High-temperature Superconductivity

Groundbreaking findings on high-temperature superconductivity. A superconductor is a material that can conduct electricity with virtually no resistance. The phenomenon was discovered by Kamerlingh Onnes in 1911 and takes place in certain metals such as mercury. In 1986, so called "high-temperature" superconductivity was discovered (i.e. superconductivity at temperatures up to 130 K (-143°C), which is considerably higher than the previous limit of 30 K, the temperature at which it was possible to obtain low-temperature superconductivity. This is a characteristic of modified copper oxides known as cuprates. Until very recently, it was believed that both types of superconductivity had fundamentally different mechanisms. However, researchers from Cornell University (US) have found evidence that the mechanisms occurring in both types of superconductor could be much closer than previously thought. Low-temperature superconductivity was previously explained by the "BCS" theory, developed in 1957 by John Bardeen, Leon Cooper and John Schrieffer according to which electrons with the opposite spin could become paired thanks to the interaction of the electrons with "phonons" i.e. the quantized vibrations in the rigid crystal lattice. Roughly speaking, once paired, the two electrons are then held together with a certain binding energy. At low temperatures this energy is higher than the energy provided by the oscillating atoms in the conductor, thus allowing the electron pair to stick together and overcome electrical resistance due to the Coulomb repulsion. However, the BCS theory could not explain the persistence of superconductivity at higher temperatures. The main hypothesis was that electron pairing in cuprates was due to magnetic interactions, but the Cornell researchers led by Seamus Davis found that in a high-temperature superconductor the distribution of electrons shared common features with the distribution of the phonons. According to Prof. Seamus Davis, this could not prove that the electron-phonon interaction was involved in the pairing, however it proved that the interaction itself could not be ignored. Other experiments provide additional evidence for the previously unsuspected continuity between both types of superconductivity.