Low-energy quasiparticle excitations in a BiCh₂-based superconductor studied by magnetic penetration depth measurements

BiCh₂-based superconductors are layered superconductors composed of an alternate stacking of the Bi-chalcogen superconducting layers and various types of blocking layers. The structure is similar to high-T_c cuprate and Fe-based superconductors, which have unconventional pairing mechanisms. Whether the mechanism of BiCh₂-based superconductivity is conventional or unconventional is still an open question, and some conflicting experimental results of superconducting gap structure have been reported. While the specific heat and thermal conductivity measurements imply a fully gapped s-wave superconducting state, angle-resolved photoemission and isotope effect studies point to an unconventional pairing mechanism, which is predicted theoretically. Moreover, two-fold-symmetric magnetoresistance for in-plane field was recently reported in LaO_0.5F_0.5BiSSe, which breaks the rotational symmetry of the tetragonal system. Here, we report on high-resolution measurements of the magnetic penetration depth in single crystals of LaO_0.5F_0.5BiSSe, which is a sensitive probe of low-energy quasiparticle excitations. By comparing with the results of previous studies, we will discuss the superconducting gap structure and the pairing mechanism in this system.