We report on a sensitive and reliable ac technique to measure the Seebeck coefficient S of materials, in particular of high Tc superconductors. The small temperature difference between the ends of the sample allows structure in the S(T) curve to be observed. This technique avoids the creation of a component proportional to dS/dT in the measured signal. Such a component has in the past led to erroneous conclusions regarding the high-temperature superconductor YBCO. A mathematical analysis shows the origin of this component and how it can be avoided. These predictions are confirmed by experiments on a YBCO single crystal.

Epitaxial n-type GaAs samples were studied before and after irradiation with 7 MeV electrons at fluences varying from 1×1013 to 5×1015/cm2. The measurements involved the Hall effect and resistivity from 15 to 300 K. Fitting of the data revealed the evolution of the density and activation energy of impurity levels as a function of fluence. At the higher fluences, the carrier concentration and mobility decrease as expected but at the lower fluences, both of these quantities increase in a manner that depends on doping and fluence levels, and on temperature. The results are consistent with the radiation-stimulated gettering effect. It is shown that the maximum observed in the Hall coefficient as a function of temperature is possibly due to a manifestation of DX centers in the absence of applied pressure.

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