From the University of Bengal, India –
We report the observation of an exceptionally large room-temperature electrical conductivity in silver and aluminum layers deposited on a lead zirconate titanate (PZT) substrate. The surface resistance of the silver-coated samples also shows a sharp change near 313 K. The results are strongly suggestive of a superconductive interfacial layer, and have been interpreted in the framework of Bose-Einstein condensation of bipolarons as the suggested mechanism for high-temperature superconductivity in cuprates.
The samples used for current-voltage measurements were (i) thin strips 2 cm × 2 mm cut off from commercial PZT discs (supplied by Central Electronics Ltd., New Delhi, India) 0.3 mm thick and with an average grain size 1 μm which were supplied in the poled state and with 0.1 mm silver coating on both faces and (ii) the same type of strips with the original silver coating removed and 4000A aluminum deposited by vacuum evaporation. The Curie temperature of the material was 3600C as specified in the manufacturer’s data sheet.
Measurements were carried out at room temperature using a four-probe arrangement with the sample placed inside a double permalloy magnetic shield, the residual magnetic field inside the enclosure being less than 10^−5 tesla. The output voltage, which was of the order of microvolts, was measured using a home-built instrumentation amplifier based on an Analog Devices AD620 chip. Data were recorded in an Agilent 54622A digital storage oscilloscope by using a sawtooth current excitation at a frequency of 20 Hz from a function generator. It was found that scanning near this rate yielded the most consistent and reproducible data, least affected by fluctuations and noise.
The experimental results reported here strongly suggest the presence of a superconducting layer near room temperature in the interface between a metal film and a PZT substrate. The data have been interpreted in the framework of the above model in terms of the experimentally-observed inhomogeneous charge patterns in high-temperature superconductors.
The fact that the results described above have been obtained from very simply-fabricated systems, without the use of any sophisticated set-up and any special attention being given to crystal purity, atomic perfection, lattice matching, etc. suggests that the physical process is a universal one, involving only an interface between a metal and an insulator with a large low-frequency dielectric constant. We note in passing that PZT and the cuprates have similar (perovskite or perovskite-based) crystal structures. This resemblance may provide an added insight into the basic mechanism of high-temperature superconductivity.
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