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High-Tc Cuprates


As previously mentioned, high-Tc cuprate superconductors were discovered in 1986. They are layered materials (come in sheets), and are characterized by the presence of copper-oxygen planes which alternate with other layers consisting of a wide variety of atoms. These "other" layers usually bond well to the copper-oxygen (CuO) planes, but are much more weakly bonded to themselves. Hence, when these samples are cleaved (mechanically broken), they will often break along the planes, but more specifically, between two non-CuO planes.

It is currently still difficult to generalize between different species of cuprate superconductors, but nevertheless, scientists typically draw phase diagrams to try to communicate and understand the physical phenomena associated with these substances. A version of the most common phase diagram is shown at the right. The antiferromagnetic regime (a Mott insulator phase) is located on the left of the diagram, at zero to very low hole doping and at moderately low temperatures. The d-wave superconducting regime (or "dome") begins at lower temperatures but does not exist for regions which have too high or too low doping. This is the only regime where superconductivity occurs. The so-called "pseudogap" phase exists between the antiferromagnetic regime and the superconducting dome. At high doping, samples form normal metals (Fermi liquids), although perhaps not the same kind of phase as a conventional metal like copper. And finally, at the top of the phase diagram, for high temperatures, we usually have a "strange" metal for which there is metallic behavior, but probably not in the well-known Fermi liquid sense. I will speak about the antiferromagnetic and the "pseudogap" phases briefly later.

What is important to understand here is that the phase diagram is only a cartoon. The points at which the onset of one phase begin (such as, at zero temperature, the onset of superconductivity) and end as well as the positions and shapes of the phase boundaries themselves will change from cuprate species to cuprates species. Sometimes, this entire diagram is scaled wildly with doping, as the optimal doping of some cuprates is quite different than others. Moreover, there is still much debate over the position, shape, and even existence of the line separating the two metal phases and the line between the metal and "pseudogap" phase. (This is why I drew them broken.) These broken lines could cross, could fall down to a point with temperature (which could lie on the superconducting dome or not), could never cross, could actually become a continuation of the superconducting lines, or a variety of other possibilities. Despite all of these problems and complications, the key features of the phase diagram remain, and it continues to be a preferred tool for at least visualizing the parameters of a particular sample or experiment and explaining key concepts about the cuprates.

A few types of cuprate superconductors common to physicists and materials scientists
Species Optimum Tc Experimental Pros Experimental Cons
La1.85Ba.15CuO4 30K
YBa2Cu3O7 92K
Bi2Sr2CaCu2O8 92K Cleaves cleanly. Does not cleave well all the time.
Ca2-xNaxCuO2Cl 25K Cleaves cleanly, with a high success rate. Difficult to grow: requires super high pressures. Moisture ruins samples in about 1 minute.
Next: Mott Insulators