In this paper we report the effect of uniaxial strain $\varepsilon$ applied along the crystalline $a$ axis on the newly discovered kagome superconductor ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$. At ambient conditions, ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ shows a charge-density wave (CDW) transition at $T_{\mathrm{CDW}}=94.5\mathrm{K}$ and superconducts below $T_c=3.34\mathrm{K}$. In our paper, when the uniaxial strain $\varepsilon$ is varied from $-0.90\%$ to $0.90\%,T_c$ monotonically increases by $\sim 33\%$ from 3.0 to 4.0 K, giving rise to the empirical relation $T_c\left(\varepsilon \right)=3.4+0.56\varepsilon +0.12{\varepsilon }^2$. On the other hand, for $\varepsilon$ changing from $-0.76\%$ to $1.26\%,T_{\mathrm{CDW}}$ decreases monotonically by $\sim 10\%$ from 97.5 to 87.5 K with $T_{\mathrm{CDW}}\left(\varepsilon \right)=94.5-4.72\varepsilon -0.60{\varepsilon }^2$. The opposite response of $T_c$ and $T_{\mathrm{CDW}}$ to the uniaxial strain suggests strong competition between these two orders. Comparison with hydrostatic pressure measurements indicate that it is the change in the $c$ axis that is responsible for these behaviors of the CDW and superconducting transitions, and that the explicit breaking of the sixfold rotational symmetry by strain has a negligible effect. Combined with our first-principles calculations and phenomenological analysis, we conclude that the enhancement in $T_c$ with decreasing $c$ is caused primarily by the suppression of $T_{\mathrm{CDW}}$, rather than strain-induced modifications in the bare superconducting parameters. We propose that the sensitivity of $T_{\mathrm{CDW}}$ with respect to the changes in the $c$ axis arises from the impact of the latter on the trilinear coupling between the $M_1^{+}$ and the $L_2^{-}$ phonon modes associated with the CDW. Overall, our paper reveals that the $c$-axis lattice parameter, which can be controlled by both pressure and uniaxial strain, is a powerful tuning knob for the phase diagram of ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$.

• Received 12 July 2021
• Accepted 4 October 2021

DOI:https://doi.org/10.1103/PhysRevB.104.144506