Following the Kondo temperature of a tunable single-molecule junction continuously from the Kondo singlet to the free spin 1/2 ground state

Martin Žonda (1,3), Oleksandr Stetsovych (2), Richard Korytár (3), Markus Ternes (4,5), Ruslan Temirov (5,6), Andrea Racanelli (5,6), F. Stefan Tautz (5,6), Pavel Jelínek (2,7), *Tomáš Novotný (3), Martin Švec (2,7)
(1) Institute of Physics, Albert Ludwig University of Freiburg, Germany, (2) Institute of Physics, Czech Academy of Sciences, Prague, Czech Republic, (3) Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic, (4) Institute of Physics II B, RWTH Aachen University, Germany, (5) Peter Grünberg Institut, Jülich, Germany, (6) Jülich Aachen Research Alliance (JARA), Jülich, Germany, (7) RCPTM, Palacký University, Olomouc, Czech Republic

We monitor the conductance evolution by varying the coupling between an organic molecule and the Ag(111) surface by lifting the molecule from the surface with the tip of a scanning tunneling microscope. Numerical renormalization group theory analysis of the junction conductance reveals that the system is tuned from the strongly coupled Kondo singlet to the free spin 1/2 ground state. In the crossover between the strong and weak coupling regimes, where the Kondo temperature TK is similar to the experimental Texp, fitting procedures generically provide ambiguous estimates of TK that tend to vary by an order of magnitude. Measuring the dependence of the junction conductance in external magnetic fields resolves this ambiguity if the Zeeman splitting due to the imposed magnetic field overcomes the thermal energy kBTexp. We demonstrate that Frota analysis overestimates TK by several orders of magnitude in the transient and weak coupling regimes. The numerical analysis also allows us to determine molecular coupling to surface and tip and their evolution during the lifting process.