Concomitant singularities of Yb-valence and magnetism at a critical lattice parameter of icosahedral quasicrystals and approximants

Abstract

Non-Fermi-liquid (NFL), a significant deviation from Fermi-liquid theory, usually emerges near an order-disorder phase transition at absolute zero. Recently, a diverging susceptibility toward zero temperature was observed in a quasicrystal (QC). Since an electronic long-range ordering is normally absent in QCs, this anomalous behaviour should be a new type of NFL. Here we study high-resolution partial-fluorescence-yield x-ray absorption spectroscopy on Yb-based intermediate-valence icosahedral QCs and cubic approximant crystals (ACs), some of which are new materials, to unveil the mechanism of the NFL. We find that for both forms of QCs and ACs, there is a critical lattice parameter where Yb-valence and magnetism concomitantly exhibit singularities, suggesting a critical-valence-fluctuation-induced NFL. The present result provides an intriguing structure–property relationship of matter; size of a Tsai-type cluster (that is a common local structure to both forms) tunes the NFL whereas translational symmetry (that is present in ACs but absent in QCs) determines the nature of the NFL against the external/chemical pressure.

Crystalline Electronic Field in Rare-Earth Based Quasicrystal and Approximant: Analysis of Quantum Critical Au-Al-Yb Quasicrystal and Approximant

“…[3][4][5][6][7] Recently, a sharp change in the Yb valence has been observed in the QC (Au 1−y Cu y ) 51 (Al 1−x Ga x ) 34 Yb 15 at x = y = 0, which gives the direct evidence of the quantum valence criticality. 8) The QC Au 51 Al 34 Yb 15 consists of the Tsai-type cluster which has concentric shell structures shown in Fig. 1.…”

Interplay of Valence and Semimetal-to-Insulator Transitions in SmS

“…Quantum critical phenomena not following conventional magnetic criticality have attracted great interest in condensed matter physics. Recent discovery of the quantum critical point (QCP) of valence transition in the Yb-based periodic crystal 1) and quasicrystal 2) opens a new class of quantum criticality originating from Yb valence fluctuations. 3) So far, the valence QCP has been identified in the metallic Yb-based compounds.…”

“…We now formulate our model for SmS partly following the model for SmB 6 . 26,27) At each site, the f and d hole is described by an orbital and spin index, denoted by the combination ζ ≡ (a, σ) with (a = Γ (1) 8 , Γ (2) 8 , Γ 7 , σ = η = ±) for f states and (a = xy, yz, zx, σ = σ =↑, ↓) for d states. The fields are given by the twelve component spinor…”