学 位 記 番 号 理工博 第

氏 名 伏屋

健

吾

所 属 理工学研究科 物理学専攻 学 位 の 種 類 博士（理学）

学 位 記 番 号 理工博 第

号 学位授与の日付 平成

年

月

日 課程・論文の別 学位規則第４条第

項該当

学 位 論 文 題 名

金属間化合物における新奇な量子秩序相（英文）

論 文 審 査 委 員 主査 教 授 青木 勇二

准教授 松田 達磨

委員 教 授 堀田 貴嗣

【論文の内容の要旨】

Studies on strongly correlated electron physics associated with f-electrons have mainly been made in intermetallic compounds including Ce and Yb ions, which have 4f¹-electron and 4f¹–hole configurations, respectively. Recently, researcher’s interest is shifting to an uninvestigated field of compounds with rare-earth ions possessing multiple f-electrons, i.e., Pr-, Eu-, and Sm. Typical examples are the unusual magnetic-field-insensitive heavy-fermion behavior in SmOs4Sb12, the magnetic-field-insensitive phase transitions and significantly enhanced Sommerfeld coefficients in SmT2Al20 (T: Ti, V, Cr, and Ta), and the metal-insulator (MI) transition under pressure in SmX (X:

S, Se, and Te).

In this study, we focus on two Sm-based intermetallic compounds SmRu4P12 and SmPt2Si2. We have succeeded in growing single crystals and have revealed novel quantum ordered phases by investigations of the magnetic, thermodynamic and electrical transport properties.

1. Magnetic-field-induced charge ordering in the filled skutterudite SmRu4P12

From studies on polycrystalline samples, it was reported that filled skutterudite compound SmRu4P12 exhibits a MI transition at 16.5 K (=TI) and a possible phase boundary at ~ 12 K (=TII) in applied fields; hereafter, the phases are called the phase I (TII<T<TI) and phase II (T<TII). Although these phases are roughly known to be antiferromagnetically (AFM) ordered phases, the detailed features and mechanisms of these orderings have not been clarified yet. We have succeeded in growing single crystals of SmRu4P12

and have performed electrical transport, magnetization, and specific heat measurements to investigate the exotic phases.

X-ray diffraction measurements performed in SPring-8 revealed that Thomson scattering intensity increases in applied fields in phase I, providing a clear evidence for the realization of "field-induced charge ordering" in phase I. In this phase, magnetization shows characteristic nonlinear magnetic-field dependence, which reflects the development of the field-induced charge ordering.

In SmRu4P12, since the site symmetry of Sm³⁺ ions is a cubic Th, the J = 5/2 multiplet of Sm ions splits into two crystalline-electric-field (CEF) states, i.e., a Γ5 (Γ7) doublet and a Γ67 (Γ8) quartet (the corresponding Oh notation is shown in the parentheses). The electronic entropy obtained by specific heat measurements exceeds Rln4 at TI. This fact indicates that the two CEF states are not energetically well separated and both are involved in the magnetic ordering.

Specific heat measurements clarified that the phase boundary between I and II has remarkable anisotropy; a crossover-like anomaly appears for H //

[001] while a sharp peak structure indicating a first-order phase transition appears for H // [111].

At low temperatures in phase II, the magnetic contribution to the specific heat shows a pronounced Tⁿ behavior with n~6 below 6 K. This behavior indicates the existence of magnon excitations with an anomalous dispersion of E ∝ √𝑘𝑘, where E and represents the excitation energy and the wave vector.

This anomalous AFM magnon dispersion may be accounted for theoretically considering the characteristics of the AFM ordered phase in the Kondo

lattice.

The electrical resistivity increases below TI, indicating an opening a gap on the large area of the Fermi surface, and changes to a weak metallic behavior below 0.8 K after showing a broad maximum at 0.8 K. This finding demonstrates that low-density carriers (with equal numbers of electrons and holes because SmRu4P12 is a compensated metal) remain in phase II. The Sommerfeld coefficient of 14.5 mJ/K²mol observed in phase II is a quite large value considering that only small carrier pockets remain in the Brillouin zone. This fact indicates the possible formation of heavy quasiparticles on the carrier pockets.

2. Partial existence of magnetically-disordered Sm ions in an antiferromagnetic state of Ising magnet SmPt2Si2

RT2X2 (R: rare earth and actinide, T: transition metal, X: Si and Ge) is a typical series of compounds, in which a variety of strongly correlated electron behaviors appear. For Sm-based compounds, however, only a few reports on systematic investigations of the physical properties using single crystals have been made.

We succeeded in growing SmPt2Si2 single crystals and found that it has a tetragonal CaBe2Ge2-type crystal structure (P4/nmm, #129). Magnetization measurements revealed that SmPt2Si2 has a perfect Ising anisotropy and the CEF ground state is │ ±³₂>. At low temperatures, there appear two magnetically ordered phases; an AFM ordered phase (I) occurring at TI = 5.1 K and a field induced magnetization plateau phase (II) possessing a ↑↑↓

magnetic structure. From pulse magnetization measurements up to 32 T, it was found that the boundary of the phase II forms a dome structure ranging from 1 to 13 T in the H-vs-T phase diagram.

In phase I, a Curie term anomalously remains in the magnetic susceptibility, indicating the partial existence of magnetically-disordered (paramagnetic) Sm ions. The realization of such inhomogeneous magnetic structures is attributable to geometric frustrations inherent in the crystal structure of 122 compounds. This is in line with the fact that the similar

magnetic properties observed in UPd²Si² is well accounted for by a magnetic frustration model (the axial-next-nearest-neighbor Ising (ANNNI) model).

The disordered Sm ions can form a Kondo sublattice through hybridization with conduction electrons,resulting in a quasiparticle mass enhancement.

This may account for the largely increased electronic specific heat coefficient of 350 mJ/K²mol observed in phase I. SmPt²Si² should provide an appropriate system in which one can study the ground-state properties of a

“partial Kondo sublattice” immersed in a magnetically ordered phase.

Both of the field-induced charge ordering found in SmRu4P12 and the heavy-fermion state formed possibly in partial Kondo sublattice in SmPt²Si² are unconventional types of strongly correlated electron states, which have not been observed in Ce- and Yb-based intermetallic compounds. The observed novel features of these quantum phases are expected to provide clues to unveil the unprecedented mechanisms of the Sm-based strongly correlated electron states.