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 from the National Institute of Chemical Physics and Biophysics in Tallinn, Estonia (NICPB) to develop the instrumentation that would enable a study “How do superconductors break time-reversal symmetry?” The work in Tallinn builds on PI-s expertise in studying strongly correlated electron systems and on the expertise of NICPB in the field of terahertz spectroscopy and low temperature physics.
Superconductors are used to build magnets for MRI machines and quantum computers, but even 109 years after the discovery of superconductivity we understand its microscopic mechanisms only in the simplest cases. This research focuses on studying the basic symmetries of superconductors in order to reveal new properties that could prove useful for building new devices including quantum computers. The symmetries observed in nature give rise to conservation laws and the properties of particles. Among the most important of these symmetries is time-reversal – breaking this symmetry leads to a variety of physical effects in condensed matter physics, especially in superconductors.
Our everyday experience shows that past and future are not symmetric: we cannot predict the future just based on our experience from the past! This is captured by the time-asymmetry of the second law of thermodynamics, which says that the entropy always increases.
In contrast, most laws of physics satisfy time-symmetry: such include Newton’s laws, Einstein’s laws, and the basic laws of quantum mechanics. Time-reversal symmetry implies that the equations of motion do not inherently contain a direction for time. Time-reversal symmetry is the motion-reversed symmetry. However, in a system with a magnetic field, the reversed motion of an electron breaks the Newton’s laws, and thus we call the time-reversal symmetry (motion-reversed symmetry) broken. Therefore, the appearance of spontaneous magnetic field is often taken as a signature of broken time-reversal symmetry.
In the context of condensed matter physics, time-reversal symmetry breaking usually implies something that behaves like a magnetic field. Conventional superconductors (lossless conductors of charge current) are robust diamagnets: materials that expel magnetic fields (through the Meissner effect). 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. For some unconventional superconductors, when these materials are being cooled into the superconducting state, something like a spontaneous magnetization appears. 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. However, in striking contrast to the unconventional A phase of superfluid 3He where broken rotational symmetry was directly observed, identification of broken time-reversal symmetry for the superconductors has presented a challenge. The ‘smoking gun’ experiments which could confirm time-reversal symmetry breaking are experiments which are sensitive to a very tiny magnetization. Such novel probes will be developed at NICPB.
Additional info:
Girš Blumberg, girsh.blumberg@kbfi.ee
Urmas Nagel, urmas.nagel@kbfi.ee, +372 5690 1425
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