B.R. Zhao

Far infrared optical properties of the pyrochlore spin ice compound Dy2Ti2O7

Near normal incidence far infrared reflectivity spectra of [111] dysprosium titanate (Dy2Ti2O7) single crystal have been measured at different temperatures. Seven phonon modes (eight at low temperature) are identified at frequencies below 1000 cm−1. Optical conductivity spectra are obtained by fitting all the reflectivity spectra with the factorized form of the dielectric function. Both the Born effective charges and the static optical permittivity are found to increase with decreasing temperature. Moreover, phonon linewidth narrowing and a phonon mode shift with decreasing temperature are also observed, which may result from enhanced charge localization. The redshift of several low frequency modes is attributed to the spin–phonon coupling. All observed optical properties can be explained within the framework of the nearest neighbour ferromagnetic (FM) spin ice model.

Influence of MgO particle doping on flux pinning in Bi2Sr2CaCu2Oy crystals

Bi2Sr2CaCu2O8+delta (Bi2212) single crystals embedded with MgO particles are successfully synthesized. The critical current density of Bi2212 single crystals with an appropriate amount of MgO doping is noticeably increased at low temperatures below 20 K with applied magnetic fields parallel to the c-axis as compared with undoped samples. Whereas at T greater than or equal to 25 K, there is no extra flux pinning in MgO-doped samples. Microstructure analyses reveal that MgO doping induces a high density of dislocations in the ab plane of Bi2212 matrix, which may be responsible for the enhanced flux pinning at low temperatures

Control of the epitaxial orientation and reduction of the interface leakage current in YMnO3/GaN heterostructures

Epitaxial ferroelectric YMnO3 (YMO) thin films with high crystalline quality were fabricated on (0?0?0?1) GaN substrates by pulsed laser deposition followed by rapid thermal annealing. X-ray diffraction and transmission electron microscopy measurements reveal that YMO layers are of (0?0?0?1) orientation with an in-plane epitaxial relation (0?0?0?1)YMO||(0?0?0?1)GaN, . The orientation of the lattice of YMO was found to be controlled by the annealing conditions. Y2O3 nanocrystallites observed near the interface were possible seeds to stabilize the orientation of the epitaxial YMO thin films. The appropriate annealing treatment could improve the crystalline quality and reduce the interface leakage current.

Electron–phonon coupling in Nb-doped SrTiO3 single crystal

Near normal incident infrared reflectivity spectra of (001) SrTi1−xNbxO3 single crystal have been measured at different temperatures in the frequency region between 100 and 6000 cm−1. The optical parameters of phonons and plasmons are obtained on the basis of the double-damping extended Drude model. The lowest frequency phonon mode at 175 cm−1 softens with decreasing temperature, and the plasmon frequency triples when the temperature decreases from 300 to 10 K. Extra absorption near the highest frequency longitudinal phonon mode shows that the effect of the polarons should be taken into account in addition to the plasmon–phonon coupling. The splitting of the last high reflectivity band, the temperature dependence of the 615 cm−1 structure and the appearance of a new weak peak-like defect mode provide evidence for the existence of small polarons.

Thickness-induced insufficient oxygen reduction in La2−xCexCuO4 ± δ thin films

A series of electron-doped cuprate La2?xCexCuO4 ? ? thin films with different thicknesses have been fabricated and their annealing times are adjusted carefully to ensure the highest superconducting transition temperature. The transport measurements indicate that, with the increase of the film thickness (<100?nm), the residual resistivity increases and the Hall coefficient shifts in the negative direction. Furthermore, the x-ray diffraction data reveal that the c-axis lattice constant c0 increases with the decrease of film thickness. These abnormal phenomena can be attributed to the insufficient oxygen reduction in the thin films. Considering the lattice mismatching in the ab plane between the SrTiO3 substrates and the films, the compressive stress from the substrates may be responsible for the more difficult reduction of oxygen in the thin films.

The role of excess Cu on La2CuO4 system

The Cu-rich La2CuO4+delta ([La]:[Cu] = 2:1 + x) samples with 0 less than or equal to x less than or equal to 0.18 are characterized using X-ray diffraction, transmission electron microscopy and electron energy loss spectrum methods. The excess Cu is identified to occupy the La site and the maximum of such substitution, i.e., the maximum of excess Cu content, x, is examined to be about 0.06. From the magnetization measurements of the samples with different excess Cu, we find that the substitution of Cu for La can remarkably enhance the three-dimensional antiferromagnetic exchange interaction, thus, promoting electronic phase separation in La2CuO4+delta system with delta < 0.01

New electric features in Cu-rich La2CuO4+δ system

A series of light oxygen doped Cu-rich La2CuO4+delta samples. with delta < 0.01 and the nominal composition of cations being La:Cu = 2:1 + x (0 less than or equal to x less than or equal to 0.18), have been investigated. Characterization shows that the samples are single phase till x greater than or equal to 0.10. TEM, electron energy loss spectroscopy, magnetization and electric transport measurements show that the excess Cu (the substitution of Cu2+ for La3+) can enhance the 3D antiferromagnetic (AF) exchange interaction and promote electronic phase separation. In the AF background the hole-rich cluster caused by phase separation becomes superconducting below 34 K, and shows ferromagnetism from the superconducting transition temperature T-c to above the Neel temperature T-N