Mixed-valence state in the dilute-impurity regime of La-substituted SmB$_6

• Main Authors: Zonno, Marta, Michiardi, Matteo, Boschini, Fabio, Levy, Giorgio, Volkaert, Klara, Curcio, Davide, Bianchi, Marco, Rosa, Priscila F. S, Fisk, Zachary, Hofmann, Philip, Elfimov, Ilya S, Green, Robert J, Sawatzky, George A, Damascelli, Andrea
• Format: Journal Article
• Language: English
• Published: 11-09-2023
• Subjects: Physics - Materials Science; Physics - Strongly Correlated Electrons
• DOI: 10.48550/arxiv.2309.05706

Nature Communications 15, 7621 (2024) 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 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, $v_{Sm}$, 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$$\,$=$\,$0.2, with
$v_{Sm}$$\,$$\sim$$\,$2; surprisingly, by further reducing $x$, a re-entrant
increase of $v_{Sm}$ develops, approaching the value of
$v_{imp}$$\,$$\sim$$\,$2.35 in the dilute-impurity limit. Such observation
departs from a monotonic evolution of $v_{Sm}$ across the whole series, as well
as from the expectation of its convergence to an integer value for
$x$$\,$$\rightarrow$$\,$0. Our ARPES and XAS results, complemented by a
phenomenological model, demonstrate an unconventional evolution of the MV
character in the Sm$_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.