Crystal electric field excitations in the quasicrystal approximant TbCd 6 studied by inelastic neutron scattering

Miyake

2016

J. Phys. Soc. Jpn.

To get insight into the mechanism of emergence of unconventional quantum criticality observed in quasicrystal Yb 15 Al 34 Au 51 , the approximant crystal Yb 14 Al 35 Au 51 is analyzed theoretically. By constructing a minimal model for the approximant crystal, the heavy quasiparticle band is shown to emerge near the Fermi level because of strong correlation of 4f electrons at Yb. We find that charge-transfer mode between 4f electron at Yb on the 3rd shell and 3p electron at Al on the 4th shell in Tsai-type cluster is considerably enhanced with almost flat momentum dependence.The mode-coupling theory shows that magnetic as well as valence susceptibility exhibits χ ∼ T −0.5 for zero-field limit and is expressed as a single scaling function of the ratio of temperature to magnetic field T/B over four decades even in the approximant crystal when some condition is satisfied by varying parameters, e.g., by applying pressure. The key origin is clarified to be due to the strong locality of the critical Yb-valence fluctuation and small Brillouin zone reflecting the large unit cell, giving rise to the extremely-small characteristic energy scale. This also gives a natural explanation for the quantum criticality in the quasicrystal corresponding to the infinite limit of the unit-cell size.Recent discovery of unconventional quantum critical phenomena in quasicrystal Yb 15 Al 34 Au 51 has attracted much attention.1, 2) The measured criticality such as magnetic susceptibility χ ∼ T −0.5 , NMR relaxation rate (T 1 T ) −1 ∼ T −0.5 , specific heat C e /T ∼ − log T , and resistivity ρ ∼ T is unconventional and quite similar to those observed in periodic crystals such as heavy-electron metals YbRh 2 Si 2 3) and β-YbAlB 4 , 4,5) which are well explained by quantum critical phenomena of Yb-valence fluctuations.6)The quasicrystal Yb 15 Al 34 Au 51 is constituted of concentric shell structures of Tsai-type cluster shown in Fig. 1. 1, 7) There also exists approximant crystal Yb 14 Al 35 Au 51 .7) The approximant crystal has periodic arrangement of the body-centered cubic (bcc) structure whose unit cell contains shell structures shown in Figs. 1(a)-1(e). Theoretical analysis of the Yb-AlAu cluster taking account of critical Yb-valence fluctuations has provided a natural explanation for robustness of quantum criticality in the quasicrystal measured under pressure and has pointed out a possibility that the same criticality appears even in the approximant crystal when pressure is applied. 8,9) On the other hand, a new theoretical framework for critical Yb-valence fluctuation under magnetic field has been developed recently.10) The theory has succeeded in explaining novel scaling discovered in β-YbAlB 4 where the magnetic susceptibility χ is expressed as a single scaling function of the ratio of temperature to magnetic field T/B over four decades. 5)Surprisingly, recent measurement of magnetic susceptibility in the quasicrystal Yb 15 Al 34 Au 51 has revealed that the same T/B-scaling behavior appears over six decades of T/B.11) This strongly s...

Section: Introductionmentioning

confidence: 96%

Origin of Quantum Criticality in Yb–Al–Au Approximant Crystal and Quasicrystal

Miyake

2016

J. Phys. Soc. Jpn.

“…In the QC, it has been an open question whether the magnetic long range order is realized 4 . Thus far, in the rare-earth based approximant crystal (AC) retaining the periodicity as well as the local atomic configuration common to the QC, the magnetic long-range orders have been observed in R (R = Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm) 5 – 8 and Au–SM–R (SM = Si, Ge, Sn, and Al, and R = Gd, Tb, Dy, and Ho) 9 , 10 . Most of these measurements were performed by using the bulk probe such as susceptibility and the detailed magnetic structures remain unresolved.…”

mentioning

confidence: 99%

“…In contrast, in the rare-earth based QC and AC, understanding about the CEF is very limited because the local symmetry around the rare-earth site is completely different from the crystallographic point group, e.g., fivefold symmetry allowed in the QC and AC is forbidden in the periodic crystals. Thus far, the CEF in the rare-earth based ACs was analyzed experimentally 8 , 11 , 30 . Recently, the full CEF Hamiltonian in the rare-earth based QC and AC has been formulated theoretically 40 .…”

mentioning

confidence: 99%

Magnetism and topology in Tb-based icosahedral quasicrystal

2021

Sci Rep

Quasicrystal (QC) possesses a unique lattice structure with rotational symmetry forbidden in conventional crystals. The electric property is far from complete understanding and it has been a long-standing issue whether the magnetic long-range order is realized in the QC. The main difficulty was lack of microscopic theory to analyze the effect of the crystalline electric field (CEF) at the rare-earth atom in QCs. Here we show the full microscopic analysis of the CEF in Tb-based QCs. We find that magnetic anisotropy arising from the CEF plays a key role in realizing unique magnetic textures on the icosahedron whose vertices Tb atoms are located at. Our analysis of the minimal model based on the magnetic anisotropy suggests that the long-range order of the hedgehog characterized by the topological charge of one is stabilized in the Tb-based QC. We also find that the whirling-moment state is characterized by unusually large topological charge of three. The magnetic textures as well as the topological states are shown to be switched by controlling compositions of the non-rare-earth elements in the ternary compounds. Our model is useful to understand the magnetism as well as the topological property in the rare-earth-based QCs and approximant crystals.

“…Which effect is predominant is material dependent. As for TbCd 6 , it was reported that in the point charge model the CEF parameter B 20 is essentially enough to explain the specific-heat [34] and neutronscattering measurements [35]. In this case, 4f level has r −3 dependence.…”

Section: Resultsmentioning

confidence: 90%

Quantum Valence Criticality in Heavy Fermions on Periodic and Aperiodic Crystals

Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2019)

Miyake

2020

Progress of theories and experiments of the quantum valence criticality is overviewed focusing on recent discoveries of direct evidence of quantum critical point of valence transitions in periodic crystal α-YbAl 1−x Fe x B 4 at x = 0.014 and quasicrystal Yb 15 (Al 1−x Ga x ) 34 (Au 1−y Cu y ) 51 at x = y = 0. The common quantum criticality as well as T/B scaling behavior in the magnetization observed in the periodic crystal β-YbAlB 4 and quasicrystal Yb 15 Al 34 Au 51 is shown to be explained from the theory of critical Yb-valence fluctuations starting from the Hamiltonians for describing the lowenergy electronic states in both systems.