Nicolas Lorente

Affiliation: CFM – Materials Physics Center

Date: Feb 28th, 2023 (17:00 – 18:00, KST) (09:00 -10:00, CET)

Title: The Kondo effect as revealed by STM measurements

Abstract: 

The ground state of a metal is, to a great degree of accuracy, well described by one-electron states. However, as soon as there is a magnetic interaction that can change the spin of the electrons, the ground state becomes a very complex state. The reason for this is the development of a multielectronic state that cannot be separated in single states. Magnetic impurities are efficient at mixing electronic spins and the new emerging ground state is the hallmark of the Kondo effect. To this respect, the scanning tunneling microscope (STM) is an excellent tool to interrogate the electronic correlations induced by the magnetic impurities. It can locally study the magnetic impurities on metallic substrate and it can reveal the properties of the electronic states essential for the Kondo state [1]. In this Colloquium, I will review the main features of the Kondo effect and how they have been revealed by STM experiments. Moreover, I will analyze some recent results obtained in my group in collaboration with experimental colleagues. In the spirit of a Colloquium talk, the exposition will be pedagogical, emphasizing physical results over formal theoretical considerations.

The first case will be the study of Manganese phthalocyanines on different metallic substrates. Manganese phathalocyanines are S=3/2 magnetic molecules that present orbital and spin degeneracies. Here, the Kondo effect is efficiently mixed with orbital excitations [2]. The second topic will be about Nickelocene molecules that 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 [3]. In this case, the competing excitations are vibrations. The joint Kondo plus vibrational excitation reveal some astonishing features [4]. 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 [5].

References :
[1] D.-J. Choi and N. Lorente, Handbook of Materials Modeling: Applications: Current and Emerging Materials, p. 467, Springer International Publishing (2020).
[2] Jens Kügel et al. Phys. Rev. Lett. 121, 226402 (2018)
[3] Benjamin Verlhac et al. Science 366, 623 (2019)
[4] Nicolas Bachelier et al. Nature Comm. 11, 1619 (2020)
[5] Léo Garnier et al. Nano Letters 20, 8193 (2020).

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