We describe an experimental setup designed to study photoconductive transients in semi-insulating materials. The method, known as photoinduced transient spectroscopy, is based on a digital signal-averaging technique. Although this digital method eliminates or reduces many of the experimental problems encountered when using an analog approach, several new problems arise. The method is described, and the difficulties are illustrated using data obtained from both Cr-doped and nominally undoped samples of GaAs.
We present measurements of a dynamically induced instability in the vortex state of an amorphous multilayer and a single alloy layer of Ta͞Ge. The critical vortex velocity shows quantitative agreement with the predictions of Larkin and Ovchinnikov calculated using parameters determined independently from the normal state resistivity data. Both samples show a weak field dependence in the critical velocity implying a velocity dependent pinning force in the vortex solid state and a broadening of the transition resulting from the velocity distribution in the liquid state. [S0031-9007(97)03058-5] PACS numbers: 74.80. Dm, 74.25.Fy, 74.60.Ge Since the discovery of the high-T c cuprates much attention has been devoted to the study of vortex motion in layered type II superconductors [1]. In the limit of low applied current, several phases have been proposed such as the vortex glass [2][3][4][5][6][7] where the vortex solid is pinned by static disorder into a state with zero linear resistivity. However in the equally interesting high current limit the dynamics of the rapidly moving vortex system and its interaction with both static and thermal disorder is less well understood. This Letter demonstrates the existence of a fundamental instability in the vortex system of a Ta͞Ge multilayer and a single layer alloy which causes a sudden jump to the normal state as the driving current is increased beyond a critical value J ء . The instability is in quantitative agreement with a prediction by Larkin and Ovchinnikov (LO) [8] of a current induced instability previously seen in both low-T c [9,10] and high-T c [11,12] materials. The analysis of this instability is extended for the first time to distinguish between the regime of collective vortex motion in the vortex solid and plastic vortex motion in the vortex liquid. We also consider for the first time the effects of static disorder in the vortex solid state and infer an interesting form for the pinning force as a function of vortex velocity near the instability.The amorphous film and multilayer of Ta͞Ge were prepared by vapor deposition in a vacuum (base pressure ,10 29 torr) using high purity sources [13]. The multilayer sample was 1250 Å thick with 25 layer pairs consisting of 25 Å of Ge and 25 Å of Ta and the single layer alloy sample was 600 Å thick. Characterization of the multilayer was by Rutherford backscattering, transmission electron microscopy, and x-ray diffraction [14,15]. The composition of the alloy sample was determined by x-ray fluorescence and was chosen such that the highest T c (2.7 K) was obtained. The multilayer had a lower T c (1.7 K) determined by the coupling between the thin superconducting layers and the existence of an alloyed region at the layer interface with a higher T c than the individual Ta layers [14].dc electrical resistivity measurements were made using a four terminal method on paths of size approximately 5 mm by 1 mm scratched into the films. A temperature stability of better than 61 mK was achieved by immersing the sample directly in l...
An experimental arrangement is described by which emission coefficient behavior associated with deep-level traps in semi-insulating GaAs can be surveyed, from the photoconductive transient behavior following a trap-filling light pulse. This method, often referred to as PITS (for photoinduced transient spectroscopy) has analogies with the transient capacitance spectroscopy usable for conductive material, but some significant differences also. The necessary experimental precautions which can maximize the probability of getting reliable data are described, and illustrated by PITS results obtained with bulk semi-insulating samples of Cr-doped GaAs. Those results include data for five kinds of trap with activation energies from 0.3 to 0.8 eV, of which some are readily identifiable and others are not. The method described in this paper relies on analog measurement of the change in sample voltage during a specified time interval of the photoconductive decay.
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