Tracy Turner

Surface relaxation of SrTiO3(001)

Surface X-ray diffraction has been used to examine the 300 K structure of SrTiO3(001)1×1 with a termination of 78% TiO and 22% SrO. The data indicate that a lateral ferroelectric distortion is absent on both terminations, consistent with a recent theoretical calculation. Although the negligible Ti atom relaxation found on the TiO termination is consistent with medium-energy ion scattering data, it differs from recent density functional theory calculations by 0.13 and 0.16 A. In contrast, the Sr displacement towards the SrO surface of 0.22±0.07 A is in good agreement with theory.

Interface structure of Si(111)-(√3 × √3)R30°-ErSi2 − x

The interface structure of epitaxial ErSi2 − x (x ≈ 0.3) on Si(111) has been measured by means of surface X-ray diffraction. The silicide consists of a stack of alternating Si and Er planes. The Si planes resemble compressed substrate bilayers, in which a regular network of vacancies releases the compressive strain. This vacancy network gives rise to a (√3 × √3)R30° reconstruction of the silicide, in which Si atoms are displaced towards the vacancies, and Er atoms are displaced away from the vacancies. The Si-Si bond length in the silicide is 2.36±0.02 A, and the nearest-neighbour ErSi distance is 2.95±0.02 A. The positions of the ato the interface region are identical to those found for two-dimensional erbium silicide on Si(111), and first-layer Er atoms are located on T4 sites on the substrate. Our preparation method (reactive deposition epitaxy) leads to rough silicide films. Nevertheless, we are able to extract the atomic occupancies of eight individual layers. All silicide layers are reconstructed, but only the silicide layer at the interface shows long-range order of the vacancy network.

Surfactants used in Ag(111) homoepitaxy : Sb, In, Pt and O2

We have investigated the effect of several surfactants on the homoepitaxial growth of Ag(111) by X-ray reflectivity experiments. Recently, we found that submonolayer amounts of Sb induce layer-by-layer growth of Ag on Ag(111). Now we have varied the amount of Sb and also used In, Pt and O2 as surfactant. We show that Sb is not unique in inducing two-dimensional growth in Ag(111) homoepitaxy: In and Pt have the same effect. For O2 the sticking coefficient to Ag(111) is too low to affect the growth significantly. At the lowest growth temperatures (<170 K), only Sb induces layer-by-layer growth.

Resonant and Core Level Photoemission Spectroscopy of the Misfit Layer Compound (SnS)1.20TiS2

(SnS)1.20TiS2 is member member of a family of misfit layer compounds with a general formula (MX)1+δTX2 in which M = Sn, Pb, Bi, Ln; T = Ti, V, Cr, Nb, Ta; X = S, Se and 0.08 < δ < 0.23. The triclinic compound (SnS)1.20TiS2 is built of double layers SnS and sandwiches TiS2 with Ti trigonally antiprismatic coordinated by S. These misfit layer compounds are synthesized at high temperature. The stability gained by the alternation of the stacking must therefore be considerable. The valence band spectrum, a constant initial state spectrum and core level spectra are presented here for (SnS)1.20TiS2 to discuss a possible charge transfer. A photoemission resonance is observed for the Ti 3d states at photon energies around 35 eV. The energy of the Ti 3p states is 35.4 eV below the Fermi level. The spectra presented here do not give any arguments for a charge transfer. Therefore we think that the stability of (SnS)1.20TiS2 does not arise from a charge transfer from one layer to another.

The Ni(100)(2×2)p4g–N reconstruction determined by surface X-ray diffraction

The ‘clock’ reconstruction of the Ni(100)(2×2)p4g–N system has been measured using surface X-ray diffraction. An in-plane displacement of the surface layer Ni atoms of dxy=0.30±0.01 A was found. This value is smaller than that measured for this system using photoelectron diffraction (PED) and surface extended X-ray absorption fine structure (SEXAFS). Possible reasons for this are discussed.

A LEED and photoemission spectroscopy study of the surface of the incommensurate misfit layer compound (SnS)1.16TaS2

The misfit layer compounds with stoichiometry (MX)1+δTX2 are composites of MX and TX2 layers, which are stacked alternatingly. This type of compound is known for M = Sn, Pb, Bi, La; T = Ti, Ta, Nb and X = S, Se. The MX layers adopt an SnS-type crystal structure. The compounds are synthesized at high temperature. Therefore the stability gained by the alternation of the stacking of the TX2 layers by MX layers must be considerable. LEED photographs and angular resolved UPS spectra are presented to show the large differences between the commensurate and the incommensurate directions in (SnS)1.16TaS2. Core level photoemission spectra indicate that there is no charge transfer from one layer to the other. Thus the stability of (SnS)1.16TaS2 cannot be explained by charge transfer.

SRS Highlights Presented at British Association Festival of Science

The British Association (BA) Festival of Science is a special opportunity for any member of the public interested in science to meet together with scientists and discuss as well as review science and its achievements. One such opportunity arose when the turn of the northwest of England came to host the Annual BA Festival of Science at the University of Liverpool in September 2008. A session of synchrotron radiation science reviewing the contributions of the SRS as well as a look to the future was held. A review of accelerator science was given by Mike Poole, Director of ASTeC at STFC. John Helliwell, Professor of Structural Chemistry at Manchester University, reviewed biology and medicine; Bob Cernik, Professor of Materials Science at Manchester University, reviewed materials science; and Tracy Turner, of the Photon Science Department of STFC, reviewed future light sources.

The electronic structure of Ti4O7 studied by resonant photoemission

We present and discuss resonant photoemission (PES) spectra of Ti4O7 taken at 50 and 300 K. The PES spectra taken in the metallic phase at 300 K show Ti 3d bands at the Fermi level and O 2p bands at higher binding energies. The intensity of the Ti 3d bands increases for photon energies above 40 eV. This effect is attributed to a constructive interference with the Ti 3p → 3d resonance which is confirmed by the corresponding constant-initial-state (CIS) spectra. The Ti 3d bands shift approximately 0.25 eV towards higher binding energies in the low temperature semiconducting phase at 50 K. This shift reflects the opening of the band gap and provides a rough estimate of the electronic contribution to the formation energy of the Ti3+-Ti3+ pairs in the low-temperature phase.

Low temperature photoemission study of La2CuO4+x single crystals

New angle-resolved photoemission results obtained using synchrotron radiation of 15–40 eV from cleaved La2CuO4+x single crystals at a temperature of about 30 K are presented. We have observed spectral enhancements at photon energies around 17 eV that are consistent with autoionization resonance excitations of the hybridized La 5p-O 2s states to O 2p empty states. The crystal surface was found to be unstable. A broad feature around 5 eV binding energy that could be caused by surface reconstruction was observed.