C. Q. Jin

Band gap and wave guiding effect in a quasiperiodic photonic crystal

A two-dimensional octagonal quasiperiodic photonic crystal composed of alumina cylinders is prepared. The transmission spectra of the quasicrystal are measured in the microwave region for the TM wave. We find that the position and width of the band gap do not depend on the incident direction, while the band structure can appear for quite a small piece of the quasicrystal. Two types of waveguide, a straight guide and a bending guide with two sharp 90° corners, are fabricated by removing three rows of cylinders. The measured transmittances show that the guiding efficiency for both waveguides is high.

117 K superconductivity in the BaCaCuO system

Abstract Superconductivity was observed at 117 K in the BaCaCuO samples synthesized at 950°C under 5.0 GPa for 1 h. The samples were multiphase. Electrical resistivity and DC magnetic susceptibility measurements revealed bulk superconductivity at Tconset = 117 K, Tczero = 107.6 K and Tcmag = 116.0 K. Crystals having primitive tetragonal unit cells with the lattice parameters of a=3.88 A , and c=15.0 A , 18.3 A and 21.6 A found by electron diffraction. The dominant peaks of the X-ray powder diffraction pattern were tentatively assigned to the hypothetical Cu-1234 ( a=3.85 A and c=18.30 A ) and Cu-1223 ( a=3.88 A and c=14.94 A ). structures. It is likely that these are new high-Tc superconducting phases.

High-pressure synthesis of orthorhombic SrIrO3 perovskite and its positive magnetoresistance

The orthorhombic SrIrO3 perovskite was synthesized under 5GPa and 1000°C. It is paramagnetic below about 170K and transfers to an unknown magnetism under higher temperature. A band type metal to insulator transition caused by a pseudogap was observed at about 44K. Interestingly a positive magnetoresistance, i.e., resistance increased with applying magnetic field, was observed in the orthorhombic SrIrO3 perovskite below about 170K.

Manifestation of ferroelectromagnetism in multiferroic BiMnO3

Multiferroic BiMnO3 with a highly distorted perovskite structure induced by the stereochemically active 6s2 electron lone pairs of Bi3+ was synthesized at a high pressure of 6 GPa. Magnetization, differential scanning calorimetry, dielectric permittivity, and in situ powder x-ray diffraction as a function of temperature were carried out, respectively. In light of comprehensive evaluation, we can conclude that the synthetic BiMnO3 ceramic displays ferromagnetic and ferroelectric orderings simultaneously, i.e., ferroelectromagnetism below its ferromagnetic Curie temperature TM∼100K.

Novel homologous series of superconducting copper oxides, Cu-12(n-1)n

Perovskite-like structures in a multi-phase BaCaCuO sample which was synthesized under high pressure and showed superconductivity at Tc=117 K have been studied by means of electron diffraction (ED) analysis and high-resolution electron microscopy (HRTEM). Based on the ED analysis, three different phases with perovskite-like structures are identified to possess tetragonal primitive unit cells with lattice parameters a=3.88 A and c=(5.1+3.3 n) A where n=3, 4 and 5. The HRTEM observations reveal structures similar to those of members of the Hg-12 (n−1)n and/or Tl-12(n−1)n series. Thus, it is most likely that there exists a novel homologous series of superconducting cuprate phases, i.e. Cu-12(n−)n, whose n=21 member corresponds to the well-known tetragonal “123” structure.

More direct evidence of the fcc arrangement for artificial opal

We present a scanning electron microscopy (SEM) investigation of artificial opals prepared by the sedimentation of suspension with sub-micrometer silica spheres. An analysis of the SEM images confirms that the fcc stacking is the most stable phase for the artificial crystals. Optical diffraction and transmission spectrum indicate a good alignment of the top (111) crystal plane. We also show that defects and dislocations of opal crystals are caused mainly by the irregular spheres. Slow sedimentation and uniform size of silica spheres may be the key factors for growing high quality opal crystals.

Pressure effects on mechanical properties of bulk metallic glass

The effects of high pressure (up to 5GPa) on the mechanical properties of a typical Zr41Ti14Cu12.5Ni10Be22.5 bulk metallic glass (BMG) have been investigated. It is found that the high-pressure pretreatment at room temperature can significantly improve the mechanical performance of the BMG. Particularly, the compressive plasticity of the BMG can be increased as large to as 12% by 4.5GPa pressure pretreatment. The origin of the pressure effect on mechanical properties is studied.

Enhancement of the superconducting critical temperature of Sr2CuO3+delta up to 95 K by ordering dopant atoms

We address the question of whether the superconducting transition temperature (Tc) of high-Tc cuprates is enhanced when randomly distributed dopant atoms form an ordered array in the charge reservoir layers. This study is possible for the Sr2CuO3+d superconductor with K2NiF4-type structure in which oxygen atoms only partially occupy the apical sites next to the CuO2 planes and act as hole-dopants. We show that remarkable Tc enhancement up to 95K in this mono CuO2 layered HTS is associated with the apical oxygen ordering, not to the hole concentration change. The result points a route toward further enhancement of Tc in cuprate superconductors.

Demultiplexer using directly resonant tunneling between point defects and waveguides in a photonic crystal

We propose a frequency selective demultiplexer that is realized by the directly resonant tunneling between waveguides and point defects in a two-dimensional photonic crystal. The multiple scattering method is used to simulate the behavior of the demultiplexer. Results show that the demultiplexing efficiency is very high when the point defects are symmetrically arranged at both sides of the central line of the inlet waveguide. The selective frequency is only determined by the defect mode of the corresponding point defect in the photonic crystal and is not affected by the other point defects. A microwave experiment results agree well with the simulation. This structure may be valuable for the design of all-optical integrated circuits.

Reflectionless multichannel wavelength demultiplexer in a transmission resonator configuration

A wave demultiplexer system with N channels in a two-dimensional photonic crystal is proposed. The demultiplexer is realized by the coupling among an ultra-low-quality factor microcavity and N resonators with high-quality factor. The coupling mode theory is employed to analyze the behavior of this system. The analytic results reveal that the reflection is fully absent at the peak frequencies for all channels. The simulations obtained by multiple-scattering method and experimental results in the microwave region show that the analysis is valid. This method might also be valuable for the design of other all-optical functional circuits.