Abstract. We used a quartz tuning fork vibrating at 30 kHz both as an acoustic near field microscope and at the same time as a microscopic Kelvin probe. One leg of the tuning fork carried a small gold electrode serving as a conducting vibrating tip. By using this instrument and the method described here it is possible to measure simultaneously both the surface topography of the sample surface and the contact potential between tip and sample. The topography is observed by operating the instrument as an acoustic near field microscope. The contact potential between the vibrating tip and the sample gives rise to a displacement current which is used here for the determination of the contact potential. In first applications of this method we demonstrate that the contact potential can be measured with a sensitivity of at least 100 mV and a local resolution of about 5 m. It seems possible to use the microscopic method described here also for investigating local potentials at low temperatures and even in high magnetic fields. For example, the microscopic study of the Hall voltages in the quantum Hall effect might be an interesting application.

Abstract, The influence of aging and temperature cycling effects on thin epitaxialYBa,Cu,O,-, films on (100) SrTiO, has been investigated over a period of 15 months using measurements of the spatial magnetlc field distribution. Two of the four investigated samples have an amorphous YBaCuO cover layer which appears to protect the samples against degrading.

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We report on a picosecond YBa2Cu3O7−δ detector for far-infrared radiation. The detector, consisting of a current carrying structure cooled to liquid-nitrogen temperature, was studied by use of ultrashort laser pulses from an optically pumped far-infrared laser in the frequency range from 25 to 215 cm−1. We found that the sensitivity (1 mV/W) was almost constant in this frequency range. We estimated a noise equivalent power of less than 5×10−7 W Hz−1/2. Taking into account the results of a mixing experiment (in the frequency range from 4 to 30 cm−1) we suggest that the response time of the detector was few picoseconds.

We report the observation of an ultrafast photoresponse of a high-T(c), film to far-infrared radiation pulses. The response of a sample, consisting of a current-carrying structured YBa(2)Cu(3)O(7-delta) film cooled to liquid-nitrogen temperature, was studied by use of ultrashort laser pulses from an optically pumped far-infrared laser in the frequency range from 0.7 to 7 THz. We found that the response time was limited by the time resolution, 120 ps, of our electronic registration equipment.