Electron and Cooper-pair transport across a single magnetic molecule explored with a scanning tunneling microscope

Abstract

A scanning tunneling microscope is used to explore the evolution of electron and Cooper-pair transport across single Mn-phthalocyanine molecules adsorbed on Pb(111) from tunneling to contact ranges. Normal-metal as well as superconducting tips give rise to a gradual transition of the Bardeen-Cooper-Schrieffer energy gap in the tunneling range into a zero-energy resonance close to and at contact. Supporting transport calculations show that in the normal-metal-superconductor junctions this resonance reflects the merging of in-gap Yu-Shiba-Rusinov states as well as the onset of Andreev reflection. For the superconductor-superconductor contacts, the zero-energy resonance is rationalized in terms of a finite Josephson current that is carried by phase-dependent Andreev and Yu-Shiba-Rusinov levels.

Quantum phase transitions and the role of impurity-substrate hybridization in Yu-Shiba-Rusinov states

“…Approaching the tip to an impurity with a YSR state induces an interaction between the tip and the impurity (e.g., attractive force 17,18 ), which manifests itself as a change in the binding energy of the YSR state. A similar behavior in agreement with this picture has been observed in a number of systems [19][20][21][22][23] . However, although this behavior has been qualitatively attributed to a decrease 21 as well as an increase 22 in impurity-substrate coupling, so far a clear quantitative connection to specific microscopic energy scales has experimentally not been confirmed.…”

“…We were able to identify on which side of the quantum phase transition the system is for all three examples. In addition, this method can easily be extended to other scenarios presented in the literature [19][20][21][22][23] .…”

“…and even driving them through the quantum phase transition, depending on their adsorption site 29,[31][32][33][34] as well as the tipsample distance [19][20][21][22][23] .…”

Local Probes of Graphene Lattice Dynamics

“…Contact is defined by the point of maximum attraction between tip and graphene, which signals a chemical bond and may readily be determined from I –Δ z characteristics (Figure 5b), as demonstrated previously for a variety of tips and surfaces. [ 159–183 ] For Δ z < z c , the exponential evolution of I differs from the one observed for Δ z > z c , and the transition between these two ranges may be abrupt or gradual depending on the contact site. [ 173 ] The tip–graphene hybridization obviously has a profound effect on the IET signal strength and will be elaborated below (Section 3.4).…”

Tuning the Coupling of an Individual Magnetic Impurity to a Superconductor: Quantum Phase Transition and Transport

“…At large tip-molecule distances, the force is attractive and the molecule is lifted from the surface. This modifies characteristic parameters in the junction and in particular reduces the exchange coupling [29][30][31]. (For a discussion of the influence of other parameters, see supplementary material [32].)…”