Jahn-Teller stabilization of a "polar" metal oxide surface: Fe3O4(001)

Using ab initio thermodynamics we compile a phase diagram for the surface of Fe3O4(001) as a function of temperature and oxygen pressures. A hitherto ignored polar termination with octahedral iron and oxygen forming a wave-like structure along the [110]-direction is identified as the lowest energy configuration over a broad range of oxygen gas-phase conditions. This novel geometry is confirmed in a x-ray diffraction analysis. The stabilization of the Fe3O4(001)-surface goes together with dramatic changes in the electronic and magnetic properties, e.g., a halfmetal-to-metal transition.Comment: 4 pages, 4 figure

Comment on "mechanisms for Pressure-Induced Isostructural Phase Transitions in EuO"

Authors thank the financial support from Projects PGC2018-101464-B-I00, PGC2018-097520-A-I00, and MALTA-Consolider Team RED2018-102612-T (Ministerio de Ciencia, Innovaci´on y Universidades) is acknowledged. V. Monteseguro acknowledges the “Beatriz Galindo” fellowship (BG20/000777) and the “Juan de la Cierva” fellowship (IJC2019-041586-I)

Chemisorption of NTCDA on Ag(1 1 1): a NIXSW study including non-dipolar and electron-stimulated effects

Abstract The adsorption of one monolayer of 1,4,5,8-naphthalene-tetracarboxylicacid-dianhydride (NTCDA) on the Ag(1 1 1)-surface was studied using the normal incidence X-ray standing waves (XSW) technique. Results regarding two key-issues are presented: Most prominent, the precise adsorbate-substrate bonding distance could be evaluated to 3.02 ± 0.02 Å (for the ''relaxed monolayer''-structure). This value is significantly smaller than a van der Waals bonding distance and clearly indicates the chemisorptive bonding character. Concordant results were obtained from both O 1s photo-and O KLL Auger electron emission. This was enabled by the development of a data analysis procedure--the second issue addressed--which takes into account non-dipolar contributions to the photoemission as well as electronstimulated Auger excitations. The latter effect adds a fraction to the total Auger yield being as high as 50% and hence may be important for any XSW study using Auger signals

A new type of reconstruction on the InSb() surface determined by grazing incidence X-ray diffraction

The (3×3) reconstruction of the InSb( ) surface has been investigated by grazing incidence X-ray diffraction and scanning tunneling microscopy. The structure is characterized by 6-atom rings on top of a slightly buckled InSb top double layer. Two types of rings have been found, an elliptic ring consisting of 4 In and 2 Sb atoms and a trigonal ring with 3 In and 3 Sb atoms. The bond angles and lengths are consistent with the concept of rehybridization and depolarization which explains the reconstructions of the (111) and (110) surfaces

Geometric and electronic structure of the Cs-doped Bi2Se3(0001) surface

Using surface x-ray diffraction and scanning tunneling microscopy in combination with first-principles calculations, we have studied the geometric and electronic structure of Cs-deposited Bi2Se3(0001) surface kept at room temperature. Two samples were investigated: a single Bi2Se3 crystal, whose surface was Ar sputtered and then annealed at ∼500∘C for several minutes prior to Cs deposition, and a 13-nm-thick epitaxial Bi2Se3 film that was not subject to sputtering and was annealed only at ∼350∘C. In the first case, a considerable fraction of Cs atoms occupy top layer Se atoms sites both on the terraces and along the upper step edges where they form one-dimensional-like structures parallel to the step. In the second case, Cs atoms occupy the fcc hollow site positions. First-principles calculations reveal that Cs atoms prefer to occupy Se positions on the Bi2Se3(0001) surface only if vacancies are present, which might be created during the crystal growth or during the surface preparation process. Otherwise, Cs atoms prefer to be located in fcc hollow sites in agreement with the experimental finding for the MBE-grown sample

Evidence for a mixed CoNiO layer at the Co/NiO(001) interface from surface x-ray diffraction

The clean and cobalt-covered NiO(001) surface was studied by surface x-ray diffraction. The NiO(001) surface prepared by annealing in air at 1000 degrees C is characterized by a strong expansion of the top interlayer spacing (approximate to 14% relative to the bulk NiO-spacing of 2.085 A) and an outward relaxation (0.28 A) of the oxygen atoms, which can be related to a high (27%) concentration of oxygen vacancy defects. In addition we find an oxygen-layer (most likely as part of the OH- anion) on top of the surface Ni atoms at a distance of 2.33 A. Deposition of 0.8 monolayers of Co induces a complex surface restructuring involving the formation of double layer CoO-islands above an oxygen deficient interfacial oxide layer ([NiCo]O-0.5) in registry with the underlying NiO single crystal. Our results support previous spectroscopic studies suggesting the presence of uncompensated interfacial spins responsible for the exchange bias