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

The case for dx2 − y2 pairing in the cuprate superconductors

There is a strong tendency for dilute holes in an antiferromagnet to phase separate. (This is a generic feature of doping into a commensurate correlated insulating state.) We review the general and model-specific theoretical arguments that support this conclusion for neutral holes. In the presence of long-range Coulomb interactions, there is frustrated phase separation leading to large-amplitude, low-energy fluctuations in the hole density at intermediate length scales, provided the dielectric constant is sufficiently large. We describe extensive experimental evidence showing that such “clumping” of the holes is an important feature of the cuprate superconductors. We also summarize theoretical results which suggest that frustrated phase separation may account for the anomalous properties of the normal state and give rise to high-temperature superconductivity.

Frustrated electronic phase separation and high-temperature superconductors

The review dwells on the problems of high-temperature superconductivity. Ideas encouraging the search for high-temperature superconductivity are elucidated. The way from cubical alloys, containing niobium, to laminated copper-containing oxides with the perovskite-type structure is shown. Properties of new laminated oxide superconductors are described.

Perovskite‐Type Oxides—the New Approach to High—Tc Superconductivity

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

Cu and Y NQR and NMR in the superconductor YBa2Cu3O7−δ

The frequencies νQ, the linewidth and the spin-lattice and spin-spin relaxation times T1 and T2, respectively, of the 63Cu nuclear quadrupole resonance (NQR) signals for chain and planar copper sites in YBa2Cu4O8 were measured between 6 and 750 K; in addition nuclear magnetic resonance (NMR) spectra were taken at 150 and 300 K. The results are compared with data known for the related structure YBa2Cu3O7. Above Tc the tensors of the electric field gradient and the anisotropic Knight shift at the copper sites differ in a subtle way from those of YBa2Cu3O7. At the Cu2 site, the derivative of νQ with respect to temperature changes its sign around 200 K. No indication for magnetic ordering at the Cu sites was found. Below Tc there is no evidence for BCS type relaxation and the Cu2 Tc rates vary like exp( T T 0 ) and the Cu1 rates like Tn with n≈1.35. Above Tc the Cu1 rates suggest the presence of quasiparticles (and phonon interactions) while the Cu2 rates can perhaps be explained by a model of Monien et al. postulating the coupling of Cu nuclei to itinerant interacting quasiparticles.

Copper NQR and NMR in the superconductor YBa2Cu4O8+x

Measurements of the temperature dependence of the anisotropy of the Knight shift and of the spin-lattice relaxation time T 1 of 63 Cu for double chain Cu1 and plane Cu2 sites in YBa 2 Cu 4 O 8 are presented. The T 1 anisotropy could be partly due to the presence of charge carriers. For B ‖ c, T 1 is field independent at high temperatures and becomes field dependent already 15 K above T c . Cu2 Knight shift data are extended up to 500 K.

Anisotropy of 63Cu knight shift and spin-lattice relaxation in YBa2Cu4O8

New measurements of the Knight shift tensor K at both Cu sites in the normal conducting state of the 82 K superconductor 1-2-4 are reported. The data are in accordance with a picture where at both Cu sites the Cu valence is close to 2+ and the hole on the plane site is quasi-localized. A significant temperature dependence at plane Cu site is found which is associated with temperature dependent antiferromagnetic fluctuations. Knight shift and spin-lattice relaxation times at Cu plane site are interpreted to be dominated by antiferromagnetic correlations with a temperature dependent correlation length.

H. Zimmermann

Papers

Cu and Y NQR and NMR in the superconductor YBa2Cu3O7−δ

Cu and Y NQR and NMR in the superconductor YBa2Cu3O7−δ

Copper NQR and NMR in the superconductor YBa2Cu4O8+x

Anisotropy of 63Cu knight shift and spin-lattice relaxation in YBa2Cu4O8

Spin susceptibility and spin dynamics at Cu sites in Y Ba2Cu4O8