Nodeless superconductivity in topologically nontrivial materials HfRuP and ZrRuAs

Topologically nontrivial electronic states are recently found in a family of noncentrosymmetric transition metal pnictides TRuX (T = Zr, Hf; X = P, As), presenting a unique platform for superconductivity to interplay with topological electronic states and asymmetric spin-orbit coupling. Here, we investigate the superconducting order parameter of HfRuP and ZrRuAs by measuring the magnetic penetration depth change Delta lambda(T) using a method based on the tunnel-diode oscillator. Both compounds show clear exponential temperature dependence in Delta lambda(T) at low temperatures, suggesting fully-gapped superconductivity. Moreover, the superfluid densities in both HfRuP and ZrRuAs can be reasonably described by an s-wave superconducting model.