S A Pauli

Paul Scherrer Institut, Aargau, Switzerland

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Publications (22)69.38 Total impact

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    ABSTRACT: An automated cluster algorithm is described, applicable to any image where a signal is to be analysed. The algorithm is employed in the context of surface X-ray diffraction data and extended to automate the data reduction process, which at present limits both the lead time to and the reliability of the retrieved structural information. A detailed evaluation of the constraints used to automate surface X-ray diffraction data analysis is provided. To overcome limitations of the algorithm and the experiment itself in certain geometries, the full field of view of area detectors is exploited to obtain orders of magnitude improvements in data collection. The method extends the surface X-ray diffraction technique to new systems and highlights the often archaic approach to the analysis of data collected with a two-dimensional detector.
    Journal of Applied Crystallography 02/2014; 47(1). · 3.34 Impact Factor
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    ABSTRACT: The application of multi-wavelength anomalous diffraction to thin films, interfaces and surface structures is presented. The method directly determines the amplitudes and phases of the complex surface structure factors from surface x-ray diffraction data, measured at three different energies around the absorption edge of one of the elements present in the film. Thereby, one is able to directly Fourier transform the data, which immediately provides meaningful and unambiguous electron-density distributions. These serve as a starting point for subsequent structural refinement. The robustness of the algorithm was evaluated on simulated data as a proof of principle. The experimental limitations and their effect on the method will be discussed as well as stability tests for the algorithm, such as the positions of the anomalous scatterers and the interfacial roughness. It will be shown that the method can be applied to real structures. The algorithm was tested on real data from a thin film of SrTiO(3) grown on NdGaO(3)(110).
    Journal of Physics Condensed Matter 06/2012; 24(30):305002. · 2.22 Impact Factor
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    ABSTRACT: We present a direct comparison between experimental data and ab initio calculations for the electrostrictive effect in the polar LaAlO(3) layer grown on SrTiO(3) substrates. From the structural data, a complete screening of the LaAlO(3) dipole field is observed for film thicknesses between 6 and 20 uc. For thinner films, an expansion of the c axis of 2% matching the theoretical predictions for an electrostrictive effect is observed experimentally.
    Physical Review Letters 07/2011; 107(5):056102. · 7.73 Impact Factor
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    ABSTRACT: Angle calculations for a (2+3)-type diffractometer are presented with comprehensive derivations for both cases of either a vertical or horizontal sample configuration. This work focuses on some particular aspects of using area detectors in surface X-ray diffraction, namely the role of the detector rotation and the direct conversion of the angle-resolved diffraction signal recorded by the detector into a two-dimensional slice through reciprocal space.
    Journal of Applied Crystallography 02/2011; 44(1):73-83. · 3.34 Impact Factor
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    ABSTRACT: The evolution of the atomic structure of LaAlO_{3} grown on SrTiO_{3} was investigated using surface x-ray diffraction in conjunction with model-independent, phase-retrieval algorithms between two and five monolayers film thickness. A depolarizing buckling is observed between cation and oxygen positions in response to the electric field of polar LaAlO_{3}, which decreases with increasing film thickness. We explain this in terms of competition between elastic strain energy, electrostatic energy, and electronic reconstructions. Based on these structures, the threshold for formation of a two-dimensional electron system at a film thickness of 4 monolayers is quantitatively explained. The findings are also qualitatively reproduced by density-functional-theory calculations.
    Physical Review Letters 01/2011; 106(3):036101. · 7.73 Impact Factor
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    ABSTRACT: Recent studies have shown evidence for the formation of a strongly correlated metal (SCM) phase in the metal-insulator transition (MIT) in vanadium dioxide; the spectral signatures of the SCM are distinct from the rutile metallic phase observed once the MIT is complete around 67C. We describe a search for a corresponding intermediate in the structural phase transition (SPT) from monoclinic to tetragonal, using both high-temperature X-ray diffraction (HTXRD) and synchrotron radiation experiments. The HTXRD experiments, with both epitaxial and highly ordered VO2 films on c-cut sapphire, showed that the hysteresis in the VO2 dielectric function derived from a Bruggeman effective-medium treatment was not congruent in temperature with that known from optical studies of the MIT. However, there was no evidence for anything other than a mixed tetragonal-monoclinic phase as the temperature was varied from below to above the critical temperature. The synchrotron experiments, carried out with samples prepared by in situ pulsed laser deposition on a-, c- and r-cut titania, on the other hand, do not rule out the existence of an intermediate structural phase, and may provide some support for such a strongly correlated structural phase.
    03/2010;
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    ABSTRACT: We present a high-resolution surface X-ray diffraction study of hexagonal boron nitride (h-BN) on the surface of Rh(111). The previously observed commensurate 13-on-12 superstructure for this system is stable in the temperature range between room temperature and 830°C. Surface X-ray diffraction measurements up to 830°C on the superstructure show no sign of a shift towards a different superstructure, demonstrating the high thermal stability and strong bonding between film and substrate. At lower temperatures, an anomalous thermal expansion behaviour of the topmost surface region of rhodium is observed, where the rhodium in-plane lattice constant remains invariant. This can be explained by the (h-BN) single-layer being compressively strained, whereby the strong bonding to the substrate causes the latter to be tensile strained.
    Surface Science - SURFACE SCI. 01/2010; 604(5).
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    ABSTRACT: The atomic structures of ultrathin YBa2Cu3O7−x (YBCO) films on SrTiO3(001) (STO) and (LaxSr1−x)(AlyTa1−y)O3(001) (LSAT) were investigated with sub-Angstrom resolution using surface x-ray diffraction and the phase-retrieval direct-method difference map using the constraints of atomicity and film shift (DCAF). The model-independent electron densities which emerge from random initializations in DCAF are exceedingly stable. The films grow with a well-defined stacking sequence even when grown on substrates with mixed terrace termination. Only very minor out-of-plane deviations from bulk YBCO are observed in the film structures, although they are perfectly strained to the substrate and are therefore tetragonal. The films are superconducting, with critical temperatures for growth on STO and LSAT of 43 K and 70 K, respectively. These results have important implications for reliable structure determination of technologically relevant complex-metal oxide surfaces and interfaces.
    Phys. Rev. B. 01/2010; 81(17).
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    ABSTRACT: The structure of epitaxially grown hexagonal boron nitride (h-BN) on the surface of a Ru(0001) single crystal was investigated using surface X-ray diffraction, which showed the system to form a commensurate 14-on-13 superstructure. This result disagrees with previous reports on superstructures of the same system and arguments based on simple thermal expansion coefficient calculations. We argue that the larger observed superstructure forms because of the strong bonding of h-BN to Ru. In comparison to h-BN/Rh(111) it can accommodate more induced lateral in-plane strain- or lock-in energy over larger regions (referred to as the holes) within the superstructure, which itself can consequently become larger.
    Surface Science 01/2010; 604(5). · 1.84 Impact Factor
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    ABSTRACT: Not Available Bibtex entry for this abstract Preferred format for this abstract (see Preferences) Find Similar Abstracts: Use: Authors Title Return: Query Results Return items starting with number Query Form Database: Astronomy Physics arXiv e-prints
    Surface Science 01/2010; 604. · 1.84 Impact Factor
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    ABSTRACT: The atomic structures of ultrathin YBaCuO{sub 7-x} (YBCO) films on SrTiO(001) (STO) and (LaSr{sub 1-x})(Al{sub y}Ta{sub 1-y})O(001) (LSAT) were investigated with sub-Angstrom resolution using surface x-ray diffraction and the phase-retrieval direct-method difference map using the constraints of atomicity and film shift (DCAF). The model-independent electron densities which emerge from random initializations in DCAF are exceedingly stable. The films grow with a well-defined stacking sequence even when grown on substrates with mixed terrace termination. Only very minor out-of-plane deviations from bulk YBCO are observed in the film structures, although they are perfectly strained to the substrate and are therefore tetragonal. The films are superconducting, with critical temperatures for growth on STO and LSAT of 43 K and 70 K, respectively. These results have important implications for reliable structure determination of technologically relevant complex-metal oxide surfaces and interfaces.
    Physical Review B 01/2010; 81(17):174520-174520. · 3.66 Impact Factor
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    ABSTRACT: We present a structural analysis of the graphene/Ru(0001) system obtained by surface x-ray diffraction. The data were fit using Fourier-series expanded displacement fields from an ideal bulk structure, plus the application of symmetry constraints. The shape of the observed superstructure rods proves a reconstruction of the substrate, induced by strong bonding of graphene to ruthenium. Both the graphene layer and the underlying substrate are corrugated, with peak-to-peak heights of (0.82 +/- 0.15) A and (0.19 +/- 0.02) A for the graphene and topmost Ru-atomic layer, respectively. The Ru-corrugation decays slowly over several monolayers into the bulk. The system also exhibits chirality, whereby in-plane rotations of up to 2.0 degrees in those regions of the superstructure where the graphene is weakly bound are driven by elastic energy minimization.
    New Journal of Physics 08/2009; · 4.06 Impact Factor
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    ABSTRACT: The conducting interface of LaAlO3/SrTiO3 heterostructures has been studied by hard x-ray photoelectron spectroscopy. From the Ti 2p signal and its angle dependence we derive that the thickness of the electron gas is much smaller than the probing depth of 4 nm and that the carrier densities vary with increasing number of LaAlO3 overlayers. Our results point to an electronic reconstruction in the LaAlO3 overlayer as the driving mechanism for the conducting interface and corroborate the recent interpretation of the superconducting ground state as being of the Berezinskii-Kosterlitz-Thouless type.
    Physical Review Letters 06/2009; 102(17):176805. · 7.73 Impact Factor
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    ABSTRACT: The availability of high-brilliance hard x-ray synchrotron radiation and the advent of novel photon counting area detectors have brought surface x-ray diffraction (SXRD) into a new era. It is now possible to record large numbers of structure factors with much improved reliability within reasonable beamtime durations. As a result, structural determination of the surfaces and interfaces of complex crystallographic systems and heterostructures has now become feasible, especially in conjunction with phase-retrieval methods. It is thereby hoped that detailed structural information will shed light on the unusual physical properties of these systems. Complex metal oxide systems investigated at the Materials Science beamline of the Swiss Light Source, including the surface of SrTiO3, the interface between LaAlO3 and SrTiO3, and the structure of YBa2Cu3O7 grown on NdGaO3, SrTiO3, and (LaSr)(AlTa)O3 will be presented as examples of what is now possible using SXRD.
    AIP Conference Proceedings. 01/2009; 1092(1).
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    ABSTRACT: We have studied the buried interface of LaAlO3/SrTiO3 heterostructures by hard X-ray photoelectron spectroscopy (HAXPES). Free charge carriers forming a two-dimensional electron gas (2DEG) at the interface between both insulators are identified from the Ti 3+ signal in the Ti 2p core level spectrum. Its distinct angle-dependence indicates that the thickness of the 2DEG is much smaller than the HAXPES probing depth of 4 nm. It is observed that the carrier density varies between differently prepared heterostructures and with increasing number of LaAlO3 overlayers. We also observe a sizeable contribution of photogenerated carriers of surprisingly long lifetimes, which needs to be corrected for in order to obtain intrinsic carrier concentrations.
    10/2008;
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    ABSTRACT: A direct method for the retrieval of electron density maps from surface x-ray diffraction data is presented and its performance is evaluated. The method, DCAF (difference map using the constraints of atomicity and film shift), is based on the difference map iteration scheme and uses, apart from the traditional constraints of atomicity, positivity and film thickness, a novel constraint, which we have named the 'film shift', whereby the real space solution is shifted up by an out-of-plane unit cell size of the underlying bulk substrate material if the topmost region of the same thickness contains insignificant electron density. This relaxes the film thickness constraint, which is necessarily loose in order to accommodate structural uncertainties at the film–substrate interface due to intermixing, roughness, and heteroepitaxial strain. DCAF's performance was evaluated by retrieval of the electron density distribution from a real data set, recorded from a five-monolayer film of LaAlO3 on SrTiO3, which resulted in an electron density in good agreement with the previously solved structure. Importantly, the stability and reproducibility of the final solution compares favorably with constraint combinations in which the film shift projection is omitted, highlighting the power of this new method. In addition, an example of a full structural solution for a three-monolayer-thick film of La1−xSrxMnO3 on SrTiO3 is presented, where DCAF electron density retrieval followed by model building and refinement was conducted. It will be shown that DCAF can be successfully applied to thin films for retrieving physically meaningful electron densities, and that it can also serve as a starting point for subsequent structure refinement.
    Journal of Physics Condensed Matter 10/2008; 20(44):445006. · 2.22 Impact Factor
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    ABSTRACT: The structure of a single layer of graphene on Ru(0001) has been studied using surface x-ray diffraction. A surprising superstructure containing 1250 carbon atoms has been determined, whereby 25 x 25 graphene unit cells lie on 23 x 23 unit cells of Ru. Each supercell contains 2 x 2 crystallographically inequivalent subcells caused by corrugation. Strong intensity oscillations in the superstructure rods demonstrate that the Ru substrate is also significantly corrugated down to several monolayers and that the bonding between graphene and Ru is strong and cannot be caused by van der Waals bonds. Charge transfer from the Ru substrate to the graphene expands and weakens the C-C bonds, which helps accommodate the in-plane tensile stress. The elucidation of this superstructure provides important information in the potential application of graphene as a template for nanocluster arrays.
    Physical Review Letters 10/2008; 101(12):126102. · 7.73 Impact Factor
  • S A Pauli, P R Willmott
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    ABSTRACT: Two-dimensional electron gases in semiconductors have found use in applications such as optoelectronics, high-power radio-frequency and magnetoelectronic devices. The ability to grow heterostructures of oxides exhibiting similar effects is a significant step towards the fabrication of all-oxide devices. Here, we give an overview of recent studies of two-dimensional electron gases formed at the interface between polar and non-polar perovskites. We discuss the proposed explanations of the origin of the conductivity and properties of the ground state.
    Journal of Physics Condensed Matter 07/2008; 20(26):264012. · 2.22 Impact Factor
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    ABSTRACT: We present new results on the application of the x-ray phase retrieval method, Coherent Bragg Rod Analysis (COBRA), to heteroepitaxial interfaces in LaAlO3 thin films grown on SrTiO3 substrates, a system known to form an interfacial quasi-2D electron gas. We observe a dilated, mixed-valence interface which modifies the electronic band structure, lowering the minimum of the conduction band below the Fermi level and thereby rendering the dilated interface conducting. In particular the COBRA measurements reveal the formation of an interfacial La,SrTiO3 layer with an accumulation of trivalent Ti at the interface which is responsible for the lattice dilation and minimizes the electrostatic energy at the TiO2-terminated SrTiO3 substrate surface. The work presented here establishes a structural basis for the formation of the conducting interface.
    03/2008;
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    ABSTRACT: Surface x-ray diffraction was used to determine the atomic structures of La1-xSrxMnO3 thin films, grown monolayer by monolayer on SrTiO3 by pulsed laser deposition. Structures for one-, two-, three-, four-, six-, and nine-monolayer-thick films were solved using the Coherent Bragg rod analysis phase-retrieval method and subsequent structural refinement. Four important results were found. First, the out-of-plane lattice constant is elongated across the substrate-film interface. Second, the transition from substrate to film is not abrupt, but proceeds gradually over approximately three unit cells. Third, Sr segregates towards the topmost monolayer of the film: we determined a Sr-segregation enthalpy of -15kJ/mol from the occupation parameters. Finally, the electronic bandwidth W was used to explain the onset of magnetoresistance for films of nine or more monolayers thickness. Resistivity measurements of the nine monolayer-thick film confirm magnetoresistance and the presence of a dead layer with mostly insulating properties.
    Physical review. B, Condensed matter 01/2008; 77(8). · 3.77 Impact Factor

Publication Stats

462 Citations
69.38 Total Impact Points

Institutions

  • 2007–2014
    • Paul Scherrer Institut
      Aargau, Switzerland
  • 2012
    • Uppsala University
      • Department of Physics and Astronomy
      Uppsala, Uppsala, Sweden
  • 2009
    • University of Michigan
      Ann Arbor, Michigan, United States