W.H. Tang

Doping tuned rectifying properties in La2−xSrxCuO4/Nb:SrTiO3 heterojunctions

Here, we carried out a study on strontium doping effect on the rectifying properties of La2−xSrxCuO4 (LSCO)/NSTO heterojunctions. The intimate relationship between the change in electronic structure of LSCO and the variation of diffusion potential (Vd) of the junction supports opportunities for detecting Fermi level shift and superconducting gap evolution of LSCO upon hole doping. The results agree well to generally perceived viewpoints obtained by other methods. Even the suppression of Fermi level shift in the underdoped regime and weak-coupling d-wave BCS superconductivity behavior in the overdoped regime of LSCO are clearly exhibited by the tuned rectifying behaviors. This work not only reveals rich properties of LSCO/NSTO but also opens an alternative route to monitor the Fermi level and superconducting gap of high-temperature superconductors.

Electronic structure and magnetism of EuX (X = O, S, Se and Te): A first-principles investigation

Electronic structures and magnetic properties of EuX (X=O, S, Se and Te) are investigated by the plane-wave pseudopotential method with explicit account for Coulomb repulsion within the 4f shell, and compared with other theoretical and experimental results. All the EuX compounds are magnetic semiconductors with noticeable indirect Γ-X gap. Ferromagnetic ordering is observed for EuO and EuS while antiferromagnetic ordering along [111] is the ground state for EuSe and EuTe. These results are consistent with the experiments, although previous calculations indicated that the ground state of EuS is antiferromagnetic. The exchange parameters are calculated, and are in fair agreement with experimental results.

Carrier tuned rectifying-like behavior in superconducting La1.8Sr0.2CuO4/La1.9Sr0.1CuO4 bilayers

A bilayer structure of superconducting overdoped La1.8Sr0.2CuO4 and underdoped La1.9Sr0.1CuO4 films was fabricated using pulsed laser deposition and wet chemical etching techniques. The difference in carrier concentrations in the two layers causes rectifying-like behavior when the films are cooled below a critical temperature Tc. This behavior can be interpreted within the framework of quenched superconducting films, with carrier migration occurring between the two layers at very low positive current. These results suggest that electric field-controlled carrier migration between two layers in a bilayer structure can yield new interfacial properties, which would be of interest for device applications.

FERROELECTRIC SWITCHING PATH IN MONODOMAIN RHOMBOHEDRAL BiFeO3 CRYSTAL: A FIRST-PRINCIPLES STUDY

Based on density-functional calculations, we have studied possible ferroelectric switching path in monodomain single crystal of rhombohedral BiFeO3, a prototypical multiferroic compound. By carefully studying the behaviors of FeO6 corner-sharing double-tetrahedrons, we find abrupt changes in total energy and oxygen atomic positions, and therefore polarizations, occur in the ferroelectric switching path of rhombohedral BiFeO3. Detailed analyses suggest that such behavior might be caused by the frustrated magnetic ordering in the paraelectric phase of rhombohedral BiFeO3, where three O atoms and the Bi atom are in the same plane perpendicular to the polarization direction. This is supported by the fact that the ferroelectric switching for paramagnetic BiFeO3 is smooth and has a much lower energy barrier than that of antiferromagnetic BiFeO3.

Tunnelling current in YBa2Cu3O7−δ/Nb-doped SrTiO3 heterojunctions

We report the fabrication of YBa2Cu3O7−δ/Nb-doped (0.8 wt%) SrTiO3 p–n junctions by pulsed electron deposition, photolithography and Ar-ion etching techniques. In the junctions, the abnormal rectifying behaviour and the Esaki tunnelling diode-like characteristic have been observed above and below the superconducting transition temperature TC of YBa2Cu3O7−δ, respectively. The tunnelling process is responsible for these behaviours and the depletion layer formed at the junction interface with very low resistance is the dominating factor. The intriguing properties in this work indicate that it is possible to realize the functions of traditional semiconductor diodes by using high-temperature superconductor-based heterojunctions.

Fabrication of shape controlled Fe{sub 3}O{sub 4} nanostructure

Abstract Shape-controlled Fe3O4 nanostructure has been successfully prepared using polyethylene glycol as template in a water system at room temperature. Different morphologies of Fe3O4 nanostructures, including spherical, cubic, rod-like, and dendritic nanostructure, were obtained by carefully controlling the concentration of the Fe3+, Fe2+, and the molecular weight of the polyethylene glycol. Transmission Electron Microscope images, X-ray powder diffraction patterns and magnetic properties were used to characterize the final product. This easy procedure for Fe3O4 nanostructure fabrication offers the possibility of a generalized approach to the production of single and complex nanocrystalline oxide with tunable morphology.