Itoh, M., M. Kotani, et al. (2009). "New Metallic Carbon Crystal." Physical Review Letters 102(5): 055703.

Recently, mathematical analysis clarified that sp(2) hybridized carbon should have a three-dimensional crystal structure (K(4)) which can be regarded as a twin of the sp(3) diamond crystal. In this study, various physical properties of the K(4) carbon crystal, especially for the electronic properties, were evaluated by first principles calculations. Although the K(4) crystal is in a metastable state, a possible pressure induced structural phase transition from graphite to K(4) was suggested. Twisted pi states across the Fermi level result in metallic properties in a new carbon crystal.