Design and Applications of Sub-Kelvin Tunnelling Microscopy

Doctoral thesis, 1995

The main part of the work presented in this thesis deals with the extension of tunnelling microscopy to the temperature interval below one kelvin. Several microscopes have been designed and manufactured, one for ultra high vacuum at room temperature and two for sub-kelvin cooling. The latest millikelvin instrument is arranged for immersion into the coolant to secure the best possible heat transfer from substrate and probe. A mechanical coarse adjustment arrangement manoeuvrable from outside the cryostat was constructed to circumvent the difficulties associated with refrigerated step-by-step mechanisms that have been experienced by other designers. A dilution refrigerator insert for an older cryostat was designed with special care to ensure mechanical stability and the easiest possible maintenance. The mixing chamber, where the actual cooling takes place, was designed to accommodate the immersion microscope. To minimise the vibration level at the instrument site the entire cryostat was mounted on a pneumatic suspension system; the calculations for this arrangement is given in an appendix. To isolate the cryostat from vibrations travelling along the pumping lines, a new type of bellows based flexible link was designed. This component, the "saddle tube compensator " is described together with the necessary formulae. In addition to instrument characterisation measurements, two specific problems are studied. First the problem of mapping penetrating magnetic flux through technical type II superconductors were studied. The surface contamination problem was partially overcome by applying a thin protective gold overlayer immediately after the surface was cleaned. It proved possible to monitor proximity induced superconductivity through this top layer. Results not forming a regular lattice but with a clear field dependence are presented from a lead bismuth alloy film sample. In another experiment one example of a mesoscopic phenomenon is studied, the conductance variations in a small section of a semi-metal thin film due to fluctuations in scattering centra. It is shown that such centra can be affected in a predictable way by scanning the microscope probe above the surface. A special, self aligning technique was developed to fabricate a mesoscopic object at the accessible part of the sample.