Fully-gapped superconductivity with rotational symmetry breaking in pressurized kagome metal CsV${}_3$Sb${}_5$

Abstract

The discovery of the kagome metal CsV${}_3$Sb${}_5$ 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 ${}^{121/123}$Sb nuclear quadrupole resonance (NQR) measurements to explore the superconducting pairing symmetry in CsV${}_3$Sb${}_5$. At ambient pressure, we found that the spin-lattice relaxation rate 1/$T_1$ exhibits a kink at $T\sim$ 0.4 $T_{\text{c}}$ within the superconducting state and follows a $T^3$ variation as temperature further decreases. This suggests the presence of two superconducting gaps with line nodes in the smaller one. As pressure increases beyond $P_{\mathrm{c}}\sim 1.85$ GPa, where the charge-density wave phase is completely suppressed, 1/$T_1$ shows no Hebel-Slichter peak just below $T_{\text{c}}$, and decreases rapidly, even faster than $T^5$, indicating that the gap is fully opened for pressures above $P_{\mathrm{c}}$. In this high pressure region, the angular dependence of the in-plane upper critical magnetic field $H_{c2}$ breaks the $C_6$ rotational symmetry. We propose the $s+id$ pairing at $P>P_{\mathrm{c}}$ which explains both the 1/$T_1$ and $H_{c2}$ behaviors. Our findings indicate that CsV${}_3$Sb${}_5$ is an unconventional superconductor and its superconducting state is even more exotic at high pressures.

25 pages, 4 figures, to appear in Nature Communications

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• Topological pair density waves in kagome superconductors
• Strain-Tuned Nodal Superconductivity in the Charge-Ordered Kagome Metal CsV${}_3$Sb${}_5$