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Register a new userStern Layers on Surfaces of RuO2(100), RuO2(110), and Pt(111): Surface X-ray Scattering Studies
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Surface X-ray scattering studies of electrochemical Stern layer are reported. The Stern layers formed at the interfaces of RuO2 (110) and (100) in 0.1 M CsF electrolyte are compared to the previously reported Stern layer on Pt(111) [Liu et al., J. Phys. Chem. Lett., 9 (2018) 1265]. While the Cs+ density profiles at the potentials close to hydrogen evolution reaction are similar, the hydration layers intervening the surface and the Cs+ layer on RuO2 surfaces are significantly denser than the hydration layer on Pt(111) surface possibly due to the oxygen termination of RuO2 surfaces. We also discuss in-plane ordering in the Stern layer on Pt(111) surface.
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Polarization dependence of resonant anomalous surface x-ray scattering (RASXS) was studied for interfaces buried in electrolytes or in high-pressure gas. We demonstrate that RASXS exhibits strong polarization dependence when the surface is only slightly modified by adsorption of light elements such as carbon monoxide on platinum surfaces. s- and p-polarization RASXS data were simulated with the latest version of ab initio multiple scattering calculations (FEFF8.2). Elementary considerations are additionally presented for the origin of the polarization dependence in RASXS.
Strain engineering vanadium dioxide thin films is one way to alter this materials characteristic first order transition from semiconductor to metal. In this study we extend the exploitable strain regime by utilizing the very large lattice mismatch of 8.78 % occurring in the VO$_2$/RuO$_2$ system along the c axis of the rutile structure. We have grown VO$_2$ thin films on single domain RuO$_2$ islands of two distinct surface orientations by atomic oxygen-supported reactive MBE. These films were examined by spatially resolved photoelectron and x-ray absorption spectroscopy, confirming the correct stoichiometry. Low energy electron diffraction then reveals the VO$_2$ films to grow indeed fully strained on RuO$_2$(110), exhibiting a previously unreported ($2times2$) reconstruction. On TiO$_2$(110) substrates, we reproduce this reconstruction and attribute it to an oxygen-rich termination caused by the high oxygen chemical potential. On RuO$_2$(100) on the other hand, the films grow fully relaxed. Hence, the presented growth method allows for simultaneous access to a remarkable strain window ranging from bulk-like structures to massively strained regions.
We have studied in-gap states in epitaxial CoFe2O4(111), which potentially acts as a perfect spin filter, grown on a Al2O3(111)/Si(111) structure by using ellipsometry, Fe L2,3-edge x-ray absorption spectroscopy (XAS), and Fe L2,3-edge resonant inelastic x-ray scattering (RIXS), and revealed the relation between the in-gap states and chemical defects due to the Fe2+ cations at the octahedral sites (Fe2+ (Oh) cations). The ellipsometry measurements showed the indirect band gap of 1.24 eV for the CoFe2O4 layer and the Fe L2,3-edge XAS confirmed the characteristic photon energy for the preferential excitation of the Fe2+ (Oh) cations. In the Fe L3-edge RIXS spectra, a band-gap excitation and an excitation whose energy is smaller than the band-gap energy (Eg = 1.24 eV) of CoF2O4, which we refer to as below-band-gap excitation (BBGE) hereafter, were observed. The intensity of the BBGE was strengthened at the preferential excitation energy of the Fe2+ (Oh) cations. In addition, the intensity of the BBGE was significantly increased when the thickness of the CoFe2O4 layer was decreased from 11 to 1.4 nm, which coincides with the increase in the site occupancy of the Fe2+ (Oh) cations with decreasing the thickness. These results indicate that the BBGE comes from the in-gap states of the Fe2+ (Oh) cations whose density increases near the heterointerface on the bottom Al2O3 layer. We have demonstrated that RIXS measurements and analyses in combination with ellipsometry and XAS are effective to provide an insight into in-gap states in thin-film oxide heterostructures.
A seemingly simple oxide with a rutile structure, RuO2 has been shown to possess several intriguing properties ranging from strain-stabilized superconductivity to a strong catalytic activity. Much interest has arisen surrounding the controlled synthesis of RuO2 films but, unfortunately, utilizing atomically-controlled deposition techniques like molecular beam epitaxy (MBE) has been difficult due to the ultra-low vapor pressure and low oxidation potential of Ru. Here, we demonstrate the growth of epitaxial, single-crystalline RuO2 films on different substrate orientations using the novel solid-source metal-organic (MO) MBE. This approach circumvents these issues by supplying Ru using a pre-oxidized solid metal-organic precursor containing Ru. High-quality epitaxial RuO2 films with bulk-like room-temperature resistivity of 55 micro-ohm-cm were obtained at a substrate temperature as low as 300 C. By combining X-ray diffraction, transmission electron microscopy, and electrical measurements, we discuss the effect of substrate temperature, orientation, film thickness, and strain on the structure and electrical properties of these films. Our results illustrating the use of novel solid-source MOMBE approach paves the way to the atomic-layer controlled synthesis of complex oxides of stubborn metals, which are not only difficult to evaporate but also hard to oxidize.
Current-induced spin torques provide efficient data writing approaches for magnetic memories. Recently, the spin splitting torque (SST) was theoretically predicted (R. Gonzalez-Hernandez et al. Phys. Rev. Lett. 126, 127701 (2021)), which combines advantages of conventional spin transfer torque (STT) and spin-orbit torque (SOT) as well as enables controllable spin polarization. Here we provide the experimental evidence of SST in collinear antiferromagnet RuO2 films. The spin current direction is found to be correlated to the crystal orientation of RuO2 and the spin polarization direction is dependent on (parallel to) the Neel vector. These features are quite characteristic for the predicted SST. Our finding not only present a new member for the spin torques besides traditional STT and SOT, but also proposes a promising spin source RuO2 for spintronics.
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