ONE of the long-standing mysteries associated with the high-temperature copper oxide superconductors concerns the anomalous suppression1 of superconductivity in La2-xBaxCuO4 (and certain related compounds) when the hole concentration x is near . Here we examine the possibility that this effect is related to dynamical two-dimensional spin correlations, incommensurate with the crystal lattice, that have been observed in La2-xSrxCuO4 by neutron scattering2–4. A possible explanation for the incommensurability involves a coupled, dynamical modulation of spin and charge in which antiferromagnetic 'stripes' of copper spins are separated by periodically spaced domain walls to which the holes segregate5–9. An ordered stripe phase of this type has recently been observed in hole-doped La2NiO4 (refs 10–12). We present evidence from neutron diffraction that in the copper oxide material La1.6-xNd0.4SrxCuO4, with x = 0.12, a static analogue of the dynamical stripe phase is present, and is associated with an anomalous suppression of superconductivity13,14. Our results thus provide an explanation of the '
 ' conundrum, and also support the suggestion15 that spatial modulations of spin and charge density are related to superconductivity in the copper oxides.

Evidence for stripe correlations of spins and holes in copper oxide superconductors

This article discusses fluctuating order in a quantum disordered phase proximate to a quantum critical point, with particular emphasis on fluctuating stripe order. Optimal strategies are derived for extracting information concerning such local order from experiments, with emphasis on neutron scattering and scanning tunneling microscopy. These ideas are tested by application to two model systems---an exactly solvable one-dimensional (1D) electron gas with an impurity, and a weakly interacting 2D electron gas. Experiments on the cuprate high-temperature superconductors which can be analyzed using these strategies are extensively reviewed. The authors adduce evidence that stripe correlations are widespread in the cuprates. They compare and contrast the advantages of two limiting perspectives on the high-temperature superconductor: weak coupling, in which correlation effects are treated as a perturbation on an underlying metallic (although renormalized) Fermi-liquid state, and strong coupling, in which the magnetism is associated with well-defined localized spins, and stripes are viewed as a form of micro phase separation. The authors present quantitative indicators that the latter view better accounts for the observed stripe phenomena in the cuprates.

How to detect fluctuating stripes in the high-temperature superconductors

Oxygen diffusion and transport properties in non-stoichiometric Ln2 − xNiO4 + δ oxides

Many transition-metal oxides show very large (“colossal”) magnitudes of the dielectric constant and thus have immense potential for applications in modern microelectronics and for the development of new capacitance-based energy-storage devices In the present work, we thoroughly discuss the mechanisms that can lead to colossal values of the dielectric constant, especially emphasising effects generated by external and internal interfaces, including electronic phase separation In addition, we provide a detailed overview and discussion of the dielectric properties of CaCu3Ti4O12 and related systems, which is today’s most investigated material with colossal dielectric constant Also a variety of further transition-metal oxides with large dielectric constants are treated in detail, among them the system La2−xSrxNiO4 where electronic phase separation may play a role in the generation of a colossal dielectric constant

/pdf/colossal-dielectric-constants-in-transition-metal-oxides-ipw2t6omi6.pdf

Colossal dielectric constants in transition-metal oxides

X-ray techniques have been used for more than a century to study the atomic and electronic structure in practically any type of material. The advent of correlated electron systems, in particular complex oxides, brought about new scientific challenges and opportunities for the advancement of conventional X-ray methods. In this context, the need for new approaches capable of selectively sensing new forms of orders involving all degrees of freedom—charge, orbital, spin, and lattice—paved the way for the emergence and success of resonant X-ray scattering, which has become an increasingly popular and powerful tool for the study of electronic ordering phenomena in solids. We review the recent resonant X-ray scattering breakthroughs in the copper oxide high-temperature superconductors, in particular regarding the phenomenon of charge order, a broken-symmetry state occurring when valence electrons self-organize into periodic structures. After a brief historical perspective on charge order, we outline the mileston...

Resonant X-Ray Scattering Studies of Charge Order in Cuprates

We report a single-crystal neutron diffraction study of the incommensurate magnetic ordering that occurs in ${\mathrm{La}}_{2}$Ni${\mathrm{O}}_{4.125}$ below 110 K. Besides the magnetic first and third harmonic Bragg peaks, we have also observed second harmonic peaks associated with charge ordering. The magnitude of the incommensurate splitting, $\ensuremath{\epsilon}$, is strongly temperaure dependent. Lock-in behavior indicates that $\ensuremath{\epsilon}$ tends to rational fractions, while regions of continuous variation suggest a devil's staircase. Analysis of these features indicates that the holes, induced by the excess oxygen, order in domain walls that form antiphase boundaries between antiferromagnetic domains.

Simultaneous ordering of holes and spins in la2nio4.125

The dimerization of the Cu ions in the spin-Peierls state of ${\mathrm{CuGeO}}_{3}$ below 14 K has been determined by neutron-diffraction measurements. In addition to superlattice reflections with indices $\frac{h}{\frac{2kl}{2}}$ ($h$, $k$, $l$, all odd) recently observed in electron and x-ray diffraction studies, we also found peaks for which $k=5$ = even. Oxygen displacements in the $a\ensuremath{-}b$ plane of about 0.01 \AA{}, accompanied by comparable shifts of the Cu ions along the chain direction $c$, can be related to the dimerized spin state in a natural fashion.

Dimerization of CuGeO3 in the spin-Peierls state

We report neutron-scattering experiments that reveal coupled incommensurate peaks arising from magnetic as well as charge ordering at low temperatures ({ital T}{le}100 K) in stoichiometric La{sub 2{minus}{ital x}}Sr{sub {ital x}}NiO{sub 4+{delta}} ({delta}=0.00) crystals. In contrast with the long-range ordering of holes and spins observed recently in La{sub 2}NiO{sub 4.125}, the correlations are short ranged in Sr-doped La{sub 2}NiO{sub 4} and the incommensurability {var_epsilon} is 0.25 and 0.12 for {ital x}=0.20 and 0.135, respectively. Based on our study and previous reports on La{sub 2{minus}{ital x}}Sr{sub {ital x}}NiO{sub 4+{delta}}, the modulation wave vector appears to be a sensitive function of net hole concentration, {ital p}={ital x}+2{delta}. The magnetic correlation length increases with doping and the incommensurability follows the simple relationship {var_epsilon}{approx}{ital p}.

Charge and spin ordering in La2-xSrxNiO4.00 with x=0.135 and 0.20.

${\mathrm{La}}_{2}$${\mathrm{NiO}}_{4.125}$ exhibits a cooperative ordering of dopant-induced holes and Ni spins below a transition temperature of 110 K. There is also an ordering of interstitial oxygens that occurs near room temperature. We present a comprehensive analysis of experimentally observed superlattice intensities in terms of structural models, justifying previous identifications. The model for the interstitial order involves a 3a\ifmmode\times\else\texttimes\fi{}5b\ifmmode\times\else\texttimes\fi{}5c unit cell with an ideal interstitial density of \ensuremath{\delta}=2/15 per formula unit. In the spin and charge ordered state, the magnetic moments are sinusoidally modulated within the ${\mathrm{NiO}}_{2}$ planes. The moments point transverse to the modulation direction, with nearest neighbors antiparallel, and have a maximum amplitude that is g80% of that observed in undoped ${\mathrm{La}}_{2}$${\mathrm{NiO}}_{4}$. A corresponding modulation of atomic positions within ${\mathrm{NiO}}_{2}$ planes involves breathing-mode distortions consistent with a modulation of the charge density. The results are compared with theoretical models and with experimental observations on related systems.

Cooperative ordering of holes and spins in La2NiO4.125.

We report neutron diffraction studies on a series of ${\mathrm{La}}_{2}$${\mathrm{NiO}}_{4+\mathrm{\ensuremath{\delta}}}$ single crystals with 0.05\ensuremath{\lesssim}\ensuremath{\delta}\ensuremath{\lesssim}0.11. At 300 K, all of the crystals have an average tetragonal structure (space group I4/mmm). On cooling below 290 K, one or more orthorhombic phases appear, characterized by incommensurate superlattice peaks at (0,k,l\ifmmode\pm\else\textpm\fi{}\ensuremath{\Delta}) with k odd, l even, and 1/4\ensuremath{\le}1/2. The positions, widths, and intensities of the superlattice peaks are quite sensitive to the cooling rate. We show that the incommensurate peaks are evidence of intercalated layers of oxygen spaced periodically along the c axis, with a one-dimensional ordering similar to the staging of intercalates in graphite. The structure of the ${\mathrm{La}}_{2}$${\mathrm{NiO}}_{4}$ lattice between the interstitial layers is of the Bmab-type; the superlattice peaks result from the ordered antiphase domain boundaries induced by the interstitial oxygens, which sit at (1/4, 1) / 4 , 1/4)-type positions. Observed orderings involve interstitial layers separated by two to four Ni-O layers. Peak shapes and positions are modelled quantitatively using the formulas of Hendricks and Teller for one-dimensional disorder in a layer lattice. Besides the one-dimensional ordering of the intercalant layers, temperature-dependent phase separations are observed. Because of the slow ordering kinetics, phase separation can be suppressed by rapid cooling.

J. E. Lorenzo

Papers

Simultaneous ordering of holes and spins in la2nio4.125

Dimerization of CuGeO3 in the spin-Peierls state

Charge and spin ordering in La2-xSrxNiO4.00 with x=0.135 and 0.20.

Cooperative ordering of holes and spins in La2NiO4.125.

Oxygen intercalation, stage ordering, and phase separation in la2nio4+delta with 0.05 delta 0.11