Nicolás Lorente, Centro de Física de Materiales (CSIC-EHU)

Jeudi 5.Novembre 2020 à 14h

Kondo physics in the absence of a degenerate impurity ground state

The Kondo effect is a consequence of the electron correlations induced by an impurity with a ground state that is degenerate. The zero-energy “spin-flip” of the electron gas induced by the impurity leads to a many-body state with properties different from an uncorrelated electron gas. As the degeneracy is lifted, the Kondo effect weakens and eventually disappears. Kondo physics does not present abrupt transitions but rather soft evolutions or crossovers between different regimes. As a consequence, it is possible to retrieve Kondo features even when the conditions for Kondo physics are not met any longer.

Manganese phathalocyanines are S=3/2 magnetic molecules that present orbital and spin degeneracies. When adsorbed on a metal surface, these properties together can give yield to a sophisticated SU (4) Kondo effect. However, Jahn-Teller distortions are also present, slightly lifting the orbital degeneracy and reducing the symmetry of the Kondo effect. The usual SU (2) Kondo effect is found. However, the orbital effect is still there. This is revealed by an electronic excitation that takes place near zero bias. The orbital character of the excitation leads to a specific excitation location on the molecule as we have recently revealed [1].

Nickelocene molecules are also magnetic, but their ground state is S=1 and the Kramers theorem does not apply. The spin degeneracy is lifted and no Kondo effect is detectable and instead spin-flip excitations are strong signals in the experimental spectra [2]. As in the case of magnanese phthalocyanines, molecular vibrations are ubiquitous in this system too. In certain circumstances, a vibration couples with tunneling electrons and a vibration can be detected in the spectra. However the spin-flip excitation is so strong that a combined vibration-spin excitation dominates the tunneling spectra. These excitations can only be described using the same type of theory as for the Kondo effect [3].

Finally, even in the case of a pure S=1/2 cobaltocene molecule, the Kondo spectra becomes strongly modified by the presence of molecular vibrations. We show that the Kondo effect can only be described if molecular vibrations are included [4].

References :

[1] Jens Kügel et al. Phys. Rev. Lett. 121, 226402 (2018)

[2] Benjamin Verlhac et al. Science 366, 623 (2019)

[3] Nicolas Bachelier et al. Nature Comm. 11, 1619 (2020)

[4] Léo Garnier et al. Submitted.

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