263 research outputs found
Prominent 5d-orbital contribution to the conduction electrons in gold
We have examined the valence-band electronic structures of gold and silver in
the same column in the periodic table with nominally filled d orbitals by means
of a recently developed polarization-dependent hard x-ray photoemission.
Contrary to a common expectation, it is found that the 5d-orbital electrons
contribute prominently to the conduction electrons in gold while the conduction
electrons in silver are to some extent free-electron-like with negligible 4d
contribution, which could be related to a well-known fact that gold is more
stable than silver in air. The 4d electron correlation effects are found to be
essential for the conduction electron character in silver.Comment: 8 pages, 4 figures, to be appeared in New J. Phys
Analysis of Metal-Insulator Crossover in Strained {SrRuO}3 Thin Films by X-ray Photoelectron Spectroscopy
The electronic properties of ultrathin epitaxial films of strontium ruthenate SrRuO3 perovskite oxide are modified by epitaxial strain, as determined by growing by pulsed laser deposition, on different the substrates. Electron transport measurements indicated that tensile strain deformation of the SrRuO3 unit cell reduces the metallicity of the material and reduces the metal-insulator-transition (MIT) temperatures. The shrinkage of the Ru-O-Ru buckling angle due to compressive strain is counterweighted by the increased overlap of the conduction Ru-4d orbitals with the O-2p ones due to the smaller interatomic distances resulting into an increased MIT temperature, i.e. a more conducting material. In the more metallic samples the core level x-ray photoemission spectroscopy lineshapes show the occurrence of an extra-peak at the lower binding energies of the main Ru-3d peaks that is attributed to screening, as observed in volume sensitive photoemission of the unstrained material
Three-Dimensional Electronic Structure of type-II Weyl Semimetal WTe
By combining bulk sensitive soft-X-ray angular-resolved photoemission
spectroscopy and accurate first-principles calculations we explored the bulk
electronic properties of WTe, a candidate type-II Weyl semimetal featuring
a large non-saturating magnetoresistance. Despite the layered geometry
suggesting a two-dimensional electronic structure, we find a three-dimensional
electronic dispersion. We report an evident band dispersion in the reciprocal
direction perpendicular to the layers, implying that electrons can also travel
coherently when crossing from one layer to the other. The measured Fermi
surface is characterized by two well-separated electron and hole pockets at
either side of the point, differently from previous more surface
sensitive ARPES experiments that additionally found a significant quasiparticle
weight at the zone center. Moreover, we observe a significant sensitivity of
the bulk electronic structure of WTe around the Fermi level to electronic
correlations and renormalizations due to self-energy effects, previously
neglected in first-principles descriptions
Depth dependence of itinerant character in Mn-substituted Sr3Ru2O7
We present a core-level photoemission study of Sr3(Ru 1-xMnx)2O7, in which we
monitor the evolution of the Ru-3d fine structure versus Mn substitution and
probing depth. In both Ru 3d3/2 and 3d5/2 core levels we observe a clear
suppression of the metallic features, i.e. the screened peaks, implying a sharp
transition from itinerant to localized character already at low Mn
concentrations. The comparison between soft and hard x-ray photoemission, which
provides tunable depth sensitivity, reveals that the degree of
localized/metallic character for Ru is different at the surface than in the
bulk.Comment: 10 pages, 4 figures, 1 tabl
Role and optimization of the active oxide layer in TiO<sub>2</sub>-based RRAM
TiO2 is commonly used as the active switching layer in resistive random access memory. The electrical characteristics of these devices are directly related to the fundamental conditions inside the TiO2 layer and at the interfaces between it and the surrounding electrodes. However, it is complex to disentangle the effects of film “bulk” properties and interface phenomena. The present work uses hard X-ray photoemission spectroscopy (HAXPES) at different excitation energies to distinguish between these regimes. Changes are found to affect the entire thin film, but the most dramatic effects are confined to an interface. These changes are connected to oxygen ions moving and redistributing within the film. Based on the HAXPES results, post-deposition annealing of the TiO2 thin film was investigated as an optimisation pathway in order to reach an ideal compromise between device resistivity and lifetime. The structural and chemical changes upon annealing are investigated using X-ray absorption spectroscopy and are further supported by a range of bulk and surface sensitive characterisation methods. In summary, it is shown that the management of oxygen content and interface quality is intrinsically important to device behavior and that careful annealing procedures are a powerful device optimisation technique
Protected surface state in stepped Fe (0 18 1)
Carbon (C) surface segregation from bulk stabilizes the Fe(0 18 1) vicinal surface by forming a c(3 root 2 x root 2 reconstruction with C zig-zag chains oriented at 45 degrees with respect to the iron surface steps. The iron surface electronic states as measured by high resolution ARPES at normal emission with polarized synchrotron radiation split in two peaks that follow distinct energy dispersion curves. One peak follows the dispersion of the carbon superstructure and is photoexcited only when the polarization vector is parallel to the steps, the second peak disperses similarly to the pristine Fe(0 0 1) surface. Such surface electronic structure is robust as it persists even after coating with an Ag overlayer. The robustness of this surface electronic structure and its similarity with that of the clean Fe(0 0 1) surface make this system of interest for magnetic and spintronic properties such as magneto tunnel junctions based on Fe/MgO interface
Electric control of magnetism at the Fe/BaTiO3 interface
Interfacial magnetoelectric coupling is a viable path to achieve electrical writing of magnetic information in spintronic devices. For the prototypical Fe/BaTiO3 system, only tiny changes of the interfacial Fe magnetic moment upon reversal of the BaTiO3 dielectric polarization have been predicted so far. Here, by using X-ray magnetic circular dichroism in combination with high-resolution electron microscopy and first principles calculations, we report on an undisclosed physical mechanism for interfacial magnetoelectric coupling in the Fe/BaTiO3 system. At this interface, an ultrathin oxidized iron layer exists, whose magnetization can be electrically and reversibly switched on and off at room temperature by reversing the BaTiO3 polarization. The suppression/recovery of interfacial ferromagnetism results from the asymmetric effect that ionic displacements in BaTiO3 produces on the exchange coupling constants in the interfacial-oxidized Fe layer. The observed giant magnetoelectric response holds potential for optimizing interfacial magnetoelectric coupling in view of efficient, low-power spintronic devices
Study of equilibrium carrier transfer in LaAlO3/SrTiO3 from an epitaxial La1 12x Sr x MnO3 ferromagnetic layer
Using x-ray magnetic circular dichroism and ab-initio calculations, we explore the La1-xSrxMnO3/LaAlO3/SrTiO3 (001) heterostructure as a mean to induce transfer of spin polarized carriers from ferromagnetic La1-xSrxMnO3 layer into the 2DEG (two-dimensional electron gas) at the LaAlO3/SrTiO3 interface. By out-of-plane transport measurements, the tunneling across the LaAlO3 barrier is also analyzed. Our results suggest small or vanishing spin-polarization for the 2DEG: magnetic dichroism does not reveal a neat signal on Ti atoms, while calculations predict, for the pristine stoichiometric interface, a small spin-resolved mobile charge of 2.5 x 10(13) cm(-2) corresponding to a magnetic moment of 0.038 mu(B) per Ti atom, tightly confined within the single SrTiO3 layer adjacent to LaAlO3. Such a small magnetization is hard to be detected experimentally and perhaps not robust enough to survive to structural disorder, native doping, or La1-xSrxMnO3 dead-layer effects. Our analysis suggests that, while some spin-diffusion cannot be completely ruled out, the use of ferromagnetic La1-xSrxMnO3 epilayers grown on-top of LaAlO3/SrTiO3 is not effective enough to induce robust spin-transport properties in the 2DEG. The examined heterostructure is nevertheless an excellent test-case to understand some fundamental aspects of the spin-polarized charge transfer in 2D wells
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