Magnetotransport Properties of Antiferromagnetic YBa_2Cu_3O_6.25 Single Crystals

In-plane and out-of-plane magnetoresistivities (MR) of antiferromagnetic YBa_2Cu_3O_6.25 single crystals were measured in magnetic fields H applied along the (ab) plane. In-plane MR is a superposition of two components: The first component is strongly in-plane anisotropic, changing sign from negative when H is parallel to the electrical current I to positive when H is perpendicular to I. The second component is positive, quadratic in H, and isotropic in the (ab)-plane. The out-of-plane MR displays a fourfold symmetry upon in-plane rotation of the magnetic field, with maxima along the easy axes of antiferromagnetic spin ordering and minima along unfavorable directions of spin orientation (45 degrees from the Cu-O-Cu bonds).Comment: 8 pages, 4 figure

Flux pinning in two-dimensional high temperature superconductors

Various types of high temperature superconducting single crystals (TI-2223, Bi-2212, and Y-123 with oxygen contents between 6.4 and 6.64) were investigated by SQUID magnetometry. Both materials show very {open_quotes}low{close_quotes} lying irreversibility lines (Hllc), which follow an exponential law-and rise rapidly below T/T{sub c}=0.5. High critical current P densities are found only at low temperatures, e.g. J{sub c} is about 2.7{circ}10{sup 10}Am{sup -2} at 1 T and 5 K in TI-2223. A comparison of the condensation energy provided by different types of defects with the thermal energy gives evidence for pinning of individual pancakes by the as-grown defect structure

Microstructural and mechanical characterization of alumina scales thermally developed on iron aluminide alloys

Several alumina-forming Fe-Al intermetallic alloys have been oxidized in oxygen for 100 h at 1000 C to understand the scaling kinetics, scale morphology, scale adhesion, and strain accommodation in the scales. Oxidation studies were conducted by thermogravimetry, followed by analyses of the surfaces of oxide scales. In addition, samples were cooled to 77 K and then fractured; then, their scale/metal interfaces were analyzed. Some of the scales were adhesion-tested by applying a tensile load to pull the scale away from the substrate. Finally, ruby fluorescence was used to measure in-plane strains in the oxide scales and values correlated with scale microstructures

X-Ray-Diffraction Study of Charge-Density-Waves and Oxygen-Ordering in YBa2Cu3O6+x Superconductor

We report a temperature-dependent increase below 300 K of diffuse superlattice peaks corresponding to q_0 =(~2/5,0,0) in an under-doped YBa_2Cu_3O_6+x superconductor (x~0.63). These peaks reveal strong c-axis correlations involving the CuO_2 bilayers, show a non-uniform increase below \~220 K with a plateau for ~100-160 K, and appear to saturate in the superconducting phase. We interpret this unconventional T-dependence of the ``oxygen-ordering'' peaks as a manifestation of a charge density wave in the CuO_2 planes coupled to the oxygen-vacancy ordering.Comment: 4 pages, 4 figure

Antiferromagnetic Excitations and Van Hove Singularities in YBa2_2Cu3_3O6+x_{6+x}

We show that in quasi-two-dimensional $d$-wave superconductors Van Hove singularities close to the Fermi surface lead to novel magnetic quasi-particle excitations. We calculate the temperature and doping dependence of dynamical magnetic susceptibility for YBCO and show that the proposed excitations are in agreement with inelastic neutron scattering experiments. In addition, the values of the gap parameter and in-plane antiferromagnetic coupling are much smaller than usually believed.Comment: REVTeX, 4 pages + 3 PostScript (compressed) figures; to appear in Phys. Rev. B (Rap. Comm.

Relaxation Effects in the Transition Temperature of Superconducting HgBa2CuO4+delta

In previous studies on a number of under- and overdoped high temperature superconductors, including YBa_{2}Cu_{3}O_{7-y} and Tl_{2}Ba_{2}CuO_{6+\delta}, the transition temperature T_c has been found to change with time in a manner which depends on the sample's detailed temperature and pressure history. This relaxation behavior in T_c is believed to originate from rearrangements within the oxygen sublattice. In the present high-pressure studies on HgBa_{2}CuO_{4+\delta} to 0.8 GPa we find clear evidence for weak relaxation effects in strongly under- and overdoped samples ($T_c\simeq 40 - 50 K$) with an activation energy $E_{A}(1 bar) \simeq 0.8 - 0.9 eV$. For overdoped HgBa_{2}CuO_{4+\delta} E_{A} increases under pressure more rapidly than previously observed for YBa_{2}Cu_{3}O_{6.41}, yielding an activation volume of +11 \pm 5 cm^{3}; the dependence of T_c on pressure is markedly nonlinear, an anomalous result for high-T_c superconductors in the present pressure range, giving evidence for a change in the electronic and/or structural properties near 0.4 GPa

Vortex Dynamics Differences Due To Twin-boundary Pinning Anisotropy In Yba 2cu 3o X At Low Temperatures For H∥ab Planes

We measured the magnetization M of a twin-aligned single crystal of YBa 2Cu 3O x (YBaCuO), with T c =91 K, as a function of temperature T and magnetic field H, with H applied along the ab planes. Isothermal M-vs-H and M-vs-time curves were obtained with H applied parallel (∥) and perpendicular (⊥) to the twin boundary (TB) direction. M-vs-H curves exhibited two minima below 38 K, which resembled similar curves that have been obtained in YBaCuO for H∥c axis. Above 12 K, the field positions of the minima for H∥TB and H⊥TB were quite similar. Below 12 K, the position of the second minimum H min occurred at a higher field value with H∥TB. Below 6 K, only one minimum appeared for both field directions. At low temperatures, these minima in the M-vs-H curves produced maxima in the critical current. It was determined that vortex lines were expelled more easily for H∥TB than for H⊥TB and, therefore, below a certain field value, that J c(H⊥TB) was larger than J c(H∥TB). At T<12 K with H∥TB, the relaxation rate for flux lines leaving the crystal was found to be different from that for flux entering the crystal. We also observed flux jumps at low temperatures, with their sizes depending on the orientation of magnetic field with respect to the TB's. © 2005 The American Physical Society.712Sarikaya, M., Stern, E.A., (1988) Phys. Rev. B, 37, p. 9373Van Bakel, G.P.E.M., Hof, P.A., Van Engelen, J.P.M., Bronsveld, P.M., De Hosson, J.Th.M., (1990) Phys. Rev. B, 41, p. 9502Liu, J.Z., Jia, Y.X., Shelton, R.N., Fluss, M.J., (1991) Phys. Rev. Lett., 66, p. 1354Swartzendruber, L.J., Roitburd, A., Kaiser, D.L., Gayle, F.W., Bennett, L.H., (1990) Phys. Rev. Lett., 64, p. 483Kwok, W.K., Welp, U., Crabtree, G.W., Vandervoort, K.G., Hulscher, R., Liu, J.Z., (1990) Phys. Rev. Lett., 64, p. 966Duran, C.A., Gammel, P.L., Wolfe, R., Fratello, V.J., Bishop, D.J., Rice, J.P., Ginsberg, D.M., (1992) Nature (London), 357, p. 474Gyorgy, E.M., Van Dover, R.B., Schneemeyer, L.F., White, A.E., O'Bryan, H.M., Felder, R.J., Waszczak, J.V., Rhodes, W.W., (1990) Appl. Phys. Lett., 56, p. 2465Oussena, M., De Groot, P.A.J., Porter, S.J., Gagnon, R., Taillefer, L., (1995) Phys. Rev. B, 51, p. 1389Oussena, M., De Groot, P.A.J., Deligiannis, K., Volkozub, A.V., Gagnon, R., Taillefer, L., (1996) Phys. Rev. Lett., 76, p. 2559Vlasko-Vlasov, V.K., Dorosinskii, L.A., Polyanskii, A.A., Nikitenko, V.I., Welp, U., Veal, B.W., Crabtree, G.W., (1994) Phys. Rev. Lett., 72, p. 3246Wijngaarden, R.J., Griessen, R., Fendrich, J., Kwok, W.K., (1997) Phys. Rev. B, 55, p. 3268Duran, C.A., Gammel, P.L., Bishop, D.J., Rice, J.P., Ginsberg, D.M., (1995) Phys. Rev. Lett., 74, p. 3712Pastoriza, H., Candia, S., Nieva, G., (1999) Phys. Rev. Lett., 83, p. 1026Herbsommer, J.A., Nieva, G., Luzuriaga, J., (2000) Phys. Rev. B, 62, p. 3534Jorge, G.A., Rodriguez, E., (2000) Phys. Rev. B, 61, p. 103Bondareko, A.V., (2001) Low Temp. Phys., 27, p. 339(2001) Phys. Rev. B, 27, p. 201Esquinazi, P., Setzer, A., Fuchs, D., Kopelevich, Y., Zeldov, E., Assmann, C., (1999) Phys. Rev. B, 60, p. 12454Mints, R.G., Brandt, E.H., (1996) Phys. Rev. B, 54, p. 12421Muller, K.-H., Andrikidis, C., (1994) Phys. Rev. B, 49, p. 1294Guillot, M., Potel, M., Gougeon, P., Noel, H., Levet, J.C., Chouteau, G., Tholence, J.L., (1988) Phys. Lett. A, 127, p. 363Salem-Sugui Jr., S., Alvarenga, A.D., Friesen, M., Veal, B., Paulikas, P., (2001) Phys. Rev. B, 63, p. 216502Bean, C.P., (1962) Phys. Rev. Lett., 8, p. 250Tinkham, M., (1996) Introduction to Superconductivity, 2nd Ed., , McGraw-Hill, New YorkDe Andrade, M.C., Dilley, N.R., Ruess, F., Maple, M.B., (1998) Phys. Rev. B, 57, pp. R708Abulafia, Y., Shaulov, A., Wolfus, Y., Prozorov, R., Burlachkov, L., Yeshurun, Y., Majer, D., Vinokur, V.M., (1995) Phys. Rev. Lett., 75, p. 2404Maley, M.P., Willis, J.O., Lessure, H., McHenry, M.E., (1990) Phys. Rev. B, 42, p. 2639Shi, D., Salem-Sugui Jr., S., (1991) Phys. Rev. B, 44, p. 7647Beasley, M.R., Labash, R., Weeb, W.W., (1969) Phys. Rev., 181, p. 682Burlachkov, L., (1993) Phys. Rev. B, 47, p. 8056Alvarenga, A.D., Salem-Sugui Jr., S., (1994) Physica C, 235, p. 2811Junod, A., (1989) Physica C, 162-164, p. 482Triscone, G., (1990) Physica C, 168, p. 40Genoud, J.Y., (1991) Physica C, 177, p. 31

One Dimensional Oxygen Ordering in YBa2Cu3O(7-delta)

A model consisting of oxygen-occupied and -vacant chains is considered, with repulsive first and second nearest-neighbor interactions V1 and V2, respectively. The statistical mechanics and the diffraction spectrum of the model is solved exactly and analytically with the only assumption V1 >> V2. At temperatures T ~ V1 only a broad maximum at (1/2,0,0) is present, while for ABS(delta - 1/2) > 1/14 at low enough T, the peak splits into two. The simple expression for the diffraction intensity obtained for T << V1 represents in a more compact form previous results of Khachaturyan and Morris[1],extends them to all delta and T/V2 and leads to a good agreement with experiment. [1] A.G.Khachaturyan and J.W.Morris, Jr., Phys.Rev.Lett. 64,76(1990)Comment: 13 pages,Revtex,3 figures available upon request but can be plotted using simple analytical functions,CNEA-CAB 92/04