Universität zu Köln

Haverkort, Maurits (2005). Spin and orbital degrees of freedom in transition metal oxides and oxide thin films studied by soft x-ray absorption spectroscopy. PhD thesis, Universität zu Köln.

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Abstract

The class of transition metal compounds shows an enormous richness of physical properties [1,2], such as metal-insulator transitions, colossal magneto-resistance, super-conductivity, magneto-optics and spin-depend transport. The theoretical description of these materials is still a challenge. Traditional methods using the independent electron approximation most of the time fail on even the simplest predictions. For example, many of the transition metal compounds, with NiO as the classical example, should be a metal according to band-structure calculations, but are in reality excellent insulators. The single band Mott-Hubbard model [3,4] explains very nicely why many correlated materials are insulating. But even the Mott-Hubbard model has some problems in understanding the band-gap found for many of the transition metal compounds [5]. With the recognition that transition metal compounds can be of the charge-transfer type or the Mott-Hubbard type [6], depending on the ratio of U and Delta, also the band-gap can be understood. Hereby U is defined as the repulsive Coulomb energy of two electrons on the same transition metal site and Delta is defined as the energy it costs to bring an electron from an oxygen site to a transition metal site. The single band Mott-Hubbard model is however, even when charge transfer effects are included, inadequate in describing the full richness found in many of the transition metal compounds [7-9]. It now becomes more and more clear that in order to describe transition metal compounds, the charge, orbital, spin and lattice degrees of freedom should all be taken into account. Especially the orbital degrees of freedom have not been considered to the full extend until recently. In the manganates, for example, orbital and charge ordering of the Mn ions play an important role for the colossal magneto-resistance of these materials [10-14]. An other example would be the metal-insulator transition in V2O3 [15-17]. The orbital occupation of the V ion changes drastically at the phase transition [15]. This change in orbital occupation will change the local spin-spin correlations which in-turn will change the effective band-width. This indicates that not only electron-electron Coulomb repulsion in a single band must be considered, but a full multi-band theory including all interactions must be considered in order to understand this prototypical Mott-Hubbard system. With the recognition that the local orbital occupation plays an important role in many of the transition metal compounds there is a need for experimental techniques that can measure the orbital occupation. This technique is soft x-ray absorption spectroscopy (XAS). For transition metal atoms one measures the local transition of a 2p core electron into the 3d valence shell. In chapter 5 to chapter 7 we used soft x-ray absorption spectroscopy to measure orbital occupations, crystal fields, and spin directions in thin films of NiO, CoO and MnO. In chapter 8 to chapter 10 we used soft x-ray absorption spectroscopy to gain insight in the importance of spin and orbital degrees of freedom in bulk crystals of Sr2CoO3Cl, LaTiO3 and VO2. [1] N. Tsuda, K. Nasu, A. Yanase, and K. Siratori, Electronic Conduction in Oxides (Springer Series in Solid-State Sciences 94, Springer Verlag, Berlin, 1991) [2] M. Imada, A. Fujimori, and Y. Tokura, Rev. Mod. Phys. 70, 1039 (1998). [3] N. F. Mott, Proc. Phys. Soc. A 62, 416 (1949). [4] J. Hubbard, Proc. Roy. Soc. London Ser. A 276, 238 (1963) [5] G. A. Sawatzky and J. W. Allen, Phys. Rev. Lett. 53, 2339 (1984). [6] J. Zaanen, G. A. Sawatzky, and J. W. Allen, Phys. Rev. Lett. 55, 418 (1985). [7] R. J. Birgeneau and M. A. Kastner, Science 288, 437 (2000). [8] Y. Tokura and N. Nagaosa, Science 288, 462 (2000). [9] J. Orenstein and A. J. Millis, Science 288, 468 (2000) [10] A. P. Ramirez, J. Phys.: Conden. Matter 9, 8171 (1997). [11] D. I. Khomskii and G. A. Sawatzky, Solid State Commun. 102, 87 (1997). [12] T. Mizokawa and A. Fujimori, Phys. Rev. B 51, 12880 (1995). [13] T. Mizokawa and A. Fujimori, Phys. Rev. B 54, 5368 (1996). [14] T. Mizokawa and A. Fujimori, Phys. Rev. B 56, 493 (1997). [15] J.-H. Park, L. H. Tjeng, A. Tanaka, J. W. Allen, C. T. Chen, P. Metcalf, J. M. Honig, F. M. F. de Groot, and G. A. Sawatzky, Phys. Rev. B 61, 11506 (2000). [16] S. Y. Ezhov, V. I. Anisimov, D. I. Khomskii, and G. A. Sawatzky, Phys. Rev. Lett. 83, 4136 (1999). [17] F. Mila, R. Shiina, F.-C. Zhang, A. Joshi, M. Ma, V. Anisimov, and T. M. Rice, Phys. Rev. Lett. 85, 1714 (2000).

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Haverkort, Maurits haverkort@ph2.uni-koeln.de UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-14558
Date:	2005
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Physics > Institute of Physics II
Subjects:	Physics
Uncontrolled Keywords:	Keywords Language NiO, CoO, MnO, LaTiO3, VO2 German NiO, CoO, MnO, LaTiO3, VO2 English
Date of oral exam:	6 June 2005
Referee:	Name Academic Title Tjeng, L. H. Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/1455