[en] 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).]. 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

[en] We present a structural analysis of the graphene/Ru(0001) system obtained by surface x-ray diffraction. The data were fitted 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 graphene and the 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 2.0⁰ in those regions of the superstructure where the graphene is weakly bound are driven by elastic energy minimization.

[en] The Paul Scherrer Institute is planning the construction of a hard-x-ray free-electron laser, the SwissFEL, by 2016, which will produce intense, ultrashort pulses of transversely coherent radiation in the wavelength range 0.1-7 nm, with future extensions to cover the range 0.08-30 nm. Special design considerations include (a) a compact construction, compatible with the status of a national facility, (b) a uniform 100 Hz repetition rate, well suited to sample manipulations and detector readout, (c) flexible wavelength tuning by the electron beam energy and undulator gaps, (d) soft x-rays at approximately 1 nm wavelength, with circular polarization and Fourier-transform-limited pulses, (e) hard x-rays of pulse duration 5-20 fs and (f) an independent source of high-energy, half-cycle terahertz pump pulses. The science case for the Swiss FEL project, which emphasizes the dynamics of condensed matter systems and the damage-free imaging of nanostructures, includes novel considerations that make optimal use of these features.

[en] The ESB instrument at the SwissFEL ARAMIS hard X-ray free electron laser is designed to perform pump-probe experiments in condensed matter and material science employing photon-in and photon-out techniques. It includes a femtosecond optical laser system to generate a variety of pump beams, a X-ray optical scheme to tailor the X-ray probe beam, shot-to-shot diagnostics to monitor the X-ray intensity and arrival time, and two endstations operated at a single focus position that include multi-purpose sample environments and 2D pixel detectors for data collection.

[en] Conditional measurements on the undriven mode of a two-mode cavity QED system prepare a coherent superposition of ground states which generate quantum beats. The continuous system drive induces decoherence through the phase interruptions from Rayleigh scattering, which manifests as a decrease of the beat amplitude and an increase of the frequency of oscillation. We report recent experiments that implement a simple feedback mechanism to protect the quantum beat. We continuously drive the system until a photon is detected, heralding the presence of a coherent superposition. We then turn off the drive and let the superposition evolve in the dark, protecting it against decoherence. At a later time we reinstate the drive to measure the amplitude, phase, and frequency of the beats. The amplitude can increase by more than fifty percent, while the frequency is unchanged by the feedback. (authors)

[en] 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-xSr_xTiO₃. By invoking electrostatic potential minimization, the ratio of Ti⁴⁺/Ti³⁺ 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

[en] 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 (1x1) relaxation and (2x1) and (2x2) reconstructions, simultaneously present at the surface. The structures are best modeled by a TiO₂-rich surface similar to that proposed by Erdman et al. [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 (1x1), 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 (2x1) and (2x2) reconstructions, to form the hot pseudo (1x1) structure. Electrostatic stability arguments provide circumstantial support for the experimentally determined TiO₂-rich surfaces

[en] Beam-induced damage on diffractive hard X-ray optics is studied by means of X-ray diffraction and scanning electron microscopy. The issue of beam-induced damage on diffractive hard X-ray optics is addressed. For this purpose a systematic study on the radiation damage induced by a high-power X-ray beam is carried out in both ambient and inert atmospheres. Diffraction gratings fabricated by three different techniques are considered: electroplated Au gratings both with and without the polymer mold, and Ir-coated Si gratings. The beam-induced damage is monitored by X-ray diffraction and evaluated using scanning electron microscopy

[en] We implement a simple feedback mechanism on a two-mode cavity QED system to preserve the Zeeman coherence of a ground state superposition that generates quantum beats on the second-order correlation function. Our investigation includes theoretical and experimental studies that show how to prevent a shift away from the Larmor frequency and associated decoherence caused by Rayleigh scattering. The protocol consists of turning off the drive of the system after the detection of a first photon and letting it evolve in the dark. Turning the drive back on after a pre-set time reveals a phase accumulated only from Larmor precession, with the amplitude of the quantum beat more than a factor of two larger than with continuous drive. (paper)

[en] We measure the quantum speed of the state evolution of the field in a weakly-driven optical cavity QED system. To this end, the mode of the electromagnetic field is considered as a quantum system of interest with a preferential coupling to a tunable environment: the atoms. By controlling the environment, i.e., changing the number of atoms coupled to the optical cavity mode, an environment assisted speed-up is realized: the quantum speed of the state re-population in the optical cavity increases with the coupling strength between the optical cavity mode and this non-Markovian environment (the number of atoms)