How do superconductors break time-reversal symmetry?

Superconductors are one of the most exciting materials discovered in the last century. It is used to build medical tomographs, crucial details of particle accelerators and quantum computers. However, even a hundred years after the discovery of superconductivity we understand its microscopic mechanisms only in the simplest cases.

The European Research Council (ERC) has awarded a competitive 2.5 million euros advanced research grant to the principal investigator Girsh Blumberg and a team of researchers at the national institute KBFI, Tallinn, Estonia. The grant ‘How do superconductors break time-reversal symmetry?’ is to develop a unique instrument to study microscopic properties of superconductors. The work builds on Blumberg’s expertise in studying strongly correlated electron systems and the competence of the team in the field of terahertz spectroscopy and low temperature physics.

Our everyday experience shows that the past and future are not the same — they are not ‘symmetric’ say the physicists. The second law of thermodynamics explains the asymmetry between the past and future. In contrast, most laws of physics satisfy time-symmetry: such include Newton’s laws from school physics, but also the laws of gravity and quantum mechanics. For example, time-reversal symmetry implies that the equations of motion do not contain a direction of time. Thus, the physicists say that the time-reversal symmetry is equivalent with the motion-reversed symmetry. However, if one puts an electric wire to the magnetic field, one discovers that the reversed motion of an electron in the wire breaks Newton’s laws. One can conclude that the motion-reversed symmetry is broken and so the time-reversed symmetry. Therefore, the appearance of spontaneous magnetic fields is often taken as a signature of broken time-reversal symmetry.

“This research focuses on the microscopic symmetries of superconductors. We really hope to fill a gap in our knowledge that hinders us from building new devices including quantum computers,” said Girsh Blumberg. He explained that conventional superconductors are robust diamagnets: materials that expel magnetic fields. It would therefore be highly unexpected if a superconducting material would support spontaneous magnetic fields. Nevertheless, such spontaneously broken time-reversal symmetry states have been suggested for unconventional superconductors, but their identification remains experimentally controversial. Particularly interesting are unconventional superconductors for which the superconducting state is protected topologically and vortices of the supercurrent can host unconventional particles (Majorana fermions) with potential use in quantum computing applications.

“The ‘smoking gun’ experiments (could confirm time-reversal symmetry breaking) are experiments which are extremely sensitive to a very tiny magnetization,” said a team member of the grant, Urmas Nagel. “I am really excited we can start to build very novel probes here, at our institute KBFI.”

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