Interplay of charge and spin fluctuations in a Hund's coupled impurity

TL;DR

This paper provides an analytical understanding of how valence fluctuations influence spin and charge dynamics in Hund's metals, revealing singular spin fluctuations that may impact superconductivity models.

Contribution

It introduces a Hund's coupled mixed valence quantum impurity model that explains the suppression of large magnetic moments and the nature of spin fluctuations in Hund's metals.

Findings

Valence fluctuations suppress large magnetic moments over a wide temperature range.

Spin susceptibility exhibits a logarithmic divergence, indicating singular spin fluctuations.

Results suggest a potential role of these fluctuations in Hund's driven Cooper pairing.

Abstract

In Hund's metals, the local ferromagnetic interaction between orbitals leads to an emergence of complex electronic states with large and slowly fluctuating magnetic moments. Introducing the Hund's coupled mixed valence quantum impurity, we gain analytic insight into recent numerical renormalization group studies. We show that valence fluctuations drastically impede the development of a large fluctuating moment over a wide range of temperatures and energy, characterized by quenched orbital degrees of freedom and a singular logarithmic behavior of the spin susceptibility $\chi_{\rm sp}''(\omega) \propto [\omega \ln(\omega/T_K^{\rm eff})^2]^{-1}$, closely resembling power-law scaling $\chi_{\rm sp}''(\omega) \sim \omega^{-\gamma}$. Such singular spin fluctuations are suspected to play an important role in future models of Hund's driven Cooper pairing.