Singlet-bipolaron formation among degenerate electronic orbitals: Softening bipolarons

Journal Article · Wed Mar 01 04:00:00 EST 2000 · Physical Review. B, Condensed Matter and Materials Physics

DOI:https://doi.org/10.1103/PhysRevB.61.6069· OSTI ID:20215631

Emin, David ^[1]

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1. Department of Physics and Astronomy, University of New Mexico, Albuquerque, New Mexico 87131-1156 (United States)

Bipolaron formation among degenerate electronic orbitals is considered. Symmetry-breaking deformations that lift the electronic degeneracy are shown to be required to stabilize a singlet with respect to a triplet. These symmetry-breaking deformations mix singlet states of the undeformed system. Mixing singlets of different energies causes the electronic energy of the lowest-energy singlet to fall nonlinearly with symmetry-breaking deformations. Through this nonlinearity, a singlet pair of the lowest energy not only fosters symmetry-breaking deformations but also reduces the stiffness constants of these symmetry-breaking distortions. The concomitant singlet-induced lowering of the vibrational frequencies reduces the vibrational free energy. Thus, singlet-induced atomic displacements and vibrational softening both contribute to lowering the free energy of a singlet bipolaron below that of a triplet and of two separated polarons. At strong enough electron-lattice coupling, stabilization of a singlet bipolaron is driven mainly by symmetry-breaking shifts of atoms' equilibrium positions. This singlet bipolaron is a variant of a conventional bipolaron whose stabilization with respect to a triplet requires symmetry-breaking deformations. However, stabilization of a singlet bipolaron at moderate coupling results primarily from singlet-induced reductions of the frequencies of symmetry-breaking vibrations. This type of bipolaron is defined as a ''softening'' bipolaron. Distinctive properties of a softening bipolaron enable it to be identified. Most prominently, softening bipolarons can lack the polaronic absorption bands that characterize conventional bipolarons. In addition, the Seebeck coefficients of softening bipolarons garner a distinctive contribution from singlet-induced softening of vibrational modes. Observation of these effects for boron carbides' singlet bipolarons suggests that they are examples of softening bipolarons. (c) 2000 The American Physical Society.

• OSTI ID: 20215631
• Journal Information: Physical Review. B, Condensed Matter and Materials Physics, Journal Name: Physical Review. B, Condensed Matter and Materials Physics Journal Issue: 9 Vol. 61; ISSN 1098-0121
• Country of Publication: United States
• Language: English

Related Subjects

75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
CHARGE CARRIERS
DEFORMATION
ELECTRONIC STRUCTURE
FREE ENERGY
ORBITS
POLARONS
SOLIDS
SYMMETRY BREAKING
THEORETICAL DATA
VIBRATIONAL STATES