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

We present detailed study of specific heat of FeSe single crystals with critical temperature $T_c=8.45$ 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 ${\Delta }_1=1.7$ meV ($2{\Delta }_1/k_B{T}_c$=4.7) and anisotropic gap with the amplitude ${\Delta }_2^{\max }=1.8$ meV ($2{\Delta }_2^{\max }/k_B{T}_c$=4.9 and anisotropy parameter $m=0.85$); (ii) two gaps with substantially different values: isotropic large gap ${\Delta }_1=1.65$ meV ($2{\Delta }_1/k_B{T}_c=4.52$) and anisotropic small gap ${\Delta }_2^{\max }=0.75$ meV ($2{\Delta }_2^{\max }/k_B{T}_c=2$) with anisotropy parameter $m=0.71$. These results are confirmed by the field behavior of the residual electronic specific heat ${\gamma }_r$.