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Odd-frequency superconductivity

This article reviews odd-frequency (odd-w) pairing with focus on superconducting systems. Since Berezinskii introduced the concept of odd frequency order in 1974 it has been viewed as an exotic and rarely occurring in nature. Here, we present a view that the Berezinskii state is in fact a ubiquitous superconducting order that is both non-local and odd in time. It appears under quite general circumstances including in bulk materials, heterostructures and dynamically driven superconducting states. It is therefore important to understand the nature of odd-w pairing. We present the properties of odd-w pairing in bulk materials, including possible microscopic mechanisms, discuss definitions of the odd-w superconducting order parameter, and the unusual Meissner response of odd-frequency superconductors. Next, we present how odd-w pairing is generated in hybrid structures of nearly any sort and focus on its relation to Andreev bound states, spin polarized Cooper pairs, and Majorana states. We overview how odd-w pairing can be applied to non-superconducting systems such as ultracold Fermi gases, Bose-Einstein condensates, and chiral spin-nematics. Due to the growing importance of dynamic o