R. Rentzsch

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.

Structure and fundamental optical absorption of CdSe films

Abstract The absorption spectra at the fundamental absorption edge of as-deposited and recrystallized CdSe films have been measured at low temperatures. The large number of crystal imperfections in as-deposited films leads to the appearance of band tails which give rise to a flat fundamental edge whose steepness is a function of the substrate temperature. After recrystallization of the films in selenium vapour the absorption edge becomes steeper and intrinsic exciton lines are observed. Below a characteristic temperature the line form of the A n = 1 exciton, which is of Lorentzian type, does not depend on temperature. This is related to the dominant mechanism of line broadening, which is due to the interaction with charged impurities. Above the characteristic temperature the A n =1 exciton line can be fitted by a symmetrical Lorentzian with a linewidth that is determined by weak exciton-phonon coupling and intraband scattering.

Local distribution of high-conductivity regions in polyamide thin films

Conducting points with ohmic conduction are observed in polyamide thin films by the atomic force microscopy method. The correlation between the distribution of highly conducting points and the roughness of the polymerfilm relief is revealed. The conducting-channel density inside the polymer is shown to depend on the substrate material.

Electron correlation effects at variable-range hopping in doped GaAs, CdTe, Ge and Si

Abstract We report low-temperature electrical resistivity and magnetoresistivity measurements of doped semiconductors below the critical concentration of the metal-insulator transition. The series of samples were chemically doped (CdTe and Si) or were obtained by the neutron transmutation doping technique (GaAs and Ge). The resistivity was measured in the temperature range T = 0.03-4.2 K. All samples show, at the lowest temperatures, variable-range hopping resistivity p(T) = p o exp [(T o/T) s ] with s = ½. Comparison of the experimentally determined T o with the heoretical single-electron value of Efros and Shklovskii, T o = T ESO = 2.86e 2/k B a B k O, shows that multiple-electron transitions are dominant at all compensations except K ≥ 0.7, where a large scale potential relief exists. In this case the single-electron approach seems to be a good approximation.

Orbital and spin effects in the low-temperature behavior of the magnetoresistance of doped CdTe crystals

An observation of the suppression of negative magnetoresistance in samples of doped CdTe that are far from the metal-insulator transition as the temperature is lowered in the temperature range 3–0.4 K was previously reported [N. V. Agrinskaya, V. I. Kozub, and D. V. Shamshur, JETP 80, 1142 (1995)]. The results of an investigation of samples that are closer to the transition in the low-temperature region below 36 mK are presented. It is discovered that the samples investigated (which do not exhibit the suppression of negative magnetoresistance at comparatively high temperatures) display this effect at low temperatures and that, as previously, the suppression of the negative magnetoresistance correlates with the transition to conduction via Coulomb-gap states. A plateau-like magnetoresistance feature is displayed at low temperatures for the sample that is closest to the metal-insulator transition. The results obtained are analyzed within existing theoretical models that take into account the role of both the orbital and spin degrees of freedom. In particular, the low-temperature feature indicated is interpreted as a manifestation of positive magnetoresistance caused by spin effects. Nevertheless, it is shown within a detailed analysis supplemented by numerical calculations that the observed suppression of the negative magnetoresistance cannot be attributed only to the appearance of spin positive magnetoresistance. Moreover, the possibility of observing spin positive magnetoresistance is determined to a certain extent specifically by the suppression of the negative magnetoresistance competing with it.

Role of electron “lakes” in the negative magnetoresistance effect in the region of Mott hopping conductivity

It is shown for doped and compensated germanium that the appearance of negative magnetoresistance under the conditions of Mott hopping conductivity may be due to the presence of a nonuniform spatial distribution of the electron density, the temperature at which the effect appears apparently being determined by the temperature at which the electron gas condenses into electron “lakes.” A “dead zone” effect was also observed in weak magnetic fields, the threshold field increasing with the nonuniformity of the electron distribution.

Low-temperature conductivity and magnetoconductivity of neutron-transmutation-doped barely metallic GaAs in the vicinity of the metal-insulator transition

Measurements of the temperature dependence of conductivity and magnetoconductivity are presented of barely metallic n-GaAs bulk crystals doped by the neutron-transmutation doping technique resulting in medium-compensated samples of fixed compensation degree in the immediate vicinity of the metal-insulator transition at low temperatures. At n to nc the temperature dependence in tau phi (the phase-breaking time) disappears and therefore at low temperatures the temperature dependence of the conductivity is determined by the electron-electron interaction and necessarily Lint is the smallest relevant length leading to a dependence sigma (T)=bT13/. The experimentally determined density of states at the Fermi level is quite different whether n=nc is reached by an exactly controlled impurity concentration or by magnetic tuning with n=nc(B)>nc(0). This is interpreted by the influence of the magnetic field on the weak-localisation contribution and a change in the density of states at the Fermi level. When n=nc is reached by a controlled impurity concentration, the experimentally obtained density of states at the Fermi level is in good agreement with the value estimated from the impurity concentration and the energy spread of donor states in the gap.

Scaling analysis of the variable range hopping in p-Ge at high compensation

Isotopic mixtures of 70 Ge and 74 Ge (Isotopically Engineered: IE-Ge) were by a neutron-transmutation doping (NTD) process provided with a high compensation of K = N As /N Ga ≅ 0.4, 0.6, 0.9. Variable range hopping (vrh) resistivity at T = 0.1-4.2K shows Q(T) = Q 0 exp(T 0 /T) 1/2 . At K = 0.4-0.6 a small change of the critical concentration of the metal-insulator transition, N c and at K = 0.9 a very strong increase of up to N c = (1.2 ± 0.2) x 10 19 cm -3 was found. We calculated the value of the localization length a from measurements of the temperature- and low-field dependencies of the positive magnetoresistance, and by a combination of a with To = 2.8 e 2 /k B a4πe 0 κ the dielectric constant K is estimated. In accordance with the predictions of a disorder-induced scaling theory, the critical indices ν ≅ 1, ζ ≅ 2. and p ≅ 3 were obtained for a, K and To, respectively. At medium compensation, K < 0.6, the value of To is much smaller than T ES predicted by Efros and Shklovskii for one-electron-transitions, which is interpreted as being due to many-particle excitations which reduce the width of the Coulomb gap. Only at K = 0.9 and at very low impurity concentration far from the range of validity of the scaling relation many-particle excitations can be neglected, resulting in a strong increase of T 0 → T ES ≅ 366 K.

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.

Development of a Si bolometer for dark matter detection

Abstract Neutron transmutation doped (NTD) Ge irradiated 12 years ago, and hence with a low background radioactivity, was used to make thermometric sensors mounted on pure silicon targets. γ-ray detection with a sensitivity in the range of 1 keV was achieved in a 5.7 g target. An excess heat capacity of the detector in the range of 20–60 mK was measured in irradiation experiments. The role of adsorbed gases was studied.