Showing papers on "Variable-range hopping published in 1979"

Negative Magnetoresistance in the Anderson Localized States

Abstract: It is shown that the mechanism of the electronic conduction due to variable-range hopping in the Anderson localized states gives rise to a large negative magnetoresistance, particularly in the vicinity of the metal-nonmetal transition. This result is mainly caused by an exponential dependence of the hopping rate on the highest occupied energy levels. The same mechanism also leads to a strong electric field-dependence of the conductivity.

Hopping conduction and the Coulomb gap; applications to Fe 3 O 4 , Ti 4 O 7 and impurity conduction in Si:P

Abstract: Proceses in which electrons move by hopping from a full to an empty localized site in a degenerate electron gas are considered. One example is impurity conduction in doped and compensated silicon (e.g. Si:P). For the conductivity a variation as A exp (−B/T1/4) was predicted some years ago if the “interatomic” Coulomb interaction between electrons on different sites is neglected. If it is not neglected, a “Coulomb gap” EC of order e2/κa or in some cases less is introduced, where a is the distance between centres, as first pointed out in 1970 by Pollak. The activation energy by single-electron hops cannot be less than EC, so only if kT≫EC should they lead to the T1/4 law. Following earlier work (Mott 1976), it is shown that many-electron hops may lead to this law at low temperatures, with the same value of B but a smaller and T-dependent value of A. Somewhat different conclusions presented by Knotek and Pollak and others due to Efros and Shklovskii are discussed. Particular attention is given to the thermopower S, predicted to behave as T1/2 dlnN(E)/dE for hopping conduction both of single electron or (probably) of many-electron types; a new suggestion is an intermediate range of T where kT≅EC in which S=(k/e) (EC/2kT).

Metal-Nonmetal Transition in Amorphous Si-Au System at Low Temperatures: Measurements of Electrical Conductivity and Thermoelectric Power

Abstract: Metal-nonmetal transition in amorphous Si–Au system has been investigated by means of electrical conductivity and thermoelectric power. It is found that the Au atoms act as acceptors and form a narrow band in pseudo-gap of amorphous Si. The metal-nonmetal transition occurs at x =14% and is assigned to the Anderson transition. In the nonmetallic region, the transport is dominated by the variable range hopping conduction in the lower temperature range and by the ambipolar conduction in the host band in the higher temperature range.

On the hopping mechanism of dispersive transport

Abstract: The hopping mechanism of dispersive transport is discussed starting directly from the hopping transport equation, in contrast to the conventional approach using the continuous-time random-walk model. The power-law decay of the transient current due to the spread of the packet of photo-injected carriers, is obtained for impurity-band hopping both in the presence of trapping and without it. The analysis shows that variable-range hopping generally cannot be ruled out as a probable mechanism of dispersive transport in disordered semiconductors.

Hopping Conductivity of Undoped ZnSe Thin Films

Abstract: Results on the temperature dependence of the dc conductivity at low and high electric field strengths, and the frequency dependence of the ac conductivity are interpreted as hopping conductivity. Near room temperature the low field conductivity is determined by nearest-neighbour hopping, and at lower temperatures by variable range hopping conductivity which is given by σ ∼ exp (−(T0/T)b), b = (1 + x)/(1 + d + x) ≈ 0.6 to 0.8. The high field limit of the current density yields j ∼ exp (−(F0/F)c). The obtained correspondence c ≈ b can be taken as an evidence of the hopping conduction mechanism. Die Ergebnisse der Temperaturabhangigkeit der Gleichstromleitfahigkeit bei kleinen und grosen elektrischen Feldstarken sowie der Frequenzabhangigkeit der Wechselstromleitfahigkeit werden als Hoppingleitfahigkeit gedeutet. Bei Temperaturen nahe Raumtemperatur wird die Leitfahigkeit bei kleinen Feldern durch Hopping uber nachste Nachbarn und bei tieferen Temperaturen durch variable range hopping der Form σ ∼ exp (−(T0/T)b), b = (1 + x)/(1 + d + x) ≈ 0,6 bis 0,8 bestimmt. Im Grenzfall hohen Feldes ergibt sich fur die Stromdichte j ∼ exp (−(F0/F)c). Die beobachtete Ubereinstmmung von c ≈ b wird als ein Beweis fur den Hoppingleitungsmechanismus genommen.

Electron hopping in Eu1–xSrxFeO3 and Nd1–xSrxCoO3

Abstract: Mossbauer and electrical resistivity measurements on Eu1–xSrxFeO3(0.0 TN. Variable range hopping arising from Anderson localization seems to occur at T < TN indicating that the electron hopping time in this regime is likely to be greater than 10–7 s. Mossbauer studies on Nd1–xSrxCoO3 show that in the Anderson localization regime, the hopping time is greater than 10–7 s in this system as well.

Magnetic and Electrical Properties in Amorphous Ge–Se–Fe, Ge–Fe and Se–Fe Films

Abstract: Magnetic and electrical properties in amorphous Ge–Se–Fe, Ge–Fe, and Se–Fe films prepared by rf-sputtering have been studied. The Ge–Se–Fe films exhibit paramagnetic and ferromagnetic resonances in the composition ranges with low and high Fe content respectively, and exhibit variable range hopping conduction for paramagnetic films and metallic conduction for ferromagnetic films. The films with intermediate Fe content behave in a manner intermediate between semiconductive and metallic conduction. Similar results are also found in the Ge–Fe and Se–Fe films except for the absence of paramagnetic resonance signals in the range with low Fe content.

Effect of electron-electron scattering on electrical conduction in inversion layers

Abstract: In a system of non-interacting electrons with disorder, such that there are both localised and mobile electron states, separated from each other in energy by a mobility edge, the contribution to the conductivity from the mobile electrons is proportional to their lifetimes, whereas that due to the hopping of localised electrons is proportional to their rate of hopping. Hence, as a scattering mechanism is switched on between localised and mobile electrons, the conductivity due to mobile electrons is decreased while that due to hopping electrons is enhanced. Which is the dominant mode of conduction would then depend on the strength of the scattering. In this article, the authors examine the Coulomb interaction between electrons as the scattering mechanism, and develop the results for the case of inversion layers. They find that the hopping mode of conduction contributes more and more as the Fermi level of the system approaches the mobility edge from below, and may finally take over. This contribution to the conductivity behaves as if it originated from an activated mobility, which is what experiments seem to indicate.

Rate equations in the hopping transport III. A completely new interpretation of the dc phonon-assisted hopping in the mobility gap

Abstract: A new interpretation of the low-frequency phonon-assisted hopping conduction in the mobility gap of disordered semiconductors which is inherently connected with the effect of broadening of electronic levels due to electron-phonon coupling is suggested. This interpretation is fully compatible with our previous reported mathematical theory of the hopping conduction. It is argued that the current understanding of the hopping transport is based on a dubious neglect of the above mentioned broadening which makes the usual theories of the dc phonon-assisted hopping conduction inconsistent.