Enhancement of superconductive critical temperatures in almost empty or full bands in two dimensions: possible relevance to beta-HfNCl, C60 and MgB2

Abstract

We examine possibility of enhancement of superconductive critical temperature in two-dimensions. The weak coupling BCS theory is applied, especially when the Fermi level is near the edges of the electronic bands. The attractive interaction depends on ${\bf k}$ due to screening. The density of states(DOS) does not have a peak near the bottom of the band, but $k$-dependent contribution to DOS (electron density on the Fermi surface) has a diverging peak at the bottom or top. These features lead to significant enhancement of the critical temperatures. The results are qualitatively consistent with the superconductive behaviors of HfNCl ($\Tc \le 25K$) and ZrNCl($\Tc \le 15K$), C$_{60}$ with a field-effect transistor configuration ($\Tc = 52K$), and MgB$_2$ ($\Tc \approx 40K$) which have the unexpectedly high superconductive critical transition temperatures.