J. J. Krajewski

Superconductivity near 70 K in a new family of layered copper oxides

A new family of high-temperature superconductors is described, with the general formula Pb2Sr2ACu3O8+δ. Although they have the planes of CuO5 square pyramids characteristic of the other copper-oxide superconductors, the new compounds belong to a distinct structural series, with wide scope for elemental substitution. Their unusual electronic configuration also gives new insight into the role of charge distribution among the structural building blocks in controlling superconductivity.

Properties of high κ gate dielectrics Gd2O3 and Y2O3 for Si

We present the materials growth and properties of both epitaxial and amorphous films of Gd2O3 (κ=14) and Y2O3 (κ=18) as the alternative gate dielectrics for Si. The rare earth oxide films were prepared by ultrahigh vacuum vapor deposition from an oxide source. The use of vicinal Si (100) substrates is key to the growth of (110) oriented, single domain films in the Mn2O3 structure. Compared to SiO2 gate oxide, the crystalline Gd2O3 and Y2O3 oxide films show a reduction of electrical leakage at 1 V by four orders of magnitude over an equivalent oxide thickness range of 10–20 A. The leakage of amorphous Y2O3 films is about six orders of magnitude better than SiO2 due to a smooth morphology and abrupt interface with Si. The absence of SiO2 segregation at the dielectric/Si interface is established from infrared absorption spectroscopy and scanning transmission electron microscopy. The amorphous Gd2O3 and Y2O3 films withstand the high temperature anneals to 850u200a°C and remain electrically and chemically intact.

Dielectric properties of Ta2O5–ZrO2 polycrystalline ceramics

The dielectric properties of (Ta2O5)1−x(ZrO2)x polycrystalline ceramics at 1 MHz and temperatures between 0 and 100u2009°C are reported for 0.0<x<0.25. The dielectric constant is moderately enhanced in the solid solution as a function of Zr content, and there is a significant decrease in the temperature coefficient of the dielectric constant (TCK). The annealing temperature of the ceramics is found to have a dramatic effect on TCK, associated with the appearance of the low temperature form of tantalum oxide at intermediate Zr concentrations, and Ta4Zr11O32 at high Zr concentrations: a bulk ceramic consisting of a mixture of the latter two phases was found to display a dielectric constant of ≈50 and a temperature coefficient of ≈−40u2009ppm/°C.

Low Temperature Coefficient Bulk Dielectrics in the Ca2Nb2O7–Ca2Ta2O7 System

Abstract The dielectric properties of bulk polycrystalline ceramics in the binary Ca 2 Ta 2 O 7 –Ca 2 Nb 2 O 7 system are reported. Measurements of the dielectric constant at 1 MHz were made at temperatures between 0 and 100°C. The large negative (for the niobate) and positive (for the tantalate) temperature coefficients for the dielectric constants in the pure materials in the vicinity of room temperature can be balanced in a two-phase region near x = 0.36 in Ca 2 Ta 2−x Nb x O 7 . At this composition, the dielectric constant is approximately 30, and the temperature coefficient of the dielectric constant is approximately 2 ppm/°C. The Q in our unoptimized polycrystalline ceramic at 1 MHz is approximately 5000.

Ca5Nb2TiO12 and Ca5Ta2TiO12: low temperature coefficient low loss dielectric materials

The 1 MHz dielectric properties for single-phase polycrystalline ceramics of the Ca[sub 5]Nb[sub 2]TiO[sub 12] and Ca[sub 5]Ta[sub 2]TiO[sub 12] perovskites are reported. In the vicinity of ambient temperature, the dielectric constants are approximately 35 and 23, respectively, and the dielectric losses are low, with Qs on the order of 5,000, the upper limit of the authors measurement technique. Ceramics with a temperature coefficient of dielectric constant (TCK) less than 5 ppm/[degree]C are reported. The dielectric properties are found to be systematically dependent on processing conditions. The data suggest that an order-disorder transition among the B-site ions plays a major role in the determination of the temperature-dependence of the dielectric constant for both compounds.

REDUCED ALKALINE EARTH TANTALATES

Among reduced ternary transition metal oxides, tantalates are rare, even though interesting electronic properties might be expected. Here, the authors report on new reduced alkaline earth tantalates Ca{sub 2}Ta{sub 2}O{sub 6}F, Sr{sub 3{minus}x}Ta{sub 5}O{sub 15}, and Ba{sub 2}Ta{sub 15}O{sub 32} grown under vacuum conditions and high temperatures. These reduced ternary tantalates are isostructural with known reduced ternary niobates, indicating a strong similarity of the crystal chemistry between the two systems.

Neutron powder diffraction study of the 12 K superconductor La3Ni2B2N3-x

Abstract The structure of superconducting La 3 Ni 2 B 2 N 3− x has been investigated by neutron powder diffraction methods at room temperature. The compound crystallizes with the symmetry of space group I4/mmm (No. 139) and lattice parameters a = 3.72512(5) and c = 20.5172(4)A. The structure is layered and is made of (LaN) (LaN) (LaN) blocks with the rock-salt configuration alternating along the c -axis with Ni 2 B 2 blocks having a fluorite-type arrangement of the atoms. The nitrogen site located on the central layer of the rock-salt block is only 90% occupied, yielding stoichiometry La 3 Ni 2 B 2 N 2.9 . Layers are connected to each other along the c -axis by La N, B N and Ni B bonds, while the connections within the layers are provided by La N and Ni Ni bonds.

Stabilization of the low temperature coefficient of dielectric constant of Ca5Nb2TiO12 by Zr doping

Microwave communications devices involve the use of filters and resonators that have a ceramic dielectric as an essential component. To meet the requirements for use in such devices, dielectric materials must satisfy stringent property restriction. The effects of small amounts of Zr substitution for Ti in the low loss, low temperature coefficient dielectric material Ca{sub 5}Nb{sub 2}TiO{sub 12} are reported. Substitutions at the 1--5% level significantly increase the range of possible firing temperatures at which polycrystalline ceramics with low temperature coefficients can be obtained. This low temperature coefficient state is at the borderline between ordered and disordered states of the Ca, Nb, and Ti ions in the B sites of the perovskite lattice. The kinetics for ordering is apparently enhanced by the Zr substitutions.

Transparent Conducting Films Grown By Pulsed Laser Deposition

We have studied the properties of two types of transparent conducting oxides grown by pulsed laser deposition (PLD) on quartz substrates. We have grown films of ITO with resistivity as low as 250 μΩ-cm and with absorption coefficient −1 throughout the visible spectrum. Even films deposited at room temperature can have ρ 3 is a recently identified transparent conducting material which is structurally distinct from ITO. Films have been grown with no intentional dopants and with either Ge substitution for Ga or Sn substitution for In. Doping concentrations as high as 10 at. % have been studied. There is no evidence for dopant segregation. Films with resistivities as low as 3 μΩ-cm have been achieved, and the absorption coefficient can be less than 500 cm −1 throughout the entire visible spectrum.

Superconductivity in Bi O and Sb O perovskites

Abstract Ba(Pb,Sb)O 3 is the latest addition to the group of superconducting oxides. When substituting for Pb in BaPbO 3 , Sb behaves crystal-chemically the same as Bi. The Sommerfeld parameter γ∞N * (0) has been estimated from measurements of the lower and upper critical fields. As for Cu O and Bi O superconductors, the transition temperature of ≈3.5K of the Sb O compound is high in comparison to superconductors with the same density of states at E F . This points to the close similarity between Bi O and Sbz.sbnd;O perovskites and distinguishes them from other superconducting oxides which have no significant O2p character at E F