Anomalous temperature behavior of the resistivity in lightly doped manganites around a metal-insulator phase transition

in JETP Letters (1999), 70(7), 481-487

An unusual temperature and concentration behavior of the resistivity ρ(T,x) in La0.7Ca0.3Mn1-xCuxO3 is observed at slight Cu doping (0 ≤x≤ 0.05). Namely, the introduction of copper results in a splitting ... [more ▼]

An unusual temperature and concentration behavior of the resistivity ρ(T,x) in La0.7Ca0.3Mn1-xCuxO3 is observed at slight Cu doping (0 ≤x≤ 0.05). Namely, the introduction of copper results in a splitting of the resistivity maximum around the metal-insulator transition temperature T0(x) into two differently evolving peaks. Unlike the original Cufree maximum, which steadily increases with doping, the second (satellite) peak remains virtually unchanged for x<xc, increases for x≥xc, and finally disappears at xm ≃ 2xc, with xc ≃ 0.03. The observed phenomenon is thought to arise from a competition between substitution-induced strengthening of the potential barriers (which hinder the charge hopping between neighboring Mn sites) and weakening of the kinetic energy of the carriers. The data are well fitted by assuming a nonthermal tunneling conductivity theory with randomly distributed hopping sites. © 1999 American Institute of Physics. [less ▲]

A new fast and non-destructive method for evaluating the superconducting properties of bulk materials

in Obradors, X.; Sandiumenge, F.; Fontcuberta, J. (Eds.) Applied Superconductivity 1999. Volume 1 : Large Scale Applications (1999)

This paper aims at reporting a new, simple and low-cost experimental technique designed for investigating the electrical properties of disk-shaped superconducting samples. The principle is similar to that ... [more ▼]

This paper aims at reporting a new, simple and low-cost experimental technique designed for investigating the electrical properties of disk-shaped superconducting samples. The principle is similar to that of an induction motor. Two neighbouring and 90° out-of-phase AC magnetic fields are applied perpendicularly to the disk to be measured. As a result, the material experiences an electromagnetic torque which depends on its electrical properties. In this paper the experimental system is briefly described and illustrative measurements on different HTS are reported and discussed. [less ▲]

A sharp decrease of resistivity in La0.7Ca0.3Mn0.96Cu0.04O3: Evidence for Cu-assisted coherent tunneling of spin polarons

in JETP Letters (1999), 69(11), 858-862

Nearly a 50% decrease of the resistivity rho(T,x) is observed upon just 4% Cu doping at the Mn site of La2/3Ca1/3Mn1-xCuxO3. When the observed phenomenon is attributed to a decrease of the spin-polaron ... [more ▼]

Nearly a 50% decrease of the resistivity rho(T,x) is observed upon just 4% Cu doping at the Mn site of La2/3Ca1/3Mn1-xCuxO3. When the observed phenomenon is attributed to a decrease of the spin-polaron energy E-sigma(x) below T-C(x), all of the data are found to be well fitted by the nonthermal coherent tunneling expression rho(T,x)=rho(0)e(-gamma M2(T,x)), assuming that the magnetization in the ferromagnetic state is given by the expression M(T,x)=M-R(x)+M-0(x)tanh{T-[root(C)(x)/T](2)-1}. The best fits through all the data points suggest M-0(x)similar or equal to root 1-xM(0)(0), M-R(x)similar or equal to x root M-0(0), and E-sigma(x)similar or equal to E-sigma(0)(1-x)(4) for the explicit x dependence of the Cu-induced modifications of the Mn-spin-dominated zero-temperature spontaneous magnetization, residual paramagnetic contribution, and spin-polaron tunneling energy, respectively, with E-sigma(0)=0.12 eV. (C) 1999 American Institute of Physics. [S0021-3640(99)00811-7]. [less ▲]

Foam-like evolution in polycrystalline systems following successive 'melt and growth' cycles

in Philosophical Magazine B-Physics of condensed matter statistical mechanics electronic optical and magnetic properties (1998), 78(4), 397-408

A stochastic model of multigrain growth which allows for successive melting-growth cycles is investigated on a square lattice. Two fundamental constraints are introduced: (i) the melted mass amplitude and ... [more ▼]

A stochastic model of multigrain growth which allows for successive melting-growth cycles is investigated on a square lattice. Two fundamental constraints are introduced: (i) the melted mass amplitude and (ii) the number of cycles guide the process. The evolution of the microstructure is found to be quite similar to foam systems, i.e. topological rearrangements are observed together with the increase of the-mean grain area. A drastic crossover between two types of growth regimes is found as a function of the amplitude of the melting-growth cycles. This allows one to envisage the existence of optimal conditions for polycrystal processing. [less ▲]

Effect of BaZrO3 additions on the microstructure and physical properties of melt-textured Y-123 superconducting materials

in Materials Science & Engineering : B (1998), 53(1-2), 154-158

The effect of BaZrO3 additions on the microstructure and physical properties of melt-textured Y-123 have been investigated. The distribution of the 211 particles is strongly modified by the presence of ... [more ▼]

The effect of BaZrO3 additions on the microstructure and physical properties of melt-textured Y-123 have been investigated. The distribution of the 211 particles is strongly modified by the presence of such an additive. In fact, BaZrO3 is effective in reducing the size of the 211 particles. As a consequence, a strong interaction in terms of repulsion takes place between the 123 growth front and the 211 particles which are partially dissolved in the reactive liquid phase. New types of patterns are also reported in the microstructure of such materials and are related to the presence of BaZrO3 in the melt. [less ▲]

The Nd-123 superconducting system: From single crystal to top-seeded large grain. Physical and chemical parameters influence

in Applied Superconductivity (1998), 6(2-5), 77-85

Nd-123 signle crystals have been produced via the Aux method in alumina crucibles by controlling the chemical composition of the flux. For so doing, quenched materials have been prepared and analyzed in ... [more ▼]

Nd-123 signle crystals have been produced via the Aux method in alumina crucibles by controlling the chemical composition of the flux. For so doing, quenched materials have been prepared and analyzed in order to get information about the optimum barium: copper ratio for an unchanged neodymium concentration. Non corrosive fluxes with a low viscosity have been put into evidence and can thus be used for single crystal production. Experiments have been performed with three different chemical compositions of the Aux and by considering various amounts of neodymium oxide for each system. The single crystals have been analyzed by energy dispersive X-ray microanalysis and used for the synthesis of large Dy-123 single-domains? [less ▲]

AC susceptibility data on Dy2O3 seeded randomly oriented Dy-123 mono domains melt-textured superconductor

in Materials Science & Engineering : B (1998), 53(1-2), 198-202

Dy-123 superconducting ceramics have been grows using small Dy2O3 single-crystals as seeds. Randomly oriented large mono-domains have been observed in the vicinity of the seeding single crystal. These ... [more ▼]

Dy-123 superconducting ceramics have been grows using small Dy2O3 single-crystals as seeds. Randomly oriented large mono-domains have been observed in the vicinity of the seeding single crystal. These domains are one order of magnitude greater in volume than the seed. The AC susceptibility versus temperature curves have been measured in a polycrystalline cubic sample for different orientations of the magnetic field with respect to the sample and for amplitudes up to 20 G. Three measuring coils, each parallel to one side of the sample, were used to investigate the anisotropy of the properties. Electrical resistance versus temperature and flux profiles at 77 K were also explored. The results show good grain connectivity. A non negligible part of the magnetic shielding is due to currents flowing inside the grains. (C) 1998 Elsevier Science S.A. All rights reserved. [less ▲]

Distribution of vortex lattice melting temperatures in mixed state diagram of Bi2212 tapes

in Physica C (1998), 303(3-4), 169-176

The electrical resistivity in the mixed state of Bi2212 tape superconductors can be accurately reconstructed for values below approximately a half of the normal state value of resistivity, assuming a ... [more ▼]

The electrical resistivity in the mixed state of Bi2212 tape superconductors can be accurately reconstructed for values below approximately a half of the normal state value of resistivity, assuming a Gaussian distribution for the vortex lattice melting temperatures. The magnetic field dependences of the distribution width and of the dB(c2)/dT parameter were calculated. Moreover the vortex liquid viscosity coefficient was determined between the vortex lattice melting temperature and T-c at various magnetic fields. It is pointed out that the vortex lattice melting transition should not be confused with the irreversibility line of magnetic studies nor with the percolation temperature line for electrical resistivity. (C) 1998 Elsevier Science B,V. All rights reserved. [less ▲]

The effect of BaZrO3 addition on transport properties of Dy-based 123-211 composite materials: electrical resistivity, thermal conductivity and thermoelectric power

in Superconductor Science & Technology (1998), 11(8), 803-809

Composite samples made of 123-211 DyBaCuO-based systems with 1, 4, 10 and 15% in weight BaZrO3 additions have been synthesized. Their electrical resistivity, thermal conductivity and thermoelectric power ... [more ▼]

Composite samples made of 123-211 DyBaCuO-based systems with 1, 4, 10 and 15% in weight BaZrO3 additions have been synthesized. Their electrical resistivity, thermal conductivity and thermoelectric power have been measured. The electrical resistivity has also been measured in a magnetic field less than 1 T. The data show that BaZrO3 addition has a non-trivial effect: beside mildly reducing the critical temperature of the superconductor, it increases the scattering at small concentration, and effectively reduces weak link features in the electrical property, but increases the thermal conductivity. The field influence on T-c is shown to be T-c(O)- T-c(B) approximate to B-1/2. [less ▲]

Formation of nick instabilities due to particle clustering along crystal interfaces

in Physical Review. E : Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics (1997), 56(4), 4042-4047

The kinetic growth of crystal grains is simulated on a square lattice in the presence of mobile particles. The model is based on the Eden and the dynamic epidemic models. Trapping of the particles by the ... [more ▼]

The kinetic growth of crystal grains is simulated on a square lattice in the presence of mobile particles. The model is based on the Eden and the dynamic epidemic models. Trapping of the particles by the advancing front and a short-range repulsion of the particles by the front are both allowed. The grain boundary morphology is studied through the kink density concept and the spatial distribution of particles is discussed. Indeed, the clustering of particles along the crystal edges is found to induce the formation of so-called nick instabilities at the liquid-solid interface. The particles, when trapped near such instabilities, occur as oblique filamentary structures in the crystal matrix, just like river networks. These findings agree with recent experimental observations. The numerical laws are also explained by theoretical arguments based on different characteristic lengths. [less ▲]