Xiong Guang-Cheng

Deposition of MgB2 Superconducting Films on Different Metal Substrates

By a method of hybrid physical-chemical vapour deposition (HPCVD) on three metal substrates of stainless steel, copper and niobium, we deposit MgB2 superconducting films over 1 μm thickness. All of them have zero resistance temperatures TC(0)>36 K and critical current densities JC (10 K, 0 T) >106 A/cm2. Meanwhile, in the bending test, all the MgB2 superconducting films adhere strongly to the metal substrates without peeling off. Therefore, the MgB2 superconducting films supplied by the HPCVD method exhibit preferable electrical, magnetic and mechanical properties, and have potential applications in future.

Difference of Oxide Hetero-structure Junctions with Semiconductor Electronic Devices

Charge carrier injection performed in Pr0.7Ca0.3MnO3 (PCMO) hetero-structure junctions exhibits stable without electric fields and dramatic changes in both resistances and interface barriers, which are entirely different from behaviors of semiconductor devices. Disappearance and reversion of interface barriers suggest that the adjustable resistance switching of such hetero-structure oxide devices should associate with motion of charge carriers across interfaces. The results suggested that injected carriers should be still staying in devices and resulted in changes in properties, which guided to a carrier self-trapping and releasing picture in strongly correlated electronic framework. Observations in PCMO and oxygen deficient CeO2 devices show that oxides as functional materials could be used in microelectronics with some novel properties, in which interface is very important.Charge carrier injection is performed in Pr0.7Ca0.3MnO3 (PCMO) hetero-structure junctions, exhibiting the stability without electric fields and dramatic changes in both resistance and interface barriers, which are entirely different from behaviour of semiconductor devices. The disappearance and reversion of interface barriers suggest that the adjustable resistance switching of such hetero-structure oxide devices should associate with motion of charge carriers across interfaces. The results suggest that injected carriers should be still staying in devices and result in changes of properties, which lead to a carrier self-trapping and releasing picture in a strongly correlated electronic framework. Observations in PCMO and oxygen deficient CeO2-δ devices show that oxides as functional materials could be used in microelectronics with some novel properties, in which the interface is very important.

Relationship of Polaron Exchange with Ferromagnetic and Insulator--Metal Transitions in Doped Manganites

We report the experiment results and data analyses based on a polaron exchange model for La0.7 Ca0.3 MnO3 and Pr0.7 (Sr1-x Cax)0.3MnO3 epitaxial thin films. In the polaron exchange model with an energy balance condition, critical temperature of TC for stable ferromagnetic (FM) ordering should depend on δE as kBTC = E0 exp(- δE/kbTc), where δE denotes the potential barrier for the exchange polarons to overcome. Using the small polaron hopping model, the resistivity peak temperature TP is a function of the hopping energy Ehop. The dependence of TP on Ehop is similar to the dependence of TC on δE, which reveals that the polaron exchange relates to FM and insulator–metal transitions. The result indicates that the polaron exchange model is a simple way for describing the FM ordering, and is very helpful for understanding of complex doped manganites.

Fantastic Behaviour of High-TC Superconductor Junctions: Tunable Superconductivity

Carrier injection performed in oxygen-deficient YBa2Cu3O7(YBCO) hetero-structure junctions exhibited tunable resistance that was entirely different with behaviors of semiconductor devices. Tunable superconductivity in YBCO junctions, increasing over 20 K in transition temperature, has achieved by using electric processes. To our knowledge, this is the first observation that intrinsic property of high TC superconductors superconductivity can be adjusted as tunable functional parameters of devices. The fantastic phenomenon caused by carrier injection was discussed based on a proposed charge carrier self-trapping model and BCS theory.

Resistance Switching Characteristic and Charge Carrier Self-Trapping in Epitaxial Pr0.7(Ca1–xSrx)0.3MnO3 Thin Films

Carrier injection performed in Pr0.7Ca0.3MnO3 junctions demonstrate resistance switching (RS) characteristic with dramatic changes in both resistances and interface barriers, which suggests a charge carrier self-trapping model in strongly correlated electronic framework. Un-stable RS behaviour without electric fields in epitaxial Pr0.7(Ca1–xSrx)0.3MnO3 (PCSMO) films shows dependences on insulator–metal transition temperature, which indicates that RS process is really related to the intrinsic property of carriers. The switched resistance of epitaxial PCSMO films also depends on the amount of current pulses, which should be another evidence of the carrier self-trapping model, similarly to the dependence on the amount of self-trapped charge carriers.

Growth of n-type ZnO thin films by using mixture gas of hydrogen and argon

High-quality oxide semiconductor ZnO thin films were prepared on single-crystal sapphire and LaAlO3 substrates by pulsed laser deposition (PLD) in the mixture gas of hydrogen and argon. Low resistivity n-type ZnO thin films with smoother surface were achieved by deposition at 600°C in 1Pa of the mixture gas. In addition, ferromagnetism was observed in Co-doped ZnO thin films and rectification I−V curves were found in p-GaN/n-ZnO and p-CdTe/n-ZnO heterostructure junctions. The results indicated that using mixture gas of hydrogen and argon in PLD technique was a flexible method for depositing high-quality n-type oxide semiconductor films, especially for the multilayer thin film devices.

Spectroscopy of GdBa2Cu3O7−x high temperature superconducting films

Abstract High temperature superconducting films of GdBa 2 Cu 3 O 7− x have been studied by spectral ellipsometry and X-ray spectroscopy. Spectra of the imaginary part of the dielectric permittivity function, ϵ 2 , have been obtained in the range from 1.5 to 4.5 eV. The structural parameters, charge carrier density and stoichiometry (with respect to oxygen) of the films have been measured.

Different interdiffusion characteristics between Ag/YBa2Cu3O7−x and Al/YBa2Cu3O7−x contact interfaces

Abstract The differences between the interdiffusion characteristics of Ag YBa 2 Cu 3 O 7−x and Al YBa 2 Cu 3 O 7−x contact interfaces have been revealed by secondary ion mass spectrometry (SIMS). The different electrical properties of Ag YBa 2 Cu 3 O 7−x and YBa2Cu3O7−x films after high temperature treatment are well understood by the SIMS results.

Anomalous phonons in oriented film samples of Y-Ba-Cu-O

Abstract We prepared two epitaxial films of YBa 2 Cu 3 O 7 , in which the C -axes are oriented either perpendicular or parallel to the film surfaces. The anomalous phonon properties in the epitaxial films superconductor annealed from Ar and O 2 atmospheres were observed by means of IR reflection spectroscopy. We classified more clearly phonons or orthorhombic phase to be two modes, including E ⊥ C and E // C modes, than using polycrystalline samples. The strong coupling between the phonons and carriers reduces the IR activity of phonons. The important role of the phonon in superconductivity is shown and this may provide a crucial clue to the mechanism study.

Infrared reflectance from oriented film samples of Y-Ba-Cu-O

We have studied infrared reflectivity from epitaxial films of YBaCuO, in which the c axes are oriented either perpendicular or parallel to the film surface. We classified phonons in superconducting phase to be two modes including E⊥C and E//C modes. The strong coupling between the phonons and free carriers reduces the infrared activity of phonons. The infrared activity of phonons increases in the process that the orthorhombic phase changed into tetragonal phase. The important role of phonon in superconductivity is shown.