Large polaron mechanism

Note term comes from the pre-exponential term in Eq. 7.51, i.e. from the density of states. This result is applicable to an intrinsic semiconductor for which phonon scattering is responsible for the temperature dependence of the electronic mobility, i.e., one in which the mobility decreases with increasing temperature. Other possibilities exist, however two of the more important ones are the small and large polaron mechanisms discussed below. A polaron is a defect in an ionic crystal that is formed when an... 

Lor an intrinsic semiconducting oxide where the electronic conductivity mechanism can be described in terms of a large polaron mechanism, the temperature dependence of aei can be written by combination of Eqs. 6.47 and 6.52... 

The presence of the polar mode oriented in the direction of maximum conductivity gives grounds to postulate the feasibility of the polaronic mechanism of motion of the protons in this crystal. As was shown in the previous subsection (see also Refs. 57,58, and 191) the proton can traverse a comparatively large distance between the nearest sites of the protonic sublattice (0.4 to 0.8 nm) with the participation of vibrational quanta, that is, phonons the virtual absorption of such a quantum can appreciably increase the resonance integral of overlapping of the wave functions of the proton on the nearest sites see expression (318). 

SP), quantum mechanical tunneling (QMT) and overlapping large polaron (OLP)... 

Figure 8.12. Frequency dependence of the frequency exponent s for various models correlated-barrier hopping (CBH) (WM/kT=75 has been assumed) small polaron (SP), quantum mechanical tunneling (QMT) and overlapping large polaron (OLP)...

For species that migrate by other mechanisms than diffusion (notably itinerant electrons and holes), the diffusion coefficient is undefined, and instead one uses mobilities with temperature dependences typical of band transport with phonon-, impurity-, or large polaron-Hmited transport [8]. 

Figure 40. A schematic illustrating the difference between the superexchange mechanism and molecular wire behavior in a D-B-A dyad. Superexchange the virtual bridge states lie well above the donor level (A is large) and, consequently, the electron is never localized within the bridge instead, the electron is transferred from donor to acceptor in one coherent jump. The distance dependence behavior is exponential decay. Molecular wire behavior The virtual bridge states are energetically comparable to the donor level (A is very small). In this case, the electron may be thermally injected into the bridge and becomes localized within the bridge, whereupon it moves from the donor to the acceptor incoherently as a defect, such as a polaron. The distance dependence behavior is Ohmic (varies inversely with distance).

---