Structural features that optimize high temperature superconductivity

For example, various defects can be introduced into the blocking layer to provide the optimum carrier concentration, but defects that form in or adjacent to the CuO{sub 2} layers will lower T{sub c} and eventually destroy superconductivity. After these requirements are satisfied, the highest T{sub c}`s are observed for compounds (such as the HgBa{sub 2}Ca{sub n-1}CuO{sub 2n{plus}2{plus}x} family) that have flat and square CuO{sub 2} planes and long apical Cu-O bonds. This conclusion is confirmed by the study of materials in which the flatness of the CuO{sub 2} plane can be varied in a systematic way. In more recent work, attention has focused on how the structure can be modified, for example, by chemical substitution, to improve flux pinning properties. Two strategies are being investigated: (1) Increasing the coupling of pancake vortices to form vortex-lines by shortening or ``metallizing`` the blocking layer; and (2) the formation of defects that pin flux

Spin and orbital ordering in double-layered manganites

We study theoretically the phase diagram of the double-layered perovskite manganites taking into account the orbital degeneracy, the strong Coulombic repulsion, and the coupling with the lattice deformation. Observed spin structural changes as the increased doping are explained in terms of the orbital ordering and the bond-length dependence of the hopping integral along $c$-axis. Temperature dependence of the neutron diffraction peak corresponding to the canting structure is also explained. Comparison with the 3D cubic system is made.Comment: 7 figure

Magnetic field induced effects on the electric polarization in RMnO3 R Dy,Gd

X-ray resonant magnetic scattering studies of rare earth magnetic ordering were performed on perovskite manganites RMnO3 (R = Dy, Gd) in an applied magnetic field. The data reveal that the field-induced three-fold polarization enhancement for H || a (H approx. 20 kOe) observed in DyMnO3 below 6.5 K is due to a re-emergence of the Mn-induced Dy spin order with propagation vector k(Dy) = k(Mn) = 0.385 b*, which accompanies the suppression of the independent Dy magnetic ordering, k(Dy) = 1/2 b*. For GdMnO3, the Mn-induced ordering of Gd spins is used to track the Mn-ordering propagation vector. The data confirm the incommensurate ordering reported previously, with k(Mn) varying from 0.245 to 0.16 b* on cooling from T_N(Mn) down to a transition temperature T'. New superstructure reflections which appear below T' suggest a propagation vector k(Mn) = 1/4 b* in zero magnetic field, which may coexist with the previously reported A-type ordering of Mn. The Gd spins order with the same propagation vector below 7 K. Within the ordered state of Gd at T = 1.8 K we find a phase boundary for an applied magnetic field H || b, H = 10 kOe, which coincides with the previously reported transition between the ground state paraelectric and the ferroelectric phase of GdMnO3. Our results suggest that the magnetic ordering of Gd in magnetic field may stabilize a cycloidal ordering of Mn that, in turn, produces ferroelectricity.Comment: 8 Figures, v2: improved figure layou

Quasiparticlelike peaks, kinks, and electron phonon coupling at the pi,0 regions in the CMR oxide La2 2xSr1 2xMn2O7

Using Angle Resolved Photoemission ARPES , we present the first observation of sharp quasiparticle like peaks in a CMR manganite. We focus on the pi,0 regions of k space and study their electronic scattering rates and dispersion kinks, uncovering the critical energy scales, momentum scales, and strengths of the interactions that renormalize the electrons. To identify these bosons we measured phonon dispersions in the energy range of the kink by inelastic neutron scattering INS , finding a good match in both energy and momentum to the oxygen bond stretching phonon

The Structure of Nanoscale Polaron Correlations in La1.2Sr1.8Mn2O7

A system of strongly-interacting electron-lattice polarons can exhibit charge and orbital order at sufficiently high polaron concentrations. In this study, the structure of short-range polaron correlations in the layered colossal magnetoresistive perovskite manganite, La1.2Sr1.8Mn2O7, has been determined by a crystallographic analysis of broad satellite maxima observed in diffuse X-ray and neutron scattering data. The resulting q=(0.3,0,1) modulation is a longitudinal octahedral-stretch mode, consistent with an incommensurate Jahn-Teller-coupled charge-density-wave fluctuations, that implies an unusual orbital-stripe pattern parallel to the directions.Comment: Reformatted with RevTe

Soft X-ray resonant scattering study of single-crystal LaSr2_2Mn2_2O7_7

Soft X-ray resonant scattering studies at the Mn $L_{\texttt{II, III}}$- and the La $M_{\texttt{IV, V}}$- edges of single-crystal LaSr$_2$Mn$_2$O$_7$ are reported. At low temperatures, below $T_\texttt{N} \approx 160$ K, energy scans with a fixed momentum transfer at the \emph{A}-type antiferromagnetic (0 0 1) reflection around the Mn $L_{\texttt{II, III}}$-edges with incident linear $\sigma$ and $\pi$ polarizations show strong resonant enhancements. The splitting of the energy spectra around the Mn $L_{\texttt{II, III}}$-edges may indicate the presence of a mixed valence state, e.g., Mn$^{3+}$/Mn$^{4+}$. The relative intensities of the resonance and the clear shoulder-feature as well as the strong incident $\sigma$ and $\pi$ polarization dependences strongly indicate its complex electronic origin. Unexpected enhancement of the charge Bragg (0 0 2) reflection at the La $M_{\texttt{IV, V}}$-edges with $\sigma$ polarization has been observed up to 300 K, with an anomaly appearing around the orbital-ordering transition temperature, $T_{\texttt{OO}} \approx 220$ K, suggesting a strong coupling (competition) between them.Comment: Accepted by European Physical Journal

Competition of charge, orbital, and ferromagnetic correlations in layered manganites

The competition of charge, orbital, and ferromagnetic interactions in layered manganites is investigated by magneto-Raman scattering spectroscopy. We find that the colossal magnetoresistance effect in the layered compounds results from the interplay of the orbital and ferromagnetic double-exchange correlations. Inelastic scattering by charge-order fluctuations dominates the quasiparticle dynamics in the ferromagnetic-metal state. The scattering is suppressed at low frequencies, consistent with the opening of a charge-density wave pseudogap.Comment: 10 pages, 4 figure

Temperature-dependent Raman spectroscopy in BaRuO3_3 systems

We investigated the temperature-dependence of the Raman spectra of a nine-layer BaRuO$_3$ single crystal and a four-layer BaRuO$_3$ epitaxial film, which show pseudogap formations in their metallic states. From the polarized and depolarized spectra, the observed phonon modes are assigned properly according to the predictions of group theory analysis. In both compounds, with decreasing temperature, while $A_{1g}$ modes show a strong hardening, $E_g$ (or $E_{2g}$) modes experience a softening or no significant shift. Their different temperature-dependent behaviors could be related to a direct Ru metal-bonding through the face-sharing of RuO$_6$. It is also observed that another $E_{2g}$ mode of the oxygen participating in the face-sharing becomes split at low temperatures in the four layer BaRuO$_3$. And, the temperature-dependence of the Raman continua between 250 $\sim$ 600 cm$^{-1}$ is strongly correlated to the square of the plasma frequency. Our observations imply that there should be a structural instability in the face-shared structure, which could be closely related to the pseudogap formation of BaRuO$_3$ systems.Comment: 8 pages, 6 figures. to be published in Phys. Rev.

Coherent X-ray Scattering from Manganite Charge and Orbital Domains

We report coherent x-ray scattering studies of charge and orbital domains in manganite systems. The experiments were carried out on LaMnO_3 and Pr_{0.6}Ca_{0.4}MnO_3, with the incident photon energy tuned near the Mn K edge. At room temperature, the orbital speckle pattern of LaMnO_3 was observed to be constant over a timescale of at least minutes, which is indicative of static orbital domains on this timescale. For Pr_{0.6}Ca_{0.4}MnO_3, both charge and orbital speckle patterns were observed. The observation of the latter rules out the presence of fast orbital fluctuations, while long time series data-- on the order of several minutes-- were suggestive of slow dynamic behavior. In contrast, the charge order speckle patterns were static.Comment: 6 pages, 4 figure

Average Lattice Symmetry and Nanoscale Structural Correlations in Magnetoresistive Manganites

We report x-ray scattering studies of nanoscale structural correlations in the paramagnetic phases of the perovskite manganites La$_{0.75}$(Ca$_{0.45}$Sr$_{0.55}$)$_{0.25}$MnO$_3$, La$_{0.625}$Sr$_{0.375}$MnO$_3$, and Nd$_{0.45}$Sr$_{0.55}$MnO$_3$. We find that these correlations are present in the orthorhombic $O$ phase in La$_{0.75}$(Ca$_{0.45}$Sr$_{0.55}$)$_{0.25}$MnO$_3$, but they disappear abruptly at the orthorhombic-to-rhombohedral transition in this compound. The orthorhombic phase exhibits increased electrical resistivity and reduced ferromagnetic coupling, in agreement with the association of the nanoscale correlations with insulating regions. In contrast, the correlations were not detected in the two other compounds, which exhibit rhombohedral and tetragonal phases. Based on these results, as well as on previously published work, we propose that the local structure of the paramagnetic phase correlates strongly with the average lattice symmetry, and that the nanoscale correlations are an important factor distinguishing the insulating and the metallic phases in these compounds.Comment: a note on recent experimental work, and a new reference adde