Effect of Pressure on Superconducting Ca-intercalated Graphite CaC$_6$
  The pressure effect on the superconducting transition temperature ($T_c$) of the newly-discovered Ca-intercalated graphite compound CaC$_6$ has been investigated up to $\sim$ 16 kbar. $T_c$ is found to increase under pressure with a large relative ratio $\Delta$$T_c$/$T_c$ of $\approx$ +0.4 %/kbar. Using first-principles calculations, we show that the large and positive effect of pressure on $T_c$ can be explained in the scope of electron-phonon theory due to the presence of a soft phonon branch associated to in-plane vibrations of Ca atoms. Implications of the present findings on the current debate about the superconducting mechanism in graphite intercalation compounds are discussed. 
J. S. Kim (1), L. Boeri (1), R. K. Kremer (1), and F.S. Razavi (2)   ((1) Max-Planck-Institut fuer Festkoerperforschung, Stu...