Field-Induced Staggered Magnetic Order in La(2)NiO(4.133)

At low temperature the holes doped into the NiO(2) planes of La(2)NiO(4.133) by the excess oxygen collect in diagonal stripes that separate narrow antiferromagnetic domains. The magnetic order drops abruptly to zero at T_m = 110.5 K, but charge order remains with a period of 3a/2. We show that application of a magnetic field in the regime T > T_m induces staggered magnetic order of period 3a due to the net magnetic moment of the high-temperature bond-centered stripes, together with the odd number of Ni spins across an antiferromagnetic domain.Comment: 4 pages, Revtex, 4 epsf figs included with psfig

Dopant Structural Distortions in High-temperature Superconductors: An Active or a Passive Role?

The parent compounds of high-temperature superconductors, such as YBa2Cu3O6 and La2CuO4, are strongly interacting electron systems, rendering them insulators with Mott-Hubbard gaps of a few electronvolts. Charge carriers (holes) are introduced by chemical doping, causing an insulator-metal (IM) transition and, at low temperatures, superconductivity. the role of dopants is widely seen as limited to the introduction of holes into the CuO2 planes (i.e. occupying electronic states derived from Cu and O 2px,y atomic orbitals). Most theories of high-Tc superconductivity deal with pairing interactions between these planar holes. Local distortions around dopants are poorly understood, because of the experimental difficulty in obtaining such information, particularly at low doping. This has resulted in the neglect, in most theories, of the effect of such distortions on the chemical and electronic structure of high-Tc superconductors. Angular-resolved X-ray absorption fine structure (XAFS) spectroscopy on oriented samples is an ideal technique to elucidate the dopant distortions. Element specificity, together with a large orientation dependence of the XAFS signal in these layered structures, allows the local structure around dopants to be resolved. Results are presented here on (Sr, Ba) and Ni dopants, which substitute at the La and Cu sites, respectively, of insulating La2CuO4. the relevance of the measured local distortions for a complete understanding of the normal and superconducting properties of cuprates is discussed

Enhancement of the upper critical field of Nb3Sn utilizing disorder introduced by ball milling the elements

Nb3Sn was prepared by milling Nb and Sn powder mixtures followed by limited reactions to restrict disorder recovery. Although disorder reduced the superconducting critical temperature Tc, the concomitant electron scattering increased the upper critical field mu0Hc2 to as high as 35 T at 0 K, as determined by the Werthamer-Helfand-Hohenberg equation. Hc2 was higher for longer milling times and lower annealing temperatures. Substitution of 2% Ti for Nb did not appreciably enhance Hc2, suggesting that alloying mitigates the benefits of disorder. Since alloyed Nb3Sn wires have mu0Hc2(0) approximately 29 T, wires based on heavily milled powders could extend the field range for applications if they can be made with high current density

Xafs Study of the Low-Temperature Tetragonal Phase of La₂₋ₓBaₓCuO₄: Disorder, Stripes, and T\u3csub\u3ec\u3c/sub\u3e Suppression at X=0.125

Angular dependent x-ray absorption fine structure (XAFS) measurements of all La, Ba, and Cu sites determined the origin and nature of the intrinsic disorder present in the low temperature tetragonal (LTT) structural ground state of La2-xBaxCuO4 with x=0.125,0.15 at T=10 K. Ba doping induces major local distortions that extend as far as 5 Å from its lattice site. Despite the low Ba content the large distortions significantly affect the XAFS of the majority La and Cu sites. The distribution of the local LTT tilt angle of CuO6 octahedra was directly determined from single and multiple scattering contributions to the XAFS of La and Cu sites. This angle, which regulates the strength of electron-lattice and spin-spin coupling in this material, was found to exhibit large, random fluctuations with rms [〈(θ-〈θ〉)2〉] 1/2≈2〈θ〉, 〈θ〉 being the average tilt angle determined by diffraction techniques. These fluctuations originate in the large, random, static disorder introduced with Ba doping. Their presence has implications for the plausibility of a mobility gap causing Tc suppression at x=0.125 in only the LTT phase, and for the correlation length of the postulated charge and spin stripes

Scanning tunneling spectroscopy characterization of the pseudogap and the x = 1/8 anomaly in La2-xSrxCuO4 thin films

Using scanning tunneling spectroscopy we examined the local density of states of thin c-axis La2-xSrxCuO4 films, over wide doping and temperature ranges. We found that the pseudogap exists only at doping levels lower than optimal. For x = 0.12, close to the 'anomalous' x = 1/8 doping level, a zero bias conductance peak was the dominant spectral feature, instead of the excepted V- shaped (c-axis tunneling) gap structure. We have established that this surprising effect cannot be explained by tunneling into (110) facets. Possible origins for this unique behavior are discussed.Comment: 15 pages, 6 figure

Inversion of two-band superconductivity at the critical electron doping of (Mg,Al)B-2

Electron energy-loss spectroscopy (EELS) was combined with heat capacity measurements to probe changes of electronic structure and superconductivity in Mg(1-x)AlxB2. A simultaneous decrease of EELS intensity from sigma-band hole states and the magnitude of the sigma gap was observed with increasing x, thus verifying that band filling results in the loss of strong superconductivity. These quantities extrapolated to zero at x approximate to 0.33 as inferred from the unit cell volume. However, superconductivity was not quenched completely, but persisted with T-c< 7 K up to about x approximate to 55. Only the pi band had detectable density of states for 0.33 less than or similar to x less than or similar to 0.55, implying an inversion of the two-band hierarchy of MgB2 in that regime. Since pi-band superconductivity is active in other materials such as intercalated graphite, implications for new materials with high T-c are discussed

Reaction between YBa2Cu3O7−x and water

Reaction between water at 80 °C and YBa2Cu3O7−x (0.8<x<0.0) is studied using x‐ray absorption fine structure (EXAFS) and x‐ray diffraction (XRD). Oxygen deficient YBa2Cu3O7−x reacts with water and decomposes into BaCO3, CuO and at least one other Cu containing phase. YBa2Cu3O7 reacts slowly with water and the bulk material is modified before it decomposes. The structural and chemical modifications of this material resemble those reported for YBa2Cu3O7 reacted with hydrogen gas. We conclude that either hydrogen or water enters the bulk before decomposition.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87349/2/360_1.pd

Soft phonons and structural phase transitions in La1.875_{1.875}Ba0.125_{0.125}CuO4_{4}

Soft phonon behavior associated with a structural phase transition from the low-temperature-orthorhombic (LTO) phase ($Bmab$ symmetry) to the low-temperature-tetragonal (LTT) phase ($P4_{2}/ncm$ symmetry) was investigated in La$_{1.875}$Ba$_{0.125}$CuO$_{4}$ using neutron scattering. As temperature decreases, the TO-mode at $Z$-point softens and approaches to zero energy around $T_{\rm d2}=62$ K, where the LTO -- LTT transition occurs. Below $T_{\rm d2}$, the phonon hardens quite rapidly and it's energy almost saturates below 50 K. At $T_{\rm d2}$, the energy dispersion of the soft phonon along in-plane direction significantly changes while the dispersion along out-of-plane direction is almost temperature independent. Coexistence between the LTO phase and the LTT phase, seen in both the soft phonon spectra and the peak profiles of Bragg reflection, is discussed in context of the order of structural phase transitions.Comment: 6 pages, 8 figure