Ahmed A. A. Youssef

The Permittivity and AC Conductivity of the Layered Perovskite [(CH3)(C6H5)3P]2HgI4

The dielectric permittivity and ac conductivity of bis-(Methyltriphenyl-phosphonium)2 tetraiodomercurate (II), [(CH3)(C6H5)3P]2HgI4, has been measured in at 300-400 K and 0.11-20 kHz. The frequency dependent conductivity is interpreted in terms of the jump relaxation model, where translational and reorientational hopping takes place. The conductivity results were fitted to the law σ = σ0 + A1 (T)ωs + A2 (T) ωr, with s < 1 and r < 2. The temperature dependence of the conductivity was fitted to two relaxation processes. The activation energies ΔE1 and ΔE2 are frequency dependent and lie in the ranges of 0.8 eV and 0.2 eV. ΔE1 is associated with translational long range hopping while, DE2 is associated with localized and/or reorientational hopping. PACS Nos. 76, 77.

Ultrasonic studies of thermally assisted flux flow and intrinsic pinning in high-Tc phase Bi-Pb-Sr-Ca-Cu-O ceramics

Abstract The temperature dependence of the ultrasonic attenuation coefficient α ( T, H ) and sound velocity V s ( T, H ) were measured under magnetic fields for high- T c phase Bi-Pb-Sr-Ca-Cu-O ceramic samples aligned with the c -axis. In the case H · k , where k is the direction of sound propagation, a broad peak was observed in the α( T , 10T)-α( T , 0 T) versus T curve at T ∼ 65 K accompanied with a broad step in the V s ( T , 10 T) - V s ( T , 0 T) versus T curve. On the other hand, the peak and the step were also observed in the case of H | k , were the temperature position shifted down to T ∼50 K, and the width was narrower than in H · k . The behavior was explained in terms of thermally assisted flux flow in which flux lines are pinned by intrinsic pinning centers.

The frequency dependence of the conductivity and dielectric relaxation of [(CH2)3(NH3)2]Cu(II)Cl4

The structural, dielectric and conductive properties of [(CH2)3(NH3)2]Cu(II)Cl4 have been studied. The material shows an order-disorder transition at T1 = (333±2) K and a ferroelectric phase transitions at T2 = (434±3) K. Results of differential thermal analysis, infrared spectroscopy, X-ray diffraction, AC conductivity and permittivity measurements are reported and discussed. The conductivity results are interpreted in terms of barrier hopping at low and intermediate temperatures, and band-type conduction at high temperatures. - PACS: 81.05.-t, 77.22.-d

Preparation, Characterization and Crystal Structure of the Room Temperature Phase of [(CH3)(C6H5)3P]2[ZnBr4]: A Member of the A2BX4 Family

The compound bis(methyltriphenylphosphonium) tetrabromozincate(II), [C19H18P]2[ZnBr4], Mr = 939.640, has a monoclinic unit cell, space group P21. The lattice parameters are a = 9.7693(4) Å , b = 12.5508(4) Å , c = 16.5372(6) Å , α = 90.00°, β = 105.2670(11)°, γ = 90.00°, V = 1956.11(11) Å3, Z = 2, Dx = 1.595 mgm−3 at T = 298 K. The structure consists of one distorted [ZnBr4]2− tetrahedron and two [(CH3)(C6H5)3P]+ cations. Differential scanning calorimetry indicates a continuous second-order transition at (276±2) K that may be classified as a commensurate to incommensurate transformation. A first-order transition to a higher symmetry is associated with a four-fold rotation of the [ZnBr4]2− ion and a change of entropy ΔS = 22.92 J/(K· mol) at T = (362±3) K. Dilatometric measurements showed a decrease of the lattice parameters in the temperature range 230 - 260 K, confirmed the transition at (276±2) K, and indicated the presence of a third transition at 282 K.

Magnetic and Electric Studies of a New Cu(II) Perovskite-like Material

Thermal analysis of a lipid-like bilayer of [(CH2)9 (NH3)2]CuCl4 indicates one minor transition at 303 K and the following five major ones (with entropies in J/K-mole): The initial magnetic susceptibility at 320 Hz and a magnetic field of 160 A/m in the temperature range 75 - 290 K revealed a Curie-Weiss behavior of phase (V) and possible ferromagnetic ordering of phases (VI) and (IV) at low temperatures. The variation of the dielectric permittivity with temperature at 60.0 Hz - 100 kHz indicates large changes of the dipole moment at the transition temperatures. The conductivity is thermally activated and frequency dependent, following the universal power law: σ (ω) =σdc +A(T)ωs(T). Values of s being phase dependent. For phase (I), T > 383 K, band type conduction prevails. Hopping conduction is found in phases(II) and (III). Comparison with other Cuand Cd-containing materials is made. -PACS: 76, 77

Thermally assisted flux flow excited by ultrasound in superconducting thin films

Abstract A new ultrasonic method has been proposed for the investigation of the depinning line of superconducting thin films. The motion of the depinned flux lines is detected as the excess flux-flow voltage caused by the excitation of pinned flux lines through the ultrasonic vibration of pinning centers. When a strain related to the tilt modulus C 44 of flux-line lattice was applied to YBa 2 Cu 3 O 7−δ thin films in a parallel magnetic field, a maximum of the excess flux-flow voltage was observed below the superconducting transition temperature. To explain the maximum observed, a theory is proposed in which thermally assisted flux flow and ultrasonic attenuation due to the depinning of flux lines are taken into account. The discrepancies between the experimental and the theoretical results are discussed with the ultrasonic frequency and amplitude dependence of the depinning temperature.

Dielectric Permittivity and AC Conductivity Investigation for the New Model Lipid Bilayer Material: (CH2)10(NH3)2CdCl4

Abstract Differential thermal scanning of the new lipid-like bilayer material (CH2)10(NH3)2CdCl4 showed two structural phase transitions, with onset temperatures at T2 = (359 ± 2) K and T1 = (415± 1) K. Permittivity measurements were performed between room temperature and 450 K at 60-100 kHz. A step-like rise in permittivity at T2, associated with an order-disorder transition, is attributed to chain melting. Two anomalies at (413 ± 1) K and (430 ± 3) K, showing thermal hysteresis of -8 and 10 K, respectively, in dicate first order transitions which are associated with crystalline phase change. The AC conductivity follows an Arrhenius-type relation with the activation energy ΔE varying with the frequency / according to the relation ΔE = ΔE0 [1 -exp (f0/f)] where ΔE0,f0 and a are 0.95 eV, 52 Hz and 0.11, respectively. The frequency dependent conductivity follows the power law σ= σdc +Bωx with 0.3 <s< 1.5 for hopping conduction of hydrogen and/or chloride ions in the high temperature range, and localized hopping and/or orientational motion predominating temperatures lower than 413 K. Variations of B and s with temperature are discussed. PACS No. 76, 77

Ultrasonic studies of crystal structure dependence of flux pinning in Bi2Sr2Can − 1CunOy (n = 1, 2 and 3)

Abstract The changes in ultrasound velocity ΔVs(T, H) with magnetic field were measured for three kinds of Bi-based oxide superconductors, Bi2Sr2Can − 1CunOy (n = 1, 2 and 3) with a dopant of La or Pb. For n = 2 and 3, steps were observed in ΔVs versus T curves at a depinning temperature. However, no step was observed for n = 1, suggesting a small flux pinning force compared with n = 2 and 3. The correlation among crystal structure, coherence length and intrinsic pinning was discussed.