Topic
Variable-range hopping
About: Variable-range hopping is a research topic. Over the lifetime, 3606 publications have been published within this topic receiving 66561 citations.
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TL;DR: A simple two-dimensional hopping model for independent electrons which has strictly localized states in addition to the extended states is presented and the effect is shown to hold for a much more general class of systems.
Abstract: We present a simple two-dimensional hopping model for independent electrons which has strictly localized states in addition to the extended states. These localized states can exist either in a band gap or within the continuum. The localized states persist if the lattice periodicity is destroyed. Finally, the effect is shown to hold for a much more general class of systems.
318 citations
TL;DR: This work demonstrates the polarization of the conduction bands and indicates that, when ZnO is doped into the metallic regime, it behaves as a genuine magnetic semiconductor.
Abstract: Films of ZnO doped with magnetic ions Mn and Co and, in some cases, with Al have been fabricated with a very wide range of carrier densities. Ferromagnetic behavior is observed in both insulating and metallic films, but not when the carrier density is intermediate. Insulating films exhibit variable range hopping at low temperatures and are ferromagnetic at room temperature due to the interaction of the localized spins with static localized states. The magnetism is quenched when carriers in the localized states become mobile. In the metallic (degenerate semiconductor) range, robust ferromagnetism reappears together with very strong magneto-optic signals and room temperature anomalous Hall data. This demonstrates the polarization of the conduction bands and indicates that, when ZnO is doped into the metallic regime, it behaves as a genuine magnetic semiconductor.
315 citations
TL;DR: Transport by hopping in semiconductor or weakly coupled metal nanocrystal solids dominates transport, as in disordered semiconductors, and such behavior at finite temperature is not proof of band-like conduction.
Abstract: In nanocrystal solids, the small density of states of quantum dots makes it difficult to achieve metallic conductivity without band-like transport. However, to achieve band-like transport, the energy scale of the disorder should be smaller than the coupling energy. This is unlikely with the present systems due to the size polydispersivity. Transport by hopping may nevertheless lead to an increased mobility with decreasing temperature for some temperature range, and such behavior at finite temperature is not proof of band-like conduction. To date, at low temperature, variable range hopping in semiconductor or weakly coupled metal nanocrystal solids dominates transport, as in disordered semiconductors.
310 citations
TL;DR: In this article, the interdependence of electron hopping between redox centers and the physical motion of redox centres is investigated systematically, and Monte-Carlo simulations are employed to study the transition between static percolation and mean-field behaviors as a function of the relative rates of electron hop and physical motion.
Abstract: The interdependence of electron hopping between redox centers and the physical motion of redox centers is investigated systematically. When physical motion is either nonexistent or much slower than electron hopping, charge propagation is fundamentally a percolation process. In the opposite extreme, rapid molecular motion thoroughly rearranges the molecular distribution between successive electron hops, thereby leading to mean-field behavior. Monte-Carlo simulations are employed to study the transition between static percolation and mean-field behaviors as a function of the relative rates of electron hopping and physical motion
289 citations
TL;DR: In this article, the behavior at low temperatures can be described using Mott's variable range hopping arguments, and at high temperatures by non-adiabatic small polaron hopping between nearest neighbours.
Abstract: Dc conductivity measurements have been made between 90 and 520 K on three bulk samples of V 2 O 5 P 2 O 5 glass. Heat treatment is found to result in a reduction of the activation energy at a given temperature and this is most noticeable at low temperatures. The behaviour at low temperatures can be described using Mott's variable range hopping arguments, and at high temperatures by non-adiabatic small polaron hopping between nearest neighbours. At intermediate temperatures a simple model is used in which excitations by optical and acoustic phonons are considered to make independent contributions to the jump frequency. Mott's theory is extended to the polaron case for T> 1 4 ⊖ and is shown to be in good agreement with results. Values for r p (∼2.8 A ) the polaron radius and α(∼3.5 A −1 ) the electron decay constant are shown to be consistent with the model for small polarons. A method is suggested for obtaining α and N ( E F ) from the ac conductivity and the slope of 1nσ versus 1 T 1 4 at low temperatures. Values of N ( E ) are obtained which correlate with those obtained by the previous analysis. This implies that the disorder energy separating adjacent sites Δ 0 is large (∼0.4 eV) in these materials.
285 citations