Polaron-hopping model

Although the mechanism of conduction in lithium-doped NiO and other low-mobility semiconductors is a controversial matter, the simple polaron hopping model outlined above serves well as a basis for understanding conduction processes in many of the systems discussed later (eg Section 4.4.1). 

As temperature decreases, the behavior of experimental data deviates from that predicted by adiabatic small polaron hopping model. This deviation is accompanied by a sharp growth of both susceptibility and magnetoresistance. Further temperature lowering leads to the formation of the peaks near Tc on the R(T) and MR(I) dependences for the samples A and B. It is noteworthy that, although in the case of the sample C the resistivity at Tc is far from its maximum value, the conduction below Tc does not obey adiabatic small polaron hopping law. 

The schematic representation of the energy landscapes in the polaronic hopping model is shown in Figure 5(d), where the energy difference is equal to AE = fe+i During the time... 

The electronic band structure of a neutral polyacetylene is characterized by an empty band gap, like in other intrinsic semiconductors. Defect sites (solitons, polarons, bipolarons) can be regarded as electronic states within the band gap. The conduction in low-doped poly acetylene is attributed mainly to the transport of solitons within and between chains, as described by the intersoliton-hopping model (IHM) . Polarons and bipolarons are important charge carriers at higher doping levels and with polymers other than polyacetylene. 

Hopping Models Hole-Resting-Site and Phonon-Assisted Polaron Transport... 

In a second possibility, the polaron-like hopping model, a structural distortion of the DNA stabilizes and delocalizes the radical cation over several bases. Migration of the charge occurs by thermal motions of the DNA and its environment when bases are added to or removed from the polaron [23]. 

The hole-resting-site and polaron-like hopping models can be distinguished by the distance and sequence behavior of radical cation migration. Analysis of the hole-resting-site model leads to the prediction that the efficiency of radical cation migration will drop ca. ten-fold for each A/T base pair that separates the G resting sites [33]. 

The phonon-assisted polaron-like hopping model is unique because it is built upon an understanding of the dynamical nature of DNA in solution. The fundamental assumption of this model is that the introduction of a base radical cation into DNA will be accompanied by a consequent structural change that lowers the energy for the system. 

Liu C-S, Schuster GB (2003) Base sequence effects in radical cation migration in duplex DNA support for the polaron-like hopping model. J Am Chem Soc 125 6098-6102 Llano J, Eriksson LA (2004) First principles electrochemical study of redox events in DNA bases and chemical repair in aqueous solution. Phys Chem Chem Phys 6 2426-2433... 

The addition of Li20 to NiO leads to an increase in conductivity, as illustrated in Fig. 2.16. The lithium ion Li+ (74 pm) substitutes for the nickel ion Ni2+ (69 pm) and, if the mixture is fired under oxidizing conditions, for every added Li+ one Ni2+ is promoted to the Ni3+ state, the lost electron filling a state in the oxygen 2p valence band. The lattice now contains Ni2+ and Ni3+ ions on equivalent sites and is the model situation for conduction by polaron hopping , which is more often referred to simply as electron hopping . 

Gill (1972) was the first to suggest that charge transport in polymers occurred by polaron hopping. The application of polaron theory to transport in polymers was first described by Sahvun (1984). Schein et al. (1990), and Schein (1992). The models described by Sahvun and Schein and coworkers lead to a mobility that is a product of a Boltzmann probability of energy coincidence and the probability a carrier will hop to an adjacent site by thermal activation once... 

Several models can explain the carrier transport in organic semiconductors. However, none of them can be independently employed to explain the carrier transport phenomena and the mechanism at the same time. Among the theoretical models, the most often used models are the band transport model (Warta and Karl, 1985 Pemstich et al., 2008 Karl et al., 1991), polaron transport model (Holstein, 1959 Emin and Holstein, 1969 Marcus, 1960), hopping transport model (Vissenberg and Matters, 1998), and multiple trapping and release model (Horowitz et al., 1995 Le Comber and Spear, 1970). 

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