RESUMEN
Ferroelectric tunnel junctions (FTJs) utilizing an in-plane head-to-head ferroelectric domain wall (DW) have recently been realized, showing interesting physics and new functionalities. However, the DW state in these junctions was found to be metastable and not reversible after applying an electric field. In this work, we demonstrate that a stable and reversible head-to-head DW state can be achieved in FTJs by proper engineering of polar interfaces. Using density functional theory (DFT) calculations and phenomenological modeling, we explore the DW stability by varying stoichiometry of the La_{1-x}Sr_{x}O/TiO_{2} interfaces in FTJs with La_{0.5}Sr_{0.5}MnO_{3} electrodes and a ferroelectric BaTiO_{3} tunnel barrier. For, x≤0.4 we find that the DW state becomes a global minimum and the calculated hysteresis loops exhibit three reversible polarization states. For such FTJs, our quantum transport calculations predict the emergence of a DW tunneling electroresistance effect-reversible switching of the tunneling conductance between the highly conductive DW state and two much less conductive uniform polarization states.
RESUMEN
Recent experimental results have demonstrated ferroelectricity in thin films of SrTiO_{3} induced by antisite Ti_{Sr} defects. This opens a possibility to use SrTiO_{3} as a barrier layer in ferroelectric tunnel junctions (FTJs)-emerging electronic devices promising for applications in nanoelectronics. Here using density functional theory combined with quantum-transport calculations applied to a prototypical Pt/SrTiO_{3}/Pt FTJ, we demonstrate that the localized in-gap energy states produced by the antisite Ti_{Sr} defects are responsible for the enhanced electron tunneling conductance which can be controlled by ferroelectric polarization. Our tight-binding modeling, which takes into account multiple defects, shows that the predicted defect-assisted tunneling electroresistance effect is greatly amplified when the defect energy levels are brought to the Fermi energy by one of the polarization states. Our results have implications for FTJs based on conventional ferroelectric barriers with defects and can be employed for the design of new types of FTJs with enhanced performance.
RESUMEN
Persistent spin texture (PST) is the property of some materials to maintain a uniform spin configuration in the momentum space. This property has been predicted to support an extraordinarily long spin lifetime of carriers promising for spintronics applications. Here, we predict that there exists a class of noncentrosymmetric bulk materials, where the PST is enforced by the nonsymmorphic space group symmetry of the crystal. Around certain high symmetry points in the Brillouin zone, the sublattice degrees of freedom impose a constraint on the effective spin-orbit field, which orientation remains independent of the momentum and thus maintains the PST. We illustrate this behavior using density-functional theory calculations for a handful of promising candidates accessible experimentally. Among them is the ferroelectric oxide BiInO3-a wide band gap semiconductor which sustains a PST around the conduction band minimum. Our results broaden the range of materials that can be employed in spintronics.
RESUMEN
The spin-filter transport and magnetoresistance effects are of particular interest in the field of molecular spintronics. In this work, based on first-principles quantum transport calculations, we report on the spin-dependent transport properties of a molecular junction made of two manganese phthalocyanine (MnPc) molecules linked by single-walled carbon nanotubes. Owing to the half-metallicity of MnPc around the Fermi energy, a perfect spin-filter effect and a giant magnetoresistance effect are observed in the molecular junction. The current-voltage characteristics show nearly ohmic behavior for the junction in an anti-parallel magnetic configuration, while a very low-bias negative differential resistance effect is observed for the junction in a parallel magnetic configuration. The results are well understood from the analysis of molecular frontier orbitals, scattering states and transmission spectra. Our results provide some fundamental understanding of spin-dependent transport in molecular junctions that are useful for the design of future spintronic devices.
RESUMEN
The Salvia miltiorrhiza callus tissue cells were entrapped with 3% alginate. The immobilized cells were incubated in medium with LS + KT 0.1 + NAA 1.0 containing 3% sucrose at 25 degrees C for nearly one month. After incubation, the medium free from cells was extracted with ether 3 times. After evaporation, the residue of the ether extract was employed to determine the content on TLC and HPLC. The results showed that the incubation system mentioned above could continuously secrete the main components of S. miltiorrhiza, tanshinone IIA and cryptotanshinone, which were almost the same as the extract of S. miltiorrhiza roots. In addition, suspension incubation of callus tissue cells, the conditions of immobilization with alginate, the stability of immobilized cells and the characteristics of products were also examined.
