respectively. The $T_c$ for ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ is approximately 2.5 K, with a slight suppression in resistivity due to high probe currents.

Figure 3

Experimental ARPES data and comparison with DFT calculations. (a) A selection of constant energy maps at 80 K are compared with DFT calculations, showing excellent agreement. The hexagonal Brillouin zone is superimposed on the $E=0\mathrm{eV}$ data. (b) ARPES and DFT data tracing from $M\text{−}K\text{−}\mathrm{\Gamma }\text{−}K\text{−}M$ reveal multiple Dirac points throughout the dispersion. Surface states can be observed in the DFT data at the $M$ point, slightly above $E_F$.

Figure 4

(a) Calculated band structure of ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ along high-symmetry directions across the Brillouin zone. A continuous direct gap (shaded) is noted and high-symmetry points in the BZ are labeled. (b) Tight-binding model of ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ showing topologically protected surface states that manifest at the time-reversal invariant momentum $\overline{M}$ point.