Superconductivity in the ${\mathbb{Z}}_2$ kagome metal ${\mathrm{KV}}_3{\mathrm{Sb}}_5$

Here we report the observation of bulk superconductivity in single crystals of the two-dimensional kagome metal ${\mathrm{KV}}_3{\mathrm{Sb}}_5$. Magnetic susceptibility, resistivity, and heat capacity measurements reveal superconductivity below $T_c=0.93\mathrm{K}$, and density functional theory (DFT) calculations further characterize the normal state as a ${\mathbb{Z}}_2$ topological metal. Our results demonstrate that the recent observation of superconductivity within the related kagome metal ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ is likely a common feature across the $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A$: K, Rb, Cs) family of compounds and establishes them as a rich arena for studying the interplay between bulk superconductivity, topological surface states, and likely electronic density wave order in an exfoliable kagome lattice.

Figure 1

Crystal structure of ${\mathrm{KV}}_3{\mathrm{Sb}}_5$, the prototype structure for the $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ family of kagome metals. (a) The kagome lattice of vanadium is immediately apparent from the top-down view of the structure. Bond lengths $<3\AA$ are drawn in the isometric perspective to highlight the layered nature of ${\mathrm{KV}}_3{\mathrm{Sb}}_5$. (b) Intuitive decomposition of the structure highlights the kagome lattice and graphitelike (antimonene) layers.

Figure 2

(a)–(c) Susceptibility, electrical resistivity, and heat capacity showing behavior of stoichiometric single crystals above 2 K. All measurements show an anomaly at 78 K, coinciding with emergence of a charge density wave. Magnetization results indicate that ${\mathrm{KV}}_3{\mathrm{Sb}}_5$ is a Pauli paramagnet at high temperatures. As shown previously, the weak Curie tail at low temperature can be fit with a small concentration of impurity spins. Resistivity is low, indicating a high mobility metal. The $\lambda$-like anomaly in the heat capacity is shown, magnified, with a spline interpolation used to isolate the transition. (d)–(f) Susceptibility, electrical resistivity, and heat capacity measurements below 2 K highlight the onset of bulk superconductivity in ${\mathrm{KV}}_3{\mathrm{Sb}}_5$. A well-defined Meissner state is observed in susceptibility, which coincides with the zero-resistivity state and a sharp heat-capacity anomaly.

Figure 3

(a) The specific heat capacity difference $\mathrm{\Delta }{C}_{es}$ between the superconducting and normal states ($\mathrm{\Delta }{C}_{es}=C_{p,0T}-C_{p,3T}$) shows a sharp transition at $T_c=0.93\mathrm{K}$. Here, ${\gamma }_n=22.8\mathrm{mJ}{\mathrm{mol}}^{-1}{\mathrm{K}}^{-1}$ and ${\theta }_D=141\mathrm{K}$ have been modeled from the 3 T data set. (b) The electronic heat capacity $C_{es}=C_{p,0T}-A{T}^3$ below the transition (and above the Schottky anomaly) vanishes exponentially with temperature. Red lines show the exponential fits to the data as described in the text.

Figure 4

DFT electronic band structure for ${\mathrm{KV}}_3{\mathrm{Sb}}_5$ near the Fermi level. The continuous gap (shaded) is noted, with band indices and parity products given. Note that surface states at the $\overline{M}$ are expected to be topologically protected, in analogy to ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$.