Scanning Josephson spectroscopy on the atomic scale

Abstract

The Josephson effect provides a direct method to probe the strength of the pairing interaction in superconductors. By measuring the phase fluctuating Josephson current between a superconducting tip of a scanning tunneling microscope (STM) and a BCS superconductor with isolated magnetic adatoms on its surface, we demonstrate that the spatial variation of the pairing order parameter can be characterized on the atomic scale. This system provides an example where the local pairing potential suppression is not directly reflected in the spectra measured via quasipartcile tunneling. Spectroscopy with such superconducting tips also show signatures of previously unex- A number of novel superconducting states of matter such as those appearing in disordered superconductors, heavy fermion materials, and high-T c superconductors have been predicted to have pairing order parameters that are spatially modulated on atomic length scales. These short range spatial modulations can occur due to different mechanisms such as the inhomogeneous material properties in disordered superconductors [1][2][3][4], a momentum dependent pairing interaction, such as the Fulde-Ferrell-Larkin-Ovchinnkov (FFLO) state proposed for heavy fermion materials [5][6][7], or the interplay between different forms of electronic ordering in the pair density waves proposed for high-T c cuprates [8][9][10]. Although spectroscopic mapping with a scanning tunneling microscope (STM) can provide evidence for variations in the local density of states (LDOS) through quasi-particle tunneling, such measurements probe the superconducting order parameter only indirectly. If the Josephson effect can be measured and mapped on the atomic scale, then it would allow for direct characterization of the local pairing order parameter and high-resolution studies of novel superconducting phases [11].This goal has motivated previous efforts in the use of superconducting tips in STM [12] and has led to the local observation of thermal phase fluctuating Josephson supercurrent close to the point contact regime [13][14][15]. Subsequent measurements have mapped the Josephson effect on the nanometer scale, applying this technique to vortices [16,17] and high-T c cuprates [18,19]. A major challenge in improving the resolution of these experiments has been satisfying the competing requirements of a high junction impedance necessary for imaging and a low junction impedance allowing for the strong tip-sample coupling necessary to observe the Josephson effect despite thermal fluctuations. Extending the Josephson STM measurements to millikelvin temperatures allows for mapping of the Cooper pair current at junction resistances that are compatible with atomic resolution imaging.In this letter, we use scanning Josephson spectroscopy to probe variations of the superconducting order parameter on the scale of a single atom. We map the strength of the phase fluctuating Josephson current between a superconducting Pb tip and a Pb(110) surface with a dilute concentration of magnetic impurities us...

Disorder robustness and protection of Majorana bound states in ferromagnetic chains on conventional superconductors

“…The latter can be determined straightforwardly by measuring the LDOS using scanning tunneling spectroscopy (STS), but also direct observations of local order parameter variations have recently been demonstrated using scanning Josephson spectroscopy. 55 If the LDOS show clear coherence peaks with non-dispersive energies, then well-protected MBS should be formed at the end points of a FM chain positioned well within the topological phase.…”

Visualising the Charge and Cooper-Pair Density Waves in Cuprates

“…The next advance in the state of the art will be to achieve intra unit cell resolution imaging capabilities on a cuprate superconductor. Such resolution has recently been achieved on the surface of lead, using a lead coated tungsten tip [145]. For cuprate SJTM however, our nanoflake tip approach is unlikely to ever yield intra unit cell resolution.…”

“…Firstly, it is known that at magnetic impurities in s-wave superconductors the superconducting order parameter, Ψ, is suppressed whereas the single-particle gap, ∆, is unperturbed [134][135][136][137]. Similarly, In the face of these adversities a handful of groups have initiated efforts to develop SJTM as a direct probe of the superconducting order parameter [139][140][141][142][143][144][145]. In our group we have focussed on an implementation of SJTM using high temperature d -wave superconducting tips to search for a pair density wave in cuprate superconductors [132].…”

“…Another facet of the ultra-small size of SJTM junctions is that they can have a junction capacitance, C J , as small as ∼1fF and a shunt resistance provided by the SIS single-particle tunnelling channel of ∼ 0.1 − 100MΩ [139][140][141][142][143][144][145]. On the face of it the small capacitance would favour over-damping while the large resistance favours underdamping.…”

Imaging the spatial form of a superconducting order parameter via Josephson scanning tunneling spectroscopy

“…As the oscillations of the superconducting order parameter are expected to occur on the length scale of a few lattice constants, and their detection hence requires near atomic resolution, recent experimental efforts have focused on the development of Josephson scanning tunneling spectroscopy (JSTS) [11][12][13][14]. The idea underlying JSTS is that the Josephson current, I J , [15] flowing between a superconducting JSTS tip and a superconductor probes the order parameter of the latter [16].…”