A. Dabkowski

Comment on the origin(s) of the giant permittivity effect in CaCu3Ti4O12 single crystals and ceramics

Impedance spectroscopy has been performed on CaCu3Ti4O12 (CCTO) single crystals and ceramics using Au and InGa alloy electrodes. The extrinsic effect responsible for the giant permittivity observed at radio-frequencies near room temperature is shown to be different for single crystals and ceramics. For single crystals it is attributed to a non-ohmic electrode contact to semiconducting CCTO. For ceramics it is attributed to an internal barrier layer capacitor mechanism associated with semiconducting CCTO grains and insulating grain boundaries.

Thermal contraction behavior in Al2(WO4)3 single crystal

Abstract The anisotropic thermal expansion behavior of a single crystal of Al 2 (WO 4 ) 3 was obtained for the first time and the anisotropic contraction behavior was identified. The direct clarification of the anisotropic behavior contributes greatly towards designing the thermal expansion characteristics along every axis, realizing unique materials such as a zero expansion material.

SrLaGaO4 - Czochralski crystal growth and basic properties

Abstract Single crystals of SrLaGaO4 have been grown by the Czochralski method. This material melts congruently at 1520°C. It has the K2NiF4 structure (I4/mmm) with lattice parameters a = 3.84 A and c = 12.68 A . According to the EPMA analysis, the crystals obtained contained more SrO and Ga2O3 and less La2O3 then Expected from the stoichiometric composition. These crystals are applicable as substrates for high temperature superconducting thin films.

Growth of large-domain YBa2Cu3Ox with new seeding crystals of CaNdAlO4 and SrLaGaO4

Abstract Single crystals of CaNdAlO 4 and SrLaGaO 4 were used as seeds to grow large domains of YBa 2 Cu 3 O x for levitation applications. These crystals have high melting temperatures (> 1500°C) and similar lattice structures to that of YBa 2 Cu 3 O x . In a seeded melt-texturing method developed previously, the single crystals of CaNdAlO 4 , SrLaGaO 4 , and NdBa 2 Cu 3 O x were used as seeds for comparison. After melt processing, scanning electron microscopy analysis did not reveal any major differences in all these seeded melt-textured samples. However, the levitation forces in the samples seeded with single crystals of CaNdAlO 4 and SrLaGaO 4 increased considerably compared to that of the sample seeded with NdBa 2 Cu 3 O x . A model is proposed to describe the domain growth mechanism during seeded melt processing.

Spin ice: magnetic excitations without monopole signatures using muon spin rotation.

Theory predicts the low temperature magnetic excitations in spin ices consist of deconfined magnetic charges, or monopoles. A recent transverse-field (TF) muon spin rotation (μSR) experiment [S. T. Bramwell et al., Nature (London) 461, 956 (2009)] reports results claiming to be consistent with the temperature and magnetic field dependence anticipated for monopole nucleation-the so-called second Wien effect. We demonstrate via a new series of μSR experiments in Dy(2)Ti(2)O(7) that such an effect is not observable in a TF μSR experiment. Rather, as found in many highly frustrated magnetic materials, we observe spin fluctuations which become temperature independent at low temperatures, behavior which dominates over any possible signature of thermally nucleated monopole excitations.

Evaluation of LaSrGaO4 as a substrate for YBa2Cu3O7-δ

Abstract We present a comparison study between YBa2Cu3O7−δ thin films deposited on LaSrGaO4 substrates. The LaSrGaO4 crystals were grown, cut and polished in-house while the LaAlO3 substrates were prepared commercially. The deposited films were characterized by a variety of techniques including DC resistivity, critical current density, X-ray diffraction including rocking-curve analysis and scanning electron microscopy. Growth temperatures above 780°C on both substrates yielded highly oriented c-axis material with excellent transport properties. Because LaSrGaO4 has the advantage of being untwinned, a desirable property for microwave-device applications, we conclude that it has much promise as a potential substitute for LaAlO3.

Magnetic flux jumps in textured Bi 2 Sr 2 CaCu 2 O 8 + δ

Magnetic flux jumps in textured Bi 2 Sr 2 CaCu 2 O 8 + δ have been studied by means of magnetization measurements in the temperature range between 1.95 K and T c , in an external magnetic field up to 9 T. Flux jumps were found in the temperature range 1.95-6 K. with the external magnetic field parallel to the c axis of the investigated sample. The effect of sample history on magnetic flux jumping was studied and it was found to be well accounted for by the available theoretical models. The magnetic-field sweep rate strongly influences the flux jumping and this effect was interpreted in terms of the influence of both flux creep and the thermal environment of the sample. Strong flux creep was found in the temperature and magnetic-field range where flux jumps occur suggesting a relationship between the two. The heat exchange conditions between the sample and the experimental environment also influence the flux jumping behavior. Both these effects stabilize the sample against flux instabilities, and this stabilizing effect increases with decreasing magnetic-field sweep rate. Demagnetizing effects are also shown to have a significant influence on flux jumping.

Crystal Growth of Oxides by Optical Floating Zone Technique

Single crystals of various congruently and incongruently melting oxides have been recently grown by the floating zone (FZ) and traveling solvent floating zone (TSFZ) techniques. For the incongruently melting materials, the use of solvent with an experimentally determined composition allows the establishment of the practical steady state much faster, leading to better, more stable growth. Growth conditions for different oxides are compared. Important problems in crystal characterization and assessment of micro- and macrodefects are briefly presented.

Anderson-Mott transition induced by hole doping in Nd 1 − x TiO 3

The insulator/metal transition induced by hole doping due to neodymium vacancies of the Mott-Hubbard antiferromagnetic insulator, Nd1�xTiO3, is studied over the composition range 0.0106x 0.24310. Insulating p-type conduction is found for x 0.07110. Anderson localization in the presence of a Mott-Hubbard gap is the dominant localization mechanism for the range of 0.07410x 0.0891 samples. For x 0.0891, n-type conduction is observed and the activation energy extrapolates to zero by x 0.1. The 0.0958x 0.20310 samples are Fermi-liquid metals and the effects of strong electronic correlations are evident near the metal-to-insulator boundaries in features such as large Fermi liquid T 2 coefficients. For 0.0749x 0.1124, a weak negative magnetoresistance is found below 15 K and it is attributed to the interaction of conduction electrons with Nd 3+ magnetic moments. Combining information from our companion study of the magnetic properties of a Nd1�xTiO3 solid solution, a phase diagram is proposed. The main conclusions are that long-range antiferromagnetic order disappears before the onset of metallic behavior, and that the Anderson-Mott transition occurs over a finite range of doping levels. Our results differ from conclusions drawn from a similar study on the hole-doped Nd1�xCaxTiO3 system, which found the coexistence of antiferromagnetic order and metallic behavior and that the Mott transition occurs at a discrete doping level.

Phase diagram and crystal growth of Pb2Sr2(YxCa1-x)Cu3O8+y

Abstract Fundamental research on the crystal growth of Pb2Sr2(YxCa1-x)Cu3O8+y (PSYCCO) has been carried out by differential thermal analysis (DTA), phase diagram studies, and improved thermogravimetric analysis (ITGA). The phase diagrams of the systems PSYCCO-PbO and PSYCCO-PbO(NaCl) have been produced. These indicate that the addition of NaCl, which functions as a flux as well as separating the growth system from air, is beneficial to the crystal growth. Superconducting crystals were grown by spontaneous nucleation with and without precursor. Single crystals up to 3×2.8×2 mm3 in size have been successfully grown from PbO-NaCl flux without using a precursor. The crystals were characterized by X-ray powder analysis and by measurement of their superconducting magnetic properties in the as-grown state.