"OPTICAL TESTS FOR THE TIME REVERSAL SYMMETRY BREAKING IN UNCONVENTIONAL SUPERCONDUCTORS"

By Aharon Kapitulnik (Stanford University, Stanford, USA)

BIOGRAPHY:

Aharon Kapitulnik is the Theodore and Sydney Rosenberg Professor in Applied Physics at the Departments of Applied Physics and Physics at Stanford University. He received both, his undergraduate (1978) and graduate (1983) degrees from Tel-Aviv University in Israel.  Following a post-doctoral and a brief Assistant Professor in Residence positions at UCSB, he joined Stanford University in 1985. Kapitulnik’s research focuses on experimental condensed matter physics, while opportunistically, also apply his methods to tabletop experimental studies of fundamental phenomena in physics. His recent studies cover a broad spectrum of phenomena associated with the behaviour of correlated and disordered electron systems, particularly in reduced dimensions, and the development of effective instrumentation to detect subtle signatures of physical phenomena. Among other recognitions, his activities earned him the Alfred P. Sloan Fellowship (1986-90), a Presidential Young Investigator Award (1987-92), a Sackler Scholar at Tel-Aviv University (2006), the Heike Kamerlingh Onnes Prize for Superconductivity Experiment (2009), a RTRA (Le Triangle de la Physique) Senior Chair (2010), and the Oliver Buckley Condensed Matter Prize of the American Physical Society (2015). Aharon Kapitulnik is a Fellow of the American Physical Society, a Fellow of the American Association for the Advancement of Science (AAAS), a Fellow of the American Academy of Arts and Sciences, and a member of the National Academy of Sciences.

ABSTRACT:

The search for a material platform for topological quantum computation has recently focused on unconventional superconductors, particularly those that exhibit time reversal symmetry breaking (TRSB) and thus are capable of hosting novel phenomena, including emergent Majorana quasiparticles.  A natural place to search for chiral superconductors is among superconductors in which the pair wavefunction possesses internal degrees of freedom. Where two- (or higher) dimensional representations are allowed, the order parameter may be composed of more than one component, which can be in phase or out-of-phase with each other, hence acquiring an imaginary component that is responsible for the TRSB superconducting state. In some cases, two symmetry-distinct ordering tendencies, instead of competing, will appear together as parts of a multi-component composite order parameter. While such a scenario may be attributed to “accidental” near-degeneracy, specific examples may indicate a secondary ordering field that couples to the superconducting order parameters in a way that stabilizes the multi-component composite state. Understanding the emergence of multi-component superconductors is thus a key theoretical and experimental challenge, which bears on the more general understanding of complex condensed matter quantum systems.

In this talk we will discuss recent data on several unconventional, topological superconductors where the unique crystal structure, intrinsic magnetism, and/or topological effects result in a unique superconducting state that exhibits TRSB. The main experimental tool is the Zero-area Sagnac interferometer that we developed in our laboratory for the past fifteen years and is able to measure polar Kerr effect (PKE) with nanoradians resolution. Besides its exquisite sensitivity, an important feature of this apparatus is the ability to reject (by symmetry) all reciprocal effects. Starting with UPt3, we show that the onset of PKE below a temperature T_{Kerr} that coincides with the low temperature B-phase superconducting transition temperature T_{c-}~480mK. In contrast, no change in Kerr effect is observed through either the high temperature A-phase superconducting transition at T_{c+}∼550mK. These results indicate that TRS is broken only in the B-phase. We continue our discussion with results on the nearly ferromagnetic compound UTe2, where TRSB was inferred from observations of a spontaneous Kerr response in the superconducting state after cooling in zero magnetic field, while a finite c-axis magnetic field training was further used to determine the nature of the non-unitary composite order-parameter of this material.