Charge Density Wave-Soliton Model for Se and Te

A new microscopic theory is proposed for the electronic and lattice structures of crystal and defects of Se and Te. Consideration of the valence bond picture shows that electron correlation in valence p electrons produces a charge density wave (CDW) of vector nature, the vector CDW (VCDW), which has the period of three and keeps every atom neutral. Defects including valence alternation pairs and recombinations of helical chains may be generated as low energy solitonic excitations of the VCDW. Amorphous Se and Te may be regarded as an assembly of solitonic excitations in the VCDW. A formulation of the VCDW and its solitonic excitations is given in the HF approximation. Valence p electrons are described by an INDO type Hamiltonian with the nearest neighbour transfer and exchange interactions, intraatomic Coulomb repulsion and exchange and interatomic Coulomb repulsion. The remaining closed shell part of each atom is represented as a core with a repulsive potential. The lattice geometry is determined by the balance of the cohesive force of p electrons and the repulsion of cores. The effective intraatomic Coulomb interaction in a neutral atom has the form of a negative on-site interaction which stabilizes the VCDW. An equation is obtained which determines self-consistently the electronic and lattice structures of the VCDW and its solitonic defects. The VCDW and its defects always accompany not only an electron density modulation but also a bond order modulation so that they have lattice structures distorted from a standard cubic lattice. The trigonal lattice of the crystalline Se and Te can be explained as due to the presence of the VCDW.