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Systematic structure investigation of YBCO thin films with direct methods and surface x-ray diffraction


Schlepütz, C M. Systematic structure investigation of YBCO thin films with direct methods and surface x-ray diffraction. 2009, University of Zurich, Faculty of Science.

Abstract

Complex transition metal oxides and strongly correlated electron systems (SCES) exhibit an extraordinary wealth of physical phenomena and properties, ranging from features as diverse as ferroelectricity and colossal magnetoresistance to mulitferroicity and high-temperature superconductivity. The combination of several such materials into articial heterostructures has led to new and entirely unexpected phenomena at their interfaces, for example the formation of a high-mobility two-dimensional electron-gas at the heterointerface of the two bandgap insulators SrTiO3 and LaAlO3. These systems are both interesting from a fundamental point of view as well as for their large potential in technological applications. The complex interactions between the spin, charge, lattice, and orbital degrees of freedom in SCES are strongly aected by small atomic displacements. The exact knowledge of the atomic structure down to well below the sub-Angstrom length scale is therefore of paramount signicance both for the understanding and theoretical modeling of the physical eects, as well as for the design of nano-scaled technical devices. Surface x-ray diraction (SXRD) oers unique capabilities for the investigation of twodimensional systems such as surfaces, interfaces, and thin lms or heterostructures. The high spatial resolution of a few thousandths of an Angstrom, in combination with the high penetration power of the x-rays, allows for the three-dimensional reconstruction of the atomic structure for surfaces and even for buried interfaces. One of the main challenges for SXRD lies in the structural complexity of such systems, which often involve simultaneous atomic rearrangements over a region comprising several unit cells in depth, containing typically several dozens of individual atoms. Traditional model renement techniques are bound to fail for such complex systems. Recent developments in direct methods for SXRD, in conjunction with the enhanced data quality and quantity provided by fast, singlephoton counting area pixel detectors, can help to overcome this limitation. By iteratively reconstructing the experimentally unaccessible phases, based only on the measured diraction intensities and other a priori knowledge of the system, the recorded diraction pattern can be inverted directly to yield a three-dimensional electron-density map of the investigated structure, which can be interpreted directly in terms of an atomic structure or used as a starting point for further renement steps. In this thesis, we present SXRD studies of thin YBa2Cu3O7-x (YBCO) lms grown by pulsed laser deposition on SrTiO3 (STO), (LaxSr1-x)(AlyTa1-y)O3 (LSAT), and NdGaO3 (NGO). With a thickness of approximately 4 unit cells, the c-axis oriented YBCO lms represent one of the most complex systems investigated with direct methods and SXRD to date. This makes it an ideal testbed to evaluate dierent combinations of direct method and renement approaches in terms of their relative merits and limitations. The level of consistency between the dierent methods and for random restarts of individual algorithms gives a direct feedback about the credibility of the nal structure solutions. Using this approach, the atomic structure of YBCO both on STO and on LSAT was found to be exceedingly bulk-like in terms of the interatomic distances and bond angles, but heteroepitaxially strained to match the in-plane lattice constants of each substrate. This results in (a) a very slight contraction of the YBCO c-axis constant of less than 1%, in response to the tensile in-plane strain, and (b) the formation of a tetragonal YBCO unit cell due to the equal lengths of the substrate a- and b-axes. This change in symmetry may have important consequences for the physical properties of the lms, especially regarding the eects on the CuO chain layers of YBCO, which show a strong anisotropy in the a- and b-directions of the orthorhombic YBCO bulk unit cell. This detailed structural information is of eminent importance for the interpretation of surface-sensitive measurements, for example of the electronic structure using angle-resolved photoelectron spectroscopy, and as a starting point for theoretical calculations. Finally, the fact that the lm structures are very bulk-like may allow for a direct correlation between bulk properties and results obtained from surface-sensitive measurements, making high-quality thin lms viable substitutes for bulk samples, where they are unavailable in sucient quality.

Abstract

Complex transition metal oxides and strongly correlated electron systems (SCES) exhibit an extraordinary wealth of physical phenomena and properties, ranging from features as diverse as ferroelectricity and colossal magnetoresistance to mulitferroicity and high-temperature superconductivity. The combination of several such materials into articial heterostructures has led to new and entirely unexpected phenomena at their interfaces, for example the formation of a high-mobility two-dimensional electron-gas at the heterointerface of the two bandgap insulators SrTiO3 and LaAlO3. These systems are both interesting from a fundamental point of view as well as for their large potential in technological applications. The complex interactions between the spin, charge, lattice, and orbital degrees of freedom in SCES are strongly aected by small atomic displacements. The exact knowledge of the atomic structure down to well below the sub-Angstrom length scale is therefore of paramount signicance both for the understanding and theoretical modeling of the physical eects, as well as for the design of nano-scaled technical devices. Surface x-ray diraction (SXRD) oers unique capabilities for the investigation of twodimensional systems such as surfaces, interfaces, and thin lms or heterostructures. The high spatial resolution of a few thousandths of an Angstrom, in combination with the high penetration power of the x-rays, allows for the three-dimensional reconstruction of the atomic structure for surfaces and even for buried interfaces. One of the main challenges for SXRD lies in the structural complexity of such systems, which often involve simultaneous atomic rearrangements over a region comprising several unit cells in depth, containing typically several dozens of individual atoms. Traditional model renement techniques are bound to fail for such complex systems. Recent developments in direct methods for SXRD, in conjunction with the enhanced data quality and quantity provided by fast, singlephoton counting area pixel detectors, can help to overcome this limitation. By iteratively reconstructing the experimentally unaccessible phases, based only on the measured diraction intensities and other a priori knowledge of the system, the recorded diraction pattern can be inverted directly to yield a three-dimensional electron-density map of the investigated structure, which can be interpreted directly in terms of an atomic structure or used as a starting point for further renement steps. In this thesis, we present SXRD studies of thin YBa2Cu3O7-x (YBCO) lms grown by pulsed laser deposition on SrTiO3 (STO), (LaxSr1-x)(AlyTa1-y)O3 (LSAT), and NdGaO3 (NGO). With a thickness of approximately 4 unit cells, the c-axis oriented YBCO lms represent one of the most complex systems investigated with direct methods and SXRD to date. This makes it an ideal testbed to evaluate dierent combinations of direct method and renement approaches in terms of their relative merits and limitations. The level of consistency between the dierent methods and for random restarts of individual algorithms gives a direct feedback about the credibility of the nal structure solutions. Using this approach, the atomic structure of YBCO both on STO and on LSAT was found to be exceedingly bulk-like in terms of the interatomic distances and bond angles, but heteroepitaxially strained to match the in-plane lattice constants of each substrate. This results in (a) a very slight contraction of the YBCO c-axis constant of less than 1%, in response to the tensile in-plane strain, and (b) the formation of a tetragonal YBCO unit cell due to the equal lengths of the substrate a- and b-axes. This change in symmetry may have important consequences for the physical properties of the lms, especially regarding the eects on the CuO chain layers of YBCO, which show a strong anisotropy in the a- and b-directions of the orthorhombic YBCO bulk unit cell. This detailed structural information is of eminent importance for the interpretation of surface-sensitive measurements, for example of the electronic structure using angle-resolved photoelectron spectroscopy, and as a starting point for theoretical calculations. Finally, the fact that the lm structures are very bulk-like may allow for a direct correlation between bulk properties and results obtained from surface-sensitive measurements, making high-quality thin lms viable substitutes for bulk samples, where they are unavailable in sucient quality.

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Additional indexing

Item Type:Dissertation
Referees:Osterwalder J, Willmott P R, Keller H, Mesot J, Patterson B D
Communities & Collections:07 Faculty of Science > Physics Institute
Dewey Decimal Classification:530 Physics
Language:English
Date:2009
Deposited On:05 Mar 2010 18:54
Last Modified:07 Dec 2017 01:49
Number of Pages:259
Related URLs:http://opac.nebis.ch/F/?local_base=NEBIS&con_lng=GER&func=find-b&find_code=SYS&request=005825868

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