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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. · 2.55 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). · 2.55 Impact Factor
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P R Willmott,
S A Pauli, R Herger,
C M Schlepütz,
D Martoccia,
B D Patterson,
B Delley,
R Clarke,
D Kumah,
C Cionca,
Y Yacoby
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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. · 7.37 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. · 7.37 Impact Factor
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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. · 7.37 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. · 2.08 Impact Factor
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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⩽x⩽0.75. Synchrotron-based x-ray diffraction measurements have identified three film-to-substrate crystal orientations, with contributions from domains at 0°, ±18.43°, and ±30.96°. The relative contributions from these domains change dramatically with x. Surprisingly, the ±18.43° orientation dominates, particularly for high x, although it has the largest lattice mismatch to the underlying substrate. These results can only be explained by detailed modeling of the electrostatic forces between the ions at the film-substrate interface, and not with simplistic lattice mismatch arguments. Therefore, molecular dynamics simulations of the ionic heterogeneous interface structure of thin films of such large oxide systems have been performed in an attempt to explain the diffraction data. The simulations predict that, depending on x, an initial ultrathin layer of SrNb2O6 is favored, such that its Nb-O network forms the first ionic layer on the MgO(001) substrate. This provides the necessary template for the change in crystallographic orientation with x. Such layers were subsequently identified by further synchrotron-based x-ray-diffraction measurements. This information has important consequences for ferroelectric and electrooptic applications, on the optimal conditions required to grow thin films with large crystalline domains of the desired orientation.
Phys. Rev. B. 04/2005; 71(14).
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Synchrotron Radiation News 03/2005; 18(2):16-22.
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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: A new ablation target geometry is presented that was used to produce thin films of La1-xSrxMnO3 grown heteroepitaxially on SrTiO3 by pulsed reactive crossed-beam laser ablation. The films were grown in order to perform angle-resolved photoelectron spectroscopy, which demands that the surface be atomically flat. In situ and ex situ analysis shows that this condition was met, even after depositing to a thickness of over 100 nm.
Applied Physics A 10/2004; A79(4-6):1199-1201. · 1.63 Impact Factor
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ABSTRACT: A new ablation target geometry is presented that was used to produce thin films of La1-xSrxMnO3 grown heteroepitaxially on SrTiO3 by pulsed reactive crossed-beam laser ablation. The films were grown in order to perform angle-resolved photoelectron spectroscopy, which demands that the surface be atomically flat. In situ and ex situ analysis shows that this condition was met, even after depositing to a thickness of over 100nm.
Applied Physics A 08/2004; 79(4):1199-1201. · 1.63 Impact Factor
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ABSTRACT: A novel ablation target geometry is presented, whereby an ablation rod is divided into two or more sections and rapidly cyclically translated along its axis. In this manner, a wide range of film compositions can be achieved using the same target. The flexibility of this technique is demonstrated for the growth of quasicrystalline and complex metal oxide thin films.
Thin Solid Films · 1.89 Impact Factor
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ABSTRACT: We present the first angle-resolved photoemission spectroscopy (ARPES) results on three-dimensional manganese perovskite La0.66Sr0.34MnO3. In contrast to ARPES results on layered manganites, a finite and k-dependent spectral weight at the Fermi level was observed. We propose a complex energy band to describe the low binding energy electronic states, which may result from the temporally dynamic orbital orientation distributions and/or nanoscale charge inhomogeneities, to account for the anomalously broad ARPES features observed in the measurements. Flat sections of the Fermi surface were determined. A density wave resulting from nesting instabilities induced by the flat Fermi surface sections is manifested by the energy band folding.
Phys. Rev. B. 70(14).
