Global phase diagram of a spin-orbital Kondo impurity model and the suppression of Fermi-liquid scale

TL;DR

This paper investigates a spin-orbital Kondo impurity model relevant for Hund metals, revealing quantum critical points and a novel non-Fermi-liquid phase with universal power-law behaviors, using numerical and conformal field theory methods.

Contribution

It identifies quantum critical points and characterizes a new non-Fermi-liquid phase in the three-channel spin-orbital Kondo model, advancing understanding of Hund metal behavior.

Findings

Discovery of quantum critical points near which $T_{FL}$ vanishes.

Identification of a new non-Fermi-liquid phase with frustrated overscreening.

Universal power laws in dynamical susceptibilities explained by conformal field theory.

Abstract

Many correlated metallic materials are described by Landau Fermi-liquid theory at low energies, but for Hund metals the Fermi-liquid coherence scale $T_{\text{FL}}$ is found to be surprisingly small. In this Letter, we study the simplest impurity model relevant for Hund metals, the three-channel spin-orbital Kondo model, using the numerical renormalization group (NRG) method and compute its global phase diagram. In this framework, $T_{\text{FL}}$ becomes arbitrarily small close to two new quantum critical points (QCPs) which we identify by tuning the spin or spin-orbital Kondo couplings into the ferromagnetic regimes. We find quantum phase transitions to a singular Fermi-liquid or a novel non-Fermi-liquid phase. The new non-Fermi-liquid phase shows frustrated behavior involving alternating overscreenings in spin and orbital sectors, with universal power laws in the spin ($\omega^{-1/5}$), orbital ($\omega^{1/5}$) and spin-orbital ($\omega^1$) dynamical susceptibilities. These power laws, and the NRG eigenlevel spectra, can be fully understood using conformal field theory arguments, which also clarify the nature of the non-Fermi-liquid phase.