Coexisting Kondo hybridization and itinerant f-electron ferromagnetism in UGe2

Kondo hybridization in partially filled f-electron systems conveys a 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 second order transition T-c similar to 52 K and a crossover transition T-x similar to 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 T-c in accordance with our density functional theory + Gutzwiller calculations. As temperature is lowered below T-c, the resonance narrows and eventually splits below T-x dumping itinerant f-electron spectral weight right at the Fermi energy. Our findings suggest a Stoner mechanism forming the highly polarized ferromagnetic phase below T-x that itself sets the stage for the emergence of unconventional superconductivity at high pressure.

Automated summary

The Kondo hybridization in UGe2, a heavy fermion system, leads to the formation of two ferromagnetic phases accompanied by spin-triplet superconductivity. Scanning tunneling microscopy and spectroscopy reveal the presence of Kondo resonance and sharp 5f-electron states at high temperatures. The resonance narrows and splits, causing the itinerant f-electron spectral weight to disappear at the Fermi energy as the temperature decreases.