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Register a new userThe Pd(100)-(SQRT(5) x SQRT(5) R27^o)-O surface oxide revisited
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Combining high-resolution core-level spectroscopy (HRCLS), scanning tunneling microscopy (STM) and density-functional theory (DFT) calculations we reanalyze the Pd(100)-(SQRT(5) x SQRT(5) R27^o)-O surface oxide phase. We find that the prevalent structural model, a rumpled PdO(001) film suggested by previous low energy electron diffraction (LEED) work (M. Saidy et al., Surf. Sci. 494, L799 (2001)), is incompatible with all three employed methods. Instead, we suggest the two-dimensional film to consist of a strained PdO(101) layer on top of Pd(100). LEED intensity calculations show that this model is compatible with the experimental data of Saidy et al.
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The structure and dehydration mechanism of the proton conducting oxide Ba$_{2}$In$_{2}$O$_{5}$(H$_{2}$O)$_{x}$ are investigated by means of variable temperature Raman spectroscopy together with inelastic neutron scattering. At room temperature, Ba$_{2}$In$_{2}$O$_{5}$(H$_{2}$O)$_{x}$ is found to be fully hydrated ($x=1$) and to have a perovskite-like structure, which dehydrates gradually with increasing temperature and at around 600 $^{circ}$C the material is essentially completely dehydrated ($x=0$). The dehydrated material exhibits a brownmillerite structure, which is featured by alternating layers of InO$_{6}$ octahedra and InO$_{4}$ tetrahedra. The transition from a perovskite-like to a brownmillerite-like structure is featured by a hydrated-to-intermediate phase transition at $ca.$ 370 {deg}C. The structure of the intermediate phase is similar to the structure of the fully dehydrated material, but with the difference that it exhibits a non-centrosymmetric distortion of the InO$_{6}$ octahedra not present in the latter. For temperatures below the hydrated-to-intermediate phase transition, dehydration is featured by the release of protons confined to the layers of InO$_{4}$ tetrahedra, whereas above the transition also protons bound to oxygens of the layers of InO$_{6}$ are released. Finally, we found that the O-H stretch region of the vibrational spectra is not consistent with a single-phase spectrum, but is in agreement with the superposition of spectra associated with two different proton configurations. The relative contributions of the two proton configurations depend on how the sample is hydrated.
The structure of the $(sqrt{5}timessqrt{5})R26.6^circ$ reconstruction of LaAlO$_3$ (001) has been determined using transmission electron diffraction combined with direct methods. The structure is relatively simple, consisting of a lanthanum oxide termination with one lanthanum cation vacancy per surface unit cell. The electronic structure is unusual since a fractional number of holes or atomic occupancies per surface unit cell are required to achieve charge neutrality. Density functional calculations indicate that the charge compensation mechanism occurs by means of highly delocalized holes. The surface contains no oxygen vacancies and with a better than 99% confidence level, the holes are not filled with hydrogen. The reconstruction can be understood in terms of expulsion of the more electropositive cation from the surface followed by an increased covalency between the remaining surface lanthanum atoms and adjacent oxygen atoms.
The magnetic state of heavy metal Pt thin films in proximity to the ferrimagnetic insulator Y$_{3}$Fe$_{5}$O$_{12}$ has been investigated systematically by means of x-ray magnetic circular dichroism and x-ray resonant magnetic reflectivity measurements combined with angle-dependent magnetotransport studies. To reveal intermixing effects as the possible cause for induced magnetic moments in Pt, we compare thin film heterostructures with different order of the layer stacking and different interface properties. For standard Pt layers on Y$_{3}$Fe$_{5}$O$_{12}$ thin films, we do not detect any static magnetic polarization in Pt. These samples show an angle-dependent magnetoresistance behavior, which is consistent with the established spin Hall magnetoresistance. In contrast, for the inverted layer sequence, Y$_{3}$Fe$_{5}$O$_{12}$ thin films grown on Pt layers, Pt displays a finite induced magnetic moment comparable to that of all-metallic Pt/Fe bilayers. This magnetic moment is found to originate from finite intermixing at the Y$_{3}$Fe$_{5}$O$_{12}$/Pt interface. As a consequence, we found a complex angle-dependent magnetoresistance indicating a superposition of the spin Hall and the anisotropic magnetoresistance in these type of samples. Both effects can be disentangled from each other due to their different angle dependence and their characteristic temperature evolution.
We report new results for the elastic constants studied in Faraday and Cotton-Mouton geometry in Tb$_3$Ga$_5$O$_{12}$ (TGG), a frustrated magnetic substance with strong spin-phonon interaction and remarkable crystal-electric-field (CEF) effects. We analyze the data in the framework of CEF theory taking into account the individual surroundings of the six inequivalent Tb$^{3+}$-ion positions. This theory describes both, elastic constants in the magnetic field and as a function of temperature. Moreover we present sound-attenuation data for the acoustic Cotton-Mouton effect in TGG.
Phonons are produced when an excited vacancy in cuprous oxide (Cu$_2$O) relaxes. Time resolved luminescence was used to find the excited copper vacancy (acceptor) and oxygen vacancy (donor) trap levels and lifetimes. It was also used to determine the typical energy and number of phonons in the phonon pulses emitted by vacancies. The vacancy properties of cuprous oxide are controlled by several synthesis parameters and by the temperature. We directly demonstrate the absorption of light by oxygen vacancies with transient absorption. Copper and oxygen vacancies behave differently, in part because the two kinds of traps capture carriers from different states. For example, the copper vacancy luminescence lifetime is around 25 times greater at low temperature. However, both kinds of vacancy luminescence are consistent with a Poissonian multiple phonon emission model.
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