Exchange instabilities in an n-type silicon inversion layer

We investigate the possibility that in a semiconducting surface inversion layer the ground state has charge-density waves in each of the two equivalent subbands. The two charge-density waves are identical except that they are completely out of phase with each other. Such a state, referred to as a valley-density wave (VDW), need not be accompanied by a lattice distortion. A density-functional formalism is used to study the instability toward formation of a VDW ground state. It is found that under certain conditions the VDW state has a lower energy than the paramagnetic state. A similar conclusion is drawn on the basis of Hartree-Fock treatment of VDW state. © 1978 The American Physical Society.