Regensburg 2025 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 6: Focus Session Many-Body Phenomena in Nanomagnets: Kondo, Spinons, Spinarons and Beyond (joint session O/TT)

TT 6.3: Invited Talk

Monday, March 17, 2025, 16:00–16:30, H24

Kondo or no Kondo, that is the question — •Alexander Weismann, Neda Noei, Niklas Ide, and Richard Berndt — Institut für experimentelle und angewandte Physik, Christian-Albrechts-Universität zu Kiel, Kiel, Germany

The spin properties of individual atoms and molecules can produce distinctive spectral features in tunneling spectra near zero bias. Among these features, Kondo resonances and inelastic spin-flip excitations are often challenging to distinguish, despite their markedly different spectral line shapes. A Kondo resonance indicates a non-magnetic ground state, where the atomic spin is screened by conduction band electrons. In contrast, spin-flip excitations observed in zero-field tunneling spectra require magnetic anisotropy, which arises from spin-orbit coupling (SOC), to play a significant role. In this study, we demonstrate that the well-known Co/Cu(111) system, long believed to exhibit a Kondo resonance, instead adopts a magnetic ground state that is protected from Kondo screening by substantial magnetic anisotropy. The zero-bias anomaly in scanning tunneling spectra undergoes significant modification when Co atoms are attached to monoatomic Cu chains. Measurements conducted at 340 mK in a magnetic vector field reveal clear signatures of inelastic spin-flip excitations, with the anisotropy axis tilted away from the surface normal. The magnitude and orientation of this anisotropy are consistent with density functional theory (DFT) calculations. Moreover, quantum Monte Carlo many-body simulations confirm that the Kondo effect is suppressed when SOC is properly accounted for.

Keywords: Kondo effect; Spin excitations; Scanning Tunneling Spectroscopy; Magnetic Anisotropy