Nontrivial Topological States in BaSn_5 Superconductor Probed by de Haas–van Alphen Quantum Oscillations

We report the nontrivial topological states in an intrinsic type-II superconductor BaSn_\boldsymbol5 (T_\rmc \sim 4.4 K) probed by measuring the magnetization, specific heat, de Haas–van Alphen (dHvA) effect, and by performing first-principles calculations. The first-principles calculations reveal a topological nodal ring structure centered at the H point in the k_\rmz = \pi plane of the Brillouin zone, which could be gapped by spin-orbit coupling (SOC), yielding relatively small gaps below and above the Fermi level of about 0.04 eV and 0.14 eV, respectively. The SOC also results in a pair of Dirac points along the \varGamma–A direction, located at \sim 0.2 eV above the Fermi level. The analysis of the dHvA quantum oscillations supports the calculations by revealing a nontrivial Berry phase originating from the hole and electron pockets related to the bands forming the Dirac cones. Thus, our study provides an excellent avenue for investigating the interplay between superconductivity and nontrivial topological states.