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 Tc 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 Pc ~ 1.85 GPa, where the charge-density wave phase is completely suppressed, 1/T1 shows no Hebel-Slichter peak just below Tc, and decreases rapidly, even faster than T5, indicating that the gap is fully opened for pressures above Pc. In this high pressure region, the angular dependence of the in-plane upper critical magnetic field Hc2 breaks the C6 rotational symmetry. We propose the s + id pairing at P > Pc which explains both the 1/T1 and Hc2 behaviors. Our findings indicate that CsV3Sb5 is an unconventional superconductor and its superconducting state is even more exotic at high pressures.