1. Introduction The discovery of the iron pnictide superconductor in 2008[1] has opened a new route to high transition temperature (Tc) superconductivity. Comparing iron pnictides with high-Tc cuprates, we no-tice that they share common characteristics in terms of their "crystal structures" and "electronic phase dia-grams". Iron pnictide superconductors include two-dimensional (2D) FePn (Pn: pnictogen atom) layers separated by ionic layers as illustrated in the inset of Fig. 1. Their physical properties are thus highly two-dimensional, as in cuprate superconductors containing 2D-CuO2 planes. Concerning the latter, superconductivity in both systems emerges by suppressing magnetic orderings of the parent compounds. The electronic phase diagram of iron pnictide superconductors remarkably resembles those of cuprates as depicted in Fig. 1. The correlated metallic state neighboring a competing electronic ordering is believed to be a promising playground for unconventional superconductivity. The search for such superconductivity has been limited mostly within TM oxides or heavy fermion systems. The discovery of high-Tc superconductivity in iron pnictides might enlighten that transition-metal (TM) pnictides can be a new research field for the purpose. However, the exploration of new superconductors, triggered by the discovery of LaFeAs(O,F), has so far concentrated mostly on compounds including iron. We have therefore explored new superconductors in TM pnictides without iron.

2. Objective This study aims at exploring new unconventional superconductors in TM pnictides, from the viewpoints of "2D crystal structure" and "superconductivity in the vicinity of a competing phase". First, we have focused on TM pnictides with a ThCr2Si2-type structure (dubbed as 122 structure), which is iso-structural to iron-based superconductor (Ba, K)Fe2As2[3]. This structure is quite flexible, and a variety of TM pnictides have been reported to crystallize in this structure. We can investigate a systematic evolution of physical properties by changing TM elements within the same 122 pnictides. Next, we have searched for superconductivity in the vicinity of a competing ordered state. As a candidate for this purpose, we em-ployed RuPn where a novel metal-insulator transition (MIT) was discovered. We aim at realizing uncon-ventional superconductivity in RuPn by suppressing the electronic ordered state.

3. Superconductivity in Transition-Metal Pnictides with a ThCr2Si2-type crystal struc-ture BaRh2P2, BaIr2P2 and SrIr2As2 We discovered three new superconductors with a 122 structure, BaRh2P2, BaIr2P2 and SrIr2As2. The evidence for superconductivity can be found in the magnetizations and the resistivities at low temperatures, shown in Fig. 2(a), (b). Clear resistance drops to zero were observed, accompanied by a large diamagnetic signal indicative of superconductivity around 1.0 K, 2.1 K and 2.9 K for BaRh2P2, BaIr2P2 and SrIr2As2, respectively. In the electronic specific heat data shown in Fig. 2(c), clear jumps were seen at Tc, hallmarking the bulk superconductivity. This discovery demonstrates the presence of superconductivity over a broad range of transition metal pnictides with a 122 structure from Fe to Ir, ena-bling a systematic investigation of the superconducting properties by tuning the electronic structures.

From the electronic specific heat coefficients and Pauli paramagnetic susceptibilities, the Wilson ratio RW were estimated to be 0.98, 1.29 and 0.54 for BaRh2P2, BaIr2P2 and SrIr2As2, respectively. These values are close to or less than 1 expected for free electrons. This sharply contrasts with iron pnictide superconductors where an enhanced Wilson ratio of 11 for LaFeAsO was reported [4], possibly associated with the magnetic instability. The new 122 superconductors thus have totally different superconducting and also normal state properties from those of iron-based ones.

4. Superconductivity in the vicinity of competing ordered state in RuPn TM pnictides were found to be a rich reservoir for superconductivity as evidenced in the 122 compounds. The normal metallic states away from magnetic or other instabilities give rise to conventional superconductivity unlike iron pnictides. In order to realize exotic and also even higher-Tc superconductivity, we have searched for super-conductivity which manifests itself at the critical vicinity to a competing electronic ordering. We dis-covered a novel MIT in binary pnictides RuPn(Pn=P, As, Sb), and realized superconductivity by sup-pressing the ordering.

4-1. Metal-Insulator Transition in RuPn RuPn are known to crystallize in a MnP-type orthorhombic (Pnma) structure [5, 6], but no physical properties have been reported so far. The Ru atoms are octahedrally coordinated by pnictgen atom. RuPn6 octahedra are connected with sharing edges within bc-plane, wheres they have a face sharing network along a-axis. As a whole, this structure has a three-dimensional framework.

We discovered a MIT in RuP at 265 K, as shown in Fig.3(a). RuP displays a metallic behavior at high temperatures, while the resistivity markedly increases below the MIT temperature. Simultaneously, the magnetization shows a precipitous drop at the MIT temperature (Fig. 3(b)). The fact that the magnetization drops drastically down to negative values might suggest that paramagnetic moments are quenched into a spin-singlet insulator state. The spin-singlet formations can be found in three-dimensional system such as Tl2Ru2O7 [7]. Their spin singlet formations are attributed to the particular orbital orderings which give rise to anisotropic magnetic interactions. The orbital ordering of Ru 4d electrons might be also involved in RuP.

RuPn shows a systematic evolution in physical properties by changing Pn atoms. RuP exhibits the sharp-est transition, whereas the transition become broader in RuAs and eventually disappears in RuSb. As and Sb have more widely spreading p orbitals than P, leading to the enhanced hybridizations between Ru 4d and p orbitals of pnictogen atoms.

4-2. Superconductivity in Rh doped RuAs We attempted to achieve superconductivity by sup-pressing the novel ordering found in RuPn with carrier doping. In order to introduce electron carriers, we substituted Ru site with Rh. The Rh doping substantially affected the transition of RuAs. The transition temperatures determined by the local minimum of resistivities decreases systematically with increasing the Rh content as shown in Fig. 4(a). Beyond Rh content of 25%, the resistivity shows metallic temperature dependence without any upturns. As the low temperature ordering is removed by Rh doping, superconduc-tivity emerges at low temperatures. The superconducting transition is clearly observed for x=0.25 in the resistivity and heat capacity measurements (Fig. 4(b)). The superconductivity was also found in other dop-ing contents. The overall behavior of Rh doped RuAs is summarized in the electronic phase diagram shown in Fig. 4(c). The dome-like superconducting region in the phase diagram looks similar to those in uncon-ventional superconductors such as cuprates and iron pnictides. The superconductivity in Ru1-xRhxAs, neighboring the spin-singlet ground state might represent unique properties.

5. Summary and Perspective TM pnictides are newly developing research field for superconductivity triggered by the discovery of iron pnictide superconductors. In this study, we discovered new superconduc-tors focusing on the "2D crystal structures" and "superconductivity in the vicinity of a competing order".

First, the three new superconductors, BaRh2P2, BaIr2P2 and SrIr2As2 were discovered in the 122 structure. This discovery demonstrates the superconductivity prevails in the 122 pnictides. The difference of super-conducting properties between non-Fe and iron pnictide superconductors may imply the importance of a competing magnetism to achieve high-Tc. Second, we discovered a MIT in RuPn, likely associated with the spin-singlet formation. The ordered state in RuAs can be easily suppressed by Rh doping, and supercon-ductivity emerges. The electronic phase diagram is fairly similar to that of high-Tc superconductors, which might indicate the unconventional nature of the superconductivity. This study demonstrates that TM pnic-tides are a fertile playground for searching for new superconductors. The TM pnictides are not only an un-touched area, but posse unique properties distinct from oxides or alloys. The study in 122 pnictides shows that TM pnictides can form the same crystal structure in the wide range of TM elements. This enables a systematic evolution of physical properties by tuning electronic configurations. Furthermore, the study in RuPn indicates that the charge, spin and orbital characters of TM metals manifest themselves even in pnic-tides, as in the prominent TM oxides. The advantage in TM pnictides would be found in the fact that such collective ordered state can be easily suppressed and evolve into a metallic or even superconducting state by simple chemical substitutions. We believe that these characteristic features of TM pnictides provide us with a unique opportunity to explore further new and unconventional superconductors.

Figure 1 Electronic phase diagrams of iron pnictide superconductor[2] and high-Tc cuprate. The insets show crystal structures of representative superconductors such as BaFe2As2 and (La, Sr)2CuO4.

Figure 2 Superconducting transi-tions of BaRh2P2, BaIr2P2 and SrIr2As2, displaying (a)Meissner signals, (b)zero resistivities, and (c)electronic heat capacities.

Figure 3 (a), (b) Temperature de-pendences of resistivity and magnetic susceptibility for RuPn.

Figure 4 (a)Temperature dependences of resistivity normalized by the value at 350 K for Ru1-xRhxAs. (b)Superconducting transition of Ru0.75Rh0.25As, displaying a heat capacity jump and zero resistivity. (c)Electronic phase diagram as a function of Rh content x in Ru1-xRhxAs. .

本論文は、題目「New Transition Metal Pnictide Superconductors (新規遷移金属ニクタイド超伝導体)」に表現されるように、遷移金属ニクタイドにおいて、「二次元的結晶構造」・「秩序相との競合」というキーワードのもとに、新超伝導体を開発した研究である。論文は全六章からなる。

第一章では、研究の背景が述べられている。まず、2008年以降に発見された鉄を含む一連の超伝導体について物質開発の経緯と、その物理的・化学的特徴が紹介されている。鉄系超伝導体は、最高で50 Kをこえる超伝導転移温度Tcと、磁気相関に媒介されていると思われるエキゾチックな対形成機構という観点から大きな注目を集めている。これまでTcが50Kを超える唯一の超伝導体であった、銅酸化物高温超伝導体と新たに発見された鉄系超伝導体には共通する特徴がある。二つの高温超伝導体の共通点は、さらなる高温超伝導体探索の指針となり、本研究の動機につながっている。第一章の最後では、超伝導探索の舞台である遷移金属ニクタイドは、強相関電子系研究の中心である酸化物や、超伝導研究の中心であった合金化合物と異なる物理的、化学的性質を持ち、その中間というべきユニークな電子状態が期待されることが強調されている。遷移金属ニクタイドを超伝導探索の舞台として選択した点に本研究の特徴がある。

第二章では、研究の目的、超伝導探索の戦略が述べられている。鉄系超伝導体と銅酸化物高温超伝導体の共通点である、「二次元的結晶構造」と「電子的秩序相と競合する超伝導」に着目し、遷移金属ニクタイドにおける新超伝導の発見が研究の目的として掲げている。まず、「二次元的結晶構造」という観点での目的実現のために、鉄系超伝導体と同じ構造を有し、化合物のバリエーションが非常に豊富な、ThCr2Si2型構造を持つ化合物群に着目したことが述べられている。次に、「電子的秩序相と競合する超伝導」を達成する舞台として、これまでの研究から多彩な電子相の発現が予想される、TMPn型の物質群を選択したことが述べられている。

第三章では、「二次元的結晶構造」に着目し超伝導探索を行った結果として、ThCr2Si2型構造を持つ化合物におけるBaRh2P2, BaIr2P2, SrIr2As2の3つの超伝導の発見が述べられている。まず、抵抗測定・磁化率測定・比熱測定によりバルクの超伝導が確認されたことが説明されている。次に、BaRh2P2, BaIr2P2, SrIr2As2の三つの超伝導体と鉄系超伝導体の超伝導特性、磁気的性質の相違点が実験データに基づいて比較されている。発見された三つの超伝導体は、鉄系超伝導体に比べて、転移温度Tcが低く、磁気的な揺らぎが確認されない。鉄系超伝導体との相違点は、遷移金属が鉄からイリジウムやロジウムに変化することで、電子構造の次元性が変化することに起因していることを、第一原理計算の結果をもとに指摘している。

第四章では、RuP, RuAsにおいて金属-絶縁体転移を発見したことが述べられている。まず、磁化率と抵抗率の急激な変化から、RuPとRuAsにおける、Pauli常磁性金属状態から非磁性絶縁体状態への金属絶縁体転移が示されている。さらに、X線回折実験と電子線回折実験の結果より、RuP, RuAsにおいて2つの構造相転移が存在し、物性の変化と密接に関わっていることが指摘されている。RuSbはPauli常磁性金属であり、RuPからRuSbまでの系統的な物性の変化が、ニクタイドのp軌道の広がりに起因していると結論付けられている。また、RuP, RuAsの金属絶縁体転移の起源として、Fermi面付近のフラットバンドの電子不安定性が、第一原理計算の結果をもとに議論されている。

第五章では、RuAs, RuPに対する化学置換効果と、臨界点での超伝導の発現が述べられている。まず、RuAsに対するRh置換によるキャリアードーピングにより、低温の非磁性絶縁体相が抑制されたことが、抵抗率・磁化率測定をもとに説明されている。次に、低温の電子相が完全に抑制された臨界点において、超伝導が観測され、臨界点付近に限定された超伝導相が発現することが電子相図により示されている。RuPに関しても、同様にRh置換による非磁性絶縁体層の抑制と、臨界点付近のみ出現するの超伝導相の発現が述べられている。RuPとRuAsは実質的に同じ相転移と電子相図を有することが指摘されており、両者の秩序相の安定性とTcの比較から、秩序相の消失に伴う臨界性が超伝導発現に重要な役割を果たしていることが推測されている。また、鉄系超伝導体や銅酸化物高温超伝導体との類似性から、非従来型の超伝導発現機構の可能性が指摘されている。

第六章では、論文のまとめと今後の展望および研究の意義について述べられている。

なお、本論文は〓木英典、高山知弘との共同であるが、論文提出者が主体となって分析及び検証を行ったもので、論文提出者の寄与が十分あると判断する。

以上、本研究は鉄系超伝導体と銅酸化物高温超伝導体の共通点に着目し、遷移金属ニクタイドという化合物群に対し、固体化学・物性物理の視点に基づいて、新超伝導物質の探索を行った。実際に、新超伝導体を発見し、遷移金属ニクタイドが、新超伝導探索の舞台として有望な系であることを実証した。特に、RuP、RuAsにおける超伝導は、磁性とは異なる臨界点における超伝導という、これまでにないタイプの超伝導発現である。これらの結果は、超伝導物質科学、さらには物性物理、固体化学に貢献するところが大きい。したがって、博士(科学)の学位を授与できると認める。