Synchrotron X-ray diffraction study of a charge stripe order in 1/8-doped La1.875_{1.875}Ba0.125−x_{0.125-x}Srx_{x}CuO4_{4}

Lattice distortions associated with charge stripe order in 1/8 hole-doped La$_{1.875}$Ba$_{0.125-x}$Sr$_{x}$CuO$_{4}$ are studied using synchrotron X-ray diffraction for $x=0.05$ and $x=0.075$. The propagation wave vector and charge order correlation lengths are determined with a high accuracy, revealing that the oblique charge stripes in orthorhombic $x=0.075$ crystal are more disordered than the aligned stripes in tetragonal $x=0.05$ crystal. The twofold periodicity of lattice modulations along the c-axis is explained by long-range Coulomb interactions between holes on neighboring CuO$_{2}$ planes.Comment: 4pages, 4figures, Submitted to PR

Competition between charge/spin-density-wave orders and superconductivity in La1.875_{1.875}Ba0.125−x_{0.125-x}Srx_{x}CuO4_{4}

We have performed a series of elastic neutron scattering measurements on 1/8-hole doped La$_{1.875}$Ba$_{0.125-x}$Sr$_{x}$CuO$_{4}$ single crystals with {\it x} = 0.05, 0.06, 0.075 and 0.085. % Both charge-density-wave (CDW) and spin-density-wave (SDW) orders are found to develop simultaneously below the structural transition temperature between the low-temperature orthorhombic (LTO) and low-temperature tetragonal (LTT) or low-temperature less-orthorhombic (LTLO) phases. % In the ground state the CDW order is observed only in the LTT/LTLO phase and drastically degrades towards the LTO boundary. The {\it x}-dependence of {\it T}$_{c}$ strongly suggests a direct effect of the CDW order on the suppression of superconductivity. % Results are discussed in comparison with those from the La$_{1.6-x}$Nd$_{0.4}$Sr$_{x}$CuO$_{4}$ system within the framework of the stripe model.Comment: 4 pages, 5figure

Static and dynamic spin correlations in the spin-glass phase of slightly-doped La_{2-x}Sr_xCuO_4

Neutron scattering experiments reveal that a diagonal spin modulation, which is a one-dimensional modulation rotated away by 45$^\circ$ from that in the superconducting phase, occurs universally across the insulating spin-glass phase in La$_{2-x}$Sr$_x$CuO$_4$ (0.02$\le x\le$0.055). This establishes an intimate relation between the magnetism and the transport properties in the high-temperature copper oxide superconductors. Furthermore, it is found that the charge density per unit length estimated using a charge stripe model is almost constant throughout the phase diagram, even when the modulation rotates away by 45$^\circ$ at the superconducting boundary. However, at the lowest values for $x$ the density changes approaching 1 hole/Cu as in La$_{2-x}$Sr$_x$NiO$_4$. Magnetic excitation spectra suggest that magnetic correlations change from incommensurate to commensurate at $\omega\sim$7 meV and $T\sim$70 K, indicating a characteristic energy for the incommensurate structure of 6-7 meV.Comment: 7 pages, submitted to Phys. Rev.

Low-frequency incommensurate magnetic response in strongly correlated systems

It is shown that in the t-J model of Cu-O planes at low frequencies the dynamic spin structure factor is peaked at incommensurate wave vectors (1/2+-delta,1/2)$, (1/2,1/2+-delta). The incommensurability is connected with the momentum dependencies of the magnon frequency and damping near the antiferromagnetic wave vector. The behavior of the incommensurate peaks is similar to that observed in La_{2-x}(Ba,Sr)_xCuO_{4+y} and YBa_2Cu_3O_{7-y}: for hole concentrations 0.02<x<=0.12 we find that delta is nearly proportional to x, while for x>0.12 it tends to saturation. The incommensurability disappears with increasing temperature. Generally the incommensurate magnetic response is not accompanied by an inhomogeneity of the carrier density.Comment: 4 pages, 4 figure

Direct evidence for charge stripes in a layered cobalt oxide

Recent experiments indicate that static stripe-like charge order is generic to the hole-doped copper oxide superconductors and competes with superconductivity. Here we show that a similar type of charge order is present in La5/3 Sr1/3 CoO4 , an insulating analogue of the copper oxide superconductors containing cobalt in place of copper. The stripe phase we have detected is accompanied by short-range, quasi-one-dimensional, antiferromagnetic order, and provides a natural explanation for the distinctive hour- glass shape of the magnetic spectrum previously observed in neutron scattering mea- surements of La2−xSrx CoO4 and many hole-doped copper oxide superconductors. The results establish a solid empirical basis for theories of the hourglass spectrum built on short-range, quasi-static, stripe correlations

Bond-Stretching-Phonon Anomalies in Stripe-Ordered La(1.69)Sr(0.31)NiO(4)

We report a neutron scattering study of bond-stretching phonons in La(1.69)Sr(0.31)NiO(4), a doped antiferromagnet in which the added holes order in diagonal stripes at 45 deg to the Ni-O bonds. For the highest-energy longitudinal optical mode along the bonds, a softening of 20% is observed between the Brillouin zone center and zone boundary. At 45 deg to the bonds, a splitting of the same magnitude is found across much of the zone. Surprisingly, the charge-ordering wave vector plays no apparent role in the anomalous dispersions. The implications for related anomalies in the cuprates are discussed.Comment: 4 two-col pages, including 4 figures (2 in color); references added and updated Final version, accepted for publication in PR

Electronic Structure of Stripes in Two-Dimensional Hubbard Model

Focusing on La_{2-x}Sr_{x}CuO_{4}, we study the stripe structure by the self-consistent mean-field theory of the Hubbard model. By introducing the realistic Fermi surface topology, the SDW-gapped insulator is changed to metallic. The solitonic features of the stripe structure and the contribution of the mid-gap states are presented. We consider the band dispersion, the local density of states, the spectral weight, and the optical conductivity, associated with the solitonic structure. These results may provide important information for the experimental research of the stripe structure, such as the angle-resolved photoemission experiments. The ``Fermi surface'' shape is changed depending on the ratio of the incommensurability delta and the hole density n_h. In real space, only the stripe region is metallic when delta/n_h is large.Comment: LaTeX 12 pages (using jpsj macros) with 16 figure

Direct Observation of a One Dimensional Static Spin Modulation in Insulating La1.95Sr0.05CuO4

We report the results of an extensive elastic neutron scattering study of the incommensurate (IC) static spin correlations in La1.95Sr0.05CuO4 which is an insulating spin glass at low temperatures. The present neutron scattering experiments on the same x=0.05 crystal employ a narrower instrumental Q-resolution and thereby have revealed that the crystal has only two orthorhombic twins at low temperatures with relative populations of 2:1. We find that, in a single twin, only two satellites are observed at (1, +/-0.064, L)(ortho) and (0, 1+/-0.064, L)(ortho), that is, the modulation vector is only along the orthorhombic b*-axis. This demonstrates unambiguously that La1.95Sr0.05CuO4 has a one-dimensional static diagonal spin modulation at low temperatures, consistent with certain stripe models. We have also reexamined the x=0.04 crystal that previously was reported to show a single commensurate peak. By mounting the sample in the (H, K, 0) zone, we have discovered that the x=0.04 sample in fact has the same IC structure as the $x=0.05$ sample. The incommensurability parameter d for x=0.04 and 0.05, where d is the distance from (1/2, 1/2) in tetragonal reciprocal lattice units, follows the linear relation d=x. These results demonstrate that the insulator to superconductor transition in the under doped regime (0.05 </= x </= 0.06) in La2-xSrxCuO4 is coincident with a transition from diagonal to collinear static stripes at low temperatures thereby evincing the intimate coupling between the one dimensional spin density modulation and the superconductivity.Comment: 9 pages 8 figure

Stripes and Superconductivity in Cuprates

Holes doped into the CuO2 planes of cuprate parent compounds frustrate the antiferromagnetic order. The development of spin and charge stripes provides a compromise between the competing magnetic and kinetic energies. Static stripe order has been observed only in certain particular compounds, but there are signatures which suggest that dynamic stripe correlations are common in the cuprates. Though stripe order is bad for superconducting phase coherence, stripes are compatible with strong pairing. Ironically, magnetic-field-induced stripe order appears to enhance the stability of superconducting order within the planes.Comment: 6 pages, submitted to proceedings of ECRYS-201

Protic plastic crystal/PVDF composite membranes for Proton Exchange Membrane Fuel Cells under non-humidified conditions

Composite membranes based on the protic plastic crystal N,N-dimethylethylenediammonium triflate [DMEDAH][TFO] and poly(vinylidene fluoride) (PVDF) nanofibers have been developed for proton exchange membrane fuel cells (PEMFCs) under non-humidified conditions. The effect of addition of 5 mol% triflic acid or 5 mol% of the base N,N-dimethylethylenediamine on the thermal and transport properties of the material is discussed. The acid-doped plastic crystal reports more than double the ionic conductivity of the pure plastic crystal. The effects of doping the plastic crystal and the composites, with acid or base, on the ionic conductivity and fuel cell performance are reported. Composite membranes based on PVDF nanofibers and [DMEDAH][TFO] were tested in a single PEMFC. The results show the potential of these composite membranes to be used as electrolytes in this electrochemical application without external humidification.The authors acknowledge funding from the Australian Research Council (ARC) through its Centre of Excellence program, through the Australian Laureate Fellowship scheme for D.R.M and M.F, and Discovery Project DP140101535. In addition, M.D., A.O. and I.O acknowledge Spanish Ministry of Economy and Competitiveness for the project CTQ2015-66078-R (MINECO/FEDER, UE). M. D. is grateful to the Spanish Ministry of Education, Culture and Sport for the FPU2012-3721