Outline

The band theory of solids had been highly successful in describing metals, insulators, and their transitions. The basic distinction between metals and insulators, based on the band structure, was established in the early years of quantum mechanics, and the derived principles well explained many material characteristics including the electronic and optical properties. In 1937, however, several simple transition-metal oxides with a partially filled d-electron band were found to be insulators, and then importance of the strong Coulomb repulsion between the electrons was pointed out as a source of the eccentric insulating behavior. In subsequent theoretical approaches, N. F. Mott provided an important foundation of how the electron-electron interactions lead to the insulating phase, and this state is called the Mott insulator. With the strong on-site electron correlation U, the original band would be split into two bands with energy gap of U, and then the system would be an insulator.

One striking point of the Mott insulator is that drastic electronic state changes emerge associated with the insulator-metal (Mott) transition. In the vicinity of the Mott transition, a wide variety of unprecedented phenomena such as high-temperature superconductivity, colossal magnetoresistance, and large thermoelectric effect arises from interplay among charge, spin, and orbital degrees of freedom. While their functional effects may form a significant basis of future oxide electronics, the detailed mechanisms are still under debate. In this thesis, we focus on a couple of transition-metal oxides considered as key counterpart materials to solve the problems, and investigate their charge dynamics and Mott transition features via spectroscopy.

The respective chapters of this thesis are organized as follows. In Chapter 1, an overview is given of the previous key spectroscopic results on Mott-transition oxides. Chapter 2 gives brief descriptions about experimental setups. In Chapter 3, we take La1-xSrxVO3 as an typical example of the Mott-transition system and study relationship between the charge dynamics and thermoelectricity. Chapter 4 presents the charge dynamics observed peculiar in a doped valence-bond solid system (Ti(1-x)Vx)2O3. In Chapter 5, the charge dynamics and Mott transition features in R(2-x)SrxNiO4 with charge-ordered phases are studied. Finally, we conclude this thesis in Chapter 6, with summarizing the main results. The detailed results of this thesis are as described in the following.

Charge dynamics and thermoelectricity in a typical system (Chap. 3)

A thermoelectric effect finds versatile applications in technologies for energy issues and sustained efforts have been made to explore higher-performance thermoelectric materials. In particular, transition-metal oxides with strong electron correlation have attracted much attention as the promising candidates. One possible important factor for the enhanced thermopower is suggested to be the configurational entropy term as represented in Heikes formula. However, it is still under debate whether the entropy term becomes dominant at merely a few or several hundred kelvin. Although the thermopower in such a correlated electron system should obey the Heikes formula in the high-temperature regimes, the high-temperature crossover behaviors as predicted have not been verified experimentally so far.

In this Chapter, we focus on the vicinity of the filling-control Mott transition, where a metallic state with coherent charge transport realizes only at low temperatures and the thermopower is expected to asymptotically approach the Heikes-formula values with increasing temperature in a manner dependent on the band filling and the Coulomb correlation. Here we adopt the canonical filling-control Mott transition system La1-xSrxVO3, where the incoherent charge transport indeed appears above a few hundred kelvin in the Mott critical doping region.

By systematically measuring the high-temperature optical conductivity spectra and thermopower up to 1250 K, we have clarified generic features of how the thermoelectric response is affected by the strong electron correlation. In the vicinity of the Mott transition, the thermopower undergoes two essential crossovers asymptotically approaching the limit values obtained from the Heikes formula. By comparison with results of the dynamical mean field theory for the Hubbard model, we show that the thermopower in the Mott critical state mainly measures the entropy per charge carrier that depends on electronic degrees of freedom available at the measurement temperature. The present findings also offer important clues to the origin of the large thermopower in some correlated oxides; such an idea as focusing on the entropy count of one carrier can be useful for thermoelectric materials design at practical temperatures above room temperature.

Charge dynamics in a doped valence-bond solid system (Chap. 4)

As noted before, the insulator-metal transition has long been one of the central problems in condensed matter physics. In particular, many complex transition-metal oxides with d-electrons show the insulator-metal transition, on which the vanishing conductivity is driven either by the divergent carrier mass (sometimes terminated and associated by the onset of the long-range magnetic order) or by the decrease of the carrier density. The electron-correlation driven insulator-metal transition, e.g. in such as V2O3 and La1-xSrxTiO3, mostly shows the former type, while the insulator-metal transition in high-temperature superconducting cuprates is known to be of the latter type. The electron correlation is the key to the both types of insulator-metal transition, and hence the latter 'doped insulator' like behavior has been argued extensively in terms of novel mechanisms, such as resonating valence bond or pseudo-gap formation. Here we present another example of the carrier-density driven insulator-metal transition in the d-electron system, Ti2O3, a classic and most-simple transition-metal oxide but whose detailed charge dynamics (including optical conductivity spectrum) has scarcely been investigated so far.

In this Chapter, we systematically investigate the charge dynamics in thermally and doping induced insulator-metal transitions of (Ti(1-x)Vx)2O3 by measuring the optical and transport properties in wide temperature and doping regions. The origin of the observed "doped insulator" like characteristics, such as small Drude weight proportional to the doping level x, is proved to stem from the robust singlet formation on the Ti-Ti dimer. This is in contrast to the prototypical Mott transition in the correlated electron systems, which accompanies the large spectral weight transfer from the Mott gap excitation to the Drude component. The present results probably reveal a novel class of the insulator-metal transition in correlated oxides.

The detailed dynamics of charge carriers in the highly metallic region is also systematically investigated adopting the doped valence bond solid system (Ti(1-x)Vx)2O3. We have found that the doped holes show the ferromagnetic correlation and large negative magnetoresistance, while showing an extraordinarily large effective mass (m/m0~60-80, m0 being mass of a free electron). We have ascribed such strong mass renormalization to the interaction of doped holes with the dimeric fluctuation or resultant softened phonon on the Ti-dimer sites. The large effective mass seems to conversely favor the strong ferromagnetic correlation. Thus, (Ti(1-x)Vx)2O3 can be viewed as a novel magnetic semiconductor, which can be materialized by the strong correlation in the doped valence bond solid state.

Charge dynamics in layered nickelates with charge-ordered phases (Chap. 5)

It is now widely accepted that understanding the nature of a pseudogap is essential for clarifying the whole picture and possible origin of high-Tc superconductivity. Cuprate superconductors behave as an anomalous metal at temperatures above Tc, in which a pseudogap opens with the same momentum-space symmetry as the d-wave superconducting gap. This partially gapped Fermi surface is called 'Fermi arc', which is believed to be a hallmark of the high-Tc superconductors. However, the issue of what kind of order or fluctuation dominates the pseudogap state is still under debate.

A Layered nickelate R(2-x)SrxNiO4 (R: rare earth) has long been known as a counterpart material with typical high-Tc cuprates La2-xSrxCuO4; they have common crystal structure and show two-dimensional antiferromagnetic insulator-metal transitions with hole doping procedure. In the metallic state on the verge of the Mott transition, the strong temperature dependence of Hall effect is also common. After several-years' struggle, we could succeed for the first time in growing high-quality single crystals of R(2-x)SrxNiO4 with x exceeding 1.0; this enables us to investigate the detailed electronic structure and charge dynamics in metallic layered nickelates to be compared with those of high-Tc cuprates.

In this Chapter, we investigate the bulk (not surface-specific) nature of the momentum-resolved electronic state for the layered nickelates near the Mott transition for the first time by using the state-of-the-art angle-resolved photoemission spectroscopy with use of the VUV laser excitation. The metallic but non-superconducting layered nickelate has an x2-y2 orbital-derived large hole Fermi surface, which is accompanied with the high-energy pseudogap with the same symmetry and comparable magnitude with those of underdoped cuprates, although its antiferromagnetic interaction JAF is one order of magnitude smaller. Our findings strongly indicate that the high-energy momentum-dependent pseudogap (or Fermi arc) is not unique to the high-Tc cuprates but commonly develops in the anomalous quasi-two-dimensional metallic state near the Mott transition reflecting the real-space checkerboard-type charge correlation.

The pseudogap-related carrier dynamics and critical behavior in the insulator-metal transition is also systematically investigated. In the metallic region proximate to the insulating phase, the carrier number estimated from the Hall coefficient is strongly suppressed accompanied with evolution of the pseudogap structure in the optical conductivity spectra, while the effective mass is relatively small without critical enhancement. Our findings indicate that the checkerboard-type charge correlation dominates the charge dynamics and that the pseudogap evolution induces the insulator-metal transition by gradually vanishing the carrier number in a momentum-dependent form.

Also in this Chapter, the orbital-resolved three-dimensional Fermi surface structure of Eu(2-x)SrxNiO4 is investigated by energy-dependent soft-x-ray angle-resolved photoemission spectroscopy. In addition to a large cylindrical hole Fermi surface analogous to the cuprates, we observe a Gamma-centered 3z2-r2-derived small electron pocket. This finding demonstrates that in the layered nickelate the 3z2-r2 band resides close to the x2-y2 one in energy. The resultant multi-band feature with varying orbital character as revealed may strongly work against the emergence of the high-temperature superconductivity.

The multiorbital (x2-y2/3z2-r2) features are important also for understanding electronic structure evolution toward the insulator-metal transition. Doping variation of the hole orbital states is investigated as well, by measuring polarized soft-x-ray absorption spectra. The orbital polarization between x2-y2 and 3z2-r2, calculated applying the sum rule to total integrated intensity of the Ni L-edge spectra, increases to nearly zero above x=0.5, which indicates that the excess holes are mainly doped into the 3z2-r2 orbital. Lattice constants a and c show an increase and a decrease above x=0.5, respectively, reflecting the change of the hole orbital states, and the nonmonotonic x variation indicates the strong electron-lattice coupling characteristic of eg-orbital systems. Numbers of the Ni3+ site with occupied 3z2-r2 and x2-y2 orbitals, estimated from the preedge peak changes in O K-edge spectra, increase nearly linearly above x=0 and 0.5, respectively. The result is consistent with one deduced from the Ni L-edge spectra, and the both indicate that the checkerboard-type charge/orbital order or correlation strongly persists above x=0.5. The existence of Ni4+ sites above x=1.0 may suppress the checkerboard-type ordering and induce the insulator-metal transition at x~1. All these findings well capture the characteristic electronic structure variation toward the metallic region in the layered nickelate.

強相関電子系の特徴は、多数の電子が秩序を融解しつつも互いに強く相関しながら運動する、絶縁体金属転移(モット転移)近傍にもっとも顕著に現れる。3d遷移金属酸化物の示す高温超伝導・超巨大磁気抵抗効果・巨大熱電効果等の興味深い現象はいずれも、電子相関が強く残る異常金属領域において現れると考えられており、このような創発的電子物性における電荷・スピン・軌道自由度の働きについてはこれまでも精力的な研究が行われてきた。特に、反射分光・光電子分光をはじめとするスペクトロスコピーの手法はこのような複雑な電子状態の理解に対して多大な寄与をしてきた。一方で、これまでに発見された特徴的なスペクトル構造の起源や、提唱されている特異な伝導状態の発現可能性については、いまだ多くは未解明である。本論文では、種々の遷移金属酸化物の横断的な分光実験を行うことより、各内部自由度が強く寄与するモット転移近傍の電荷ダイナミクスについて深い実験的知見を得ることに成功している。本論文は6章から構成されており、以下にその概要を述べる。

第1章では、本研究の背景、特に、モット転移における電子状態の基本的な変化や共通したスペクトル構造の特徴について詳しく述べている。

第2章では、実験手法、特に、遠赤外-紫外反射分光、角度分解光電子分光、X線吸収分光等のスペクトロスコピーの手法について説明している。

第3章では、典型的なフィリング制御型モット転移系であるペロブスカイト型バナジウム酸化物(La(1-x)SrxVO3)を例にとり、電荷ダイナミクスと熱電応答の関係を詳細に調べている。特に、輸送・反射分光測定を行い、(1) 転移組成近傍を中心として数百K程度の温度からインコヒーレントな伝導状態が現れること、(2)同時にエントロピー項が支配的となるためにゼーベック係数に各サイトのもつ内部自由度に起因したクロスオーバーが現れること、等を明らかにした。以上の振る舞いは、熱電応答に対する強相関効果の普遍的な性質であると考えられ、ゼーベック係数の高温極限値(ハイケス公式の予測する値)への漸近がバンド幅やクーロンエネルギーのエネルギースケールと比べて非常に低温から実際に現れることを示している。

第4章では、コランダム型構造を持つチタン酸化物(Ti2O3)を例にとり、ドープされた原子価結合固体系における絶縁体金属転移と電荷ダイナミクスについて調べている。本系ではTi-Tiダイマー内においてS=1/2のスピンが強固なシングレット結合を形成し、基底状態は非磁性絶縁体となっている。本論文では、輸送・反射分光測定を中心として、(1)温度変化及びVドープによって生じるいずれの金属相においてもシングレット状態が残ること、(2)それぞれの絶縁体金属転移は、シングレット結合の弱化による僅かなバンドシフト及びホールドーピングによって生じる少数のキャリアにより引き起こされること、等を明らかにした。また、(3)基底状態のシングレット結合が徐々に弱まる高Vドープ金属領域において、特異な重い電子状態を発見し、原子価結合固体系に特徴的なダイマー内の強い電子格子結合に由来するポーラロン状態が実現している可能性を示した。

第5章では、層状ペロブスカイト型ニッケル酸化物(R(2-x)SrxNiO4、RはLa, Nd, Eu等の希土類元素)を例にとり、電荷秩序不安定性の強い系における絶縁体金属転移と電荷ダイナミクスについて調べている。本系は高温超伝導を示す層状銅酸化物の対照物質としても有名な系であり、そのモット転移近傍の電子状態には古くから興味が持たれてきた。本論文では、レーザー高分解能角度分解光電子分光測定を主に行い、(1)超伝導銅酸化物と共通の大きなホールフェルミ面及び高エネルギーのキンク構造を持つこと、(2)運動量に強く依存した擬ギャップ構造が実空間の電荷相関によって現れ、モット転移近傍の電荷ダイナミクスとその臨界的な挙動を支配していること、等を明らかにした。更に、3次元軟X線角度分解光電子分光・X線吸収分光の結果をもとに、(3)本系が銅酸化物とは対照的な多バンド・多軌道状態を有することを明らかにした。

第6章では、本研究によって得られた成果についての総括を行っている。

以上をまとめると、本論文では、モット転移近傍に位置する代表的な強相関遷移金属酸化物について横断的な分光実験を行うことで、特徴的なスペクトル構造の起源や、内部自由度の強く関与する伝導状態・電荷ダイナミクスについて明らかにした。本研究は、綿密に計画された比較対照実験のもと、モット転移近傍での各内部自由度の振舞いについて基礎的な特徴を明らかにすることに成功しており、今後の研究展開を図る上でも非常に重要な知見が得られたと言える。よって本論文は物性科学・物理工学の発展に寄与するところ大であり、博士(工学)の学位請求論文として合格と認められる。