# Detecting hidden Cooper pair symmetries

When two electrons attract instead of their usual tendency to repel each other, they form composite entities known as Cooper pairs. Cooper pairs form the building blocks of superconductors and hence understanding their properties both theoretically and experimentally is essential in finding new superconductors with novel functionalities. Depending on the degrees of freedom of constituent electrons like their spin, orbital and time symmetries, the Cooper pairs give rise to different forms of superconductors. While experimental probes exist for determining the spin or orbital symmetries of a Cooper pair, a direct signature of the time symmetry of Cooper pairs has been lacking. We show that a commonly used experimental tool namely scanning tunneling microscopy can be used to directly detect the time symmetry of Cooper pairs. In particular, we show that the quantum interference patterns generated in a superconductor due to the omnipresence of impurities have distinct features due to the time, or equivalently frequency symmetry of Cooper pairs. The proposed method is shown to be the only direct probe for the time symmetry of odd frequency Cooper pairs which are odd in time or frequency and are long believed to be hidden.

The article was published in Physical Review Letters and also selected as an Editors’ Suggestion.

## Publication

Debmalya Chakraborty and Annica M. Black-Schaffer (2022), Quasiparticle Interference as a Direct Experimental Probe of Bulk Odd-Frequency Superconducting Pairing, Phys. Rev. Lett., Published 7 December 2022

DOI: https://doi.org/10.1103/PhysRevLett.129.247001

Synopsis in the *Physics Magazine* of the American Physical Society (APS).