Evidence of nodal superconductivity in LaFeSiH

Unconventional superconductivity has recently been discovered in the first iron-based superconducting silicide LaFeSiH. By using the complementary techniques of muon spin rotation, tunneling diode oscillator and density functional theory, we investigate the magnetic penetration depth and thereby the superconducting gap of this novel high-temperature superconductor. We find that the magnetic penetration depth displays a sub-$T^2$ behavior in the low-temperature regime below $T_c/3$, which evidences a nodal structure of the gap (or a gap with very deep minima). Even if the topology of the computed Fermi surface is compatible with the $s_\pm$-wave case with accidental nodes, its nesting and orbital-content features may eventually result in a $d$-wave state, more unusual for high-temperature superconductors of this class.