McMillan band structure

In Fig. 14 we have plotted also the expected ratio Tc/TF(BCS) for the critical temperature in the single o band of MgB2 calculated by the standard BCS approximations using the McMillan formula, the density of states and electron phonon coupling obtained by band structure theory that gives for MgB2 Tc(5CS)=20K [148],... 

The quantity Xei-pi, enters two important physical relations. It gives the electron-phonon enhancement of the low-temperature electronic heat capacity as Q = jT = ybandT(l + Xei ph), where JiwiT is the heat capacity obtained from the electron band structure without regard to the electron-phonon interaction. The other important relation, of particular interest here, is the connection between Xei.ph and the transition temperature T. An accurate calculation of requires the solution of the Eliashberg equations, which explicitly take into account the shape of a F(co). In many practical applications one instead obtains F from a semiempirical relation containing a F(co) in the form of the average Xei.ph, Eq. (25). The most frequently used such relation is that due to McMillan (31). It reads... 

---