H. Özkan

Pressure dependence of the elastic constants of nonmetamict zircon

The single crystal elastic constants of nonmetamict zircons have been measured as a function of pressure to 12 kb at room temperature and also as a function of temperature between 25 and 300° C at atmospheric pressure. The pressure derivatives of the elastic constants are: C11=10.78, C33=5.88, C44=0.99, C66=−0.31, C12=3.24, C13=6.20.The anomalous negative behaviour of C66 versus pressure could be associated with a high pressure phase transition. The pressure and temperature derivatives of the isotropic elastic wave velocities and elastic moduli for nonmetamict zircon are calculated from the present single crystal data by the Voigt, Ruess, and Hill approximations and compared with the values of some other oxides and silicates. The pressure derivative of the isotropic adiabatic bulk modulus is relatively high (dKS/dP=6.50), and the pressure derivative of the shear modulus is relatively low, (dG/dP=0.78), compared to the corresponding values for some other oxides and silicates. The Debye temperature, ϑD, and the high temperature limit of the Grüneisen parameter, γHt, calculated from the elastic constants and their pressure derivatives, agrees well with the Debye temperature and the thermal Grüneisen parameter, γth, calculated from the thermal expansion, heat capacity, and compressibility data.

Enhancement in the high-Tc phase of BSCCO superconductors by Nb addition

Pure and Nb2O5 added Bi1.6Pb0.4NbxSr2Ca2Cu3O? superconductors (x varies up to 0.30) were synthesized by the solid state reaction method. Structural and superconducting properties of the samples were investigated. The fraction of the high-Tc phase (2223) increases with Nb addition up to x = 0.10, for which a nearly single 2223 phase was obtained. The critical temperature of the samples increases from 96 to 104?K with doping up to x = 0.20 and a further increase in Nb decreases the Tc sharply. Similarly, there is a considerable enhancement in the critical current density (jc) of the samples in the same doping range. Both AC susceptibility and transport measurements showed that the optimum Nb concentration is x = 0.20. The out-of-phase component of the AC susceptibility measurements showed that inter-grain coupling was enhanced up to the same doping level. Nb additions above x = 0.30 decrease the fraction of the high-Tc phase and degrade the superconducting properties. Present measurements indicate that x = 0.20 is the optimum Nb concentration in the BSCCO superconductor to enhance phase purity, inter-grain coupling and superconducting parameters (Tc and jc).

Thermally stimulated current analysis of shallow levels in TlGaS2 layered single crystals

We have carried out thermally stimulated current measurements on as-grown TlGaS2 layered single crystals in the temperature range of 10–60 K with various heating rates. We found experimental evidence for the presence of three trapping centres in TlGaS2, located at 6, 12 and 26 meV. We have determined the trap parameters using various methods of analysis, and these agree well with each other. The retrapping process is negligible for these levels, as confirmed by the good agreement between the experimental results and the theoretical predictions of the model that assumes slow retrapping.

Temperature dependence of the Raman-active phonon frequencies in indium sulfide

The temperature dependence of the Raman-active mode frequencies in indium sulfide was measured in the range from 10 to 300 K. The analysis of the temperature dependence of the Ag intralayer optical modes show that Raman frequency shift results from the change of harmonic frequency with volume expansion and anharmonic coupling to phonons of other branches. The pure-temperature contribution (phonon‐phonon coupling) is due to three- and four-phonon processes. q 1999 Elsevier Science Ltd. All rights reserved.

Thermally stimulated currents in layered semiconductor Tl4In3GaS8

We have carried out thermally stimulated current measurements on as-grown Tl4In3GaS8 layered single crystals in the temperature range 10–90 K with different heating rates of 0.10–0.30 K s−1. The data were analysed by curve fitting, heating rates and isothermal decay methods. The results were in good agreement with each other. Experimental evidence was found for one trapping centre in Tl4In3GaS8 crystal with an activation energy of 12 meV. The capture cross section and concentration of the traps were found to be 5.4 × 10−25 cm2 and 3.3 × 1014 cm−3, respectively. Analysis of the thermally stimulated current data at different light excitation temperatures leads to a value of 8 meV/decade for the trap distribution.

Effects of B2O3 addition on the properties of melt-processed YBa2Cu3O7−δ

Samples of YBa2Cu3O7??, having about 30%?Y2BaCuO5, were prepared with a melt-process method by adding B2O3 up to 0.5?wt% to the oxides. The structural and superconducting properties of the samples were studied by x-ray diffraction, x-ray fluorescence, scanning electron microscope, magnetization and magneto-optic techniques. B2O3 additions during the melt-processing enlarge intergrain regions that weaken the conduction links between the grains. Tc drops by 1.0?6.0?K, and at 50?K Jc decreases from about 1.6 ? 104 to 1.8 ? 103?A?cm?2 with B2O3 additions up to 0.5?wt%. A?secondary peak was observed in the Jc versus field plot of the boron-free sample that decreases with boron additions. At 50?K the intragrain Jc values of the boron-free sample were about 12% higher than the intergrain Jc value, and such differences increase with B2O3 additions. These observations, together with the enlargements of the intergrain regions with B2O3 additions, suggest that most of the boron atoms take part and reside at grain boundaries.

Structural and electrical characterization of Ag3Ga5Te9 and Ag3In5Se9 crystals

X-ray powder diffraction studies revealed that Ag 3 Ga 5 Te 9 and Ag 3 In 5 Se 9 crystallize in orthorhombic and tetragonal systems, respectively. The temperature dependent conductivity and Hall effect measurements have been carried out between 65-480 K. Ag 3 Ga 5 Te 9 exhibits p-type conduction with a room temperature conductivity of 4.3 × 10 -4 (Ω. cm) -1 and mobility less than 1 cm 2 /V S. Ag 3 In 5 Se 9 was identified to be n-type with room temperature conductivity 7.2 × 10 -5 (Ω. cm) -1 and mobility 20 cm 2 /V. s. From temperature dependence of the conductivity three different impurity ionization energies were obtained for both compounds. The anomalous behavior observed in the temperature dependence of mobility was attributed to the different features of the microstructure.

Effect of thermal neutron irradiation in boron-doped melt-textured YBCO superconductors

Abstract Y 1.6 Ba 2.3 Cu 3.3 O x superconductors with different amounts of boron doping have been synthesized using the MPMG technique. Undoped and boron-doped samples were irradiated with thermal neutrons to study the effect of defects produced by the fission reaction, B(n,α)Li, on the pinning and the critical current. We observed that the pinning and the critical current density were improved as a result of thermal neutron irradiation. This improvement was slightly stronger for the boron-doped samples compared to the undoped ones.

Dispersive optical constants and temperature tuned band gap energy of Tl2InGaS4 layered crystals

The optical properties of Tl2InGaS4 layered single crystals have been studied by means of transmission and reflection measurements in the wavelength range of 400–1100 nm. The analysis of the room temperature absorption data revealed the presence of both optical indirect and direct transitions with band gap energies of 2.35 and 2.54 eV, respectively. Transmission measurements carried out in the temperature range of 10–300 K revealed that the rate of change of the indirect band gap with temperature is γ = −4.70 × 10−4 eV K−1. The absolute zero value of the band gap energy was obtained as Egi(0) = 2.45 eV. The dispersion of the refractive index is discussed in terms of the Wemple–DiDomenico single-effective-oscillator model. The refractive index dispersion parameters: oscillator energy, dispersion energy, oscillator strength and zero-frequency refractive index were found to be 5.73 eV, 31.46 eV, 11.72 × 1013 m−2 and 2.55, respectively. From x-ray powder diffraction study, the parameters of the monoclinic unit cell were determined.

Dissipative behavior and γ-irradiation of silver-sheathed Bi1.8Pb0.4Sr2Ca2Cu3Ox tape

Abstract Dissipative mechanisms in Bi 1.8 Pb 0.4 Sr 2 Ca 2 Cu 3 O x tapes were investigated using magnetoresistance measurements and V – I characteristics. A considerable broadening of the resistive transition in applied magnetic field was observed up to a field of 0.5 T. The temperature and magnetic field dependence of the resistance R ( T , B ) were fitted to the Arrhenius relation and from which the magnetic field dependence of the pinning energy U 0 ( B )∼ B − α with α ≈0.9 was derived. The variations of the critical current densities J c with temperature and magnetic field before and after γ-irradiation were also studied. The temperature dependence of J c ( T ) was fitted to the relation J c ( T )∼(1− T / T c ) n with n =1.9 and discussed in terms of thermally activated flux flow (TAFF). However, J c ( B ) results were explained based on the weak links, grain boundaries, and Josephson junctions in which it shows a power law behaviour J c ( B )∼ B −0.5 . At relatively low temperatures and magnetic fields, the critical current density was increased with a γ-irradiation up to a dose of 100 MR.