A new crystalline phase of four-fold coordinated silicon and germanium

We have performed first-principles calculations for body-centered tetragonal (bct) Si and Ge consisting solely of four-fold coordinated elements. The structural optimization has been carried out based on the local density approximation (LDA) in the density functional theory (DFT). For total- energy minimized structures, quasi- particle spectra have been calculated using GW approximation. We find that the bct Si and Ge are new stable crystalline phases reachable under tensile stress with moderate magnitude. We also find that the bct Ge is a semimetal with the carrier density of 2 x 10(19) cm(-3), whereas the bct Si is a semiconductor with the indirect band gap of 0.5 eV. The calculated density of states of the bct Si and Ge show characteristic features which are discriminated from those of the diamond structures. Effective masses of conduction- band electrons and valence-band holes are found to be relatively light compared with those of the diamond Si and Ge. The origins of reduction or closure of band gaps are discussed.