High-T c superconducting thin film/GaAs MESFET hybrid microwave oscillator

A high-T _c superconducting (HTSC) thin film/GaAs MESFET hybrid microwave oscillator operated at 10.6 GHz has been designed, fabricated and characterized. Microstrip line structures were used throughout the circuit with superconducting thin film YBa₂Cu₃O_7-δ(YBCO) as the conductor material. The YBCO thin films were deposited on 15 mm×10 mm×0.5 mm LaAlO₃ substrates. The oscillator was common-source, series feedback type using a GaAs-MESFET (NE72084) as the active device and a superconducting microstrip resonator as the frequency stabilizing element. By improving the unloaded quality factorQ ₀ of the superconducting microstrip resonator and adjusting the coupling coefficient between the resonator and the gate of the MESFET, the phase noise of the oscillator was decreased. At 77 K, the phase noise of the oscillator at 10 kHz offset from carrier was −87 dBc/Hz. Project supported by the National Center for Research and Development on Superconductivity of China.     

Phase transition in layered perovskite-like manganate Ca3Mn2O7 under high pressure

In situ high pressure energy dispersive X-ray diffraction measurements on the layered perovskite-like manganate Ca₃Mn₂O₇ powder under pressures were performed by using the diamond anvil cell with synchrotron radiation. The results show that the structure of layered perovskite-like manganate Ca₃Mn₂O₇ is unstable under pressure due to the easy compression of NaCl-type blocks. The structure of Ca₃Mn₂O₇ underwent two phase transitions under pressures in the range of 0–35 GPa. One was at about 1.3 GPa with the crystal structure changing from tetragonal to orthorhombic. The other was at about 9.5 GPa with the crystal structure changing from orthorhombic back to another tetragonal.     

Heating effect of BaTiO3 in microwave field and microstructure of BaTiO3

Microwave equipment at 2 450 MHz was employed to prepare BaTiO₃. The heating effect of the system in the microwave field, which was influenced by several factors including dielectric properties of synthesis system and thermal insulate structures, was discussed in detail. The heating rates of the synthesis system were mainly determined by BaCO₃ and TiO₂ at low temperature and by TiO₂ and BaTiO₃ at high temperature. The results show that the heating effects in microwave field are greatly different from those in conventional furnace. The reaction of BaCO₃ and TiO₂ only lasts for 3 min at 1 100°C, and the fine, narrow-distributed and well-crystallized powders were prepared. Project supported by the National Natural Science Foundation of China.     

Phonon spectroscopy and transport properties of Y1-x Nd x Sr2Cu2.7Mo0.3O7-δ

The Raman and infrared spectra, resistivity as well as thermoelectric power of Y_1-xNd_xSr₂Cu_2.7Mo_0.3 O₇ δ(.x =0, 0.1. 0.2. 0.5, 0.8 and 1.0) are studied carefully. It was found that the structure characteristics of RSr₂Cu_2.7Mo_0.3O₇ δ cuprates are different from those of RBa₂Cu₃0₇ δ. The variations of the microstructure in Y_1-x Nd_xSr₂Cu_2.7Mo_0.3O₇ δ with Nd-doping affect the carrier distribution. so as a result the superconductivity changes. Furthermore, it is pointed out that the microstructure variations with the large rare earth ionic substitution cause the widely existing rare earth ionic size effect in HTSC. Project supported by the National Natural Science Foundation of China and the National Center for R & D Superconductivity.     

Field dependence of magnetoresistance in epitaxial single-crystal La2/3Ca1/3MnO3-δ thin film

Epitaxial La_2/3Ca_1/3MnO₃ thin films were prepared on NdGaO₃(1lO) substrates by d.c. magnetron sputtering method. The measurements of magnetoresistance ρ(H) upon magnetic field at different temperatures were carried out in the field range of 0–8 T. It is found that ρ(H) obeys the following relations: when the temperature (T) is higher than the Curie temperature below and whenT is far WowT _c. It is suS8ested that the negative magnetoresistive effect is mainly due to enhancement of the magnetoconductance. Project supported by the National Natural Science Foundation of China (Grant No. 19504012) and the Chinese Academy of Sciences.     

Microstructure of a-SiOx:H

A set of a-SiO_x:H (0.52 <x< 1.58) films are fabricated by plasma-enhanced-chemical-vapor-deposition (PECVD) method at the substrate temperature of 250°C. The microstructure and local bonding configurations of the films are investigated in detail using micro-Raman scattering, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). It is found that the films are structural inhomogeneous, with five phases of Si, Si₂O:H, SiO:H, Si₂O₃:H and SiO₂ that coexist. The phase of Si is composed of nonhydrogenated amorphous silicon (a-Si) clusters that are spatially isolated. The average size of the clusters decreases with the increasing oxygen concentration x in the films. The results indicate that the structure of the present films can be described by a multi-shell model, which suggests that a-Si cluster is surrounded in turn by the subshells of Si₂O:H, SiO:H, Si₂O₃:H, and SiO₂.     

M?ssbauer study on Sn-doped (Lal1-x,Srx)2Cu1-x,Snx,O4 superconductors under a new concept

¹¹⁹Sn M?ssbauer research is carried out on (La_1-xSr_x),Cu_1-xSn_xO₄(x = 0.075 and 0.110) superconductors which are designed under a new concept. The M?ssbauer spectra results show that Sn occupies Cu position in Sn⁴⁺ state, and there is no Sn²⁺ ion occupying La position. The local lattice deformation near Sn⁴⁺ site is small, but displays an increasing tendency with Sn doping. For La₂CuO₄ matrix, the simultaneous dopings of Sr/Sn induce holes and electrons on CuO₂ layer in a new mechanism which influences superconductivity. Under a new mechanism of extra oxygen, the extra oxygen effect of Sn-doping on superconductivity is discussed     

Fundamental relation and automorphism group of very thin Hv-groups

The purpose of this paper is computing the fundamental relations and automorphism groups of very thin H_v-groups. In this regards, we first investigate some basic properties of H_v-groups and then we show that any given group is isomorphic to the fundamental group of a nontrivial H_v-group. The main properties of very thin H_v-groups are also investigated and it is proved that every finite very thin H_v-group is proper.     

Atomic scale epitaxial growth of BaTiO3 thin films by laser molecular beam epitaxy

By using laser molecular beam epitaxy (L-MBE), atomic scale epitaxid growth of BaTiO₃(BTO) thin films on SrTiO₃ (STO) substrates is achieved. Measurements of reflection high energy electron diffraction (RHEED), X-ray diffraction (XRD), scanning electron microscopy, and transmission electron microscopy reveal that the BTO films arec-axis oriented single crystals with smooth surface. The multi-layer ferroelectric/superconducting heterostructures are also prepared and the ferroelectric properties of BTO films are studied. The results show that by using L-MBE technique, high quality BTO films and improved device performance can be obtained. Project supported by the National Natural Science Foundation of China, the National Department of Finance, and the National Center for R and D on Superconductivity of China.     

Epitaxial growth and electrical transport properties of La0.5Sr0.5Co03 thin films prepared by pulsed laser deposition

Epitaxial growth of the La_0.5Sr_{0.5 3}(LO) thin films has been realized on Lin3, SrTiC₃ and MgO substrates by pulsed laser deposition. The epitaxial growth behavior and the electrical transport properties of these films were studied systematically. The temperature dependencies of the resistivity of the film have been determined. Studies indicate that close dependencies exist between the crystal structures and the electrical transport properties of the epitaxial LSCO films, and that the epitaxial thin films are of low resistivity and metallic conductive features. The epitaxial films deposited on the LaA10₃ substrates at about 700 °C possess the optimal properties compared with the others. Discussions of the dependencies and the mechanisms of the epitaxial structures on the electrical transport properties of the LSCO films have been made. Project supported by the National Natural Science Foundation of China (Grant No. 19574003 and No. 19674001).     

Fibrous Filler-Matrix Interaction in Oxide Composites

The interaction between a fibrous alumina filler and a hydrated zirconia matrix during the fabrication of ceramic composites is studied. The influence of the crystal structure and porosity of the filler on the properties of the ZrO₂-Al₂O₃ composite material is investigated. It is found that, as a result of addition of alumina fibers of cubic and monoclinic structures (- and -phases), the ZrO₂ hydrogel interacts with fibers and solid ZrO₂ solutions of tetragonal structure are formed. These solutions exist over the range of 600 to 1200°C and, at annealing above 1200°C, due to crystallographic transitions the of - and -phases, they break down, namely Al₂O₃ transforms into -corundum and ZrO₂ into a monoclinic modification. However, the material does not fail and a stable porous ceramic composite is formed. To manufacture a composite with improved mechanical characteristics, fibrous alumina fillers with the structure of -corundum must be used.     

Diamond nucleation on surface of C60 thin layers

Diamond nucleation on the surface of C₆₀ thin layers and intermediate layer of Si substrates are studied by scanning electron microscopy (SEM). The cross-section SEM images of diamond films show that diamond grains really nucleate on the surface of C₆₀ thin layers. The SEM images of diamond nucleating sites show the nucleating aggregation of diamond on C₆₀ surfaces. The preferential oriented diamond films are observed. The plasma pretreatment of C₆₀ sublimating layers is a key factor for diamond nucleation.     

Synthesis and characterization of C3N4 hard films

C₃N₄ films have been synthesized on both Si and Ft substrates by microwave plasma chemical vapor deposition (MPCVD) method. X-ray spectra were calculated for single phase α-C₃N₄ and β-C₃N₄ respectively. The experimental X-ray spectra of films deposited on both Si and Pt substrates showed all the strong peaks of α-C₃N₄ and β-C₃N₄ so the films are mixtures of α-C₃N₄ and β-C₃N₄. The N/C atomic ratio is in the range of 1.0–2.0. X-ray photoelectron spectroscopy (XPS) analysis indicated that the binding energy of Is and N ls are 286.2 eV and 399.5 eV respectively, corresponding to polarized C-N bond. Fourier transform infrared absorption (FT-IR) and Raman spectra support the existence of C-N covalent bond in the films. Nano-indentation hardness tests showed that the bulk modulus of a film deposited on Pt is up to 349 GPa     

Mechanism of microwave synthesized BaTiO3

The difference of intermediate products, microstructure and element concentration in the particles between microwave synthesized samples and conventional samples was responsible for the existence of non-thermal effect in the microwave field. The diffusions of Ba²⁺, Ti⁴⁺ in the microwave field were enhanced, so that the diffusion of Ti⁴⁺ could not be neglected as in the conventional solid state reactions. The influences of the microwave field were mainly expressed as diffusion coefficient and the driving force of ionic motion. The intermediate phase Ba₂TiO₄ which occurred in the conventional solid reaction was not found during microwave syntheses. The quantity analyses based on XRD experimental data show that the reaction dynamics in microwave is suitable for the Carter equation. The activity energy for reaction of BaCO₃ and TiO₂ in the microwave field was 42.26 kj/mol, which was only one fifth of the conventional reaction. Project supported by the National Natural Science Foundation of China.     

High quality YBCO superconductive thin films fabricated by laser molecular beam epitaxy

High quality YBa₂.Cu₃O_{6 +x}(YBCO) superconductive thin films have been fabricated on the SrTiO₃(100) substrate using laser molecular beam epitaxy (laser-MBE). The active oxygen source was used, which made the necessary ambient oxygen pressure be 2–3 orders lower than that in pulsed laser deposition (PLD). T_c0 is 85–87 K, and J_c, 1.0 × 10⁶ A/cm². Atomic force microscopy (AFM) measurements show that no obvious particulates can be observed and the root mean square roughness is 7.8 nm. High stability DC superconducting quantum interference devices (DC-SQUID) was fabricated using this YBCO thin film.     

Capillarity driven spreading of power-law fluids

We investigate the spreading of thin liquid films of power-law rheology. We construct an explicit travelling wave solution and source-type similarity solutions. We show that when the nonlinearity exponent λ for the rheology is larger than one, the governing dimensionless equation h_t + (h^λ+2|h_xxx|^λ−1h_xxx)_x=0 admits solutions with compact support and moving fronts. We also show that the solutions have bounded energy dissipation rate.     

A phenomenological description of temperature dependence of magnetoresistance in La2/3Ca1/3MnO3-δ thin films

The electrical resistance in zero magnetic field and magnetoresistance in different external magnetic fields have been measured in a temperature range of 77—300 K. It is found that the temperature dependence of magnetoresistance can be well described by a phenomenofogical formula of

Asymptotic behavior of eigenvalues and eigenfunctions of the Fourier problem in a thick multilevel junction

A spectral boundary-value problem is considered in a plane thick two-level junction Ω_ε formed as the union of a domain Ω₀ and a large number 2N of thin rods with thickness of order ε = O(N ⁻¹). The thin rods are split into two levels depending on their length. In addition, the thin rods from the indicated levels are ε-periodically alternating. The Fourier conditions are given on the lateral boundaries of the thin rods. The asymptotic behavior of the eigenvalues and eigenfunctions is investigated as ε → 0, i.e., when the number of thin rods infinitely increases and their thickness approaches zero. The Hausdorff convergence of the spectrum is proved as ε → 0, the leading terms of asymptotics are constructed, and the corresponding asymptotic estimates are justified for the eigenvalues and eigenfunctions. Published in Ukrains’kyi Matematychnyi Zhurnal, Vol. 58, No. 2, pp. 195–216, February, 2006.     

Preparation, structure, magnetic properties and special magnetoresistance effect of Lao.67Sr0.33Mn03+δ/Pr0.7Cao.3 Mn03+δ/Lao.67Sro.33Mn03+δ trilayered thin films

The La_0,67Sr_0.33Mn0_{3 +δ}/Pr_0.7Ca_o.3Mn0_3+δ/La_0.67Sr_0.33Mn0_3+δ(LPL) trilayered films on (100)LaA-1O₃ substrates are prepared by using direct current (DC) magnetron sputtering method. The results obtained by means of X-ray powder diffractometer show that all films are the high quality epitaxial films. The results gained by SQUID magnetometer indicate that there is a magnetic coupling in the LPL trilayered films. The resistivities of LSMO, PC-MO and LPL films are measured using standard four-probe method and analyzed log_p-1/T curve. From the results it is concluded that the middle-layered PCMO which is ferromagnetic may play a role of intra-magnetic field, which weakens the paramagnetism of LSMO film, lowersp _max and enlargesT _p which is the transition temperature from metal to insulator, just as the applied magnetic field does. And the middle-layered PCMO may induce the change of the density of states in the LSMO’s gap. The two reasons above make the resistivity andT _p of the samples in zero field change with the thickness of PCMO layers. Project supported by the Chinese Academy of Sciences and the Foundation of State Science and Technology Commission of China.