R. Herger

Paul Scherrer Institut, Villigen, AG, Switzerland

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Publications (25)48.57 Total impact

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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: 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. · 2.36 Impact Factor
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    ABSTRACT: We present angle-resolved photoemission spectroscopy (ARPES) results for thin films of the three-dimensional manganese perovskite La1-xSrxMnO3. We show that the temperature of the transition (Tc) from the paramagnetic insulating to the ferromagnetic metallic state is closely related to details of the electronic structure, particularly to the spectral weight at the k-point, where the sharpest step at the Fermi level was observed. We found that this k-point is the same for all the samples, despite their different values of Tc. The change of Tc is discussed in terms of kinetic energy optimization. Our ARPES results suggest that the change of the electronic structure for the samples having different transition temperatures is different from the rigid band shift.
    Journal of Physics Condensed Matter 06/2008; 20(22):2001-. · 2.36 Impact Factor
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    ABSTRACT: We present angle-resolved photoemission spectroscopy (ARPES) results for thin films of the three-dimensional manganese perovskite La(1-x)Sr(x)MnO(3). We show that the temperature of the transition (T(c)) from the paramagnetic insulating to the ferromagnetic metallic state is closely related to details of the electronic structure, particularly to the spectral weight at the k-point, where the sharpest step at the Fermi level was observed. We found that this k-point is the same for all the samples, despite their different values of Tc. The change of Tc is discussed in terms of kinetic energy optimization. Our ARPES results suggest that the change of the electronic structure for the samples having different transition temperatures is different from the rigid band shift.
    Journal of Physics Condensed Matter 06/2008; 20(22):222001(1-5). · 2.36 Impact Factor
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    ABSTRACT: We present angle-resolved photoemission spectroscopy results for thin films of the three- dimensional manganese perovskite La1 xSrxMnO3. We show that the transition temperature (Tc) from the paramagnetic insulating to ferromagnetic metallic state is closely related to details of the electronic structure, particularly to the spectral weight at the k-point, where the sharpest step at the Fermi level was observed. We found that this k-point is the same for all the samples, despite their different Tc. The change of Tc is discussed in terms of kinetic energy optimization. Our ARPES results suggest that the change of the electronic structure for the samples having different transition temperatures is different from the rigid band shift. PACS numbers:
    Journal of Physics Condensed Matter 04/2008; 20(22). · 2.36 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: We present an angle- and spin-resolved photoemission study of half-metallic three-dimensional perovskite La2/3Sr1/3MnO3 thin films grown with pulsed laser deposition. The experiments are performed in the (100) mirror plane. The experimental results are related to GGA+U calculations through simulations of photoemission spectra, which take into account the final state broadening in surface-perpendicular momentum and the initial-state broadening in energy. We demonstrate that due to the three-dimensional nature of La2/3Sr1/3MnO3 , these intrinsic mechanisms of the photoemission process give rise to deviations of the photoemission spectrum from the spectral function corresponding to the strict momentum conservation. In the spin-integrated data, we identify ghost spectral intensity coming from states with different momenta and, in particular, nonvanishing intensity filling the whole interior of the central Fermi surface spheroid. In the spin-resolved data, we find experimental polarization values near the Fermi surface of 55% at the normal emission and 80% at off-normal emission. Despite a seeming contradiction to the half-metallic picture, such a reduction in spin contrast is reproduced by our simulations as a result of the broadening mechanisms and is consistent with the half-metallic nature of La2/3Sr1/3MnO3 .
    Physical review. B, Condensed matter 01/2008; 77(16).
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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).
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    ABSTRACT: The complete atomic structure of a five-monolayer film of LaAlO3 on SrTiO3 has been determined for the first time by surface x-ray diffraction in conjunction with the coherent Bragg rod analysis phase-retrieval method and further structural refinement. Cationic mixing at the interface results in dilatory distortions and the formation of metallic La(1-x)SrxTiO3. By invoking electrostatic potential minimization, the ratio of Ti{4+}/Ti{3+} across the interface was determined, from which the lattice dilation could be quantitatively explained using ionic radii considerations. The correctness of this model is supported by density functional theory calculations. Thus, the formation of a quasi-two-dimensional electron gas in this system is explained, based on structural considerations.
    Physical Review Letters 11/2007; 99(15):155502. · 7.94 Impact Factor
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    ABSTRACT: We report the first complete determination, using surface x-ray diffraction, of the surface structure of TiO2-terminated SrTiO3(001), both at room temperature in vacuum, and also hot, under typical conditions used for thin film growth. The cold structure consists of a mixture of a (1x1) relaxation and (2x1) and (2x2) reconstructions. The latter disappear over several minutes upon heating. The structures are best modeled by a TiO2-rich surface similar to that proposed by Erdman et al. [Nature (London) 419, 55 (2002).10.1038/nature01010]. Both reconstructions have been shown by density functional theory to be energetically favorable. The calculated (1x1) surface energy is higher, indicating that it may be a disordered mixture of the reconstructions. Atomic displacements are significant down to three unit cells, which may have important implications on possible surface ferroelectric phenomena in SrTiO3.
    Physical Review Letters 03/2007; 98(7):076102. · 7.94 Impact Factor
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    ABSTRACT: We have characterized for the first time in situ the growth of vanadium dioxide nanoparticulate films prepared by pulsed-laser deposition, using a five-circle x-ray diffractometer, in order to provide structural information as the films are grown. A vanadium metal target was ablated in the presence of a pulsed N2O reactive gas source, and films were grown on Si(001) and Al2O3(0001) substrates. Optical measurements confirmed that the films deposited in this way exhibit the well-known VO2 metal-insulator transition at approximately 70 °C. The VO2 films grown at room temperature on silicon substrates are amorphous and extremely smooth. These become considerably rougher upon thermal annealing, as the VO2 phase crystallizes out in the form of hemispherical islands. These films also contain traces of a V2O5 nanoparticle phase in the first few monolayers, although the degree of VO2 crystallinity in the nanoparticles is quite high. In contrast, the VO2 nanoparticles grown on the sapphire substrates are in the form of preferentially oriented rodlike islands, with a maximum rod width of about 100 nm. This in-plane texture is in keeping with the relatively good lattice match between film and substrate, indicating good potential for the growth of VO2 epitaxial films and single-crystal nanoparticles on such substrates.
    Journal of Applied Physics 01/2007; 102. · 2.21 Impact Factor
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    ABSTRACT: The hexagonal boron-nitride ‘nanomesh’ surface reconstruction on Rh(111) [Corso et al., Science 303 (2004) 217–220] has been investigated using surface X-ray diffraction utilizing synchrotron radiation. This unique structure has been found to be stable under ambient atmosphere which provides an important basis for technological applications like templating and coating. The previously suggested (12×12) periodicity of this reconstruction has been unambiguously confirmed and structural features are discussed in the light of the X-ray diffraction results.
    Surface Science 01/2007; 601(2). · 1.84 Impact Factor
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    ABSTRACT: We report on the structural determination of the surface of TiO2 -terminated SrTiO3(001) using surface x-ray diffraction. The detailed analysis of two surface diffraction data sets are presented, one (cold) taken at room temperature in vacuum, and the other (hot) under typical conditions used for thin film growth. 49 different combinations of possible surface terminations are described for the cold structure, from which the final structure was chosen, consisting of a weighted mixture of a (1×1) relaxation and (2×1) and (2×2) reconstructions, simultaneously present at the surface. The structures are best modeled by a TiO2 -rich surface similar to that proposed by Erdman [Nature (London) 419, 55 (2002)]. The reconstructions are energetically favorable according to density functional theory. They disappear within several minutes upon heating to the hot conditions, forming a termination very similar to the cold (1×1) , but more puckered and higher in energy. Six additional models, suggested by direct methods and the literature, to describe the hot surface are also discussed. Direct methods confirm the TiO2 -rich termination and the atomic positions of the hot surface. The atomic coordinates for the two TiO2 -rich surfaces exhibit significant displacements down to three unit cells, which may have important implications on possible surface ferroelectric phenomena in SrTiO3 . Surface energy considerations suggest a temperature-induced order-disorder transition, produced by a mixing of the (2×1) and (2×2) reconstructions, to form the hot pseudo (1×1) structure. Electrostatic stability arguments provide circumstantial support for the experimentally determined TiO2 -rich surfaces.
    Physical review. B, Condensed matter 01/2007; 76(19).
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    ABSTRACT: A novel energetic smoothing mechanism in the growth of complex metal-oxide thin films is reported from in situ kinetic studies of pulsed laser deposition of on , using x-ray reflectivity. Below 50% monolayer coverage, prompt insertion of energetic impinging species into small-diameter islands causes them to break up to form daughter islands. This smoothing mechanism therefore inhibits the formation of large-diameter 2D islands and the seeding of 3D growth. Above 50% coverage, islands begin to coalesce and their breakup is thereby suppressed. The energy of the incident flux is instead rechanneled into enhanced surface diffusion, which leads to an increase in the effective surface temperature of DeltaT approximately 500 K. These results have important implications on optimal conditions for nanoscale device fabrication using these materials.
    Physical Review Letters 06/2006; 96(17):176102. · 7.94 Impact Factor
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    ABSTRACT: The use of an area detector in grazing-incidence X-ray experiments lends many advantages in terms of both speed and reliability. Here a discussion is given of the procedures established using the PILATUS pixel detector developed at the Swiss Light Source for optimizing data acquisition and analysis of surface diffraction data at the Materials Science beamline, especially with regard to reflectivity measurements, crystal truncation and fractional order rods, and grazing-incidence diffraction experiments.
    Acta Crystallographica Section A Foundations of Crystallography 08/2005; 61(Pt 4):418-25. · 2.24 Impact Factor
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    ABSTRACT: The intense flux delivered by third-generation synchrotron sources has opened up exciting new possibilities in surface diffraction (SD) studies [1–3]. Nonetheless, practical SD experiments are plagued by several technical problems. Because the crystalline surface is typically probed down to a depth of approximately 1 or 2 nm when using subcritical-angle incident X-ray beams, the scattering volume is only about 10 cm. This is further exacerbated by the fact that the most interesting information on the surface structure is generally obtained from those regions of diffraction features known as crystal truncation rods (CTRs) where scattering is weakest, i.e., in between Bragg peaks.
    Synchrotron Radiation News 03/2005; 18(2):16-22.
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    ABSTRACT: A novel ultra-high vacuum chamber for growing films via pulsed laser deposition and studying the growth processes in situ using hard synchrotron radiation is presented. The chamber is mounted onto a 5-circle surface diffractometer and contains a large cylindrical-section beryllium window, which allows an extensive range of reciprocal space to be probed. The chamber is primarily used to perform surface diffraction measurements, for which much of the most valuable information derives from the weakest signals. With this in mind, we have employed a photon-counting area pixel detector system, which reduces data acquisition times by one to two orders of magnitude and concomitantly increases the potential volume of data that can be acquired for any given sample before the surface degrades due to surface contamination or radiation damage. Examples are presented of initial results obtained using this system.
    Applied Surface Science 01/2005; 247(1):188-196. · 2.11 Impact Factor
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    ABSTRACT: The seeding and morphology of quasicrystalline TixNiyZr1-x-y thin films grown by pulsed-laser deposition on Al2O3(0001) have been investigated in situ, using in-plane x-ray diffraction and reflectivity. with synchrotron radiation. The crystallinity of the final films was further studied ex situ using a laboratory x-ray source. Local icosahedral order becomes established after a film thickness of 1.5 nm, which is followed by an abrupt change in the growth rate after a film thickness of approximately 5 nm, as long-range quasicrystalline order begins to become energetically favorable. The films grow two-dimensionally even up to thicknesses of over 100 nm and are highly textured, consisting of columnar grains with one of their fivefold symmetry axes perpendicular to the substrate surface.
    Physical Review B. 01/2005; 71(9).
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    ABSTRACT: One of the primary challenges of condensed matter physicists and materials scientists is the discovery and/or design of novel materials and their detailed characterization [1]. One can argue that this scientific odyssey began with the discovery of superconductivity in ceramic cuprates in 1984 [2], and more recently, colossal magnetoresistance in the manganates [3]. One of the consequences of this has been a concerted effort to produce high-quality thin films of systems such as YBa2Cu3O7 (YBCO), La1-xSrxMnO3 (LSMO), the ruthenates, vanadates, and other complex metal oxides, driven both by technological applications, and a desire to better understand the underlying physics of these fascinating systems.
    Synchrotron Radiation News 01/2005; 18(4):37-42.
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    ABSTRACT: We report an experimental and theoretical study of the strong dependence of the crystalline texture of thin films of SrxBa1-xNb2O6 grown on MgO(001) on the Sr-content x between 0.35
    Physical review. B, Condensed matter 01/2005; 71(14).