Abstract and subjects
Homogeneous mixed-valence (MV) behaviour is one of the most intriguing phenomena of
f
-electron systems. Despite extensive efforts, a fundamental aspect which remains unsettled is the experimental determination of the limiting cases for which MV emerges. Here we address this question for SmB
6
, a prototypical MV system characterized by two nearly-degenerate Sm
2+
and Sm
3+
configurations. By combining angle-resolved photoemission spectroscopy (ARPES) and x-ray absorption spectroscopy (XAS), we track the evolution of the mean Sm valence,
v
S
m
, in the Sm
x
La
1−
x
B
6
series. Upon substitution of Sm ions with trivalent La, we observe a linear decrease of valence fluctuations to an almost complete suppression at
x
= 0.2, with
v
S
m
~ 2; surprisingly, by further reducing
x
, a re-entrant increase of
v
S
m
develops, approaching the value of
v
i
m
p
~ 2.35 in the dilute-impurity limit. Such behaviour departs from a monotonic evolution of
v
S
m
across the whole series, as well as from the expectation of its convergence to an integer value for
x
→ 0. Our ARPES and XAS results, complemented by a phenomenological model, demonstrate an unconventional evolution of the MV character in the Sm
x
La
1−
x
B
6
series, paving the way to further theoretical and experimental considerations on the concept of MV itself, and its influence on the macroscopic properties of rare-earth compounds in the dilute-to-intermediate impurity regime.
This study reveals a non-monotonic evolution of the mixed-valence character in the Sm
x
La
1−
x
B
6
series, with near-complete suppression of valence fluctuations in the intermediate substitution regime, followed by a re-emergent mixed-valence behavior in the dilute-impurity limit.