  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.
