"Development of a piezo-driven stage for scanning Andreev re?ection spectroscopy"
Development of a piezo-driven stage for scanning Andreev re?ection spectroscopy
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Point contact Andreev reection spectroscopy is employed to determine relevant properties of superconductors like the superconducting energy gap, the Bardeen- Cooper-Schrieer (BCS) coherence length and the Fermi energy. An established method of implementing these measurements is to generate a point contact on a specic point on the surface of the superconductor using a sharp tip and recording the I-V curve of the point contact. For these measurements it is necessary to cool the point contact, so that the superconductor enters the superconducting state. A temperature of 2K is suciently low to investigate conventional BCS supercon- ductors like NbN, NbTiN or NbSe2. Furthermore, it is necessary to be able to apply an external magnetic eld as high as the critical eld strength of the super- conductor to the point contact. To perform measurements at dierent location on the sample i.e. to scan the sample, a piezoelectric positioning system, working at a temperature of 2K and a magnetic eld up to 6T, is needed, however such a scanning point contact Andreev reection spectroscopy setup is not commercially available. For sample positioning, the exact behavior of commercial piezo actu- ators under these conditions is unknown which is why a measurement setup to characterise a piezoelectric actuator is presented in this thesis. To measure the expansion of the actuator, which is located inside a Janis 7T SVT cryostat, an interferometer is employed while the actuator is controlled by a piezo amplier. With this setup, it is possible to accurately quantify the maximum expansion, as well as the hysteresis of a piezoelectric actuator at a temperature of 2K and magnetic elds up to 6T. In this thesis, a piezoelectric actuator with a maximum expansion of 30 m at room temperature is characterised. At 2K, the actuators maximum expansion is 1.2 m and the width of the hysteresis loop amounts to (9.1 3.3)nm. In the nal point contact Andreev reection spectroscopy setup,the low hysteresis at 2K makes it possible to position the sample over the range of 1.2 m without a closed control loop.