Specific heat of FeSe: Two gaps with different anisotropy in superconducting state

© 2017 Elsevier B.V. We present detailed study of specific heat of FeSe single crystals with critical temperature Tc=8.45 K at 0.4-200 K in magnetic fields 0-9 T. Analysis of the electronic specific heat at low temperatures shows the coexistence of isotropic s-wave gap and strongly anisotropic extended s-wave gap without nodes. It was found two possibilities of superconducting gap parameters which give equally description of experimental data: (i) two gaps with approximately equal amplitudes and weight contribution to specific heat: isotropic δ1=1.7 meV (2δ1/kBTc=4.7) and anisotropic gap with the amplitude δ2max=1.8 meV (2δ2max/kBTc=4.9 and anisotropy parameter m=0.85); (ii) two gaps with substantially different values: isotropic large gap δ1=1.65 meV (2δ1/kBTc=4.52) and anisotropic small gap δ2max=0.75 meV (2δ2max/kBTc=2) with anisotropy parameter m=0.71. These results are confirmed by the field behavior of the residual electronic specific heat γr