Small bipolarons in boron carbides: Pair breaking in semiclassical hopping
A pair of charge carriers can be bound within a common potential well produced by displacing atoms from their carrier-free equilibrium positions. These two self-trapped carriers together with the associated atomic-displacement pattern is referred to as a bipolaron. If the self-trapped carriers` wavefunction is primarily confined to a single structural unit (atom, bond or molecule), the bipolaron is termed small. If however the self-trapped carriers` wavefunction extends over multiple structural units, the bipolaron is called large. Small bipolarons form in crystals if the energy lowering due to the carriers` self-trapping exceeds the electronic-transfer energy associated with an electronic carrier`s intersite motion. Small-polaron and -bipolaron formation is also often induced by disorder. Interest in bipolarons focuses on situations in which they are energetically stable with respect to dissociating into two individual carriers. Stability is achieved when the additional lowering of the atomic-displacement-related energy arising from two carriers sharing a common site overwhelms their mutual Coulomb repulsion. Self-trapped carriers only move when the positions of the atoms whose displacements produce self-trapping change. When atoms move so as to shift small-polaronic self-trapped carriers between adjacent sites, changes of the self-trapped carriers` energies always exceed their intersite electronic transfer energy. Since small-polaronic self-trapped carriers thereby lose coherence as they move, their transport is described as hopping incoherently between localized states. This report discusses the electronic structure of boron carbides and describes features that make them ideal for studying small bipolaron hopping. The effect on conductivity is discussed.
- Research Organization:
- Sandia National Labs., Albuquerque, NM (United States)
- Sponsoring Organization:
- USDOE, Washington, DC (United States)
- DOE Contract Number:
- AC04-94AL85000
- OSTI ID:
- 111912
- Report Number(s):
- SAND--95-1976C; CONF-9508166--2; ON: DE95017893
- Country of Publication:
- United States
- Language:
- English
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