Ultrafast photoinduced valence transition in the heavy-fermion compound YbInCu₄

Upon cooling to a temperature below 42K, the heavy-fermion compound YbInCu₄ undergoes an isostructural transition from a semimetallic, paramagnetic phase with an Yb valence of +3 to a metallic, non-magnetic phase with an Yb valence of +2.85. Here, we combine HHG and FEL-based time-resolved photoemission spectroscopy techniques to gain novel insights into the valence transition in the (sub-)surface region. Through the combination of time-resolved ARPES and time-resolved XPS, we are able to directly track different characteristic spectroscopic signatures of the photoinduced valence change in the (transient) valence and core electronic structure on the femtosecond to picosecond timescale. Our results constitute the first direct experimental observation of an ultrafast photoinduced valence change in a heavy-fermion compound.