Superconducting junctions probed by scanning tunneling spectroscopy

Staff: Clemens Winkelmann, Hervé Courtois

Quantum nano-electronic effects in mesoscopic devices have so far been mostly probed by electron transport measurements. Spatially-resolved tunneling spectroscopies on such devices can however provide deeper insights to the physics at play, for example by correlating transport properties to the local electronic density of states. We present experiments on superconducting Josephson junctions performed in a new dilution-refrigerated combined STM-AFM setup operating at down to 70 mK and up to 2 T. This home-built scanning probe microscope has an excellent tunnel spectroscopic resolution (< 60 µeV) and low tip-sample vibrations (< 4 pm). Coarse displacement of the relative tip-sample position, with a µm-level control and over millimeters, enables us to reach a single object. We combine simultaneous AFM and STM inspection on a single sample. This allows us to study and correlate in situ properties accessible by one or the other tool only. Moreover, an individual electronic device can be located by AFM even if it sits on an insulating substrate, before being locally investigated through tunneling spectroscopy in the STM mode.

Spatially-resolved tunneling spectroscopy on a Josephson junction can allow for correlating the Josephson supercurrent to local properties such as the quasi-particle density of states and energy distribution. At a clean superconductor (S) - normal conductor (N) interface, the Andreev reflection creates electron pair that diffuse in N over a fraction of a µm at low temperatures. The related proximity superconductivity modifies locally the conductor’s electronic properties, including its local conductivity or its density of states. The figure shows a single, current-biased Al-Cu-Al Josephson junction on which we have performed local tunnel spectroscopies as a function of position, bias current and applied out-of-plane magnetic field. When increasing distance from S, the induced gap in the density of states in N decreases. The data further strikingly show the analogy between the roles played by bias current and applied magnetic field, both contributing on an equal footing to pair breaking. This behavior is in full agreement with the quasi-classical theory for inhomogenous superconductivity, described by the Usadel equations. These data are the first tunnel spectroscopies on an individually-biased quantum device [1] .

STM and STS on a superconducting junction

(a) Low temperature AFM image of an Al-Cu-Al proximity Josephson junction. (b) Local tunnel spectra at equilibrium as a function of position along the while line in (a). (c,d) Out-of-equilibrium local spectroscopies at the place indicated by a cross in (a), as a function of bias current (c) and the magnetic field (d). Transport measurements determined the critical current to be 4 µA. All data are taken at T < 100 mK.

[1] A subKelvin scanning probe microscope for the electronic spectroscopy of an individual nano-device, T. Quaglio, F. Dahlem, S. Martin, A. Gérardin, C.B. Winkelmann, and H. Courtois, Rev. Sci. Instrum. 83, 123702 (2012)

- Details on the AFM-STM technique

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