Keiichiro Imura

Valence Change Driven by Constituent Element Substitution in the Mixed-Valence Quasicrystal and Approximant Au–Al–Yb

Quantum criticality has been considered to be specific to crystalline materials such as heavy fermions. Very recently, however, the Tsai-type quasicrystal Au51Al34Yb15 has been reported to show unusual quantum critical behavior. To obtain a deeper understanding of this new material, we have searched for other Tsai-type cluster materials. Here, we report that the metal alloys Au44Ga41Yb15 and Ag47Ga38Yb15 are members of the 1/1 approximant to the Tsai-type quasicrystal and that both possess no localized magnetic moment. We suggest that the Au-Al-Yb system is located near the border of the divalent and trivalent states of the Yb ion; we also discuss a possible origin of the disappearance of magnetism, associated with the valence change, by the substitution of the constituent elements.

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

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 conduc- tion 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.

Effect of nominal composition on transport, optical, magnetic, and thermodynamic properties of SmS single crystals

We have grown single crystals of SmS and investigated effects of nominal composition on the electrical resistivity, optical reflectivity, magnetoresistance, dc magnetization, and specific heat. We report that nominally stoichiometric and S-excess samples have an energy gap ( E g ) of about 950 (±100) K while Sm-excess samples possess E g of about 1500 (±100) K, and further that these values of E g correlate with the lattice constant. These findings were interpreted as the chemical pressure effect. As an external magnetic field is increased, E g monotonically decreases albeit in the nonmagnetic black phase, which can be an origin of reported negative magnetoresistance. The low-temperature specific heat under pressure shows the presence of an intrinsic γ-term in the golden phase in addition to a spurious γ-coefficient arising from secondary phases such as Sm 3 S 4 .

Pressure-Driven Quantum Criticality and T/H Scaling in the Icosahedral Au–Al–Yb Approximant

We report on ac magnetic susceptibility measurements under pressure of the Au–Al–Yb alloy, a crystalline approximant to the icosahedral quasicrystal that shows unconventional quantum criticality. In describing the susceptibility as χ(T)−1 − χ(0)−1 ∝ Tγ, we find that χ(0)−1 decreases with increasing pressure and vanishes to zero at the critical pressure \(P_{\text{c}} \simeq 2\) GPa, with γ (\( \simeq 0.5\)) unchanged. We suggest that this quantum criticality emerges owing to critical valence fluctuations. Above Pc, the approximant undergoes a magnetic transition at \(T \simeq 100\) mK. These results are contrasted with the fact that, in the quasicrystal, the quantum criticality is robust against the application of pressure. The applicability of the so-called T/H scaling to the approximant is also discussed.

Superconductivity of Au–Ge–Yb Approximants with Tsai-Type Clusters

We report the emergence of bulk superconductivity in Au64.0Ge22.0Yb14.0 and Au63.5Ge20.5Yb16.0 below 0.68 and 0.36 K, respectively. This is the first observation of superconductivity in Tsai-type crystalline approximants of quasicrystals. The Tsai-type cluster center is occupied by Au and Ge ions in the former approximant, and by an Yb ion in the latter. For magnetism, the latter system shows a larger magnetization than the former. To explain this observation, we propose a model that the cluster-center Yb ion is magnetic. The relationship between the magnetism and the superconductivity is also discussed.

Anticorrelation between polar lattice instability and superconductivity in the Weyl semimetal candidate MoTe2

The relation between the polar structural instability and superconductivity in a Weyl semimetal candidate

Localized Electron Magnetism in the Icosahedral Au–Al–Tm Quasicrystal and Crystalline Approximant

{\mathrm{MoTe}}_{2}

Pressure–Temperature Phase Diagram of Golden SmS

has been clarified by finely controlled physical and chemical pressure. The physical pressure as well as the chemical pressure, i.e., the Se substitution for Te, enhances the superconducting transition temperature

A new method for determining the valence of lanthanide compounds: Lγ4 emission spectroscopy

{T}_{\mathrm{c}}

Near-field spectroscopic investigation of dual-band heavy fermion metamaterials

at around the critical pressure where the polar structure transition disappears. From the heat capacity and thermopower measurements, we ascribe the significant enhancement of