http://sces.phys.utk.edu/mostcited/mc1_1114.pdf

Colossal Magnetoresistant Materials: The Key Role of Phase Separation

Recent computational studies of models for manganese oxides have revealed a rich phase diagram, which was not anticipated in early calculations in this context performed in the 1950s and 1960s. In particular, the transition between the antiferromagnetic insulator state of the hole-undoped limit and the ferromagnetic metal at finite hole density was found to occur through a mixed-phase process. When extended Coulomb interactions are included, a microscopically charged inhomogeneous state should be stabilized. These phase separation tendencies, also present at low electronic densities, influence the properties of the ferromagnetic region by increasing charge fluctuations. Experimental data reviewed here by applying several techniques for manganites and other materials are consistent with this scenario. Similarities with results previously discussed in the context of cuprates are clear from this analysis, although the phase segregation tendencies in manganites appear stronger.

http://science.sciencemag.org/content/sci/283/5410/2034.full.pdf

Phase Separation Scenario for Manganese Oxides and Related Materials

The perovskites and perovskite-related structures exhibit several features of technical as well as fundamental interest. Technically useful properties include oxide-ion conduction with/without electronic conduction, oxidation catalysis, ferroic displacements in classic and relaxor ferroelectrics, half-metallic ferromagnetism and high-temperature superconductivity. Of more fundamental interest is the ability to tune, by chemical substitution on the large-cation subarray, transition-metal oxides through the crossover on the transition-metal array from localized dn configurations to itinerant d-electron behaviour without/with changing the valence state of that array. The localized-electron configurations may exhibit cooperative Jahn–Teller distortions that introduce anisotropic exchange interactions. At crossover, bond-length fluctuations may segregate into an ordered array of alternating covalent and ionic bonding in a single-valent perovskite; multicentre polarons or correlation bags may replace small polarons in a mixed-valent system. Bond-length fluctuations at crossover give vibronic conduction and suppression of the phonon contribution to the thermal conductivity; the fluctuations may order, to give high-temperature superconductivity, or transform to quantum–critical-point behaviour at lowest temperatures. Crossover of σ-bonding electrons in the presence of localized spins associated with π-bonding electrons gives rise to the colossal magnetoresistance phenomenon above a ferromagnetic Curie temperature.

https://iopscience.iop.org/article/10.1088/0034-4885/67/11/R01/pdf

Electronic and ionic transport properties and other physical aspects of perovskites

The propensity of systems of charge and spin to form, under certain conditions, ‘stripe’ phases has recently attracted much attention, as it has been suggested that dynamically fluctuating stripe phases may be of central importance for an understanding of the physics of high-temperature superconductors1,2,3,4,5. A related phenomenon — static charge stripes — characterizes6 the insulating antiferromagnetic ground state of the manganese oxides, a class of materials which (like the copper oxide superconductors) have a perovskite structure, and are notable for their extraordinary electronic and magnetic properties, such as colossal magnetoresistance and charge ordering7,8. Here we report a different pattern of charge localization in the charge-ordered phase of the manganese oxide La1−xCaxMnO3 (x ⩾ 0.5). This pattern takes the form of extremely stable pairs of Mn3+O6 stripes, with associated large lattice contractions (due to the Jahn–Teller effect), separated periodically by stripes of non-distorted Mn4+O6 octahedra. These periodicities, which adopt integer values between 2 and 5 times the lattice parameter of the orthorhombic unit cell, correspond to the commensurate carrier concentrations (x = 1/2, 2/3, 3/4 and 4/5): for other values of x, the pattern of charge ordering is a mixture of the two adjacent commensurate configurations. These paired Jahn–Teller stripes appear therefore to be the fundamental building blocks of the charge-ordered state in the manganese oxides, and so may be expected to have profound implications for the magnetic and transport properties of these materials.

Pairing of charge-ordered stripes in (La,Ca)MnO 3

https://repository.library.northeastern.edu/files/neu:331186/fulltext.pdf

A survey of the Van Hove scenario for high-tc superconductivity with special emphasis on pseudogaps and striped phases

X-ray diffuse scattering from single crystals of La{sub 2{minus}{ital x}}Sr{sub {ital x}}CuO{sub 4} ({ital x}=0.10,0.15,0.20) was studied in the vicinity of several Bragg peaks near the {ital c}{sup *} axis as a function of temperature. While the diffuse scattering at room temperature is dominated by phonon scattering, at low temperatures residual nonphonon diffuse scattering is clearly seen. The intensity of the residual nonphonon scattering depends upon temperature and composition in a characteristic manner. It is suggested that the nonphonon scattering is due to local lattice distortions induced by charge carriers in these compounds.

Temperature-dependent x-ray diffuse scattering from single crystals of La 2 − x Sr x CuO 4

Local lattice distortions suggesting nonuniform charge distributions were found to be a common feature of manganites and cuprates in the metallic state by neutron scattering studies The atomic pair-density function (PDF) determined by pulsed neutron powder diffraction showed that in La1-xSrxMnO3 doped holes form lattice polarons which persist even in the metallic state In La185Sr015CuO4 the energy width of the LO phonons around (π,0) determined by inelastic neutron scattering reflects magnetic satellites indicative of spin charge stripe formation The implications of such nonuniform charge distribution for metal-insulator transition and superconductivity are discussed

Nonuniform metallic state in manganites and cuprates

An inelastic-neutron-scattering study performed on a single crystal of superconducting ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$Cu${\mathrm{O}}_{4}$ reveals that the highest longitudinal phonon mode at wave vector ($\ensuremath{\pi}, 0$) (planar oxygen bond-stretching mode), of which the frequency is strongly affected by hole doping, has a very wide and asymmetric line shape. This line shape is considered anomalous and cannot be explained by normal electron-phonon coupling (Fermi surface nesting) or a macroscopic inhomogeneous hole distribution.

Wide and asymmetric oxygen bond-stretching phonons in La1.85Sr0.15CuO4.

Significant changes in the local atomic structure and lattice dynamics were observed by pulsed neutron inelastic scattering measurements on YBa2Cu4O8 both around the superconducting transition temperature and around the spin-gap temperature. These observations and earlier results of numerical calculations on the enhancement of electron-lattice interaction by electron correlation lead to a novel picture of unconventional lattice-induced superconductivity related to antiferroelectric instability.

R. J. McQueeney

Papers

Temperature-dependent x-ray diffuse scattering from single crystals of La 2 − x Sr x CuO 4

Nonuniform metallic state in manganites and cuprates

Wide and asymmetric oxygen bond-stretching phonons in La1.85Sr0.15CuO4.

Dynamic local lattice distortion in YBa2Cu4O8

Polarons and Bipolarons in High- Tc Superconductors and Related Materials: Experimental evidence of local lattice distortion in superconducting oxides