Abstract
The enigmatic pseudogap phase in underdoped cuprate high- superconductors has long been recognized as a central puzzle of the problem. Recent data show that the pseudogap is likely a distinct phase, characterized by a medium range and quasistatic charge ordering. However, the origin of the ordering wave vector and the mechanism of the charge order is unknown. At the same time, earlier data show that precursive superconducting fluctuations are also associated with this phase. We propose that the pseudogap phase is a novel pairing state where electrons on the same side of the Fermi surface are paired, in strong contrast with conventional Bardeen-Cooper-Schrieffer theory which pairs electrons on opposite sides of the Fermi surface. In this state the Cooper pair carries a net momentum and belongs to a general class called pair density wave. The microscopic pairing mechanism comes from a gauge theory formulation of the resonating valence bond (RVB) picture, where spinons traveling in the same direction feel an attractive force in analogy with Ampere’s effects in electromagnetism. We call this Amperean pairing. Charge order automatically appears as a subsidiary order parameter even when long-range pair order is destroyed by phase fluctuations. Our theory gives a prediction of the ordering wave vector which is in good agreement with experiment. Furthermore, the quasiparticle spectrum from our model explains many of the unusual features reported in photoemission experiments. The Fermi arc, the unusual way the tip of the arc terminates, and the relation of the spanning vector of the arc tips to the charge ordering wave vector also come out naturally. Finally, we propose an experiment that can directly test the notion of Amperean pairing.
- Received 17 April 2014
DOI:https://doi.org/10.1103/PhysRevX.4.031017
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Published by the American Physical Society
Popular Summary
Since the discovery of high-temperature superconductivity in cuprates in 1986, significant effort has been expended to understand what is special about these materials. Besides superconductivity, these materials behave in a very strange way under certain doping conditions and exhibit a phenomenon called “pseudogap” behavior that is associated with the onset of a charge-density wave. There is no agreement as to what causes this behavior, and there have been debates about whether it is caused by superconducting fluctuations up to temperatures as high as 180 K or whether the pseudogap is the result of some unknown competing phase. We propose that in some sense, both causes are correct: The pseudogap phase is a competing phase that is a different kind of fluctuating superconductor.
Unlike conventional superconductors, this novel superconductor contains Cooper pairs that are formed by electrons on the same side of the Fermi surface. The pairing arises because spinons traveling in the same direction feel an attractive force. As a result of this arrangement, which we call “Amperean pairing,” the pairing state carries a finite momentum. We show that this state, previously developed for quantum spin liquids, explains many of the features of the pseudogap state seen in photoemission experiments that have remained mysterious up until now. We propose new experiments to test this idea, and we also speculate that it may be possible to stabilize this Amperean pair state to form a true superconducting state with an even higher transition temperature than what is achievable today.
Our studies lay the groundwork for explaining the pseudogap state in cuprates and synthesizing new superconducting materials with higher transition temperatures.
