We present an experimental review of the nature of the pseudogap in the cuprate superconductors. Evidence from various experimental techniques points to a common phenomenology. The pseudogap is seen in all high-temperature superconductors and there is general agreement on the temperature and doping range where it exists. It is also becoming clear that the superconducting gap emerges from the normal state pseudogap. The d-wave nature of the order parameter holds for both the superconducting gap and the pseudogap. Although an extensive body of evidence is reviewed, a consensus on the origin of the pseudogap is as lacking as it is for the mechanism underlying high-temperature superconductivity.

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The pseudogap in high-temperature superconductors: an experimental survey

This critical review presents an overview of the various classes of oxide materials exhibiting fast oxide-ion or proton conductivity for use as solid electrolytes in clean energy applications such as solid oxide fuel cells. Emphasis is placed on the relationship between structural and mechanistic features of the crystalline materials and their ion conduction properties. After describing well-established classes such as fluorite- and perovskite-based oxides, new materials and structure-types are presented. These include a variety of molybdate, gallate, apatite silicate/germanate and niobate systems, many of which contain flexible structural networks, and exhibit different defect properties and transport mechanisms to the conventional materials. It is concluded that the rich chemistry of these important systems provides diverse possibilities for developing superior ionic conductors for use as solid electrolytes in fuel cells and related applications. In most cases, a greater atomic-level understanding of the structures, defects and conduction mechanisms is achieved through a combination of experimental and computational techniques (217 references).

Oxide-ion and proton conducting electrolyte materials for clean energy applications: structural and mechanistic features

Inelastic light scattering is an intensively used tool in the study of electronic properties of solids. Triggered by the discovery of high-temperature superconductivity in the cuprates and by new developments in instrumentation, light scattering in both the visible (Raman effect) and x-ray part of the electromagnetic spectrum has become a method complementary to optical (infrared) spectroscopy while providing additional and relevant information. The main purpose of the review is to position Raman scattering with regard to single-particle methods like angle-resolved photoemission spectroscopy, and other transport and thermodynamic measurements in correlated materials. Particular focus will be placed on photon polarizations and the role of symmetry to elucidate the dynamics of electrons in different regions of the Brillouin zone. This advantage over conventional transport (usually measuring averaged properties) provides new insights into anisotropic and complex many-body behavior of electrons in various systems. Recent developments in the theory of electronic Raman scattering in correlated systems and experimental results in paradigmatic materials such as the A15 superconductors, magnetic and paramagnetic insulators, compounds with competing orders, as well as the cuprates with high superconducting transition temperatures are reviewed. An overview of the manifestations of complexity in the Raman response due to the impact of correlations and developing competing orders is presented. In a variety of materials, observations which may be understood and a summary of important open questions that pave the way to a detailed understanding of correlated electron systems, are discussed.

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Inelastic light scattering from correlated electrons

R educing the operating temperature in the 500‐750 C range is needed for widespread use of solid oxide fuel cells (SOFCs). Proton-conducting oxides are gaining wide interest as electrolyte materials for this aim. We report the fabrication of BaZr0:8Y0:2O3 (BZY) proton-conducting electrolyte thin films by pulsed laser deposition on different single-crystalline substrates. Highly textured, epitaxially oriented BZY films were obtained on (100)-oriented MgO substrates, showing the largest proton conductivity ever reported for BZY samples, being 0:11 S cm 1 at 500 C. The excellent crystalline quality of BZY films allowed for the first time the experimental measurement of the large BZY bulk conductivity above 300 C, expected in the absence of blocking grain boundaries. The measured proton conductivity is also significantly larger than the conductivity values of oxygen-ion conductors in the same temperature range, opening new potential for the development of miniaturized SOFCs for portable power supply.

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High proton conduction in grain-boundary-free yttrium-doped barium zirconate films grown by pulsed laser deposition

Magnetic oxides show a variety of extrinsic magnetotransport phenomena: grain-boundary, tunnelling and domain-wall magnetoresistance. In view of these phenomena the role of some magnetic oxides is outstanding: these are believed to be half-metallic having only one spin-subband at the Fermi level. These fully spin-polarized oxides have great potential for applications in spin-electronic devices and have, accordingly, attracted intense research activity in recent years. This review is focused on extrinsic magnetotransport effects in ferromagnetic oxides. It consists of two parts; the second part is devoted to an overview of experimental data and theoretical models for extrinsic magnetotransport phenomena. Here a critical discussion of domain-wall scattering is given. Results on surface and interfacial magnetism in oxides are presented. Spin-polarized tunnelling in ferromagnetic junctions is reviewed and grain-boundary magnetoresistance is interpreted within a model of spin-polarized tunnelling through natural oxide barriers. The situation in ferromagnetic oxides is compared with data and models for conventional ferromagnets. The first part of the review summarizes basic material properties, especially data on the spin polarization and evidence for half-metallicity. Furthermore, intrinsic conduction mechanisms are discussed. An outlook on the further development of oxide spin-electronics concludes this review.

Extrinsic magnetotransport phenomena in ferromagnetic oxides

The influence of partial isomorphous substitutions at the Cu and Ba sites of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ on the phonon spectrum and the structure has been investigated systematically by Raman scattering and neutron diffraction. For partial substitution of Sr for Ba the vibrational frequency of the apex oxygen, O(4), increases as expected from the observed compression of the structure in the region between the ${\mathrm{CuO}}_{2}$ planes and the CuO chains. Substitutions of trivalent ions such as Co, Fe, and Al for Cu change the coordination of the apex oxygen in a way consistent with an elimination of holes in the ${\mathrm{CuO}}_{2}$ planes. The O(4) vibrational frequency decreases rapidly with increasing trivalent substitution. The changes cannot be explained in a simple way by the bond-length changes observed in the neutron-diffraction experiments. A remarkable correlation is found between ${\mathit{T}}_{\mathit{c}}$ and the O(4) Raman mode for trivalent substitutions on the Cu(1) site and for oxygen deficiency. We suggest that this correlation as well as the rapid decrease of ${\mathit{T}}_{\mathit{c}}$ and the structural changes we observe are caused by a hole charge transfer from the ${\mathrm{CuO}}_{2}$ planes to the CuO chains.

Charge-transfer and compression effects of isomorphous substitutions in YBa2Cu3O7.

Solid-state sintering has been used to prepare the perovskite BaZr0.9Sc0.1O3-delta. Analysis of X-ray powder diffraction data shows that an increase of the unit cell parameter, a, was observed after deuteration. Rietveld analysis of room-temperature neutron powder diffraction data confirmed cubic symmetry (space group Pm-3m). Dynamic thermogravimetric analysis indicates that the hydration process occurs below 335 degrees C and approximately 58% of the theoretical number of protonic defects can be filled. The presence of protons/deutrons is seen from the strong O-H/O-D stretch band in the infrared spectrum of the hydrated/deuterated samples. The proton conductivity of a prehydrated sample was investigated under dry and wet Ar atmospheres

Crystal Structure and Proton Conductivity of BaZr0.9Sc0.1O3−δ

Etude de YBa 2 Cu 3-x Co x O 7- δ pour O≤x≤0.6. La structure et les proprietes supraconductrices sont soigneusement caracterisees par diffraction de rayons-X, susceptibilite magnetique, et par mesures de resistivite. Etude des variations des frequences: de vibration de valence symetrique de O(4); de cisaillement de Cu(2)-O(2),O(3); de deformation de torsion de Cu(2)-O(2),O(3); en fonction de la composition-x. Les dependances des frequences et des intensites de ces modes de vibration avec la substitution du cobalt rappellent le comportement des modes correspondants pour YBa 2 Cu 3 O 7- δ pur, de deficience croissante en oxygene. On propose que la dependance de ce mode est directement dependante de la concentration en porteurs de charge libres

Raman spectra, superconductivity, and structure of Co-substituted YBa2Cu3O7- delta.

We employ inelastic light scattering to study the dependence of the lattice and charge excitations in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ compounds on doping and temperature. The phonons reveal strong local lattice distortions in the paramagnetic state, which gradually vanish below the ferromagnetic transition. We identify charge excitations in the metallic state. They exhibit a dependence on the doping level and symmetry selection rules typical for a plasmonlike excitation. Their energy scale of 100 meV requires a low-carrier-density component of the plasma outlining the importance of electronic interactions in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}.$

S.-G. Eriksson

Papers

Charge-transfer and compression effects of isomorphous substitutions in YBa2Cu3O7.

Crystal Structure and Proton Conductivity of BaZr0.9Sc0.1O3−δ

Raman spectra, superconductivity, and structure of Co-substituted YBa2Cu3O7- delta.

Lattice and charge excitations in La 1 − x Sr x MnO 3

The effect of thallium and oxygen stoichiometry on structure and [Tc] in Tl-2201 and Tl-2212