Turning up the heat on superconductors
In 1911, Heike Kamerlingh Onnes, a Dutch physicist, cooled mercury to the temperature of liquid helium, just a few degrees above absolute zero (around – 270 degrees Celsius). Something amazing was observed: its resistance to current flow amazingly disappeared at 4.2 Kelvin. Since then, a number of materials, for instance cuprates, magnesium diboride, iron pnictides, have been discovered that become superconducting at higher temperatures. Nevertheless, even the material with the highest critical temperature (Tc) 138 Kelvin, corresponding to around – 135 degrees Celsius, is still far from room temperature (+20 Celsius) conductivity. EU-funded scientists launched the 'Search for novel mechanisms to increase the critical temperature of a superconductor' (NANOHIGHTC) project to deepen understanding leading to the controlled design of new, enhanced nano-structured superconductors. The project surpassed its original objectives, leading to 22 publications in esteemed peer-reviewed scientific journals and numerous international collaborations. Studies of finite size effects in various types of superconductors led to a quantitative formalism describing them in the limit in which mean field theory is applicable. Mean field theory essentially reduces a many-body problem to a one-body problem through averaging of particle-particle interactions. The team showed that an increase of up to 30 % in the Tc is possible in some materials. Further research helped develop a framework describing deviations from mean field. Highlights of results include identification of the minimum grain size for superconductivity to exist as a function of coupling strength in strongly coupled nano-scale superconductors. A second line of research investigated non-equilibrium dynamics and thermalisation, the process by which particles reach thermal equilibrium or uniform temperature. An interacting quantum system should eventually reach thermal equilibrium following a non-equilibrium perturbation (quantum quench). However this is not always observed. The team identified a parameter window in which the system does not thermalise after a quench and also described a novel route to thermalisation. NANOHIGHTC successfully developed new routes to enhanced superconductivity and increased Tc. Despite the enormous interest generated by superconducting materials, detailed theoretical understanding of mechanisms has proven a barrier to greater development and industrialisation. Scientists have begun to break down that barrier, paving the way to novel materials and applications in the near future.
Keywords
Superconductors, critical temperature, mean field theory, thermalisation
