Spatial confinement of electrons and their interactions as well as confinement of the spin dimensionality often yield drastic changes of the electronic and magnetic properties of solids. Novel quantum transport and optical phenomena, involving electronic spin degrees of freedom in semiconductor heterostructures, as well as a rich variety of exotic quantum ground states and magnetic excitations in complex transition metal oxides that arise upon such confinements, belong therefore to topical problems of contemporary condensed matter physics.
In this work electron spin systems in reduced dimensions are studied with Electron Spin Resonance (ESR) spectroscopy, a method which can provide important information on the energy spectrum of the spin states, spin dynamics, and magnetic correlations. The studied systems include quasi onedimensional spin chain materials based on transition metals Cu and Ni. Another class of materials are semiconductor heterostructures made of Si and Ge.
Part I deals with the theoretical background of ESR and the description of the experimental ESR setups used which have been optimized for the purposes of the present work. In particular, the development and implementation of axial and transverse cylindrical resonant cavities for high-field highfrequency ESR experiments is discussed. The high quality factors of these cavities allow for sensitive measurements on μm-sized samples. They are used for the investigations on the spin-chain materials. The implementation and characterization of a setup for electrical detected magnetic resonance is presented.
In Part II ESR studies and complementary results of other experimental techniques on two spin chain materials are presented. The Cu-based material Linarite is investigated in the paramagnetic regime above T > 2.8 K. This natural crystal constitutes a highly frustrated spin 1/2 Heisenberg chain with ferromagnetic nearest-neighbor and antiferromagnetic next-nearestneighbor interactions. The ESR data reveals that the significant magnetic anisotropy is due to anisotropy of the g-factor. Quantitative analysis of the critical broadening of the linewidth suggest appreciable interchain and interlayer spin correlations well above the ordering temperature. The Ni-based system is an organic-anorganic hybrid material where the Ni2+ ions possessing the integer spin S = 1 are magnetically coupled along one spatial direction. Indeed, the ESR study reveals an isotropic spin-1 Heisenberg chain in this system which unlike the Cu half integer spin-1/2 chain is expected to possess a qualitatively different non-magnetic singlet ground state separated from an excited magnetic state by a so-called Haldane gap. Surprisingly, in contrast to the expected Haldane behavior a competition between a magnetically ordered ground state and a potentially gapped state is revealed.
In Part III investigations on SiGe/Si quantum dot structures are presented. The ESR investigations reveal narrowlines close to the free electron g-factor associated with electrons on the quantum dots. Their dephasing and relaxation times are determined. Manipulations with sub-bandgap light allow to change the relative population between the observed states. On the basis of extensive characterizations, strain, electronic structure and confined states on the Si-based structures are modeled with the program nextnano3. A qualitative model, explaining the energy spectrum of the spin states is proposed.:Abstract i
Contents iii
List of Figures vi
List of Tables viii
1 Preface 1
I Background and Experimental 5
2 Principles of ESR 7
2.1 The Resonance Phenomenon . . . . . . . . . . . . . . . . . . . 7
2.2 ESR Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.2.1 The g -factor . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2.2 Relaxation Times . . . . . . . . . . . . . . . . . . . . . . 12
2.2.3 Lineshape Properties . . . . . . . . . . . . . . . . . . . . 13
2.3 Effective Spin Hamiltonian . . . . . . . . . . . . . . . . . . . . . 15
2.4 Spin-Orbit Coupling . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.5 d-electrons in a Crystal Field . . . . . . . . . . . . . . . . . . . . 17
2.6 Interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.6.1 Dipolar Coupling . . . . . . . . . . . . . . . . . . . . . . 23
2.6.2 Exchange Interaction . . . . . . . . . . . . . . . . . . . . 23
2.6.3 Superexchange . . . . . . . . . . . . . . . . . . . . . . . 24
2.6.4 Symmetric Anisotropic Exchange . . . . . . . . . . . . 25
2.6.5 Antisymmetric Anisotropic Exchange . . . . . . . . . . 25
2.6.6 Hyperfine Interaction . . . . . . . . . . . . . . . . . . . 26
3 Experimental 27
3.1 Setup for Experiments at 10GHz . . . . . . . . . . . . . . . . . 27
3.2 Implementation of an EDMR Setup . . . . . . . . . . . . . . . . 29
3.2.1 Basic Characterization . . . . . . . . . . . . . . . . . . . 31
3.3 High Frequency Setup . . . . . . . . . . . . . . . . . . . . . . . . 31
3.3.1 MillimeterWave Vector Network Analyzer . . . . . . . 33
3.3.2 Waveguides and Cryostats . . . . . . . . . . . . . . . . . 34
3.4 Development of the Resonant Cavity Setup . . . . . . . . . . . 35
3.4.1 Mode Propagation . . . . . . . . . . . . . . . . . . . . . 38
3.4.2 Resonant CavityModes . . . . . . . . . . . . . . . . . . 40
3.4.3 Resonant Cavity Design . . . . . . . . . . . . . . . . . . 41
3.4.4 Resonant Cavity Sample Stick . . . . . . . . . . . . . . . 45
3.4.5 Experimental Characterization . . . . . . . . . . . . . . 47
3.4.6 Performing an ESR Experiment . . . . . . . . . . . . . . 53
II Quasi One-Dimensional Spin-Chains 57
4 Motivation 59
5 Quasi One-Dimensional Systems 61
5.1 Magnetic Order and Excitations . . . . . . . . . . . . . . . . . . 63
5.2 Competing Interactions . . . . . . . . . . . . . . . . . . . . . . . 64
5.3 Haldane Spin Chain . . . . . . . . . . . . . . . . . . . . . . . . . 66
6 Linarite 69
6.1 Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
6.2 Magnetization and ESR . . . . . . . . . . . . . . . . . . . . . . . 71
6.3 NMR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
6.4 Summary and Conclusion . . . . . . . . . . . . . . . . . . . . . 81
6.5 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
7 The Ni-hybrid NiCl3C6H5CH2CH2NH3 83
7.1 Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
7.2 Susceptibility andMagnetization . . . . . . . . . . . . . . . . . 85
7.3 ESR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
7.4 Further Investigations . . . . . . . . . . . . . . . . . . . . . . . . 95
7.5 Summary and Conclusion . . . . . . . . . . . . . . . . . . . . . 96
8 Summary 99
III SiGe Nanostructures 101
9 Motivation 103
10 SiGe Semiconductor Nanostructures 107
10.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
10.1.1 Silicon and Germanium . . . . . . . . . . . . . . . . . . 107
10.1.2 Epitaxial Growth of SiGe Heterostructures . . . . . . . 109
10.1.3 Strain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
10.1.4 Band Deformation . . . . . . . . . . . . . . . . . . . . . 112
10.2 Sample Structure and Characterization . . . . . . . . . . . . . 114
11 Modelling of SiGe/Si Heterostructures 119
11.1 Program Structure . . . . . . . . . . . . . . . . . . . . . . . . . . 120
11.2 Implementation of Si/Ge System . . . . . . . . . . . . . . . . . 121
11.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
11.3.1 Single Quantum Dot . . . . . . . . . . . . . . . . . . . . 123
11.3.2 Multiple Quantum Dots . . . . . . . . . . . . . . . . . . 127
11.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
11.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
12 ESR Experiments on Si/SiGe Quantum Dots 135
12.1 ESR on Si Structures . . . . . . . . . . . . . . . . . . . . . . . . . 135
12.2 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . 137
12.2.1 Samples grown at 600◦C . . . . . . . . . . . . . . . . . . 138
12.2.2 Samples grown at 700◦C . . . . . . . . . . . . . . . . . . 139
12.2.3 T1-Relaxation Time . . . . . . . . . . . . . . . . . . . . . 143
12.2.4 Effect of Illumination . . . . . . . . . . . . . . . . . . . . 145
12.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
12.3.1 Quantum Dots . . . . . . . . . . . . . . . . . . . . . . . 149
12.3.2 Assignment of ESR Lines . . . . . . . . . . . . . . . . . . 150
12.3.3 Relaxation Times . . . . . . . . . . . . . . . . . . . . . . 153
12.3.4 Donors in Heterostructures . . . . . . . . . . . . . . . . 153
12.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
13 Summary and Outlook 157
Bibliography 163
Acknowledgements 176
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:25680 |
| Date | 31 August 2011 |
| Creators | Lipps, Ferdinand |
| Contributors | Büchner, Bernd, Schäpers, Thomas, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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