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Optical spectroscopy of orbital and magnetic excitations in vanadates and cuprates [Elektronische Ressource] / vorgelegt von Eva Vera Benckiser

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193 pages
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Ajouté le : 01 janvier 2007
Lecture(s) : 9
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Optical Spectroscopy of Orbital and Magnetic Excitations in Vanadates and Cuprates Eva Benckiser
Optical Spectroscopy of
Orbital and Magnetic Excitations
in Vanadates and Cuprates
Eva BenckiserOptical Spectroscopy of Orbital and Magnetic Excitations in Vanadates
and CupratesCover: Wind-blown sand patterns on the beach at Farewell Spit (New Zealand).Optical Spectroscopy of Orbital
and Magnetic Excitations in
Vanadates and Cuprates
Inaugural - Dissertation
zur
Erlangung des Doktorgrades
der Mathematisch-Naturwissenschaftlichen Fakultät
der Universität zu Köln
vorgelegt von
Eva Vera Benckiser
aus Stuttgart
Köln, im Oktober 2007Berichterstatter: Prof. Dr. M. Grüninger
Prof. Dr. A. Freimuth
Vorsitzender der
Prüfungskommission: Prof. Dr. G. Meyer
Tag der letzten mündlichen Prüfung: 21. November 2007’The ability to reduce everything
to simple fundamental laws
does not imply the ability
to start from those laws
and reconstruct the universe.’
(P. W. Anderson [1])
for my parentsContents
1. Introduction 1
2. Optical Spectroscopy 7
2.1. Some Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1.1. Anisotropic Samples: the Dielectric Tensor . . . . . . . . . . . 8
2.2. Electric-Dipole Transitions and Selection Rules. . . . . . . . . . . . . 10
2.2.1. Group-Theoretical Considerations . . . . . . . . . . . . . . . . 11
2.3. Electronic Excitations . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.4. Low-Energy Excitations in Correlated Insulators . . . . . . . . . . . . 14
2.4.1. Phonons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.4.2. Orbital Excitations: Local Crystal-Field Excitations vs. Or-
bitons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.4.3. MagneticExcitations: Phonon-AssistedMultimagnonAbsorp-
tion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3. Experimental Setup 27
3.1. Fourier Transform Spectroscopy . . . . . . . . . . . . . . . . . . . . . 27
3.1.1. Measurement of the Transmittance and Reflectance . . . . . . 29
3.1.2. Problems to Take Care of . . . . . . . . . . . . . . . . . . . . 30
3.1.3. Applying an External Magnetic Field . . . . . . . . . . . . . . 33
3.2. Alignment and Preparation of the Samples . . . . . . . . . . . . . . . 33
3.2.1. Sample Alignment . . . . . . . . . . . . . . . . . . . . . . . . 34
3.2.2. Lapping & Polishing . . . . . . . . . . . . . . . . . . . . . . . 34
4. Orbital Excitations in Orbitally Ordered Vanadates 35
4.1. The Compounds RVO (R=Y,Ho,Ce) . . . . . . . . . . . . . . . . . 353
4.1.1. YVO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393
4.1.2. HoVO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503
4.1.3. CeVO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503
4.2. Experimental Details . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.2.1. Samples of RVO . . . . . . . . . . . . . . . . . . . . . . . . . 513
4.2.2. Samples of VOCl . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.2.3. Samples of Dy(Sc,V)O . . . . . . . . . . . . . . . . . . . . . . 553
4.2.4. Resistivity of YVO . . . . . . . . . . . . . . . . . . . . . . . . 563
4.2.5. Transmittance of YVO . . . . . . . . . . . . . . . . . . . . . 603
viiContents
4.2.6. Transmittance of HoVO . . . . . . . . . . . . . . . . . . . . . 613
4.2.7. Tance of CeVO . . . . . . . . . . . . . . . . . . . . . 653
4.2.8. Reflectance of YVO . . . . . . . . . . . . . . . . . . . . . . . 663
4.3. Results: Optical Conductivity of RVO . . . . . . . . . . . . . . . . . 703
4.3.1. Optical Conductivity of YVO . . . . . . . . . . . . . . . . . . 703
4.3.2. Absorption spectrum of HoVO and CeVO . . . . . . . . . . 853 3
4.3.3. Spin-forbidden transitions in YVO . . . . . . . . . . . . . . . 893
4.3.4. Absorption of YVO in a Magnetic Field . . . . . . . . . . . . 923
4.3.5. Estimate of 10Dq in Dy(Sc,V)O . . . . . . . . . . . . . . . . 923
4.3.6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
5. Magnetic Excitations in Quasi-1D Quantum Spin Systems 97
5.1. Low-Dimensional Quantum Spin Systems . . . . . . . . . . . . . . . . 97
5.2. Spin-1/2 Chains in (Sr,Ca)CuO . . . . . . . . . . . . . . . . . . . . . 1022
5.2.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
5.2.2. Experimental Details . . . . . . . . . . . . . . . . . . . . . . . 108
5.2.3. Results: Optical Conductivity of Sr Ca CuO . . . . . . . . 1151-x x 2
5.3. The 5-Leg Spin Ladders in La Cu O . . . . . . . . . . . . . . . . . 1298 7 19
5.3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
5.3.2. Experimental Details . . . . . . . . . . . . . . . . . . . . . . . 131
5.3.3. Results: Magnetic Excitations in La Cu O . . . . . . . . . . 1348 7 19
6. Conclusion 141
A. Appendix 145
A.1. Cluster Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
3A.2. Configuration Mixing of the T Ground State . . . . . . . . . . . . . 1491
3+A.3. Assignment of the Dy f-f Transitions in DyScO . . . . . . . . . . . 1493
3+A.4. Ast of the Ce f-f Transitions in CeVO . . . . . . . . . . . 1513
3+A.5. Assignment of the Ho f-f Transitions in HoVO . . . . . . . . . . . 1533
A.6. Near-Infrared Photoinduced Absorption in SrCuO . . . . . . . . . . 1552
A.7. Madelung Potentials at O(1) and O(2) in SrCuO . . . . . . . . . . . 1562
References 159
List of Publications 175
Acknowledgements 177
Supplement 179
Offizielle Erklärung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
Kurzzusammenfassung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183
viii1. Introduction
In many transition-metal (TM) oxides the strong electron-electron correlations play
acrucialrolegivingrisetofascinatingphysicsandarichvarietyofnovelphenomena.
BecauseoftheirpartlyfilleddshellsmanyTMoxidesareexpectedtobemetals,but
found experimentally to be insulators [2]. To account for both, the energy gain due
to delocalization of the electrons and the Coulomb repulsion between two electrons
on the same site, the Hubbard model was introduced [3, 4]. The most important
difference to a conventional band insulator is that the atomic degrees of freedom,
spin and orbital, still survive in such Mott insulators [5]. The degrees of freedom for
electrons, localized on a specific atomic site, give rise to versatile order phenomena,
like charge, spin, and orbital order. In particular, these order offer
the possibility of new collective excitations, like charge-density waves, spin waves
(magnons), and orbital waves (orbitons). The investigation of these low-energy
excitations in undoped Mott insulators is the subject of this thesis.
First, we focus on orbital excitations in orbitally ordered vanadates RVO with3
R=rare-earth ion. The undoped RVO compounds are Mott-Hubbard insulators3
3+with a pseudo-cubic structure. The octahedrally coordinated V ions exhibit two
electrons occupying the open 3d shell. Due to the crystal field, the five atomic 3d
levels are split into a low-lying triply degenerate t level and a high-lying doubly2g
degenerate e level. In the ground state both electrons occupy the t orbitals. Twog 2g
mechanisms are discussed in the literature which give rise to a lifting of the orbital
degeneracy of partly filledt levels. First, the orbital degeneracy may be lifted by a2g
coupling to phonons, i.e. due to the so-called (collective) Jahn-Teller (JT) effect [6].
Second, a lifting of the degeneracy by superexchange processes has been suggested
[7]. In real crystals one may expect that both mechanisms are present, but with
different strength. Independent from the respective mechanism, the formation of
an orbitally ordered ground state is expected. However, a dominant superexchange
may also give rise to exotic ground states. For instance, an orbital liquid state in
LaTiO [8] and an orbital Peierls state in YVO [9, 10] have been predicted. The3 3
latter is of particular interest, because the excitations of a dimerized orbital ground
state may show some peculiarities.
From the ground state properties it is impossible to decide which mechanism is
themostrelevantone. However, whenconcerningtheelementaryexcitationsofsuch
anorbitally-orderedsystem,thecouplingtothelatticeplaysacrucialrole. IftheJT
effect is dominant, the orbital excitations are local crystal-field excitations, i.e. tran-
sitions of electrons between local crystal-field levels. Such local orbital excitations
have been investigated in detail in many TM compounds over the last decades and
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