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Because the CuO2 planes are the universal element in superconductors with Tc above ~40 K, understanding their fundamental properties is central to the problem of understanding the origin of high Tc. Here we discuss some of the contributions made by infrared measurements in this area. We emphasize reflectivity measurements of single domain Y1Ba2Cu3O7 crystals, where one can distinguish between chain and plane contributions to the infrared conductivity. For the CuO2 planes at low temperature σ 1s(ω) is very small (~0) up to roughly 500 cm-1 (8kTc), where it rises rapidly, suggesting a gap of width 8kTc in the in-plane electronic-excitation spectrum. In the normal state σ 1(ω) drops unusually slowly with ω; the ~1/ω dependence of σ 1(ω) can he directly related to the ~1/T dependence of σ dc(i.c, to the linear T resistivity), as well as to other spectroscopic properties. (Describing the conductivity in terms of a scattering rate which is linear in both frequency and temperature can help elucidate these relationships.) Examining the temperature dependence of σ(ω) in the superconducting state, one finds that the gap does not collapse as T→Tc as expected in BCS theory, but instead fills-in in a highly unconventional manner. Relationships between the infrared conductivity, the penetration depth and nuclear relaxation rates suggest a phenomenology for Y1Ba2Cu3O7 which is fundamentally different from that of BCS superconductors.

The structural, superconducting and normal-state properties of La2MO4+δ (M=Cu, Ni, Co) were studied by neutron powder diffraction, thermogravimetric analysis and resistivity measurements. La2CuO4+δ was found to undergo phase separation near room temperature into antiferromagnetic La2CuO4 and superconducting La2CuO4.08. Single-phase, metallic, superconducting La2CuO4.08 was prepared under 25 kbar of O2 at 500 °C. The excess oxygen present as O= ions provides a doping mechanism analogous to alkaline-earth metal ion substitution.

Although the discovery of heavy fermion behavior in uranium compounds has attracted a great deal of attention, relatively little work has been done which is sufficiently systematic to allow an assessment of the relationship of such behavior to more common phenomena, such as mixed valence, narrow-band effects, etc. In this paper we report bulk property measurements for a number of alloys which form a part of such a systematic study. The approach has been to take relatively simple and well-understood materials and alter their behavior by alloying to produce heavy fermion or Kondo behavior in a controlled way.

There have been a number of studies of the critical fields and critical currents in the two copper oxide superconducting families La2−xSrxCuO4 and YBa2Cu3O7−δ which show unexpected and unusual properties. Resistive measurements of the superconducting transition in zero and finite field often show long tails extending to low temperatures, and the critical field curves constructed from such data often show unusual upward curvature, especially near Tc. Initially most of the critical field studies were done on polycrystalline sintered samples. With such samples there is always a lingering question as to whether the measured properties are intrinsic to the material or are due to poor connections between particles or to the averaging of strongly anisotropic behavior over many crystallite orientations. With the advent of measurements on single crystal samples in several laboratories it is now possible to compare measurements on polycrystals and single crystals to determine the intrinsic behavior and to assess the degree of sample dependence which cannot be attributed to particle connections or to anisotropy. In this review we characterize the typical behavior of polycrystalline samples and compare it to results on single crystals. We find that much of the behavior seen in polycrystals also appears in single crystals and that among single crystal samples there is significant sample dependence. Nevertheless, there is sufficient agreement on the upper critical field values to define approximately the slope at Tc, from which the coherence length can be derived. We combine this information with new measurements of the lower critical field at low temperature to derive the anisotropic magnetic penetration depth. Finally, we compare the derived characteristic length with relevant unit cell dimensions and with the scale of the microstructure to infer information about the dimensionality of the superconductivity and the pinning forces in YBa2Cu3O7−δ.

Magnetization measurements were performed on single crystals of YBa2Cu3O7-δ to study the effect of fast neutron irradiation on flux pinning ana the critical magnetization current. Neutron irradiation up to a fluence of 8. 16×1017 cm−2 (E > 0. 1 MeV) always enhances the magnitude of critical magnetization currents significantly, while the transition temperatures decrease at a rate of 2. 6 K per 1018 neutrons cm−2. The temperature and field direction dependence of these enhancement factors are discussed in detail.

The crystal structure1 of YBa2Cu3O7-δ is composed of chains and planes of Cu-O bonds which are arranged in layers perpendicular to the c direction. The one-dimensional chains are oriented along the b direction and are located at the top and bottom of the unit cell shown in Fig. 1. The planes are composed of a corrugated two-dimensional network of Cu-O bonds oriented along the a and b directions and located near the center of the unit cell just above and below the Y site. The metallic character of YBa2Cu3O7-δ is provided by strong overlap between neighboring Cu and O ions in the chains and planes. There is almost no metallic character along the c direction because there are no oxygen atoms along the cell edges between the Cu2 sites. Thus the Cu-O plane just above the Y site does not overlap the plane just below the Y site. In the layer of chains there is virtually no metallic character in the a direction because there are no oxygen atoms between the Cul sites along a. The layer of chains and the layer of planes are connected by the bridging O4 site which overlaps both the neighboring Cul and Cu2 sites. However, this O4 is displaced considerably toward the Cul site so that it is bound much more strongly to the chains than to the planes. Thus, the coupling between the chains and planes is relatively weak, and the system may be thought of as composed of two nearly independent subsystems showing one- and two-dimensional character.2

Ternary compounds exhibiting superconductivity have been under intensive investigation in the last few years, and some of the related properties were reviewed in the last proceedings of this conference series by several authors.1–3 Among the more stimulating phenomena is the observation that many of these materials exhibit superconductivity even when one of the constituents is strongly magnetic, such as a rare earth (RE) ion. This is in sharp contrast to the behavior usually found in binary compounds, where very small concentrations of magnetic ions are adequate to totally suppress the superconducting transition. In the ternary materials, the presence of large concentrations of magnetic ions has also provoked renewed interest in questions concerning the ways in which superconductivity and magnetism affect one another, and whether the two types of order can simultaneously occur. Much of the work carried out in the past on the coexistence question utilized pseudobinary systems, and it is now generally believed that those experiments showed the coexistence of superconductivity with spin-glass type ordering, rather than with true, long-range magnetism.4 The ternary compounds, being structurally homogeneous, will not show that behavior, and so are much more ideally suited to stimulate and test theoretical predictions on the interaction between magnetism and superconductivity.

An important and interesting feature of the high temperature superconductor YBa2Cu3O7-δ is the dramatic effect of oxygen vacancies on its superconducting properties. 1–10 As oxygen is systematically removed from the structure, the orthorhombic distortion decreases and the system undergoes a structural transition to a tetragonal phase. Simultaneously, TC falls monotonically, with superconductivity disappearing at approximately the stoichiometry where the orthorhombic to tetragonal structural transition occurs. The curve of TC versus δ shows two regions of stable superconductivity where TC is not a strong function of δ. For δ near zero, there is a nearly flat region where TC is close to 90 K and for.35 < δ