Electrons at the border of localization generate exotic states of matter across all classes of strongly correlated electron materials and many other quantum materials with emergent functionality. Heavy electron metals are a model example, in which magnetic interactions arise from the opposing limits of localized and itinerant electrons. This remarkable duality is intimately related to the emergence of a plethora of novel quantum matter states such as unconventional superconductivity, electronic-nematic states, hidden order and most recently topological states of matter such as topological Kondo insulators and Kondo semimetals and putative chiral superconductors. The outstanding challenge is that the archetypal Kondo lattice model that captures the underlying electronic dichotomy is notoriously difficult to solve for real materials. Here, I will present a microscopic model for the heavy-fermion antiferromagnet CeIn₃. As pointed out in this seminar, we succeeded – for the first time – to design an ab-initio theory with quantitative power. The underlying multi-orbital periodic Anderson model of CeIn₃ embedded with input from ab-initio band structure calculations was perturbatively reduced to a simple Kondo-Heisenberg model, which captures the magnetic interactions quantitatively. This tractable Hamiltonian was validated via high-resolution neutron spectroscopy that reproduces accurately the magnetic soft modes in CeIn₃, which are believed to mediate unconventional superconductivity. The presented study paves the way for a quantitative understanding of metallic quantum states such as unconventional superconductivity, not only in heavy-fermion materials, but in all kinds of strongly correlated materials.

cf. W. Simeth, Z. Wang, E. A. Ghioldi, D. M Fobes, A. Podlesnyak, N. H. Sung, E. D. Bauer, J. Lass, S. Flury, J. Vonka, D. G. Mazzone, C. Niedermayer, Y. Nomura, R. Arita, C. D. Batista, P. Ronning & M. Janoschek in Nature Communications 14, 8239 (2023)

A major discovery in condensed matter physics is the discovery of High-Temperature Superconductivity in cuprates (copper oxides), which still hold the record for the highest critical temperature at ambient pressure. They feature layers of CuO₂ planes, believed to be responsible for their electronic properties, involving strong electronic correlations. Doping gives rise to a very complex phase diagram, going from an antiferromagnetic insulating phase to a pseudo-gap, superconductivity, and a strange metal phase, along with coexisting and/or competing orders of charge and spin. However, a comprehensive theoretical explanation of the Cooper pairing mechanism in these materials is still a debated subject, in which magnetic excitation (paramagnons) are promising candidates. The hole-doped Ca₂CuO₂Cl₂ copper oxychloride serves as an excellent compound to investigate all these phases on common ground. Its stable and simple I4/mmm 1-layer structure and strong 2D character make it very suitable for theoretical calculations, allowing direct comparison with experimental work.

In this talk I will discuss the magnetic excitation measured by Resonant Inelastic X-ray scattering (RIXS) up to the optimal doping. The paramagnon exhibits a similar dispersion with doping, along the (h,0) direction, similar to all cuprates, and a softening along the (h,h) direction, as also measured in other cuprates. Along the (h,h) direction, the bimagnon weakens in the underdoped phase, while a charge continuum seems to arise at higher doping. Raman spectroscopy confirm that the bimagnon become weaker with doping. The paramagnon band-with have the same energy as a waterfall feature in the electronic bands, as measured in Angle resolved Photo-Emission Spectroscopy (ARPES), suggesting a link between the two phenomena. This is indeed supported by cluster-DMFT calculations, which suggest a spin-polaron band emerge at such energy scale.