Design and Characterization of a millikelvin dual-tip Josephson STM

We describe the design and characterization of a dual-tip Josephson STM that operates at millikelvin temperatures. We report an effective noise temperature for the STM on the order of 200 mK.\footnote{A. Roychowdhury et. al., arXiv:1311.1855 (2013)} In addition to the expected phase diffusive super current in the ultra-small Nb-Nb junction formed by one tip and the sample,\footnote{M. Ivanchenko and L.A Zil'berman, Sov. Phys. JETP, 28, 1272 (1969)} our high resolution spectroscopy at mK temperatures reveals resonant coupling between the STM junction and the electromagnetic environment it is embedded in, as predicted by P(E) theory.\footnote{G. Ingold and H. Grabert, Phys. Rev. B., 50, 395 (1994)} We have for the first time, observed Shapiro-like steps in this limit by measuring the response of the P(E) supercurrent to microwave radiation as a function of amplitude. Fits to theory\footnote{G. Falci, V. Bubunja and G. Schon, Z. Phys. B., 85, 451 (1991)} indicate that the coupling of an ultra-small Josephson junction to its environment/circuit may be used to a) directly measure dissipation channels associated with circuit resonances and b) calibrate the frequency dependent microwave attenuation in cryogenic circuits as seen by the junction.