B. R. Zhao

Thickness and dielectric constant of dead layer in Pt/(Ba0.7Sr0.3)TiO3/YBa2Cu3O7−x capacitor

Pt/(Ba0.7Sr0.3)TiO3 (BST)/YBa2Cu3O7−x capacitors were prepared and investigated for the dead-layer (DL) thickness (td) and the DL dielectric constant (ed). Based on the series capacitor model, the td/ed ratio of 0.066 nm and the bulk BST ferroelectric-layer dielectric constant of 1370 were obtained through the measurements of the capacitance–voltage characteristics. The td×ed value of 120 nm was obtained through the measurements of the current–voltage characteristics. Combining these data, the DL thickness and the DL dielectric constant are respectively estimated to be 2.8 nm and 42.6.

Leakage current of Pt∕(Ba0.7Sr0.3)TiO3 interface with dead layer

Leakage current of Pt∕(Ba0.7Sr0.3)TiO3 (BST)∕YBa2Cu3O7−δ capacitors on a (001) SrTiO3 substrate was studied. By modeling a low-dielectric constant layer, a so-called dead layer, between the Pt∕BST interface as a parasitic capacitor in series with the bulk layer capacitor, the leakage current of Pt∕BST interface was well analyzed based on the modified Schottky emission equation. Furthermore, a two-step schematic energy band diagram is proposed to explain the carrier transport through the Pt∕BST interface.

Experimental evidence of the "dead layer" at Pt/BaTiO3 interface

The Pt∕BaTiO3 (BTO) interface was investigated by angle-resolved x-ray photoelectron spectroscopy and x-ray reflectivity technique. It was shown that there exists a transition layer of about 9A at the Pt/BTO interface with electron density lower than that of the BTO film. The transition layer shows a higher binding energy of Ba 3d than that of the bulk BTO. Moreover, neither the interdiffusion of BTO and Pt nor the oxidation of Pt near the interface had been observed. We consider that this layer is caused by “interface-induced relaxation.” This relaxation layer is believed to be the origin of the “dead layer” effect.

Raman scattering and thermogravimetric analysis of iodine-doped multiwall carbon nanotubes

Iodine-doped multiwall carbon nanotubes ~I-MWNTs! were characterized by means of Raman scattering and thermogravimetric analysis. The results show that multiwall carbon nanotubes ~MWNTs! can be effectively doped by iodine and exchange electrons with iodine. Iodine atoms form charged polyiodide chains inside tubes of different inner diameter, which is similar to the iodine-doped single-wall carbon nanotubes ~I-SWNTs!, but can not intercalate into the graphene walls of MWNTs. The Raman scattering behavior of I-MWNTs exhibits some differences from that of I-SWNTs and the low-dimensional conductive hydrocarbon-iodine complex ‘‘peryleneiI 2.92 .’’

The study of critical current in YBaCuO thin films

The magnetron sputtering method has been used for fabricating YBa2Cu3O7−y superconducting thin films. Their critical current and superconducting phase with relation to temperature have been investigated. Experimental results show, for some samples, the proportionality between critical current Ic and temperature as (1−T/Tc)3/2 in the vicinity of Tc and the typical proximity tunneling effect. While at temperatures much lower than Tc, the current‐temperature relation corresponds well with the Ambegaokar–Baratoff theory.

Surface pattern and large low-field magnetoresistance in La0.5Ca0.5MnO3 films

Formation of an ordered surface structure in La0.5Ca0.5MnO3 films due to the mismatch of the thermal expansion coefficient between the film and the substrate has been investigated. The surface pattern consists of grain chains located on regular orthogonal cracks. The cracks serve as weak-link grain boundaries, and unusually enhanced low-field magnetoresistance (−14.4% in 400 Oe at 90 K) has been observed, which may be explained by spin-polarized tunneling across the grain boundaries.

Metallic oxide p-I-n junctions with ferroelectric as the barrier

The authors report the formation of the metallic oxide p-I-n junctions with the ferroelectric (Ba,Sr)TiO3 (BST) as the barrier. The junctions with different thicknesses of BST are investigated. With appropriate thickness, the junctions possess definite parameters, such as the negligible reversed current density (⩽10−7A∕cm2), large breakdown voltage (>7V), and ultrahigh rectification (>2×104) in the bias voltage ⩽2.0V and temperature range from 5to300K. It is under consideration that the built-in field V0, the ferroelectric reversed polarized field Vrp, and the resistivity of the BST layer together decide the transport properties of the junctions.

SYNTHESIS AND PROPERTIES OF HIGH-TC YBA2CU3O7-Y ORIENTED THIN FILMS

The a‐ and c‐axis‐oriented YBa2Cu3O7−y thin films were prepared by rf magnetron sputtering technique. The characterization of the thin films was carried out by inductively coupled plasma atomic emission spectroscopy, x‐ray diffraction, and scanning Auger profile techniques. The Tc, Tc(H), Jc(H), and Hc2 ‐T(near Tc) were measured by the standard dc four‐probe method. It was found that very strong anisotropy of this material was manifested in its superconducting properties, especially for c‐axis‐oriented thin films. It was also found that Jc depends very strongly upon stoichiometric composition 123, preferential orientation of the film, and the characteristic of the film‐substrate interface.

High-Tc superconductor Bi2Sr2CaCu2O8+δ tunnel junction with Zn counterelectrode

Using Zn thin film as counterelectrode, Bi2Sr2CaCu2O8+δ tunnel junctions were prepared. Temperature-dependent tunneling spectra of these planar junctions provided measurements of energy gap, 2Δ, and transition temperature, Tc. The spectra also allowed the observation of pseudogap in normal state and showed strong evidence of d-wave symmetry of the superconducting order parameter. The present junction may find its way in the electronics applications.

Tunneling spectroscopy of Bi2Sr2CaCu2O8+δ/Zn junctions

Abstract We present tunneling spectroscopy on as-grown Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) single crystals taken with Bi2212/Zn tunnel junctions. The measured temperature-dependent tunneling conductance show a sharp quasi-particle peak at the gap voltage at low temperature, and the suppression of conductance in gap region at T above T c , revealing the opening of a pseudogap. The observed opening temperature of the pseudogap, T ∗ , is above 270K for the investigated as-grown Bi2212. The plots of normalized zero bias conductance versus T, G(0,T), provide T c of the region investigated in the crystals, and the G(0,T) can be fitted well to a T 2 law for T below T c , which provides a new evidence of d-wave symmetry pairing in Bi2212.