Theoretical investigations on ferroelectric tunnel junctions (FTJs) with asymmetric electrodes and a composite barrier are presented. A large tunneling electroresistance effect exists for the Pt/SrTiO3/BaTiO3/SrRuO3 junction; on the other hand, exchange of the dielectric and ferroelectric layer stacking sequence can seriously degrade the performance. These correlations are rationalized by the proposed concept of an asymmetry factor, defined as the ratio between the average barrier heights of FTJs for two opposite polarization orientations. We show that a large asymmetry factor is beneficial to FTJs. This work may provide a way to enhance the performance of FTJs by structure engineering.

In our previous research (Zhao et al., 2016), we focus on the transport processes from hot electrons to Kα X-ray emission in a copper foil and nanobrush target when the electron refluxing effect is not taken into account. In this work, considering the refluxing effect, the transport of hot electrons in a solid target is studied by adding the electric fields both at the front and rear surfaces of the target with Monte Carlo code Geant4. Simulation results show that the electron refluxing has an important influence on Kα photon yield and the size of Kα radiation source. Kα yield from the 10-μm-thick target with the electron refluxing effect is 2.7–3.7 times more than that without the refluxing for the electron temperatures from 0.4 to 1.4 MeV. The laser-to-Kα photon energy conversion efficiency ${\rm \eta} _{L \to K_{\rm \alpha}} $ with the refluxing effect is always higher than that without the refluxing, and both of them decrease gradually with laser strength Iλ2. Considering the electron refluxing effect or not, the variations of Kα yield with the target thickness d are very different. A critical thickness of the target dc (~30 μm) is achieved to confirm whether the refluxing effect is valid for the target. For the target with the thickness d less than dc, the refluxing effect can enhance Kα yield with several times, while for the target with the thickness d larger than dc, the refluxing effect is not so effective. The full-width at half-maximum increases from 23 to 56 µm after including the refluxing effect by the electron beam with the radius of 10 µm and the temperature of 400 keV.

Although the strong coupling of polarization to spontaneous strain in ferroelectrics would impart a flux-closure with severe disclination strains, recent studies have successfully stabilized such a domain via a nano-scaled multi-layer growth. Nonetheless, the detailed distributions of polarizations in three-dimensions (3D) and how the strains inside a flux closure affect the structures of domain walls are still less understood. Here we report a 3D polarization texture of a 4-fold flux closure domain identified in tensile strained ferroelectric PbTiO3/SrTiO3 multilayer films. Ferroelectric displacement analysis based on aberration-corrected scanning transmission electron microscopic imaging reveals highly inhomogeneous strains with strain gradient above 107/m. These giant disclination strains significantly broaden the 90° domain walls, while the flexoelectric coupling at 180° domain wall is less affected. The present observations are helpful for understanding the basics of topological dipole textures and indicate novel applications of ferroelectrics through engineering strains.

Solar radio fine structures (FSs) may be as an important diagnostics stool to draw the evolution map of the flare loop in the initial phase of solar flares. Also, it may be an important signature of the initial phase of CMEs. Here we analyzed a series of solar radio bursts with drift pulsation structures (DPS) and FSs during the former part of the 23rd solar activity cycle. Found they were associated with CMEs, and got some important statistic conclusions.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Six binary metal systems were selected to study the possibility of forming amorphous alloys by ion mixing or thermal annealing in multilayered Films, i.e. the Ta-Cu, Zr-Nb, Zr-Ta, Y-Zr, Y-Mo and Y-Ta systems, featuring positive heats of formation (ΔHf) ranging from +3 to +40 kJ/mol. Firstly, the interfacial free energy consisting of a chemical and an elastic terms was calculated and added to the energetic state of the multilayers. It was found that the excess interfacial free energy increased with increasing the fraction of the interfacial atoms in the multilayers, and could raise the multilayers to an energy level intersecting with or being higher than thai of the amorphous phase possessing a typical convex shape. It is therefore possible to produce amorphous alloys in such systems, if the multilayered filins included enough fraction of the interfacial atoms. The multilayered samples were then designed and prepared accordingly and both ion mixing and thermal annealing under appropriate conditions resulted in the formation of a number of new amorphous alloys, confirming the above prediction based on the interfacial free energy concern. It is noted that the success of synthesising amorphous alloys by solid-state reaction in the immiscible systems develops a new glass forming technique, namely interface-generated spontaneous amorphization, which has a great potential to produce new and relatively thick amorphous films, e.g. a Ta72Cu28 amorphous film of 800 nm thick was obtained.

A reverse martensitic phase transformation was observed in Nb-enriched Nb-Co multilayers induced by room temperature 200 ke V xenon ion mixing. Further experiments revealed that this bcc-fcc transition proceeds in two steps, i.e., bcc-hcp and hcp-fcc. A crystallographic model is proposed to explain the two-step transition through shearing and sliding, which are mediated by irradiation-induced defects and strain in the films. In addition, the existence of the hcp and fcc metastable states in the Nb-Co system was confirmed by high-temperature solid state interdiffusion of the corresponding multilayers.

In the Nb-rich Nb-Fe(Co,Ni) multilayered films, two metastable crystalline (MX) phases of hep and fee structures were formed by room temperature 200 keV xenon ion mixing. The phase formation was through a two-step transition, Le. a first shearing of bcc—>hcp and then a sliding of hep—>fcc. The free energy curves of the observed MX phases were calculated and included into the free energy diagram together with those of the terminal solid solutions, the amorphous phase and the equilibrium intermetaШc compounds. An independent comparative thermal annealing experiment was conducted with the corresponding as-deposited multilayers and the results certified the emerging sequence of the MX phases being exactly the same as the calculated diagram predicted. Moreover, a crystallographic model was figured out for the bec—>hcp—>fcc transition and the resultant lattice parameters agreed well with the experimental ones.