J.F. Qu

Effect of particle size on the structure and magnetic properties of La0.6Pb0.4MnO3 nanoparticles

We studied the effect of particle size on the structure and magnetic properties of La0.6Pb0.4MnO3 compounds with particle diameters varying from 5 to 100 nm. With decreasing particle size, the increase of the rhombohedral distortion narrows the bandwidth W-O2p and hence results in the decrease of ferromagnetic transition temperature. At the same time, the magnetic domain structure evolves from multidomain to single domain, and finally a superparamagnetic behavior is detected. The critical single domain size determined by experiment is about 25 nm, and the critical superparamagnetic behavior size obtained by calculation is 5.8 nm. It is found that the variation of coercivity is related to the evolution of magnetic domain states with particle size, and the saturation magnetization decreases significantly due to the increasing contribution of the surface effect with decreasing particle size. (c) 2006 American Institute of Physics.

I‐V characteristics of La1.84Sr0.16CuO4∕Nb-doped SrTiO3 heterojunction

Current-voltage (I‐V) characteristics of the La1.84Sr0.16CuO4∕1.0wt% Nb-doped SrTiO3 heterojunction were measured in the temperature range from 5to290K under magnetic fields up to 14T. All I‐V curves show a fine rectifying property. A visible reduction of the diffusion potential (Vd) can be observed at the superconducting transition temperature due to the opening of the superconducting gap in La1.84Sr0.16CuO4. The reduction of Vd from its original track, ΔVd, is found to be of similar temperature and field dependencies as those of the superconducting gap. This suggests an alternative technique for detecting the superconductivity related features in superconducting films.

Ultrasonic study on magnetic-field-induced stripe order in La1.88Sr0.12-xBaxCuO4

The ultrasonic sound velocities and attenuations of La1.88Sr0.12-xBaxCuO4 (0 <= x <= 0.12) under magnetic fields of up to 14 T show two important features of charge stripe order in the system. The first is that the ultrasonic characteristics indicate the existence of local stripe order associated with low temperature tetragonal structure domains for samples with x <= 0.04. The second is that the local stripe order can be enhanced by magnetic fields. The results indicate a strong interaction between charge stripes and crystal lattice in the system.

Transport and stability properties of the charge ordered state for Sm1-xCaxMnO3 (0.4 <= x <= 0.8)

The structure and transport properties of Sm1-xCaxMnO3 with 0.4less than or equal toxless than or equal to0.8 were investigated. With decreasing temperature from 350 K, cooperative Jahn-Teller (JT) distortion develops, leading to the formation of a charge ordered (CO) state and anomalies in the lattice parameters and resistivities. At low temperature the magnetoresistance effect originates from the polarization of the localized t(2g) spins under the magnetic field and decreases with the increase in Ca2+ content x, whereas the stability of the CO state at the CO transition temperature T-CO strongly depends on both Ca2+ content and cooperative JT distortion. These interesting phenomena sufficiently confirm the close relationship among charge, lattice, spin, and orbital degrees of freedom

Effect of Temperature on La1.85Sr0.15CuO4 Single Crystal Growth by Floating Zone Method

Superconducting single crystals of La1.85Sr0.15CuO4 have been grown at various temperatures without single crystal seeds by the traveling-solvent floating-zone method. In order to avoid the formation of bubbles during the crystal growth process, a flowing atmosphere of 2 atm oxygen or 1 atm air was applied in different temperature range. It was found that the crystal quality could be improved by raising the growth temperature in a certain range, and the orientation changed from (110) to (100) at higher temperature. X-ray diffraction results showed that the full-width at half-maximum of the best as-prepared crystal was 0.086°. The crystals grown in 2 atm oxygen showed a superconducting transition temperature (Tc) of 37.3 K, while the crystals grown in floating air showed a Tc of about 35 K and it could be improved to 36.5 K by annealing in flowing oxygen.

In-plane substitution effect on the local structure of La1.88Sr0.12CuO4

The effect of in-plane substitution on the local structure has been studied by ultrasonic method in a series sample of La1.88Sr0.12Cu1−yZnyO4 (y=0.00,0.01,0.02,0.03). It is found that a small amount of Zn doping greatly promotes the formation of the local low-temperature tetragonal phase, whereas further doping is harmful to it, the tendency of which is to show a marked resemblance to the Zn-concentration dependence of the magnetic transition temperature TN reported by muon-spin-relaxation (μSR) study. Our results also provide additional evidence that the slight in-plane nonmagnetic doping introduces local pinning centers to stabilize the charge stripes in La1.88Sr0.12CuO4.

Electrochemical fabrication of BixTe1- x(0.4 < X < 0.7) nanowire arrays

Large scale and highly ordered thermoelectric BixTe1-x (0.4 ≤ x ≤ 0.7) nanowire arrays were successfully fabricated by cathodic electrolysis into porous anodic alumina membrane (AAM) templates in aqueous solution. The structure of the nanowires was characterized by X-ray diffraction and selected-area electron diffraction (SAED). Field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) results show that the nanowires are smooth and uniform with the diameters of about 50 nm and lengths up to tens of micrometers. Energy dispersive spectroscopy (EDS) was used to check the exact stoichiometry of as-prepared samples. The results reveal that the atomic ratio between Bi and Te can be modulated effectively by controlling the concentration of the electrolyte solution. The synthesis of high quality BixTe1-x nanowires with controllable x is significant for optimizing the thermoelectric performance.

Solution Phase Synthesis of Superconducting La2CuO4 Microspheres

In order to exploit new strategies for fabricating high-temperature superconductors with special morphologies and investigate the effects on their superconductivity, we successfully utilized a facile polyol process for fabricating a large amount of La2CuO4 (LCO) microspheres using La(CH3COO)3·1.5H2O and Cu(CH3COO)2·H2O as raw materials and ethylene glycol as both complex agent and solvent. The well crystallized LCO microspheres can be obtained by annealing the La-Cu-complex precipitation formed after refluxing process at 750 oC for 10 hours. Field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) results indicated that the as-prepared sample had an average diameter of about 800 nm. The powder X-ray diffraction (XRD) patterns showed that the LCO microspheres were of pure phase. The magnetization measurement of the sample annealed at 900 oC in pure oxygen for 20 hours showed that the superconducting transition temperature (Tc) of the LCO microspheres was about 38 K, which in agreement with that of bulk counterparts.