Nodeless superconductivity in the kagome metal CsV3Sb5

The recently discovered kagome metal series $A$V$_3$Sb$_5$ ($A$=K, Rb, Cs) exhibits topologically nontrivial band structures, chiral charge order and superconductivity, presenting a unique platform for realizing exotic electronic states. nature of the superconducting state corresponding pairing symmetry are key questions that demand experimental clarification. Here, using technique based on tunneling diode oscillator, magnetic penetration depth $\Delta\lambda(T)$ CsV$_3$Sb$_5$ was measured down to 0.07 K. A clear exponential behavior in with marked deviations from $T$ or $T^2$ temperature dependence is observed at low temperatures, indicating deficiency nodal quasiparticles. Temperature superfluid density specific heat can be described by two-gap $s$-wave consistent presence multiple Fermi surfaces CsV$_3$Sb$_5$. These results evidence nodeless superconductivity under ambient pressure, constrain allowed symmetry.