We present a scanning tunneling microscope ͑STM͒ designed to operate between 275 mK and room temperature, in magnetic fields up to 14 T and in ultrahigh vacuum ͑UHV͒. The system features a compact STM connected to an UHV compatible 3 He refrigerator fitting into a bottom loading cryostat with a superconducting magnet. In this configuration, the cryostat is sitting on top of the UHV chamber, resulting in a very short distance between the STM access and the experimental position. It further enables proper thermal anchoring of the entire STM setup, allowing millikelvin temperatures to be reached in true UHV conditions. We achieve a hold time of about 40 h at 275 mK and a turnaround time of 10 h between room and base temperature. We demonstrate atomic resolution and present tunneling spectra obtained at 275 mK on the high-T c superconductors Bi 2 Sr 2 CaCu 2 O 8ϩ␦ and YBa 2 Cu 3 O 7Ϫ␦ .
The results of an experimental study using torsional oscillators of the superfluidity of 4He films adsorbed on hydrogen plated graphite are reported. The evolution of superfluidity with the growth of the film shows considerable structure arising from the atomic layering of the film. There is evidence that the superfluidity of a single fluid layer is strongly suppressed, possibly due to the influence of the periodic potential arising from the underlying solid layer. The behavior of two fluid layers is quite distinct, but shows similarities to that of thicker films on heterogeneous substrates; we suggest that the usual theory of the superfluidity of two-dimensional 4He should be extended to account for superfluid onset temperatures in such a film.
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