Nodal superconducting state in clean single crystals of FeSe

Among iron-based superconductors, the binary ``11'' family offers the possibility to investigate systems consisting of just the iron arsenic/selenium layers without the intermediate layers which are present in the ``111'', ``122'' and ``1111'' families. This simplest iron based superconductor may therefore yield vital information about the origin of superconductivity in the iron pnictides/chalcogenides. Here we measured the penetration depth and thermal conductivity in very clean single crystals of FeSe~[1] with RRR $\textgreater$ 200. Presence of line nodes is evident by the quasi $T$-linear dependence of the penetration depth. Moreover, a large residual thermal conductivity, which is much larger than that expected for $d$-wave symmetry, suggests that nodes are accidental and nearly vanishing. The field dependence of thermal conductivity suggests a possible field induced phase transition in the superconducting state. \\ ~[1] A.E. B\"{o}hmer et al., Phys. Rev. B {\bf 87}, 180505(R) (2013).