X.-G. Li

Fabrication of large-area single crystal bismuth nanowire arrays

Bi nanowire arrays have been successfully synthesized using potentiostatic electrochemical deposition into the channels of an anodic alumina membrane (AAM). The morphology and structure of Bi nanowires have been characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD). The results demonstrate that Bi nanowire arrays are compact, high filling rate and have a large area. The individual single crystal Bi nanowires are dense and continuous with uniform diameters (∼60 nm) throughout the entire length. The optimum synthesis conditions of Bi single crystal nanowire arrays were also discussed.

Hydrothermal synthesis of single-crystalline La0.5Ca0.5MnO3 nanowires at low temperature

Single-crystalline La0.5Ca0.5MnO3 nanowires have been successfully prepared by a hydrothermal method at low temperature. X-Ray diffraction (XRD) results show that the La0.5Ca0.5MnO3 nanowires have an orthorhombic perovskite structure without any other impurity phase. Transmission electron microscopy (TEM) images demonstrate that the as-synthesized samples are made up of a large quantity of nanowires with lengths ranging from several to several tens of micrometers and uniform diameter (∼80 nm). High-resolution transmission electron microscopy (HRTEM) images reveal that the nanowires with very clean surfaces grow along [100]. The result of magnetic measurements indicates that the nanowires have an enhanced Tc due to the shrinkage of the unit cell volume.

Crossover of magnetoresistance from negative to positive in the heterojunction composed of La0.82Ca0.18MnO3 and 0.5 wt % Nb-doped SrTiO3

We report the magnetotransport properties of the heterojunction composed of La0.82Ca0.18MnO3 (LCMO) and 0.5 wt % Nb-doped SrTiO3 (SNTO). At temperature below 120 K, the heterojunction only exhibits a negative magnetoresistance (MR) independent of bias current. At temperatures above 120 K, with increasing bias current a crossover of MR from negative to positive is observed. This intriguing crossover of MR can be ascribed to the dominant spin character changing from the majority spin carriers to the minority spin carriers at the LCMO-SNTO interface due to their occupation in the complicated bands of La0.82Ca0.18MnO3. (c) 2006 American Institute of Physics.

Two-dimensional van der Waals nanocomposites as Z-scheme type photocatalysts for hydrogen production from overall water splitting

The Z-scheme is an efficient route for hydrogen production from photocatalytic water splitting; however, there is no theoretical design for two-dimensional (2D) systems. In the present work, the 2D van der Waals (vdW) MoSe2/graphene/HfS2 and MoSe2/N-doped graphene/HfS2 nanocomposites are proposed to be promising candidates for Z-scheme photocatalysts and verified by density functional theory calculations. The fine control of the n-type doping on graphene can enhance the efficiency of solar energy utilization. Furthermore, the 2D vdW MoSe2/HfS2 nanocomposite is shown to be a direct Z-scheme system for photocatalytic water splitting without redox mediators, which is more easily synthesized in experiment.

Effects of solid solutions on the superconducting properties of Gd–Ba–Cu–O superconductors

The superconducting properties of the nominal Gd1+2xBa2+xCu3+xO7?? (x = 0, 0.1, 0.2, 0.3, 0.4) single-domain bulks which consist of GdBa2Cu3O7?? (Gd123), Gd2BaCuO5 (Gd211) and Gd1+yBa2?yCu3O7??, have been investigated through magnetization and resistivity measurements. With increasing Gd211 content x, the trapped magnetic field reaches a maximum for the sample x = 0.2. The second peak in the critical current density Jc versus magnetic field B curve shifts toward the high magnetic regime for the sample x = 0.2 compared with that for the sample x = 0, but decreases to low magnetic field for the samples x>0.2. Two superconducting transitions in resistivity ?ab indicate that the low Tc clusters, i.e.?Gd1+yBa2?yCu3O7?? solid solutions, are interspersed in the GdBa2Cu3O7?? superconducting matrix. The ?Tc pinning resulting from the compositional fluctuations should be responsible for the enhancement of Jc and the trapped field for the sample x = 0.2.

High transport critical current in MgB2/Fe wire by in situ powder-in-tube process

Abstract MgB 2 /Fe wires and MgB 2 /Ta/Cu wires were prepared through the in situ powder-in-tube method by using Mg and B powders at ambient pressure. The phase compositions, microstructural features are investigated by X-ray diffraction and scanning electron microscope. The transport critical current properties are measured by the standard four-probe technique. It is found that no impurity phase is formed in MgB 2 /Fe wire. At the same time, there are almost no reaction between iron and magnesium according to the results of X-ray diffraction analyses. In addition, it is found that MgB 2 grains in Fe clad are fined by comparing the micrograph of Fe-clad wire and Ta-clad wire. The high transport critical current density ( J c ) of 6.0xa0×xa010 4 A/cm 2 (5 K, 4 T) and 5.2xa0×xa010 3 A/cm 2 (15 K, 6 T) are obtained in the MgB 2 /Fe wire.

Structural, transport and magnetic properties of RuSr2Sm1.4Ce0.6Cu2O10−δ

yThe structural, transport and magnetic properties of RuSr2Sm1.4Ce0.6Cu2O10-delta have been studied. The increase of oxygen content results in a decrease of lattice parameters and an increase of superconducting transition temperature. For Raman spectra, the hardening of the 650 cm(-1) mode with decreasing temperature is due to the thermal contraction of lattice. Magnetic measurements exhibited a ferromagnetic transition around 150 K, indicating the coexistence of superconductivity and ferromagnetism in this compound

Energy dissipation in Bi2Sr2CaCu2O8+δ single crystal

Abstract The in-plane resistivity of Bi 2 Sr 2 CaCu 2 O 8+ δ single crystal in different magnetic fields was measured for magnetic field H parallel and perpendicular to the CuO 2 plane. The irreversibility line H irr ⊥ c ( T ) for H parallel to the CuO 2 plane locates in higher field and higher temperature region in the H – T phase diagram than H irr ∥ c ( T ) for H perpendicular to the CuO 2 plane. H irr ∥ c ( T ) is dominated by surface barrier pinning, while H irr ⊥ c ( T ) shows different temperature dependencies above and below 2 T, which may indicate different energy dissipation above and below the field. The field dependence of activation energy above 2 T in H ⊥ c configuration could be explained by vortex–antivortex expansion in CuO 2 plane.

Effect of stripe order on the vortex phase transition in La1.44Nd0.4Sr0.16CuO4 films

The vortex phase transition in La1.44Nd0.4Sr0.16CuO4 epitaxial films with different thicknesses under a compressive strain were studied systematically by measuring the current?voltage (IV) characteristics in magnetic fields perpendicular to the CuO2 planes. For a strongly compressed thin film, in which the static stripe order is destroyed, the IV curves show a good three-dimensional (3D) vortex glass (VG) scaling collapse in various magnetic fields, indicating a 3D VG phase transition. By applying the standard scaling procedure to the data of a strain-released thick film with static stripe order, neither a 3D nor a quasi-2D VG model works well. Further investigation shows that, in order to achieve a good scaling collapse, the Kosterlitz?Thouless correlation length should be used for the isothermals above Tg. The scaling of isothermals below Tg can also be optimized if the VG correlation length and adjusted static critical exponent ? are used with D = 2 only, indicating that a quasi-2D VG phase might exist. Our findings reveal that the nature of the vortex phase transition is completely altered in the presence of the stripe order.

Visualizing high-temperature spin dynamics in La1−xCaxMnO3 from a mapping of EPR linewidth and g-factor

We report the measurements of electron paramagnetic resonance (EPR) on powder samples of La1?xCaxMnO3 (LCMO) at the commensurate carrier concentrations of x = N/8 (N = 1,2,3,4,5,6, and 7) within the temperature range 100?K?T?450?K. It is found that the mapping of the EPR linewidth ?H and the g-factor shows an electron?hole asymmetry in the paramagnetic insulating regime of the LCMO phase diagram. The drop of linewidth ?H with decreasing temperature in the mapping resembles the contour of the ferromagnetic transition curve in the LCMO phase diagram, while the isothermal ?H becomes narrowest at x = 3/8 due to the predominant double exchange interaction. By comparing the small-polaron model with spin?spin relaxation mechanism, we demonstrate that the latter should dominate the paramagnetic insulating regime of the LCMO phase diagram.