Breaking a symmetry in a material usually means a phase transition. Now, Mikael Fogelström of Chalmers University of Technology in Goteborg, Sweden, have demonstrated the breaking of time-reversal symmetry in a class of high-temperature superconductors, a process observed when a material becomes magnetic. [Nature Physics, 2015; DOI: 10.1038/nphys3383]. The research could have implications for improving our understanding of how superconductivity arises and ultimately lead to the design of new materials.
It is common knowledge that conventional superconductors are strong diamagnets and exclude magnetic fields, a phenomenon - the Meissner effect - that gives rise to the well-known "levitation" effects of magnets placed above a superconductor. Given this fact, Fogelström and colleagues considered that it would be surprising if the superconducting ground state were itself able to support a spontaneous magnetic fields. Such behavior would represent a time-reversal symmetry-broken state, which has been proposed in high-temperature superconductors but not yet observed unequivocally in any experiment.
The Chalmers team has now demonstrated this effect in a d-wave superconducting state, which they accommodate experimental observations that are currently in conflict. The state is manifest as peculiar partial vortices forming a necklace-like pattern around the perimeter of the material, in which neighboring vortices have opposite current circulation.
Theoreticians had for many years mused on whether or not high-temperature superconductors, - a class of superconductors that become superconducting at relatively high temperatures - also break time-reversal symmetry and produce spontaneous magnetization. Electron transport experiments showed that this was indeed the case. But experiments undertaken to measure spontaneous magnetization showed no such effect. However, the Chalmers team's computational results suggest that there is an alternative route to breaking time-reversal symmetry in high-temperature superconductors. "We maintain that this has probably already been observed and that the two sets of experiments do not contradict one another," says co-investigator Tomas Löfwander.
Computational allowed the researchers to investigate cases where the ring of a superconducting crystal affects the force of the superconducting phase. The periodic necklace of vortices they found to form spontaneously below a critical temperature has a pitch on the scale of a few dozen nanometers, the team says. "We believe that new results with what are known as nanosquids, which are magnetometers with extremely good resolution, will be able to give immediate experimental verification of our results," concludes Mikael Fogelström.
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the bestselling science book "Deceived Wisdom".