Revealing the competition between charge density wave and superconductivity in CsV3Sb5 through uniaxial strain

In this paper we report the effect of uniaxial strain applied along the crystalline a axis on the newly discovered kagome superconductor CsV3Sb5. At ambient conditions, CsV3Sb5 shows a charge-density wave (CDW) transition at TCDW=94.5K and superconducts below Tc=3.34K. In our paper, when the uniaxial strain is varied from -0.90% to 0.90%,Tc monotonically increases by ∼33% from 3.0 to 4.0 K, giving rise to the empirical relation Tc()=3.4+0.56+0.122. On the other hand, for changing from -0.76% to 1.26%,TCDW decreases monotonically by ∼10% from 97.5 to 87.5 K with TCDW()=94.5-4.72-0.602. The opposite response of Tc and TCDW 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 Tc with decreasing c is caused primarily by the suppression of TCDW, rather than strain-induced modifications in the bare superconducting parameters. We propose that the sensitivity of TCDW with respect to the changes in the c axis arises from the impact of the latter on the trilinear coupling between the M1+ and the L2- 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 CsV3Sb5.