Perovskite-type 3d transition-metal (TM) oxides have been attracting great interest in these decades because of their intriguing physical properties such as metal-insulator transition, colossal magnetoresistance (CMR), and ordering of spin, charge, and orbitals [1]. Photoemission spectroscopy has greatly contributed to the understanding of the electronic structures of these materials. Since the local electronic structure of 3d TM oxides has become basically established according to the cluster-model analyses, it has now become the most important issue to observe how the band structure is formed from the local electronic structure. Therefore, the determination of band dispersions by angle-resolved photoemission spectroscopy (ARPES) is highly required. High-quality samples are an essential part of the photoemission studies of the perovskite-type 3d TM oxides especially for ARPES. However, bulk single crystals of these materials are sometimes difficult to grow, and even when single crystals are available, ARPES has rarely been performed because many of them do not have cleavage planes, which are required to perform ARPES measurements. Recently, high-quality single-crystal thin films grown by the pulsed laser deposition have become available for such materials, and a setup has been developed for their film growth followed by in-situ photoemission measurements [2].

　Thin films have a lot of advantages which bulk samples do not have. The first one is the availability of clean single-crystal surfaces. Owing to this advantage, we succeed in observing the detailed electronic structures of La1-xSrxFeO3 (LSFO), Nd1-xSrxMnO3 (NSMO), and Pr1-xCaxMnO3 (PCMO) by photoemission and x-ray absorption spectroscopy (XAS), and also in determining the electronic band dispersions by ARPES. The second advantage is the controllability of the electronic structures by the epitaxial strain effects from the substrates. One can control the electronic structures of these systems by growing their thin films on perovskite substrates with various lattice parameters, for example SrTiO3 (STO), (LaAlO3)(0.3)-(SrAl(0.5)Ta(0.5)O3)(0.7) (LSAT), and LaAlO3 (LAO) [3]. The third advantage is the possibility to study the novel physical properties at the interfaces of heterostructures. For example, at the interface of a band insulator STO and a Mott insulator LaTiO3 (LTO), metallic conductivity occurs due to the delocalization of Ti 3d electrons of LTO [4].

　In the present thesis, by utilizing the above advantages of epitaxial thin films, we could address several important and unresolved issues in the electronic properties of TM oxides and their heterostructures. In Chapter 4, we discuss the origin of the wide insulating region in LSFO. In Chapter 5, we investigate the strain effects on the electronic structures in R1-xAxMnO3 (R = La, Nd, Pr and A = Sr, Ca). In Chapter 6, we study the electronic reconstruction at the interface between a band insulator LAO and a Mott insulator LaVO3 (LVO) by combining extremely bulk-sensitive hard x-ray and relatively surface-sensitive soft x-ray photoemission spectroscopy.

In-situ photoemission study of La1-xSrxFeO3 epitaxial thin films

　LSFO has attracted much interest because it undergoes a pronounced charge disproportionation around x = 2/3. Another striking feature of LSFO is that the insulating phase is unusually wide in the phase diagram. We have performed an in-situ photoemission study of LSFO thin films grown on STO (001) substrates. From the valence-band photoemission and O 1s XAS studies, it has been found that the rigid-band model, in which the Fermi level (EF) is shifted according to the band filling, does not work in the near-EF region, that is, doped holes do not simply enter the top of the e(g↑) band but enter localized states split off from the top of the e(g↑) band. We have also measured the temperature dependence of the photoemission and XAS spectra, and observed gradual changes of the spectra with temperature not only for x = 0.67 but also for x = 0.2 and 0.4, suggesting that a local charge disproportionation occurs over a wider temperature and composition range. We have also determined its band structure by ARPES. Figure 1 shows the comparison of the ARPES spectra taken at the photon energy of 74 eV (a) and tight-binding (TB) band-structure calculation (b). By TB band-structure calculations, the experimental results have been successfully reproduced. However, in experiment there is a downward energy shift of about 1 eV compared with calculation, which we attribute to a polaronic effect. Thus, we conclude that the insulating behavior of LSFO is caused by the strong localization of doped holes by electron-phonon interaction and/or short-range charge ordering.

In-situ photoemission and x-ray absorption study of R1-xAxMnO3 epitaxial thin films

　Hole-doped perovskite manganese oxides R1-xAxMnO3, where R is a rare-earth atom and A is an alkaline-earth atom, have attracted much attention because of their remarkable physical properties such as CMR and the ordering of spin, charge, and orbitals. PCMO, where the band width is the smallest, has a particularly stable charge-ordered state in a wide hole concentration range. We have performed an in-situ photoemission study of PCMO thin films grown on LAO (001) substrates. The present thin films were with compressive strain from the LAO substrates, which suppresses charge ordering. Figure 2 shows the valence-band photoemission spectra near EF of PCMO thin films grown on LAO substrates. The line shapes were almost independent of x, and no new states appeared near EF with hole doping. From the present spectra near EF, we conclude that our PCMO thin films were complete insulators without any ferromagnetic fluctuations, in sharp contrast to the bulk photoemission results by Ebata et al. [5]. These results are considered as the spectroscopic evidence for the suppression of charge ordering due to the compressive strain effects from the LAO substrates. We have also investigated the orbital states of La1-xSrxMnO3 thin films grown on STO, LSAT, and LAO substrates by measuring linear dichrosim (LD) in XAS. The present results showed that the O 1s LD spectra clearly reflect the change of orbital states under strain from the substrates.

Photoemission study of LaAlO3/LaVO3 interfaces

　We investigated the electronic structure of superlattices consisting of a band insulator LAO and a Mott insulator LVO by combining hard x-ray and soft x-ray photoemission spectroscopy and observed how electrons behave if we confine electrons in the layers of Mott insulators. From the V 3d band photoemission spectra, a Mott-Hubbard gap of LVO has been found to remain open at the interface between LAO and LVO, indicating that this interface is insulating unlike the STO/LTO interfaces [4]. We have found that the valence of V in LVO was partially converted from V(3+) to V(4+) at the interface. We have constructed a model for the V valence distribution and succeeded in explaining the experimental results. We attribute this highly asymmetric valence change to the electronic reconstruction to eliminate polar catastrophe as shown in Fig. 3.

Fig. 1: Comparison of the ARPES spectra of La(0.6)Sr(0.4)FeO3 taken at 74 eV (a) and tight-binding calculation (b). (a)is the plot of second derivatives of the energy distribution curves, where dark parts correspond to energy bands.

Fig. 2: Valence-band photoemission spectra near EF of Pr1-xCaxMnO3 thin films grown on LaAlO3 substrates. Energy positions have been shifted by considering the chemical potential shift. The inset shows the result of bulk samples taken from Ref. [5].

Fig. 3: Electronic reconstruction to eliminate polar catastrophe in the case of LaAlO3/LaVO3/SrTiO3.

　層状ペロブスカイト構造をもつ3d遷移金属酸化物は金属-絶縁体転移、巨大磁気抵抗、スピン・電荷・軌道秩序などに代表される興味深い物性の宝庫として、固体物理学研究の中でも重要な位置を占めている。その中で、光電子分光実験はこの物質群の電子状態を解明する上で重要な役割を果たしている。特に、角度分解光電子分光(ARPES)はバンド構造を決定する上で欠くことのできない実験手法である。ARPES実験のためには高品位の単結晶試料が必要であるだけでなく、劈開性に富む物質でなければ再現性の良い結果を得ることは難しい。本研究では、新しく開発されたパルスレーザー蒸着法を駆使して、ペロブスカイト3d遷移金属酸化物の単結晶薄膜試料を作成し、それをin situで光電子分光測定することに成功した。これによって、劈開性のある単結晶育成が困難な物質に対しても光電子分光実験が可能になっただけでなく、様々な格子定数をもつ基板上にエピタキシャル成長させることで、歪みという新たなパラメータを導入して物性を制御できることも実証された。さらに本研究では、性質の異なる薄膜を組み合わせたヘテロ構造で新たな物性をもつ物質開発が可能であることも示された。

　本論文は全7章からなり、第1章では序論として研究背景が述べられたあと、第2章では光電子分光の一般原理、ARPESおよびX線吸収分光(XAS)測定の原理、さらに本研究で使用した実験装置の概略が述べられている。第3章では、バルクSrTiO3およびSrVO3薄膜を例題にしたARPESデータと強結合近似バンド計算(TB計算)の比較検討がなされている。第4章ではLa(1-x)SrxFeO3薄膜試料の光電子分光およびXAS測定の結果が示され、TB計算との定量的な比較を通じた金属-絶縁体転移のミクロな解明が試みられている。第5章ではLaAlO3(LAO)基板上に成長させたR(1-x)AxMnO3(R:希土類元素、A:アルカリ土類元素)薄膜の光電子分光実験の結果とその解釈が示されている。第6章ではLAOおよびLaVO3の超格子構造の光電子分光測定の結果とヘテロ界面での価数分布の考察がなされている。最後に第7章で論文全体のまとめと今後の研究の展望が述べられている。

　La(1-x)SrxFeO3はペロブスカイト3d遷移金属酸化物の中でもx〓2/3という例外的に高いホール濃度で金属-絶縁体転移を起こす物質として、その機構に興味が集まっていた。本研究では、単結晶薄膜試料を用意することで初めて定量的な議論に耐えるXASおよび光電子分光測定に成功し、ドープしたホールがe(g↑)バンド上部に入るのではなく、同バンドから分離した局在バンドを形成することを初めて示すことに成功した。すなわち、これが高いホール濃度域まで絶縁相が安定化する主原因であることが分かった。この局在ホール状態はTB計算では再現されず、計算は一様に1eV程度高いバンド構造を与えることから、ポーラロン効果、すなわち電子-格子相互作用や短距離電荷秩序が重要であることを推論した。

　Pr(1-x)CaxMnO3(PCMO)はペロブスカイト3d遷移金属酸化物の中でもバンド幅が最も狭く、広いホール濃度域で電荷秩序相が安定することが知られている。本研究ではLAO基板上にエピタキシャル成長させたPCMO薄膜について光電子分光測定を行い、格子不整合による圧縮性の歪みが原因となって単純な絶縁相の領域が拡大し、バルク試料で重要であった強磁性揺らぎや電荷秩序は強く抑制されることが初めて示された。

　以上のように、本研究は層状ペロブスカイト構造をもつ3d遷移金属酸化物の高品位な単結晶薄膜試料をパルスレーザー蒸着法で作成し、その電子状態を角度分解光電子分光、X線吸収分光法等で測定し、その結果を強結合近似バンド計算と比較することで、金属-絶縁体転移におけるホール局在バンド形成、歪みによる電荷秩序の抑制、さらには超格子構造における価数分布について新たな知見をもたらしたものであり、その成果は高く評価できる。なお、本論文は藤森淳氏、高田恭孝氏、尾嶋正治氏、川崎雅司氏、鯉沼秀臣氏、浜田典昭氏、永崎洋氏、Mikk Lippmaa氏、Harold Y.Hwang氏、Zhi-Xun Shen氏らを始めとする多くの研究者との共同研究であるが、論文提出者が主体となって実験の遂行と解析、理論計算そして解釈を行ったものであり、論文提出者の寄与が十分であると判断する。

　したがって、博士(理学)の学位を授与できると認める。