Superconductivity reveals its secrets
The electronic properties in high-temperature superconductors imply the absence of conventional Fermi liquid behaviour — the standard model of electrons in metals. However, copper oxides (cuprates) approach conventional metallic behaviour at high concentrations of holes. Holes, namely empty spots with positive charge are left in the crystal lattice of semiconductors when electrons are freed with increasing temperature. Recently, researchers working on the 'Unconventional superconductors: from synthesis to understanding' (USSU) project studied the anomalous properties of several cuprate compounds. There are different phases of these materials, including insulating, anomalous metallic, superconducting and normal metallic states. To understand how these phases arise and coexist would allow to elucidate the underlying physics that give rise to them. This was the key impetus which motivated the USSU project team to understand the phase diagram of cuprates. One of the main challenges that they faced was the fact that there are only a few compounds that can be chemically doped through all phases. Another challenge lay in understanding the details and importance of the 'pseudogap' on the underdoped side of the superconducting phase. Much effort was spent on understanding whether this phase promotes superconductivity. The USSU study focused on hole-doped cuprates because they exhibit the highest transition temperature. The Fermi liquid-like behaviour of high-quality crystals of Hg1201 (HgBa2CuO4+delta;) was perhaps the most ground-breaking discovery. The researchers concluded that even more structurally complex cuprates, like Y123 (YBa2Cu3O7-x) and YBa2Cu4O8 (Y124) and Tl2Ba2CuO6 (Tl-2201), are in effect nodal Fermi liquids. When the pseudogap was removed, comparisons of their anomalous properties to those of their hole-doped counterparts enabled common traits to be identified. AS well as traits not associated with the pseudogap, the USSU team demonstrated experimentally resistance was dependent on temperature. The Fermi liquid-like normal metal state resistivity that surrounds the superconducting phase was also experimentally confirmed. With these experiments, the USSU researchers discovered a striking quantum scaling of the physical properties with temperature and hole concentration. This is not seen in normal metals due to the upper energy limit of the electron system — the Fermi energy. It is also a telltale sign for a dramatic shift in the phase diagram of several other cuprates. Other research areas as well as superconductors' applications may benefit from the project’s findings. A deeper understanding of high-temperature superconductivity in cuprates has the potential to enable crystallographic engineering of new compounds. New compounds could harbour higher transition temperatures useful for power, electronic and communications applications at room temperature.
Keywords
Superconductor, electricity, cuprates, Fermi liquid, electrons, holes, crystal, electronics, communications