Electrical transport and percolation in magnetoresistive manganite / insulating oxide composites: case of La0.7Ca0.3MnO3 / Mn3O4

We report the results of electrical resistivity measurements carried out on well-sintered La0.7Ca0.3MnO3 / Mn3O4 composite samples with almost constant composition of the magnetoresistive manganite phase (La0.7Ca0.3MnO3). A percolation threshold (fc) occurs when the La0.7Ca0.3MnO3 volume fraction is ~ 0.19. The dependence of the electrical resistivity as a function of La0.7Ca0.3MnO3 volume fraction (fLCMO) can be described by percolation-like phenomenological equations. Fitting the conducting regime (fLCMO > fc) by the percolation power law returns a critical exponent t value of 2.0 +/- 0.2 at room temperature and 2.6 +/-0.2 at 5 K. The increase of t is ascribed to the influence of the grain boundaries on the electrical conduction process at low temperature.Comment: 7 pages, 3 figures, accepted for publication in Phys. Rev.

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

An unusual temperature and concentration behavior of resistivity in $La_{0.7}Ca_{0.3}Mn_{1-x}Cu_xO_3$ has been observed at slight $Cu$ doping ($0\leq x \leq 0.05$). Namely, introduction of copper results in a splitting of the resistivity maximum around a metal-insulator transition temperature $T_0(x)$ into two differently evolving peaks. Unlike the original $Cu$-free maximum which steadily increases with doping, the second (satellite) peak remains virtually unchanged for $x<x_c$, increases for $x\ge x_c$ and finally disappears at $x_m\simeq 2x_c$ with $x_c\simeq 0.03$. The observed phenomenon is thought to arise from competition between substitution induced strengthening of potential barriers (which hamper the charge hopping between neighboring $Mn$ sites) and weakening of carrier's kinetic energy. The data are well fitted assuming a nonthermal tunneling conductivity theory with randomly distributed hopping sites.Comment: 10 REVTEX pages, 2 PostScript figures (epsf.sty); to be published in JETP Letter

Quasiparticle contribution to heat carriers relaxation time in DyBa2_2Cu3_3O7−x_{7-x} from heat diffusivity measurements

It is shown that the controversy on phonons or electrons being the most influenced heat carriers below the critical temperature of high-T$_c$ superconductors can be resolved. Electrical and thermal properties of the same DyBa$_2$Cu$_3$O$_{7-x}$ monodomain have been measured for two highly different oxygenation levels. While the oxygenated sample DyBa$_2$Cu$_3$O$_{7}$ has very good superconducting properties ($T_c=90$ K), the DyBa$_2$Cu$_3$O$_{6.3}$ sample exhibits an insulator behavior. A careful comparison between measurements of the {\bf thermal diffusivity} of both samples allows us to extract the electronic contribution. This contribution to the relaxation time of heat carriers is shown to be large below $T_c$ and more sensitive to the superconducting state than the phonon contribution.Comment: 13 pages, 6 figure

Unusual thermoelectric behavior of packed crystalline granular metals

Loosely packed granular materials are intensively studied nowadays. Electrical and thermal transport properties should reflect the granular structure as well as intrinsic properties. We have compacted crystalline $CaAl$ based metallic grains and studied the electrical resistivity and the thermoelectric power as a function of temperature ($T$) from 15 to 300K. Both properties show three regimes as a function of temperature. It should be pointed out : (i) The electrical resistivity continuously decreases between 15 and 235 K (ii) with various dependences, e.g. $\simeq$ $T^{-3/4}$ at low $T$, while (iii) the thermoelectric power (TEP) is positive, (iv) shows a bump near 60K, and (v) presents a rather unusual square root of temperature dependence at low temperature. It is argued that these three regimes indicate a competition between geometric and thermal processes, - for which a theory seems to be missing in the case of TEP. The microchemical analysis results are also reported indicating a complex microstructure inherent to the phase diagram peritectic intricacies of this binary alloy.Comment: to be published in J. Appl. Phys.22 pages, 8 figure

Strongly Enhanced Current Densities in Superconducting Coated Conductors of YBa2Cu3O7-x + BaZrO3

There are numerous potential applications for superconducting tapes, based on YBa2Cu3O7-x (YBCO) films coated onto metallic substrates. A long established goal of more than 15 years has been to understand the magnetic flux pinning mechanisms which allow films to maintain high current densities out to high magnetic fields. In fact, films carry 1-2 orders of magnitude higher current densities than any other form of the material. For this reason, the idea of further improving pinning has received little attention. Now that commercialisation of conductors is much closer, for both better performance and lower fabrication costs, an important goal is to achieve enhanced pinning in a practical way. In this work, we demonstrate a simple and industrially scaleable route which yields a 1.5 to 5-fold improvement in the in-field current densities of already-high-quality conductors

Exponential and power law distribution of mass clusters in a (magnetic-like) deposition model of elongated grains in 2D piles

A generalized so called magnetically controlled ballistic rain-like deposition (MBD) model of granular piles has been numerically investigated in 2D. The grains are taken to be elongated disks whence characterized by a two-state scalar degree of freedom, called ''nip'', their interaction being described through a Hamiltonian. Results are discussed in order to search for the effect of nip flip (or grain rotation from vertical to horizontal and conversely) probability in building a granular pile. The characteristics of creation of + (or $-$) nip's clusters and clusters of holes (missing nips) are analyzed. Two different cluster-mass regimes have been identified, through the cluster-mass distribution function which can be exponential or have a power law form depending on whether the nip flip (or grain rotation) probability is large or small. Analytical forms of the exponent are empirically found in terms of the Hamiltonian parameters.Comment: submitted to Int.J. Mod. Phys. C; 16 figures; 79 reference

Glutathione S-transferase M1-null genotype as risk factor for SOS in oxaliplatin-treated patients with metastatic colorectal cancer

Background: Oxaliplatin is used as a neo-adjuvant therapy in hepatic colorectal carcinoma metastasis. This treatment has significant side effects, as oxaliplatin is toxic to the sinusoidal endothelial cells and can induce sinusoidal obstruction syndrome (SOS), which is related to decreased overall survival. Glutathione has an important role in the defence system, catalysed by glutathione S-transferase (GST), including two non-enzyme producing polymorphisms (GSTM1-null and GSTT1-null). We hypothesise that patients with a non-enzyme producing polymorphism have a higher risk of developing toxic injury owing to oxaliplatin. Methods: In the nontumour-bearing liver, the presence of SOS was studied histopathologically. The genotype was determined by a semi-nested PCR. Results: Thirty-two of the 55 (58%) patients showed SOS lesions, consisting of 27% mild, 22% moderate and 9% severe lesions. The GSTM1-null genotype was present in 25 of the 55 (46%). Multivariate analysis showed that the GSTM1-null genotype significantly correlated with the presence of (moderate-severe) SOS (P=0.026). Conclusion: The GSTM1-null genotype is an independent risk factor for SOS. This finding allows us, in association with other risk factors, to conceive a potential risk profile predicting whether the patient is at risk of developing SOS, before starting oxaliplatin, and subsequently might result in adjustment of treatment