Observation of Coexistence of Yu-Shiba-Rusinov States and Spin-Flip Excitations

Abstract

We investigate the spectral evolution in different metal phthalocyanine molecules on NbSe2 surface using scanning tunnelling microscopy (STM) as a function of the coupling with the substrate. For manganese phthalocyanine (MnPc), we demonstrate a smooth spectral crossover from Yu-Shiba-Rusinov (YSR) bound states to spin-flip excitations. This has not been observed previously and it is in contrast to simple theoretical expectations. We corroborate the experimental findings using numerical renormalization group calculations. Our results provide fundamental new insight on the behavior of atomic scale magnetic/SC hybrid systems, which is important, for example, for engineered topological superconductors and spin logic devices.

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.…”

“…For the following data analysis, we assume that the parameters that are more related to the intrinsic properties of the impurity E J and E U are constant as function of tip-sample distance, whereas the impurity-substrate coupling Γ s can vary. This is a sensible assumption of some generality, which has been used before in a somewhat different context for YSR states in molecules adsorbed on a superconducting surface 23,29 . However, we have to keep in mind that in a self-consistent treatment E J becomes a function of Γ s , which may lead to small corrections.…”

“…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] .…”

Impurity-induced resonant spinon zero modes in Dirac quantum spin liquids

“…We note that this equivalence holds only for S = 1/2 impurities, as higher S impurities would generate a free degree of freedom in each site even in the limit J J μν i j . We also note that given that the magnetic ad-atoms on top can be moved with a scanning tunnel microscope [59][60][61][62][63][64][65][66][67][68][69][70][71][72][73], this would allow us to engineer models with an arbitrary number of vacancies in the effective spinon model.…”

“…Other paradigmatic examples are carbon vacancies [36,[51][52][53][54][55][56] and hydrogen ad-atoms [57,58] in graphene, giving rise to a divergent density of states [36,52] and magnetism [37,53,58]. Along this line, recent experimental advances have demonstrated the possibility of single-atom manipulation in a variety of systems by means of scanning probe techniques [59][60][61][62][63][64][65][66][67][68][69][70][71][72][73]. This motivates the question of whether single-atom manipulation [74] can detect unique features of quantum spin-liquid states.…”