B. Sandow

Electronic Correlation Effects and the Coulomb Gap at Finite Temperature

We have investigated the effect of the long-range Coulomb interaction on the one-particle excitation spectrum of n-type germanium, using tunneling spectroscopy on mechanically controllable break junctions. At low temperatures, the tunnel conductance shows a minimum at zero bias voltage due to the Coulomb gap. Above 1 K, the gap is filled by thermal excitations. This behavior is reflected in the variable-range hopping resistivity measured on the same samples: up to a few degrees Kelvin the Efros-Shklovskii lnR infinity T(-1/2) law is obeyed, whereas at higher temperatures deviations from this law occur. The type of crossover differs from that considered previously in the literature.

On the Sign and Temperature Dependence of Magnetoresistance in Variable‐Range Hopping in Highly Compensated n‐GaAs: The Role of Spin‐Effects

The magnetoresistance of neutron-transmutation-doped (NTD) n-type GaAs has been measured in the variable range hopping regime at temperatures, T = 0.03 to 1.6 K. The negative hopping magnetoresistance (NHM) in small magnetic fields is strongly influenced by at least two positive hopping magnetoresistance (PHM) contributions including the effects of spin alignment. Three new effects are reported: (i) The log R versus T -1/2 plots in fixed magnetic field show two well-pronounced linear parts with different slopes forming a kink at a certain temperature T c which shifts to higher temperatures as the applied magnetic field is increased, occurring at B e /T c h 9 T/K. (ii) At the lowest temperatures, we observed with decreasing temperatures a steep decrease and disappearance of the negative hopping magnetoresistance, followed by only (iii) a positive hopping magnetotesistance with two different slopes with log o(B)/o(0) α B 2 . We interpret this behavior as due to the interplay between the (coherent) quantum interference, with a negative sign and different incoherent mechanisms, including spin-effects, which dominate the magnetoresistance, at different temperatures.

Does a magnetic field suppress the Coulomb gap

We used electron-tunnelling spectroscopy to investigate the Coulomb correlation in n - type Germanium. The dopant concentration was smaller than the critical concentration for the metal-insulator (Anderson) transition. The tunnelling conductance, which probes the electronic density of states, was found to depend strongly on both voltage and temperature. At low temperatures it shows a conductance minimum at the Fermi energy as expected for the Coulomb correlation gap. Applying a magnetic field up to B = 4T at T = 0.1 K reduces the magnitude of the tunneling conductance, but does not significantly change the shape of the spectra. At higher fields, the conductance minimum disappears, suggesting a suppression of the Coulomb gap. This could be due to the field-induced confinement of the electron wave functions, that strongly reduces the overlap between the localized electron states.

Regimes of charge transport across semiconductor junctions

Abstract Charge transport across break junctions of n-doped Ge has been investigated. We have found several different regimes of transport. Although all junctions reflect the density of states of the samples, to extract the bulk density of states only few junctions in a rather restricted range of parameters turned out to be useful.

Esr of heavily neutron-transmutation-doped germanium

Integrated electron spin resonance is used to monitor the magnetic properties of the electron spin in a sample of NTD Ge using temperatures down to 40 mK and uniaxial stresses up to 0.36 GPa. The sample density is just below the critical density at ambient stress for the metal-non-metal transition. The electron spin susceptibility under stress shows no temperature variation; a small broadening as T is lowered is matched by a similar decrease of intensity. Further, <110> uniaxial stress enhances the intensity of the esr line. The contrast with Si∶P is discussed. A large ‘stress-tuning’ effect is inferred.

On the negative magnetoresistance puzzle at variable range hopping at low temperatures

In this work, we report the results of magnetoresistance studies of variable range hopping down to 30 mK in isotopically engineered Ge with low compensation, and in n-CdTe crystals. Experimentally we find a decrease and disappearance of the negative magnetoresistance with decreasing temperature down to 200 mK, in weak magnetic fields. Such behaviour is in disagreement with the quantum interference theory of Nguen, Spivak and Shklovskii.