Coexisting Kondo hybridization and itinerant f-electron ferromagnetism in UGe2

TL;DR

This study investigates the coexistence of Kondo hybridization and ferromagnetism in UGe2, revealing how electronic states evolve with temperature and how they relate to ferromagnetic phases and superconductivity.

Contribution

It provides direct experimental evidence of temperature-dependent electronic state changes in UGe2, combining STM spectroscopy with DFT+Gutzwiller calculations to elucidate ferromagnetism and Kondo effects.

Findings

Kondo resonance forms above Tc and narrows with decreasing temperature.

Splitting of f-electron states occurs below Tx, indicating ferromagnetic ordering.

Results support a Stoner mechanism for ferromagnetism in UGe2.

Abstract

Kondo hybridization in partially filled f-electron systems conveys significant amount of electronic states sharply near the Fermi energy leading to various instabilities from superconductivity to exotic electronic orders. UGe2 is a 5f heavy fermion system, where the Kondo hybridization is interrupted by the formation of two ferromagnetic phases below a 2nd order transition Tc ~ 52 K and a crossover transition Tx ~ 32 K. These two ferromagnetic phases are concomitantly related to a spin-triplet superconductivity that only emerges and persists inside the magnetically ordered phase at high pressure. The origin of the two ferromagnetic phases and how they form within a Kondo-lattice remain ambiguous. Using scanning tunneling microscopy and spectroscopy, we probe the spatial electronic states in the UGe2 as a function of temperature. We find a Kondo resonance and sharp 5f-electron states near the chemical potential that form at high temperatures above Tc in accordance with our density functional theory (DFT) + Gutzwiller calculations. As temperature is lowered below Tc, the resonance narrows and eventually splits below Tx dumping itinerant f-electron spectral weight right at the Fermi energy. Our findings suggest a Stoner mechanism forming the highly polarized ferromagnetic phase below Tx that itself sets the stage for the emergence of unconventional superconductivity at high pressure.