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- Bielecki, Johan, 1982
(author)
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Optical Spectroscopy on Correlated Electron Materials
- 2010
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Licentiate thesis (other academic/artistic)abstract
- Strongly correlated materials poses a great challenge to our microscopic understanding of matter, mainly due to the inability of perturbative methods to predict the properties of such materials. This inability stems from strong correlations between electronic, lattice and spin degrees of freedom, leading to a variety of complex and exotic phenomena such as (unconventional) superconductivity, metal-insulator transitions, non-Fermi liquid behavior and colossal magnetoresistance.In this thesis, optical spectroscopy investigations on two kinds of strongly correlated matter is presented: materials exhibiting colossal magnetoresistance (manganites) and superconductivity (iron-pnictides). The research has been done from a fundamental point of view, with the goal to advance our understanding of the fascinating interplay between electrons, phonons and spin fluctuation that is encountered in these materials. The electron-phonon coupling in the newly discovered iron-pnictide class of high temperature superconductors have been studied using Raman spectroscopy assisted by theoretical calculations with the goal to elucidate the role of phonons as the mediating boson between electrons in a Cooper-pair. Our results indicate that the phononic spectra can, to good accuracy, be explained by neglecting completely the electron-phonon coupling. This provides further credentials to the widespread opinion that magnetic fluctuations constitute the attractive force responsible for the destabilization of the Fermi-sea leading to the condensation into superconducting Cooper-pairs. Further analysis shows that, through F doping on the O site, ejecting electrons into the quasi-two dimensional Fe-As plane actually leads to hole-doping at the Fe site, in accord with previous first principle calculations.The fundamental dynamics between the electrons, lattice and spins in two manganite samples have been studied using sub-picosecond time resolved pump-probe reflectance spectroscopy: The parent compound LaMnO3 and the doped La0.7Ca0.3MnO3 exhibiting colossal magnetoresistance. Even though LaMnO3 is a semiconductor and La0.7Ca0.3MnO3 is a double-exchange metal at low temperatures, the observed femtosecond dynamics are very similar. We introduce a phenomenological model that can explain all observed features in both samples using a combination of thermal (>1ps) and non-thermal (
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| 2. |
- Bielecki, Johan, 1982, et al.
(author)
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Two-component heat diffusion observed in LaMnO3 and La0.7Ca0.3MnO3
- 2010
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In: Physical Review B Condensed Matter. - : Americal Physical Society. - 0163-1829 .- 1095-3795. ; 81:6, s. art. no. 064434-
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Journal article (other academic/artistic)abstract
- We investigate the low-temperature electron, lattice, and spin dynamics of LaMnO3 (LMO) and La0.7Ca0.3MnO3 (LCMO) by resonant pump-probe reflectance spectroscopy. Probing the high-spin d-d transition as a function of time delay and probe energy, we compare the responses of the Mott insulator and the double-exchange metal to the photoexcitation. Attempts have previously been made to describe the subpicosecond dynamics of colossal magnetoresistance manganites in terms of a phenomenological three-temperature model describing the energy transfer between the electron, lattice, and spin subsystems followed by a comparatively slow exponential decay back to the ground state. However, conflicting results have been reported. Here we first show clear evidence of an additional component in the long-term relaxation due to film-to-substrate heat diffusion and then develop a modified three-temperature model that gives a consistent account for this feature. We confirm our interpretation by using it to deduce the band gap in LMO. In addition, we also model the nonthermal subpicosecond dynamics, giving a full account of all observed transient features both in the insulating LMO and the metallic LCMO.
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| 3. |
- He, Wenxiao, 1985, et al.
(author)
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Biomimetic nanocrystalline apatite resembling bone
- 2010
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In: European Cells and Materials: 3rd International NanoBio Conference 2010, ETH Zurich, Switzerland August 24-27 2010. - 1473-2262. ; 20:suppl. 3
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Conference paper (peer-reviewed)
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