SINGLET PAIRING AMONGST DEGENERATE ELECTRONIC STATES: 'SOFTENING BIPOLARONS'

摘要

Holes in boron carbides pair to form singlet bipolarons on twelve-atom icosahedral structures. Transport occurs by carriers hopping between icosahedra. However, the optical conductivity and the Seebeck coefficients differ qualitatively from expectations for small bipolarons. These novel features are attributed to the singlets being formed amongst four-fold degenerate icosahedral orbitals. In particular, carriers in highly degenerate states can form a novel type of bipolaron, a "softening bipolaron." Distinctive features of softening bipolarons result from singlets' softening the symmetry-breaking vibrational modes to which they are coupled.