Abstract
The thermal conductivity of the iron-based superconductor FeSe was measured at temperatures down to 75 mK in magnetic fields up to 17 T. In a zero magnetic field, the electronic residual linear term in the limit, , is vanishingly small. The application of a magnetic field causes an exponential increase in initially. Those two observations show that there are no zero-energy quasiparticles that carry heat and therefore no nodes in the superconducting gap of FeSe. The full field dependence of has the classic two-step shape of a two-band superconductor: a first rise at very low field, with a characteristic field , and then a second rise up to the upper critical field . This shows that the superconducting gap is very small (but finite) on one of the pockets in the Fermi surface of FeSe. We estimate that the minimum value of the gap, , is an order of magnitude smaller than the maximum value, .
- Received 22 March 2016
DOI:https://doi.org/10.1103/PhysRevLett.117.097003
© 2016 American Physical Society