Fully-gapped superconductivity with rotational symmetry breaking in pressurized kagome metal CsV3Sb5

The discovery of the kagome metal CsV₃Sb₅ has generated significant interest in its complex physical properties, particularly its superconducting behavior under different pressures, though its nature remains debated. Here, we performed low-temperature, high-pressure ^121/123Sb nuclear quadrupole resonance (NQR) measurements to explore the superconducting pairing symmetry in CsV₃Sb₅. At ambient pressure, we found that the spin-lattice relaxation rate 1/T₁ exhibits a kink at T ~ 0.4 T_c within the superconducting state and follows a T³ variation as temperature further decreases. This suggests the presence of two superconducting gaps with line nodes in the smaller one. As pressure increases beyond P_c ~ 1.85 GPa, where the charge-density wave phase is completely suppressed, 1/T₁ shows no Hebel-Slichter peak just below T_c, and decreases rapidly, even faster than T⁵, indicating that the gap is fully opened for pressures above P_c. In this high pressure region, the angular dependence of the in-plane upper critical magnetic field H_c2 breaks the C₆ rotational symmetry. We propose the s + id pairing at P > P_c which explains both the 1/T₁ and H_c2 behaviors. Our findings indicate that CsV₃Sb₅ is an unconventional superconductor and its superconducting state is even more exotic at high pressures.