Heteroepitaxial growth of T-Nb2O5 on SrTiO3

There is a growing interest in exploiting the functional properties of niobium oxides in general and of the T-Nb2O5 polymorph in particular. Fundamental investigations of the properties of niobium oxides are, however, hindered by the availability of materials with sufficient structural perfection. It is expected that high-quality T-Nb2O5 can be made using heteroepitaxial growth. Here, we investigated the epitaxial growth of T-Nb2O5 on a prototype perovskite oxide, SrTiO3. Even though there exists a reasonable lattice mismatch in one crystallographic direction, these materials have a significant difference in crystal structure: SrTiO3 is cubic, whereas T-Nb2O5 is orthorhombic. It is found that this difference in symmetry results in the formation of domains that have the T-Nb2O5 c-axis aligned with the SrTiO3 s in-plane directions. Hence, the number of domain orientations is four and two for the growth on (100)s- and (110)s-oriented substrates, respectively. Interestingly, the out-of-plane growth direction remains the same for both substrate orientations, suggesting a weak interfacial coupling between the two materials. Despite challenges associated with the heteroepitaxial growth of T-Nb2O5, the T-Nb2O5 films presented in this paper are a significant improvement in terms of structural quality compared to their polycrystalline counterparts

Approaching the High Intrinsic Electrical Resistivity of NbO2 in Epitaxially Grown Films

NbO2 is a promising candidate for resistive switching devices due to an insulator-metal transition above room temperature, which is related to a phase transition from a distorted rutile structure to an undistorted one. However, the electrical resistivity of the NbO2 thin films produced so far has been too low to achieve high on-off switching ratios. Here, we report on the structural, electrical, and optical characterization of single-crystalline NbO2 (001) thin films grown by pulsed laser deposition on MgF2 (001) substrates. An annealing step reduced the full width at half maximum of the NbO2 (004) x-ray Bragg reflection by one order of magnitude, while the electrical resistivity of the films increased by two orders of magnitude to about 1k Omega cm at room temperature. Temperature-dependent resistivity measurements of an annealed sample revealed that below 650K, two deep-level defects with activation energies of 0.25eV and 0.37eV dominate the conduction, while above 650K, intrinsic conduction prevails. Optical characterization by spectroscopic ellipsometry and by absorption measurements with the electric field vector of the incident light perpendicular to the c-axis of the distorted rutile structure indicates the onset of fundamental absorption at about 0.76eV at room temperature, while at 4K, the onset shifts to 0.85eV. These optical transitions are interpreted to take place across the theoretically predicted indirect bandgap of distorted rutile NbO2

Stability of ZnSe-Passivated Laser Facets Cleaved in Air and in Ultra-High Vacuum

Catastrophic optical mirror damage (COMD) is one of the main failure mechanisms limiting the reliability of GaAs based laser diodes. Here, we compare the facet stability of ZnSe-passivated ridge-waveguide lasers (RWLs) that are cleaved in air and subsequently cleaned using atomic hydrogen with RWLs that are cleaved in ultra-high vacuum. RWLs cleaved in ultra-high vacuum show a superior performance and reach power densities up to 58 MW/cm 2 under extended continuous wave operation at 1064 nm. This is attributed to the reduction of defects at the interface between ZnSe and the cleaved facet as evidenced by transmission electron microscopy and X-ray diffraction

Interface formation of two- and three-dimensionally bonded materials in the case of GeTe-Sb2Te3 superlattices

GeTe–Sb2Te3 superlattices are nanostructured phase-change materials which are under intense investigation for non-volatile memory applications. They show superior properties compared to their bulk counterparts and significant efforts exist to explain the atomistic nature of their functionality. The present work sheds new light on the interface formation between GeTe and Sb2Te3, contradicting previously proposed models in the literature. For this purpose [GeTe(1 nm)–Sb2Te3(3 nm)]15 superlattices were grown on passivated Si(111) at 230 °C using molecular beam epitaxy and they have been characterized particularly with cross-sectional HAADF scanning transmission electron microscopy. Contrary to the previously proposed models, it is found that the ground state of the film actually consists of van der Waals bonded layers (i.e. a van der Waals heterostructure) of Sb2Te3 and rhombohedral GeSbTe. Moreover, it is shown by annealing the film at 400 °C, which reconfigures the superlattice into bulk rhombohedral GeSbTe, that this van der Waals layer is thermodynamically favored. These results are explained in terms of the bonding dimensionality of GeTe and Sb2Te3 and the strong tendency of these materials to intermix. The findings debate the previously proposed switching mechanisms of superlattice phase-change materials and give new insights in their possible memory application

Modulation of van der Waals and classical epitaxy induced by strain at the Si step edges in GeSbTe alloys

The present work displays a route to design strain gradients at the interface between substrate and van der Waals bonded materials. The latter are expected to grow decoupled from the substrates and fully relaxed and thus, by definition, incompatible with conventional strain engineering. By the usage of passivated vicinal surfaces we are able to insert strain at step edges of layered chalcogenides, as demonstrated by the tilt of the epilayer in the growth direction with respect of the substrate orientation. The interplay between classical and van der Waals epitaxy can be modulated with an accurate choice of the substrate miscut. High quality crystalline GexSb2Te3+x with almost Ge1Sb2Te4 composition and improved degree of ordering of the vacancy layers is thus obtained by epitaxial growth of layers on 3–4° stepped Si substrates. These results highlight that it is possible to build and control strain in van der Waals systems, therefore opening up new prospects for the functionalization of epilayers by directly employing vicinal substrates

Reconciling the theoretical and experimental electronic structure of NbO2

Metal-insulator transition materials such as NbO2 have generated much excitement in recent years for their potential applications in computing and sensing. NbO2 has generated considerable debate over the nature of the phase transition, and the values for the band gap/band widths in the insulating phase. We present a combined theoretical and experimental study of the band gap and electronic structure of the insulating phase of NbO2. We carry out ab-initio density functional theory plus U calculations, directly determining U and J parameters for both the Nb 4d and O 2p subspaces through the recently introduced minimum-tracking linear response method. We find a fundamental bulk band gap of 0.80 eV for the full DFT+U+J theory. We also perform calculations and measurements for a (100) oriented thin film. Scanning tunnelling spectroscopy measurements show that the surface band gap varies from 0.75 eV to 1.35 eV due to an excess of oxygen in and near the surface region of the film. Slab calculations indicate metallicity localised at the surface region caused by an energy level shift consistent with a reduction in Coulomb repulsion. We demonstrate that this effect in combination with the simple, low cost DFT+U+J method can account for the band widths and p-d gap observed in X-ray photoelectron spectroscopy experiments. Overall, our results indicate the possible presence of a 2D anisotropic metallic layer at the (100) surface of NbO2.Comment: 11 pages, 5 figures, plus 3 pages of Supporting Informatio

Twinned-domain-induced magnonic modes in epitaxial LSMO/STO films

The use of periodic magnetic structures to control the magneto-dynamic properties of materials-Magnonics-is a rapidly developing field. In the last decade, a number of studies have shown that metallic films can be patterned or combined in patterns that give rise to well-defined magnetization modes, which are formed due to band folding or band gap effects. To explore and utilize these effects in a wide frequency range, it is necessary to pattern samples at the sub-micrometer scale. However, it is still a major challenge to produce low-loss magnonic structures with periodicities at such length scales. Here, we show that for a prototypical perovskite, La0.7 Sr0.3MnO3, the twinned structural order can be used to induce a magnetic modulation with a period smaller than 100 nm, demonstrating a bottomup approach for magnonic crystal growt

Effects of mental practice embedded in daily therapy compared to therapy as usual in adult stroke patients in Dutch nursing homes: design of a randomised controlled trial

<p>Abstract</p> <p>Background</p> <p>Mental practice as an additional cognitive therapy is getting increased attention in stroke rehabilitation. A systematic review shows some evidence that several techniques in which movements are rehearsed mentally might be effective but not enough to be certain. This trial investigates whether mental practice can contribute to a quicker and/or better recovery of stroke in two Dutch nursing homes. The objective is to investigate the therapeutic potential of mental practice embedded in daily therapy to improve individually chosen daily activities of adult stroke patients compared to therapy as usual. In addition, we will investigate prognostic variables and feasibility (process evaluation).</p> <p>Methods</p> <p>A randomised, controlled, observer masked prospective trial will be conducted with adult stroke patients in the (sub)acute phase of stroke recovery. Over a six weeks intervention period the control group will receive multi professional therapy as usual. Patients in the experimental group will be instructed how to perform mental practice, and will receive care as usual in which mental practice is embedded in physical, occupation and speech therapy sessions. Outcome will be assessed at six weeks and six months. The primary outcome measure is the patient-perceived effect on performance of daily activities as assessed by an 11-point Likert Scale. Secondary outcomes are: Motricity Index, Nine Hole Peg Test, Barthel Index, Timed up and Go, 10 metres walking test, Rivermead Mobility Index. A sample size of the patients group and all therapists will be interviewed on their opinion of the experimental program to assess feasibility. All patients are asked to keep a log to determine unguided training intensity.</p> <p>Discussion</p> <p>Advantages and disadvantages of several aspects of the chosen design are discussed.</p> <p>Trial registration</p> <p>ISRCTN27582267</p