The physical mechanisms responsible for the formation of a two-dimensional electron gas at the interface between insulating srTio 3 and LaAlo 3 have remained a contentious subject since its discovery in 2004. opinion is divided between an intrinsic mechanism involving the build-up of an internal electric potential due to the polar discontinuity at the interface between srTio 3 and LaAlo 3 , and extrinsic mechanisms attributed to structural imperfections. Here we show that interface conductivity is also exhibited when the LaAlo 3 layer is diluted with srTio 3 , and that the threshold thickness required to show conductivity scales inversely with the fraction of LaAlo 3 in this solid solution, and thereby also with the layer's formal polarization. These results can be best described in terms of the intrinsic polar-catastrophe model, hence providing the most compelling evidence, to date, in favour of this mechanism.

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)Å and (0.19 ± 0.02)Å 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 o in those regions of the superstructure where the graphene is weakly bound are driven by elastic energy minimization.

Surface x-ray diffraction was used to determine the atomic structures of La 1−x Sr x MnO 3 thin films, grown monolayer by monolayer on SrTiO 3 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 −15 kJ/ 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.

The interaction and energy transfer of a laser ablation plasma of Ti with a pulsed N2 supersonic expansion are investigated using time-of-flight quadrupole mass spectroscopy and Langmuir probe techniques. The Ti ablation target and the exit nozzle of the pulsed gas source are positioned so that the plasma plume and gas pulse interact near to their respective origins, where the number density is still high, which hence results in strong coupling of the nascent plasma with the gas pulse. The timing between the gas pulse and ablation plume is shown to be critical in determining the scattering processes and the chemical nature of the films grown by this method, an example of which is presented. The degree of ionization of the plasma when crossed with the gas pulse compared to that for expansion into vacuum increases from less than 10−3 to 0.28±0.11, which is attributed to collision-induced ionization of Ti atoms. Further increasing the N2 number density quenches the ion signal. The effective bimolecular cross section for scattering of Ti with the high density N2 pulse is about 4 times larger than that with a static background of low pressure N2, while the fractional depletion of the N2 pulse by the Ti plume depends on the N2 number density in the pulse, indicating that at these high local pressures, collective effects prevail. We propose a simple model for the resulting evolution of the plasma based on electrostatic considerations.

We present an angle-and spin-resolved photoemission study of half-metallic three-dimensional perovskite La 2/3 Sr 1/3 MnO 3 thin films grown with pulsed laser deposition. The experiments are performed in the ͑100͒ mirror plane. The experimental results are related to GGAϩU 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 La 2/3 Sr 1/3 MnO 3 , 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 La 2/3 Sr 1/3 MnO 3 .

No abstract

No abstract

Copper (II) oxide films of high chemical and crystal purity have been grown on MgO(100) substrates using pulsed laser ablation (248 mn) of copper in conjunction with a pulsed oxidizing gas source (NzO). A novel experimental geometry is presented, in which the gas pulse and ablation plume cross close to their respective origins, where the particle and energy densities are highest. The properties of the films have been analyzed by x-ray diffraction and scanning electron microscopy. Growth rates are typically 0.025-0.035 &laser pulse within the substrate temperature region of 50 to 600 "C. Results in the region of 350 "C are presented. Analysis by time-of-flight mass spectroscopy has identified the predominant gas-phase species during ablation and transportation to the growing surface.