A Coaxially Integrated Photonic Orbital Angular Momentum Beam Multiplexer

We demonstrate an integrated photonic orbital angular momentum beam multiplexer consisting of four nested arc waveguide gratings. Well-defined OAM mode emissions over wide bandwidth of 1-nm enables simultaneous wavelength division multiplexing and OAM multiplexing

High-pressure study of the Raman modes in YBa2(Cu0.96Ni0.04)4O8

URL:http://link.aps.org/doi/10.1103/PhysRevB.60.4363 DOI:10.1103/PhysRevB.60.4363We present a study of the Raman phonons in YBa2(Cu0.96Ni0.04)4O8 under hydrostatic pressure in the superconducting phase. A comparison with our earlier work on the undoped YBa2Cu4O8 shows that the pressure coefficients of two vibrational modes involving oxygen atoms differ significantly from those of the corresponding modes in the undoped material. These are the O(1) chain mode which shifts 33% faster and the O(2)-O(3) in-phase mode which shifts 23% slower than the undoped counterparts. The other Raman modes in the Ni-doped sample shift in a manner similar to the undoped material. The observed behavior of the O(1) chain and O(2)-O(3) in-phase modes in YBa2(Cu0.96Ni0.04)4O8 under pressure and the softening of the Cu(1) Ag mode frequency with increasing Ni doping suggest that the Ni atoms substitute for the Cu atoms in the chain, which in turn decreases the compressibility in the vicinity of the Cu(Ni) chain atom.This work was supported by U.S. Department of Energy Grant No. DE-FG02-90ER45427 through the Midwest Superconductivity Consortium. D.J.P. thanks the U.S. Department of Education for support through Grant No.P200A50259

Structural Evolution of Ammonia Borane for Hydrogen Storage

We have studied the crystal structure of fully deuterated BH3NH3 using powder neutron diffraction at different temperatures. It is evident that an order-disorder phase transition occurs around 225 K. At low temperature, the compound crystallizes in the orthorhombic structure with space group Pnm21 and gradually transforms to a high temperature tetragonal structure with space group I4 mm above 225 K. At 16 K, the BD3-ND3 unit stacks along the c axis with a tilt angle of about 16° between the N-B bond and the c axis. As the temperature is increased, the BD3-ND3 groups start to reorient along the c axis and the deuterium atoms become disordered, leading to the tetragonal phase transition

Crystal and Electronic Structures of LiNH₂

The crystal structure of LiNH2 was reinvestigated using powder neutron diffraction with high sensitivity. The compound crystallizes in the tetragonal space group I4 with lattice parameters α = b= 5.034 42 (24) Å, c = 10.255 58 (52) Å. It is found that H atoms occupy 8g1(0.2429, 0.1285, 0.1910) and 8g2 (0.3840, 0.3512, 0.1278) sites. The bond lengths between the nearest nitrogen and hydrogen atoms are 0.986 and 0.942 Å, respectively. The bond angle between H-N-H is about 99.97°. These results are significantly different from those of previous experiments. The electronic structure was calculated according to the revised structural data. The calculated density of states and charge density distribution show strong ionic characteristics between the ionic Li+ cation and the covalent bonded [NH2]- anion

Crystal and Electronic Structures of the Complex Hydride Li₄BN₃H₁₀

The crystal structure of Li4BN3H10 was investigated using powder neutron diffraction with high sensitivity. The compound crystallizes in the cubic space group 213 with lattice parameters a=10.645 19(52) Å with an ordered arrangement of [NH2]−1 and [BH4]−1 anions in a molar ratio of 3:1. The bond lengths between the nearest nitrogen and hydrogen atoms are 1.04(4) and 1.14(4) Å. The bond angle between H(1)-N-H(2) is about 126(6)°, while those between H(3)-B-H(3) and H(3)-B-H(4) are about 109(6)°-110(7)°. There are three different Li sites surrounded by [NH2]−1 and [BH4]−1 anions in distorted tetrahedral configurations. The Li(3)-B and Li(3)-N bond distances are about 1.72(3) and 2.32(2) Å, respectively, while the Li(1)-N and Li(2)-N distances are both around 2.09 Å. The strong bonding of Li(3) to the [BH4]−1 and the weaker Li(3)-[NH2]−1 bond are evidenced by the presence of the LiBH4 moiety in a projection of the crystal structure onto the a-b plane. First-principle calculations have been performed based on the structural data. Analyses of the density of states and charge density indicate that H(1) and H(2) strongly interact with N, and H(3) and H(4) interact with B to form [NH2]−1 and [BH4]−1, respectively. It is confirmed that Li(1) and Li(2) are strongly bonded to N and Li(3) is strongly bonded to B. These results are significantly different from some of the previous studies

Anomalous Magnetic Ordering in PrBa₂Cu₄O₈ and CmBa₂Cu₃O₇

A review of temperature-dependent magnetization data for nonsuperconducting PrBa2Cu4O8 and CmBa2Cu3O7 suggests that the failure of each to superconduct is related to the presence of Pr and Cm on their respective Ba sites. This defect is manifested, in each case, by short c-axis lattice parameters and anomalous high-temperature magnetic ordering which has been incorrectly attributed to ordering of the entire magnetic sublattice. Instead, it is shown that the anomalous high-temperature ordering as seen in the magnetization data is consistent with the ordering of magnetic ions substituted on the Ba site

Generation of photonic orbital angular momentum superposition states using vortex beam emitters with superimposed gratings

An integrated approach to produce photonic orbital angular momentum (OAM) superposition states with arbitrary OAM spectrum has been demonstrated. Superposition states between two vector OAM modes have been achieved by integrating a superimposed angular grating in one silicon micro-ring resonator, with each mode having near equal weight. The topological charge difference between the two compositional OAM modes is determined by the difference between the numbers of elements in the two original gratings being superimposed, while the absolute values of the topological charge can be changed synchronously by switching WGM resonant wavelengths. This novel approach provides a scalable and flexible source for the OAM-based quantum information and optical manipulation applications