Microscopic Modelling of Correlated Low-dimensional Systems

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Chapter 1: Introduction 3 terials. An understanding of these properties allow us to design and model novel systems with desired properties. First principles simulations give a unique connection between microscopic and macroscopic properties which, when combined with experimental tools, can deliver insight and deeper understanding of the relation between the atomic arrangement and the observed phenomena. To perform controlled ab initio calculations on reliable structures, we elaborate on a novel two-step approach, which combines classical with quantum-mechanical methods, to syste- matically prepare model structures for metal-organic or polymeric coordination compound systems and relax them to their equilibrium configuration. This procedure allows not only to study the ground state properties of compounds but also to modify their constituents. We show that this procedure is very effective, having the advantage of including quantum effects while diminishing the computational effort and increasing the accuracy. Neglecting these quantum effects can suppress interesting properties and can give rise to wrong conclu- sions. Our theoretical procedure has been successfully applied to different kind of complex metal-organic materials in this work. Within the class of metal-organic compounds, we are particularly interested in hydroquinone-derived linkers connecting Cu 2+ ions. The advantage of using hydroquinone linkers, is that they can be chemically modified in a way that influences the coordination geometry of the Cu 2+ ions while keeping the magnetic exchange at a moderate strength. We studied the influence of organic linkers in the magnetic correlation between the metal centers in two representatives of a family of hydroquinone-based low-dimensional quantum-spin systems, namely Cu 2+ -2,5-bis(pyrazol-1-yl)-1,4-dihydroxybenzene (CuCCP) which behaves as 1D spin chain system and a coupled-dimer system TK91. Additionally we have intro- duced theoretically, systematic changes to the CuCCP polymer in order to achieve desirable electronic and magnetic properties in the modified new structures. This study allows for a gradual understanding of the properties of these systems and provides a guide to systematic synthesis in the laboratory. We have also extended our study to investigate microscopically not only the influence of the components and the dimensionality but also the effects of the geometrical arrangement and the interplay between different energy scales in the magnetic phenomena observed in zero-, one- and two-dimensional materials. For this purpose, we considered three different systems containing 3d transition metal ions which have a small magnetic interaction parameter between the magnetic centers, presenting very interesting properties when one apply an
Chapter 1: Introduction 4 external perturbation such as a magnetic field or changes in the pressure or temperature. The Cu4OCl6daca4 system shows characteristics of a quasi zero-dimensional material whose magnetic building blocks are almost isolated tetrahedra of Cu ions surrounded by an oc- tahedra of Cl atoms. Due to this geometric arrangement, it is possible to obtain different ground state properties depending on the electronic state of the metal ions and the nature of their magnetic interaction. This compound and members of the same family of compounds exhibit anomalous magnetic behavior at low temperatures that cannot be accounted for by simple Heisenberg isotropic exchange. Our calculations provide a route to understand the experimental data collected for Cu4OCl6daca4 compound; this allow us to compare our results with several scenarios proposed in the literature which attempt to explain the anomalous low temperature behavior of the magnetization. We show that these previous works fail to explain the experimental observations. One intensively debated class of materials which have potential applications as optical switches, sensors or memory devices are the spin-crossover polymer systems, which involve transition metal ions linked by organic ligands. These systems show a sharp transition triggered by variation of temperature, pressure or by light irradiation between a low-spin (LS) state and a metastable high-spin (HS) state, which in many cases is accompanied by a change in color and a thermal hysteresis loop. These are signals of the large magnitude of the interactions between the active sites or cooperativity of the system. The origin of this transition and its cooperativity has been mainly discussed in the framework of elastic models, and only recently the possible role of magnetic exchange was suggested. However, there has been to date no conclusive microscopic study where all important interactions are incorporated and hence the origin of the large cooperativity has not been completely settled. The main difficulty in performing ab initio calculations for these kind of materials lies in the lack of accurate crystal structures. In the present work, we overcome the inavailability of structural data by predicting a crystallographic structure for a one-dimensional Fe(II) spin-crossover crystal using known experimental constraints and the two step procedure we have developed. We analyse the LS-HS phase transition and show, contrary to common belief, that there exists an interplay between magnetic exchange and elastic properties that is responsible for the large cooperativity in these systems. We corroborate the quality of our designed structure by comparing with magnetic experiments performed on a real sample. One particularly appealing material is K2V3O8, which behaves as an ideal 2D Heisenberg antiferromagnet. Although it does not have any organic component, it exhibits a very
Page 1 and 2: Microscopic Modelling of Correlated
Page 3 and 4: I declare that I wrote this work my
Page 5 and 6: Abstract v description of their low
Page 7 and 8: Contents vii 5 Results and Discussi
Page 9 and 10: List of Figures ix 4.7 (above) (a)
Page 11 and 12: List of Figures xi 5.16 Cu Wannier
Page 13 and 14: List of Tables 4.1 Bond lengths and
Page 15 and 16: List of Abbreviations 1D . . . . .
Page 17 and 18: Dedicated to Fernando-Andres Salgue
Page 19: Chapter 1: Introduction 2 are being
Page 23 and 24: Chapter 2 Method Our understanding
Page 25 and 26: Chapter 2: Method 8 system at any t
Page 27 and 28: Chapter 2: Method 10 The functional
Page 29 and 30: Chapter 2: Method 12 and to use it
Page 31 and 32: Chapter 2: Method 14 2.2.2 Orbital-
Page 33 and 34: Chapter 2: Method 16 linearized for
Page 35 and 36: Chapter 2: Method 18 and relativist
Page 37 and 38: Chapter 2: Method 20 the fact that
Page 39 and 40: Chapter 2: Method 22 The basis func
Page 41 and 42: Chapter 2: Method 24 • If these b
Page 43 and 44: Chapter 2: Method 26 LAPW and LMTO
Page 45 and 46: Chapter 2: Method 28 2.3 An overvie
Page 47 and 48: Chapter 2: Method 30 2.4 Effective
Page 49 and 50: Chapter 2: Method 32 HSO = λ(L ·
Page 51 and 52: Chapter 3 Crystal structures of the
Page 53 and 54: Chapter 3: Crystal structures of th
Page 55 and 56: Chapter 3: Crystal structures of th
Page 57 and 58: Chapter 4 Low dimensional spin syst
Page 59 and 60: Chapter 4: Low dimensional spin sys
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Chapter 4: Low dimensional spin sys
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Chapter 5 Results and Discussion 5.
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Chapter 5: Results and Discussion 7
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Chapter 6 Summary and Outlook The t
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Chapter 6: Summary and Outlook 150
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Chapter 6: Summary and Outlook 152
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Appendix A: Atomic coordinates for
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Appendix A: Atomic coordinates for
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Appendix B: Atomic coordinates for
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Appendix C Atomic coordinates for t
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Bibliography [1] F. H. Allen, O. Ke
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Bibliography 170 [34] R. C. Dickins
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Bibliography 172 [75] M. D. Lumsden
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Bibliography 174 [116] R. L. Wither
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Zusammenfasung Niedrigdimensionale
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Zusammenfasung 178 Quantenspinssyst
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Zusammenfasung 180 Clustern zeigen,
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Experience: Curriculum vitae Name:
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I would like to thank: Acknowledgme
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Microscopic Modelling of Correlated Low-dimensional Systems

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