Unified understanding of the valence transition in the rare-earth monochalcogenides under pressure

Abstract

Valence instability is a key ingredient of the unusual properties of f electron materials, yet a clear understanding is lacking as it involves a complex interplay between f electrons and conduction states. Here we propose a unified picture of pressure-induced valence transition in Sm and Yb monochalcogenides, considered as model system for mixed valent 4f -electron materials. Using high-resolution x-ray absorption spectroscopy, we show that the valence transition is driven by the promotion of a 4f electron specifically into the lowest unoccupied (LU) 5d t2g band. We demonstrate with a promotional model that the nature of the transition at low pressures is intimately related to the density of states of the LU band, while at high pressures it is governed by the hybridization strength. These results set a new standard for the generic understanding of valence fluctuations in f -electron materials. PACS numbers: 71.20.Eh, 78.70.Ck, 75.20.Hr Valence fluctuations play an essential role in some of the f -electron systems most exciting behaviors, such as quantum criticality and unconventional superconductivity [1,2]. But their understanding has proven challenging to capture in a unified theory because they arise from subtle many-body interactions between the f electrons and the conduction states [3,4]. Pressure on the other hand is an efficient way to act on f -electron interactions and localization and thus can serve as a window onto the f -electron physics. Notably, the pressure-induced valence transition of the lanthanide monochalcogenides is a paradigmatic illustration of f delocalization phenomena and their tremendous diversity. For instance, SmS undergoes a first-order transition to near-trivalency coinciding with the onset of magnetic ordering [5], whereas the transition of YbS into an intermediate-valent correlated metal is extremely sluggish [6]. These diverse behaviors, while establishing a severe test bed for theoretical understanding of f -electron systems, have been overlooked for the past decades. Here, we address them in the light of a direct measurement of the electronic structure of Sm and Yb monochalcogenides (SmS, SmSe, SmTe, YbS, YbSe) under pressure performed using highresolution x-ray absorption spectroscopy in the partial fluorescence yield (PFY-XAS) mode. Our data reveal that the valence discontinuity and concomitant closing of the gap under pressure are caused by the specific filling of the lowest unoccupied (LU) 5d t 2g band by a 4f electron, offering a unified picture of electron delocalization and intermediate valency. Using a promotional model, we show that both the steepness and the amplitude of the valence transition of the Sm compounds increase with the density of states (DOS) of this LU band. When exceeding a critical DOS, the transition becomes first order for SmS. The model fails to describe the prolonged transition of the Yb compounds, which suggests that at high pressures the valence transition is impeded by enhanced hybridization. Ultimately, we suggest that our analysis can serve ...

Pressure-Driven Topological Phase Transition in the Yb Chalcogenides YbO and YbS

“…However, YbO is a semiconductor with a 0.32 eV band gap, and YbS is an insulator with a 1.40 eV band gap as determined by experiment. 29,30 While the MBJLDA calculation is crucial for the noninteracting topological insulators, 36 and describes accurately the correlation effects of the 5d states, it can't describe well those of the more localized 4 f states, whose treatment still requires an additional Hubbard U value. 37,38 The MBJLDA+U calculations with U of ∼ 3.0 eV render a band gap of ∼0.24 eV for YbO, while YbS needs a larger Hubbard U=7.0 eV to obtain a band gap of 1.26 eV.…”

“…The insulator-metal transition under high pressure is consistent with the experimental result. 29,30 The novelty of this work is that we also predict that this transition is a topological phase transition, i.e., the insulating and metallic phases have different band topologies. The mechanism for this topological phase transition is pressure-driven band inversion at the time-reversal invariant momentum (TRIM) point X between the 4f -dominating band and the 5d conduction band owing to the increasing bandwidth of 5d-derived states under high pressure.…”

“…1). Experimentally, both YbO and YbS are insulators with band gaps of 0.32 and 1.40 eV respectively, at ambient pressure, 29,30 and YbO is on the margin of a mixed valence state. 24 Our MBJLDA+U calculations predict that, at ambient pressure, both YbO and YbS are trivial insulators with band gaps of about 0.24 and 1.26 eV, respectively, and both of them will undergo a topological phase transition to a nontrivial topological metallic state under high pressure.…”

“…In particular, for the chalcogenides of Eu, Sm, and Yb, 25 a fluctuating valence state is accessible under ambient or high pressure. [26][27][28][29][30][31] These rare-earth compounds providing a high probability of finding more TKIs, although there are few TKIs predicted in rare-earth pnictides and chalcogenides with a rock-salt structure at present. 32 In this work, we employ first-principles calculation based on the density functional theory (DFT) with the modified Becke-Johnson local density approximation plus Hubbard U (MBJLDA+U) to study the band topology of the Yb monochalcogenides YbO and YbS with a rock-salt structure (shown in Fig.…”

Robust ferromagnetism in the compressed permanent magnetSm2Co17

“…The position of the white line suggests that the Sm ions in Sm 2 Co 17 have a valence close to 3 + . 50 With pressure, the intensity of the white line decreases. The pressure-induced reduction in the white-line intensity of lanthanide elements is understood to be a consequence of 6s−5d charge transfer that reduces the number of empty 5d states 51 .…”