Superconductivity based on a model with three overlapping energy bands and a nonphonon mechanism

A study is made of the thermodynamic properties of a three-band superconductor with an electron (or hole) mechanism of superconductivity. Interband carrier interaction is considered as the main reason for the occurrence of superconducting pairs. Expressions are obtained for the order parameters $\Delta_n(T)$ ($n=1,2,3$), the specific heat $C_S$, and the thermodynamic critical field $H_c$ in the region of low temperatures and also the temperature $T_c$ of the superconducting transition, the jump in the specific heat ($C_S-C_N$) at the point $T=T_c$, and $2\Delta_n(0)/T_c$. It is shown that a characteristic feature in the behavior of $T_c$, ($C_S-C_N$), and $\Delta_n(0)$ as functions of the carrier concentration $n$ is the presence of a smeared step. It is also shown that $(C_S-C_N)/C_N\leqslant1{,}43$ and that this ratio depends strongly on the relationship between the densities of the electron states of the considered bands.