Probing Unconventional Superconductivity Using Synchrotron Radiation

The phenomenon of superconductivity is characterized by the complete loss of electrical resistivity and expulsion of magnetic field below a characteristic temperature, Tc. Superconductivity was first discovered by Kamerlingh Onnes and associates in 1911, enabled by the capability to cool down by the liquefaction of helium—a prime example of how new scientific discoveries are made by the advancement of experimental techniques. The microscopic mechanism for this phenomenon took a few decades to formulate and, by 1957, the Bardeen–Cooper–Schrieffer (BCS) theory had been developed, where superconductivity is understood to arise from the pairing of electrons into Cooper pairs mediated by electron-phonon coupling. While most elements on the periodic table become superconductors in one form or another, making the phenomenology of superconductivity more common than we realize, the Tc for almost all of them are only a few Kelvins. These materials later became known as conventional superconductors, in which superconductivity can be accounted for by the BCS theory. A major breakthrough in the field came in 1986 with the discovery of high-temperature superconductivity in copper oxides (a.k.a. cuprates), whose Tcs surpassed liquid nitrogen temperatures. Two aspects of the cuprates quickly emerged that set them apart from previous studies and were to become recurring: (1) the superconducting pairing temperature being too high to be accounted for by electron-phonon coupling in the BCS formalism; and (2) superconductivity appearing in close proximity to other symmetry-breaking electronic phases. It was soon clear that a new theory beyond BCS was needed to explain the pairing of this new type of unconventional superconductivity. In the years that followed, as the ever-expanding puzzles in the cuprates drew the attention of a large portion of the condensed matter physics community, experimental techniques based on synchrotron radiation were utilized to study a variety of aspects of the cuprates’ unconventional superconductivity. At the same time, in a beneficial cycle, the cuprates problem also fueled some of the development and expansion of techniques at synchrotron facilities, paving the way for future investigations of unconventional superconductivity beyond the cuprates. In 2008, two decades after the discovery of cuprate superconductors, a new class of unconventional superconductor was discovered, amongst a large material family, all containing iron. These became known as the iron-based superconductors (FeSCs). Benefiting from all the technical advancements that had already been successfully applied to the cuprate puzzle, the mature techniques helped facilitate a rapid development of the understanding of the FeSCs. Concurrently, their multi-orbital nature and their ubiquitous nematic phases have also driven researchers at synchrotrons to extend new capabilities at beamlines. In this special issue, we collect contributions from research groups to give perspectives of how synchrotron radiation techniques have made and keep making important contributions to the understanding of various classes of unconventional superconductors, including the cuprates, FeSCs, nickel-based superconductors, and heavy fermion superconductors. Techniques such as resonance photoemission spectroscopy (RPES), angle-resolved photoemission spectroscopy (ARPES), resonance elastic and inelastic X-ray scattering (REXS/ RIXS), inelastic X-ray scattering (IXS), and thermal diffuse scattering (TDS) have been intensely used to elucidate important aspects, such as the superconducting pairing symmetry, the lattice dynamics, the nature of the pseudogap phase in the cuprates, and the order parameters of competing intertwined phases in Synchrotron Radiation News ISSN 0894-0886 is published bi-monthly. Coden Code: SRN EFR