Xianggang Qiu

Leakage mechanism of (Ba0.7Sr0.3)TiO3 thin films in the low-temperature range

Pt/(Ba0.7Sr0.3)TiO3(BST)/YBa2Cu3O7−x(YBCO) capacitors on SrTiO3 substrates were fabricated, and the temperature dependence of their current–voltage characteristics was studied in the temperature range from 100 to 300 K. It was found that the leakage current strongly depends on the voltage polarity and temperature. In the case of a Pt electrode negatively biased at a high electric field (>400 kV/cm), the Pt/BST interface acts as a Schottky barrier with the barrier height of 0.47 eV in the temperature range from 200 to 300 K. When the YBCO electrode was biased negatively and the electrical field was maintained higher than 64 kV/cm, the leakage mechanism shows space-charge-limited-current behavior.

Temperature- and field-dependent leakage current of Pt/(Ba0.7Sr0.3)TiO3 interface

Pt/(Ba0.7Sr0.3)TiO3(BST)/YBa2Cu3O7−x(YBCO) capacitors on SrTiO3 substrates were fabricated. The temperature and field dependence of the leakage current of Pt/BST interface were studied in the temperature range from 100 K to 320 K and in an electric field up to 3 MV/cm. For a middle electric field ( 1.8 MV/cm), the leakage mechanism is governed by the Fowler–Nordheim tunneling in the whole measured temperature range.

Analysis of atomic force microscopic results of InAs islands formed by molecular beam epitaxy

Atomic force microscopy (AFM) measurements of nanometer-sized islands formed by 2 monolayers of InAs by molecular beam epitaxy have been carried out and the scan line of individual islands was extracted from raw AFM data for investigation. It is found that the base widths of nanometer-sized islands obtained by AFM are not reliable due to the finite size and shape of the contacting probe. A simple model is proposed to analyze the deviation of the measured value From the real value of the base width of InAs islands

Enhanced dielectric properties of Ba1-xSrxTiO3 thin film grown on La1-xSrxMnO3 bottom layer

The Ba{sub 0.7}Sr{sub 0.3}TiO{sub 3} (BST)/La{sub 0.67}Sr{sub 0.33}MnO{sub 3} (LSMO) heterostructure has been fabricated by pulsed-laser deposition on (001) SrTiO{sub 3} single crystal substrate. The crystallization and surface morphology of the heterostructure have been characterized by x-ray diffraction and atomic force microscopy. The composition distribution along the depth of the heterostructure is analyzed by Auger electron spectroscopy. It is founded that the LSMO bottom layer shows a marked effect on the dielectric properties of the BST films. Comparing with BST/YBCO (YBCO-YBa{sub 2}Cu{sub 3}O{sub 7}) heterostructure grown by similar process, the dielectric loss exhibits much lower and less frequency dispersive, especially in the high frequency region. The loss tangent at 100 kHz is about 0.012 and the figure of merit factor is about 25 under the condition of 200 kV/cm. Ferroelectric hysteresis measurement shows that the remanent polarization and coercive field of the heterostructure are 3.4 {mu}C/cm{sup 2} and 39 kV/cm, respectively. These parameters are all better than BST/YBCO heterostructure. The good surface morphology and the element diffusion between BST and LSMO layers may be suggested to be responsible for the above-improved parameters of BST/LSMO heterostructure.The Ba0.7Sr0.3TiO3(BST)∕La0.67Sr0.33MnO3(LSMO) heterostructure has been fabricated by pulsed-laser deposition on (001) SrTiO3 single crystal substrate. The crystallization and surface morphology of the heterostructure have been characterized by x-ray diffraction and atomic force microscopy. The composition distribution along the depth of the heterostructure is analyzed by Auger electron spectroscopy. It is founded that the LSMO bottom layer shows a marked effect on the dielectric properties of the BST films. Comparing with BST∕YBCO (YBCO—YBa2Cu3O7) heterostructure grown by similar process, the dielectric loss exhibits much lower and less frequency dispersive, especially in the high frequency region. The loss tangent at 100kHz is about 0.012 and the figure of merit factor is about 25 under the condition of 200kV∕cm. Ferroelectric hysteresis measurement shows that the remanent polarization and coercive field of the heterostructure are 3.4μC∕cm2 and 39kV∕cm, respectively. These parameters are all better than...

Positive temperature coefficient of resistivity in Pt/(Ba0.7Sr0.3)TiO3/YBa2Cu3O7-x capacitors

Pt∕(Ba0.7Sr0.3)TiO3(BST)∕YBa2Cu3O7-x(YBCO) capacitors were fabricated on (001) LaAlO3 substrates. The leakage current density-electric field characteristics were measured in the temperature range from 20 to 300K. In the electric field region of >0.7MV∕cm for positively biased Pt electrode and >1.4MV∕cm for negatively biased Pt electrode, the leakage current shows space-charge-limited-current behavior from 20 to 250K. Meanwhile a positive temperature coefficient of resistivity (PTCR) in the temperature range from 20 to 150K was observed and attributed to the strong negative temperature dependence of the dielectric constant of BST thin films.

Effects of geometrical symmetry on the vortex in mesoscopic superconductors

Abstract. We first systematically study the multivortex states in mesoscopic superconductors via self-consistent Bogoliubov-de Gennes equations. Our work focuses on how the geometrical symmetry affects the penetration and arrangement of vortices in mesoscopic superconductors and find that the key parameter determining the entrance of the vortex is the current density at the hot spots on the edge of sample. Through determining the spatial distribution of hot spots, the geometrical symmetry of the superconducting sample influences the nucleation and entrance of vortices. Our results propose one possible experimental approach to control and manipulate the quantum states of mesoscopic superconductors with their topological geometries, and they can be easily generalized to the confined superfluids and Bose-Einstein condensates.

Abnormal temperature dependence of dielectric constant in (Ba0.7Sr0.3)TiO3 thin films

Pt∕(Ba0.7Sr0.3)TiO3∕YBa2Cu3O7−x capacitors were fabricated on LaAlO3 substrates. Dielectric and ferroelectric properties were measured in the temperature range from 20 to 250K. It was found that the dielectric constant decreased with decreasing temperature from 250 to ∼150K but then increased as the temperature continued to decrease to 20K. Considering this phenomenon along with the irregular temperature dependence of spontaneous polarization, it is indicated that such abnormal behavior originates in a second-order structural phase transition in the (Ba1−xSrx)TiO3 thin film in the vicinity of 150K.