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

Dense, highly ordered monolayers can be prepared by the adsorption of octadecylphosphonic acid (ODPA) onto nonporous ZrO2, TiO2, and zirconated silica powders. ODPA reacts strongly with Al2O3 to form a bulk (aluminoalkyl)phosphonate. The infrared spectra of the ODPA monolayers indicate a degree of conformational order comparable to self-assembled monolayers on planar substrates. From the solid-state 31P and 13C NMR spectra, the strength of the surface interaction and the degree of conformational order decrease in the following order:  zirconated silica > ZrO2 > TiO2. Although the inner methylenes are primarily in an all-trans conformation, the chain termina are highly disordered. In the case of zirconated silica, there is evidence for the formation of some bulk zirconium alkylphosphonate. Proton line width measurements from 2D WISE NMR experiments reveal that considerable chain mobility is present in the ODPA monolayers on ZrO2 and TiO2, in contrast to bulk crystalline phases.

Self-Assembled Monolayers of Alkylphosphonic Acids on Metal Oxides

Organophosphorus acids and their derivatives (salts, esters) are quite complementary of organosilicon coupling molecules for the preparation of hybrid organic–inorganic materials, by sol–gel processing or surface modification. Organosilicon compounds are best suited for the anchoring of organic groups to silicon-containing inorganic matrices or supports, such as silica, silicates, silicon carbide, etc., whereas organophosphorus coupling molecules appear well adapted to matrices or supports based on metals or transition metals: oxides, hydroxides, as well as carbonates and phosphates. The different reactivity of organophosphorus coupling molecules leads to different structures and stabilities of the final hybrid materials and may provide decisive advantages in the sol–gel synthesis of homogeneous hybrids, the preparation of surface monolayers or the selective surface modification of nanopatterned supports.

Hybrid materials from organophosphorus coupling molecules

Organic−inorganic hybrid films present special challenges and opportunities with respect to potential applications, as well as for the observation of interesting physical phenomena. Although the often disparate characteristics (e.g., solubility and thermal stability) of the two components provide a potential barrier toward forming workable and convenient film deposition strategies, recent results have demonstrated that not only is it possible to develop suitable techniques for these materials but relatively simple processes are often sufficient to yield high quality films. In the current review, some of these recent results on organic−inorganic hybrid films are discussed, with selected examples chosen from among the deposition of sol−gel materials, self-assembled hybrids, Langmuir−Blodgett films, and artificially layered materials. In addition to discussing film-deposition techniques, an effort will also be made, where appropriate, to indicate how the resulting films might be useful for applications such ...

Thin-Film Deposition of Organic−Inorganic Hybrid Materials

In materials with strong local Coulomb interactions, simple defects such as atomic substitutions strongly affect both macroscopic and local properties of the system. A nonmagnetic impurity, for instance, is seen to induce magnetism nearby. Even without disorder, models of such correlated systems are generally not soluble in 2 or 3 dimensions, and so few exact results are known for the properties of such impurities. Nevertheless, some simple physical ideas have emerged from experiments and approximate theories. Here, we first review what we can learn about this problem from 1D antiferromagnetically correlated systems. We then discuss experiments on the high Tc cuprate normal state which probe the effect of impurities on local charge and spin degrees of freedom, and compare with theories of single impurities in correlated hosts, as well as phenomenological effective Kondo descriptions. Subsequently, we review theories of impurities in d-wave superconductors including residual quasiparticle interactions, and compare with experiments in the superconducting state. We argue that existing data exhibit a remarkable similarity to impurity-induced magnetism in the 1D case, implying the importance of electronic correlations for the understanding of these phenomena, and suggesting that impurities may provide excellent probes of the still poorly understood ground state of the cuprates.

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Defects in correlated metals and superconductors

We report /sup 63/Cu nuclear spin-lattice relaxation data for Ba/sub 2/YC/sub 3/O/sub 7-//sub delta/ in the superconducting state obtained by nuclear quadrupole resonance in zero applied magnetic field. The temperature dependences of the relaxation rates reveal striking differences in the excitation spectra for quasiparticles on the chain-forming Cu1 and planar Cu2 lattice sites, corresponding to substantially different pairing energies for electrons on the planes and chains. Values of the gap ratio 2..delta../kT/sub c/ 8.3 and 2.4 are obtained for the Cu1 and Cu2 sites, respectively.

Evidence for two pairing energies from nuclear spin-lattice relaxation in superconducting Ba2YCu3O7- delta.

Structural and electronic transformations of liquid selenium at high temperature and pressure: A 77 Se NMR study

{sup 63}Cu NMR shifts and spectra are presented for both Cu(1) and Cu(2) sites in a specimen of YBa{sub {ital z}}Cu{sub 3}O{sub 6.64} ({ital T}{sub {ital c}}{similar to}60 K). For {bold H} along the {ital c} axis, the Cu(2) shift is not distinguishable from that reported for YBa{sub 2}Cu{sub 3}O{sub 7.0}. For {bold H} in the {ital a}-{ital b} plane, the spin component of Cu(2) shift declines sharply as {ital T}{r arrow}{ital T}{sub {ital c}}. The Cu(1) shift behaves similarly, showing also a dramatic broadening at low temperatures in clearly nonmetallic behavior. Combined with anomalous {ital T}{sub 1} data reported earlier for the 60-K phase of YBa{sub 2}Cu{sub 3}O{sub 6+{ital x}}, these results are shown to be consistent with strong and monotonically increasing antiferromagnetic correlations as {ital T} is decreased.

63Cu NMR shift and linewidth anomalies in the Tc=60 K phase of Y-Ba-Cu-O.

Nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) studies of copper nuclei are reported for high-quality single-phase powder samples of Ba/sub 2/YCu/sub 3/O/sub 7-//sub delta/. The high-field /sup 63/Cu NMR spectrum shows well-defined axial symmetry for the chain-forming Cu(1) site and evidence of appreciable disorder in the Cu(2) planes. Spontaneous oxygen loss was found to degrade the sharp NMR spectrum of the Cu(1) site. Spin-lattice relaxation rates reveal strikingly different electron dynamics for the two sites. For neither site do the rates exhibit conventional Bardeen-Cooper-Schrieffer behavior near T/sub c/.

Nuclear magnetic resonance and nuclear quadrupole resonance study of copper in Ba2YCu

We report ${}^{27}\mathrm{Al}$ and ${}^{69}\mathrm{Ga}$ impurity and ${}^{67}\mathrm{Zn}$ host species NMR experiments on ZnO powders containing Al or Ga dopants in the range 0.03--3 at. %. Impurity resonance shift and nuclear spin-lattice relaxation data are analyzed to yield values of the impurity nucleus chemical shift, Knight shift, and hyperfine coupling. The electric quadrupole coupling of dilute ${}^{69}\mathrm{Ga}$ in ZnO is determined to be ${(e}^{2}{qQ/h)}_{69}=3.9\ifmmode\pm\else\textpm\fi{}0.3$ MHz. The concentration dependence of the Knight shifts and relaxation rates indicate rapid development of an impurity band as the dopant concentration falls below about 0.5 at. %. At all concentrations investigated, Knight shifts and relaxation rates satisfy the Korringa relation; electron correlation and localization effects are essentially absent. The shapes of ${}^{69}\mathrm{Ga}$ spin echoes reveal that a second, more distorted gallium site is present with a relative concentration that increases with the ratio of oxygen to total metal content. Magnetic hyperfine coupling of carriers to the distorted sites is very weak and the apparent carrier density at substitutional sites decreases with the fraction of observed substitutional sites. These results imply that the distorted sites are part of a diamagnetic carrier trap involving gallium complexed with a defect. We suggest that the complex consists of two gallium ions and an oxygen interstitial, i.e. ${\mathrm{Ga}}_{2}^{3+}{\mathrm{O}}_{i}^{2\ensuremath{-}}.$

William W. Warren

Papers

Evidence for two pairing energies from nuclear spin-lattice relaxation in superconducting Ba2YCu3O7- delta.

Structural and electronic transformations of liquid selenium at high temperature and pressure: A 77 Se NMR study

63Cu NMR shift and linewidth anomalies in the Tc=60 K phase of Y-Ba-Cu-O.

Nuclear magnetic resonance and nuclear quadrupole resonance study of copper in Ba2YCu

27 Al and 69 Ga impurity nuclear magnetic resonance in ZnO:Al and ZnO:Ga