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Martin Fischer «  Jihaan Fath-Allah »  Michael Herzinger
Jihaan Fath-Allah
Tuning the electron localization in transition-metal oxide compounds: Fe3O4 and TiOCl under extreme conditions
Supervisor: Prof. Dr. C.A. Kuntscher [Experimental physics II]
Date of oral examination: 07/30/2013
174 pages, english
This thesis is focused on how the physical properties of transition metal oxides change due to applying external pressure and low temperature. The main experimental techniques used are infrared spectroscopy and X-ray powder diffraction(XRD). Two of the transition metal oxide compounds, namely TiOCl and Fe3O4, are investigated. For TiOCl, both experimental techniques were used to investigate the electronic and structural properties of this compound. For Fe3O4, the infrared spectroscopy studies: first, under high pressure at room temperature and second, under high pressure and low temperature were carried out. TiOCl is a low-dimensional Mott-Hubbard insulator. Recently, application of pressure on this compound revealed strong changes in its electronic and structural properties. Based on these results, the possibility of pressure induced insulator-to-metal transition was suggested. According to the pressure-dependance of XRD data obtained in this project on TiOCl, two structure phase transitions were observed at Pc1 and Pc2. The results obtained are then compared with the literature data. The observed structure phase transition coincides with the suggested closure of the Mott- Hubbard gap. The results of pressure-dependant transmittance at low temperature on single crystal of TiOCl in the mid-infrared range show similar effects to the one which were previously observed at room temperature, where the orbital excitation shifts linearly to higher frequencies with increasing pressure at certain temperature, while it hardly shifts to higher frequencies during cooling down. Above Pc1 the optical conductivity spectrum shows huge growth of the spectral weight which could be attributed to the closure of the Mott-Hubbard gap, while very small changes are observed in the overall optical conductivity spectrum during cooling down. Magnetite is a strongly electronically correlated system and shows many interesting physical properties, including the Verwey transition at TV =122K, which has attracted much interest since its discovery in 1939. Within the present work, the pressure effect on the optical properties of magnetite was studied: first at room temperature then at low temperature, to characterize the electronic and vibrational properties of magnetite. In the first part of the work, our goal was to confirm the observed polaronic character in the infrared frequency range via several theoretical models, then to check the changes in the spectral features at around 6 GPa, where earlier pressure studies observed a crossover transition around this pressure. In the secondpart of the work, our aim was to check the pressure-dependance of the Verwey transition temperature in the far-infrared range. A decrease of the overall reflectance spectra down to TV was observed. The Verwey transition temperature decreased with increasing pressure. A pressure-temperature phase diagram based on our optical results and experimental findings of earlier measurements of magnetite is proposed.