Mohammad Mehbod

“Tweed” structure of Fe-doped YBa2Cu3O7−δ

Abstract Fe-doped YBa 2 Cu 3 O 7−δ single crystals have been prepared by a flux method from an Y-Ba-Cu-O precursor with copper partially substituted by iron. Electron diffraction and microscopy were applied to study the domain structure of the material. Superposed on a broad twin band texture, as also observed in undoped material, all samples reveal a finer “tweed” structure of overlapping lenticular domains oriented along [120] and [1 1 0]. This suggests a two-step formation process of the domain structure. The displacement field, corresponding with the tweed structure, resembles that of microtwinning in undoped, quenched YBa 2 Cu7 3 O 7−δ samples. The randomly dispersed Fe-ions act as pinning centers or fragment the structure by the induction of twin interfaces. Monte Carlo simulations support this idea.

Antiferromagnetism in YBa2Cu3O6+x: Ga and Zn substitutions I. 89Y NMR determination of the Néel temperature

Abstract The Neel temperature T N of YBa 2 (Cu 1- y M y ) 3 O 6+ x for pure (0 x x 89 Y NMR width with decreasing T . A rather weak variation of T N and of the NMR width in the antiferromagnetic state has been detected upon Zn or Ga doping. This leads us to conclude that a majority of Zn atoms are substituted on the Cu (1) site. In the absence of paramagnetic moments on the Cu (1) layer, the magnetic interactions between CuO 2 bilayers might be assigned to dipole-dipole couplings, except if the correlation length for the 2D bilayers has a much steeper T dependence than for La 2 CuO 4 . A detailed analysis of the hole doping in the pure compounds is attempted.

Electrical properties of poly(2-vinylpyridine)-iodine films

Abstract The d.c. electrical conductivity of thick films of poly(2-vinylpyridine)-iodine is measured as a function of iodine concentration and temperature. The results are interpreted in terms of electronic hopping between ionic centers dispersed at random in the polymer.

Conductivity and electron density of undoped model compounds of poly(phenylene vinylene)

Fourteen derivatives of trans,trans-1,4-bis[2-phenylethenyl]benzene were synthesized by Wittig reactions as model compounds of poly(paraphenylene vinylene). Structure, configurational homogeneity and absence of ionic impurities were controlled by mass spectrometry, infrared and neutron activation analysis, respectively. Crystallographic unit cell parameters were obtained from X-ray powder patterns and measurements of electrical conductivities were performed on undoped samples. The data of four more compounds containing one or more thiophene rings instead of phenyl rings were added from the 3iterature. If NO2 and Cl groups are excluded from the electron count a good linear correlation is found between the logarithm of the conductivity and the non-σ electron density (e A -3). The position of the substituents, on the central or on the terminal ring, also plays a role in as much as it affects the molecular volume of the compound but not the non-σ electron density. The correlation between the logarithm of the conductivity and the absorption coefficient of the longest wavelength of UV absorption identifies the π electrons in the chromophore as the principal charge carriers.

The incommensurate structure of (Sr, Ca)14Cu24O41: a study by electron diffraction and high-resolution microscopy

The modulated structure in (Sr, Ca)14Cu24O41 has been studied using electron diffraction and high-resolution microscopy. The structure can be considered as consisting of two interpenetrating substructures. The first sheet-like substructure is shown to be hardly modulated while the second substructure, consisting of c-oriented chains, contains most of the modulation. High-resolution electron microscopy allows either separate imaging of the two substructures or identification of the misfit between them.

Superconductivity and microstructure in Fe-doped Bi-Sr-Ca-Cu-O compounds

Abstract We have studied the influence of Fe impurities in the superconducting compound Bi-Sr-Ca-Cu-O prepared from two basic nominal compositions Bi 2 Sr 2 Ca(Cu 1− x Fe x ) 2 O 8+δ and Bi 4 Sr 3 Ca 3 (Cu 1− x Fe x ) 4 O 16+δ . The superconducting and secondary phases have been analyzed by SEM, EPMA, HREM and X-ray diffraction. The composition of the doped superconducting phase is best described by the formula: Bi 2+ y (Sr, Ca) 3 (Cu 1− x Fe x ) 2− y O 8+δ , where y increases as a function of x ( x Annealing of the samples in a poor oxygen atmosphere improves the T c values of the doped samples. The downward shift of T c for increasing Fe content follows a linear relationship with a slope of 5 K/(Fe/Fe+Cu) at.%. The importance of the Bi-O-Cu-O electron coupling in the superconductivity mechanism is discussed.

Thermally activated flux motion in YBa2(Cu1−xZnx)3O7−δ epitaxial thin films: influence of magnetic field and Zn doping

Abstract High quality c -axis oriented epitaxial YBa 2 (Cu 1− x ) 3 O 7− δ ( x = 0, 0.02, 0.04) thin films were deposited by DC magnetron sputtering on MgO (100) substrates. The broadening of the resistive transition has been investigated in a magnetic field range from 0.03 to 20 T for H ⊥ c -axis in and H ∥ c -axis in order to study the influence of different microstructures on the pinning properties. The deduced field dependence of the activation energy follows a power law: U 0 ( H ) ∼ H − α with a pronounced anisotropic behaviour. We observe crossovers of the α-values from α 1 = 0.13 to α 2 = 0.5 (planar pinning) for H ⊥ c -axis and from α 1 = 0.3 to α 2 = 1 (point defect pinning) for H ∥ c -axis. For both orientations the α-values obtained in the high field region can be explained in the framework of collective pinning. The results are discussed taking account of the influence of the Zn substitution on the flux pinning in epitaxial YBa 2 Cu 3 O 7−δ thin films.

STM study of YBa2Cu3O7−δ and Bi2Sr2CaCu2O8+° cleaved surfaces

Abstract Room-temperature scanning tunneling microscopy has been used to study the cleavage process in YBa 2 Cu 3 O 7-° and Bi 2 Sr 2 CaCu 2 O 8+δ single crystals. Many stepped structures were observed on their cleaved surfaces, and close examination showed that some steps were smaller than the unit cell in the case of YBa 2 Cu 3 O 7-δ and then one half of the unit cell for Bi 2 Sr 2 CaCu 2 O 8+δ . We believe that the steps are related to collisions of cleavage fronts with intergrowth defects or stacking faults which are typical in layered structures.

Thermal-conductivity of pure or iron-doped yba2cu3o7-delta with or without an excess of cuo

The thermal conductivities of YBa2(Cu1-xFex)3O7- delta +y% CuO ceramics for 0<x<0.03 and 0<y<5 have been measured. The thermal conductivities of all samples exhibit a minimum in the vicinity of the critical temperature Tc and a maximum near Tc/2. These results are interpreted with the help of an electronic model, i.e. supposing that the main contribution to the thermal conductivity below Tc is due to electron scattering in the CuO2 planes. Within a simple two-fluid model derived from kinetic theory, taking into account the temperature dependence of the electronic relaxation time and the normal charge carrier concentration, we obtain theoretical curves which reproduce the experimental results quite well. We also take into account the porosity of the samples (in term of an intergrain contribution) to derive the electronic thermal conductivity. The parameter values of the model and the observed minimum are explained in terms of physical properties. In particular, the contribution of superconductivity fluctuations to the thermal conductivity is shown to be negligible with respect to other mechanisms.

Thermal conductivity of twinned YBa2Cu3O7-x and tweeded YBa2(Cu0.95Fe0.05)3O7-x in a magnetic field: Evidence for intrinsic proximity effect

Abstract The behavior of the thermal conductivity κ of twinned YBa2Cu3O7-x and tweeded YBa2(Cu0.95Fe0.05)3O7-x polycrystals in a magnetic field up to 5 T is presented. Data as a function of field are shown to depart from the usual linear behavior of thermal magnetoresistivity (TMR) κ−1(B) for conventional superconductors. A set of stretched exponential laws, of the form κ−1(B) - κ−1 (O) = CBexp (-pNq) been found to describe our data with q decreasing with temperature from 1 to 1 2 and from 1 2 to 1 4 for pure and iron-doped samples, respectively, in contrast to the unique interpolation formula (q = 1 4 ) for TMR proposed recently by Richardson et al. [1] to fit their data on twinned and untwinned single crystals. Such a complex behavior of TMR in the high fiels region is supposed to originate from extended defect mediated intrinsic proximity effect in both samples. The proposed interpretation is based on incorporating two types of phonon scatterers, free (unpinned) and pinned (by the defect cores) vortices, into a conventional picture of phonon-vortex scattering mechanism.