Impact of ambient oxygen on the surface structure of α-Cr2O3(0001)

Surface x-ray diffraction has been employed to quantitatively assess the surface structure of α-Cr2O3(0001) as a function of oxygen partial pressure at room temperature. In ultrahigh vacuum, the surface is found to exhibit a partially occupied double layer of chromium atoms. At an oxygen partial pressure of 1×10−2 mbar, the surface is determined to be terminated by chromyl species (CrO), clearly demonstrating that the presence of oxygen can significantly influence the structure of α-Cr2O3(0001)

Revising the kinematics of 12GHz CH3OH masers in W3(OH)

We derive accurate proper motions of the CH3OH 12 GHz masers towards the W3(OH) UCHII region, employing seven epochs of VLBA observations spanning a time interval of about 10 yr. The achieved velocity accuracy is of the order of 0.1 km/s, adequate to precisely measure the relative velocities of most of the 12 GHz masers in W3(OH), with amplitude varying in the range 0.3 - 3 km/s. Towards W3(OH), the most intense 12 GHz masers concentrate in a small area towards the north (the northern clump) of the UCHII region. We have compared the proper motions of the CH3OH 12 GHz masers with those (derived from literature data) of the OH 6035 MHz masers, emitting from the same region of the methanol masers. In the northern clump, the two maser emissions emerge from nearby (but likely distinct) cloudlets of masing gas with, in general, a rather smooth variation of line-of-sight and sky-projected velocities, which suggests some connection of the environments and kinematics traced by both maser types. The conical outflow model, previously proposed to account for the 12 GHz maser kinematics in the northern clump, does not reproduce the new, accurate measurements of 12 GHz maser proper motions and has to be rejected. We focus on the subset of 12 GHz masers of the northern clump belonging to the "linear structure at P.A. = 130-140 degree", whose regular variation of LSR velocities with position presents evidence for some ordered motion. We show that the 3-dimensional velocities of this "linear distribution" of 12GHz masers can be well fitted considering a flat, rotating disk, seen almost edge-on.Comment: 32 pages, 10 figures; accepted in ApJ (Main Journal

Structural and electrical properties of indium oxide thin films grown by pulsed laser deposition in oxygen ambient

We report results of structural, optical and electrical transport studies of indium oxide (IO) thin films grown by Pulsed Laser Deposition (PLD) under various oxygen gas pressures and using different substrates at 350° C. We find that the morphology and electrical resistivity of these films which are highly transparent changes drastically as O2pressure increases into mbar range, irrespective of substrate. A systematic increase in resistivity, coming mainly from a drop in the electron concentration, is observed as oxygen pressure varies from 0.0004 to 1 mbar. This could permit modulation of IO thin–films’ electrical parameters by more than three orders of magnitude suggesting that PLD grown films could be an attractive material for optoelectronic applications

Surface x-ray-diffraction study of the Rh(111)+(2×2)−3CO structure

We have studied the geometry of the high-coverage Rh(111)+(2×2)−3CO structure by surface x-ray diffraction. Analysis of the in-plane data set reveals three evenly separated CO molecules per (2×2) unit cell. The evaluation of the crystal truncation rods shows that one CO molecule resides in an on-top site while the other two CO molecules occupy hollow sites. The intensity modulations of the out-of-plane fractional order rods provide geometrical information about distances between the C and O atoms and on the buckling of the CO overlayer

Trigonometric Parallaxes of Massive Star Forming Regions: II. Cep A & NGC 7538

We report trigonometric parallaxes for the sources NGC 7538 and Cep A, corresponding to distances of 2.65 [+0.12/-0.11] kpc and 0.70 [+0.04/-0.04] kpc, respectively. The distance to NGC 7538 is considerably smaller than its kinematic distance and places it in the Perseus spiral arm. The distance to Cep A is also smaller than its kinematic distance and places it in the Local arm or spur. Combining the distance and proper motions with observed radial velocities gives the location and full space motion of the star forming regions. We find significant deviations from circular Galactic orbits for these sources: both sources show large peculiar motions (> 10 km/s) counter to Galactic rotation and NGC 7538 has a comparable peculiar motion toward the Galactic center.Comment: 21 pages, 8 figures; to appear in the Astrophysical Journa

Structure of a model TiO2 photocatalytic interface

The interaction of water with TiO2 is crucial to many of its practical applications, including photocatalytic water splitting. Following the first demonstration of this phenomenon 40 years ago there have been numerous studies of the rutile single-crystal TiO2(110) interface with water. This has provided an atomic-level understanding of the water-TiO2 interaction. However, nearly all of the previous studies of water/TiO2 interfaces involve water in the vapour phase. Here, we explore the interfacial structure between liquid water and a rutile TiO2(110) surface pre-characterized at the atomic level. Scanning tunnelling microscopy and surface X-ray diffraction are used to determine the structure, which is comprised of an ordered array of hydroxyl molecules with molecular water in the second layer. Static and dynamic density functional theory calculations suggest that a possible mechanism for formation of the hydroxyl overlayer involves the mixed adsorption of O2 and H2O on a partially defected surface. The quantitative structural properties derived here provide a basis with which to explore the atomistic properties and hence mechanisms involved in TiO2 photocatalysis

Magnetostructural coupling, magnetic ordering, and cobalt spin reorientation in metallic P r0.5 S r0.5Co O3 cobaltite

In half-doped Pr0.50A0.50CoO3 metallic perovskites, the spin-lattice coupling brings about distinct magnetostructural transitions for A=Ca and A=Sr at temperatures close to ~100 K. However, the ground magnetic properties of Pr0.50Sr0.50CoO3 (PSCO) strongly differ from Pr0.50Ca0.50CoO3 ones, where a partial Pr3+ to Pr4+ valence shift and Co spin transition makes the system insulating below the transition. This paper investigates and describes the relationship between the Imma¿I4/mcm symmetry change [Padilla-Pantoja, García-Muñoz, Bozzo, Jirák, and Herrero-Martín, Inorg. Chem. 53, 12297 (2014)] and the original magnetic behavior of PSCO versus temperature and external magnetic fields. The FM1 and FM2 ferromagnetic phases, above and below the magnetostructural transition (TS1~120K) have been investigated. The FM2 phase of PSCO is composed of [100] FM domains, with magnetic symmetry Im'm'a (mx¿0, mz=0). The magnetic space group of the FM1 phase is Fm'm'm (with mx=my). Neutron data analyses in combination with magnetometry and earlier reports results agrees with a reorientation of the magnetization axis by 45° within the ab plane across the transition, in which the system retains its metallic character. The presence below TS1 of conjugated magnetic domains, both of Fm'm'm symmetry but having perpendicular spin orientations along the diagonals in the xy plane of the tetragonal unit cell, is at the origin of the anomalies observed in the macroscopic magnetization. A relatively small field µ0H[¿z]¿30mT is able to reorient the magnetization within the ab plane, whereas a higher field (µ0H[¿z]¿1.2T at 2 K) is necessary to align the Co moments perpendicular to the ab plane. Such a spin reorientation, in which the orbital and spin components of the Co moment rotate joined by 45°, was not observed previously in analogous cobaltites without praseodymium

Electronic and structural reconstructions of the polar (111) SrTiO3 surface

Polar surfaces are known to be unstable due to the divergence of the surface electrostatic energy. Here we report on the experimental determination, by grazing incidence x-ray diffraction, of the surface structure of polar Ti-terminated (111) SrTiO3 single crystals. We find that the polar instability of the 1 x 1 surface is solved by a pure electronic reconstruction mechanism, which induces out-of-plane ionic displacements typical of the polar response of SrTiO3 layers to an electron confining potential. On the other hand, the surface instability can be also eliminated by a structural reconstruction driven by a change in the surface stoichiometry, which induces a variety of 3 x 3 (111) SrTiO3 surfaces consisting in an incomplete Ti (surface)-O-2 (subsurface) layer covering the 1 x 1 Ti-terminated (111) SrTiO3 truncated crystal. In both cases, the TiO6 octahedra are characterized by trigonal distortions affecting the structural and the electronic symmetry of several unit cells from the surface. These findings show that the stabilization of the polar (111) SrTiO3 surface can lead to the formation of quasi two-dimensional electron systems characterized by radically different ground states which depend on the surface reconstructions

A Quantitative Structural Investigation of the 0.1 wt % Nb-SrTiO3(001)/H2O Interface

Surface X-ray diffraction has been employed to elucidate the structure of the interface between a well-characterized (001) surface of 0.1 wt % Nb–SrTiO3 and liquid H2O. Results are reported for the clean surface, the surface in contact with a drop of liquid water, and the surface after the water droplet has been removed with a flow of nitrogen. The investigation revealed that the clean surface, prepared via annealing in 1 × 10–2 mbar O2 partial pressure, is unreconstructed and rough on a short length scale. The surface is covered with large terraces, the topmost layer of which is either TiO2 or SrO with an area ratio of about 7/3. For the surface in contact with water, our results reveal that associative H2O adsorption is favored for the TiO2-terminated terrace whereas adsorption is dissociative for the SrO-terminated terrace, which validates recent first-principles calculations. After removal of the water droplet, the surface largely resembles the water-covered surface but now with a disordered overlayer of water present on the surface