Energy gaps in high-transition temperature cuprate superconductors

摘要

The spectral energy gap is an important signature that defines states ofquantum matter: insulators, density waves, and superconductors have verydifferent gap structures. The momentum resolved nature of angle-resolvedphotoemission spectroscopy (ARPES) makes it a powerful tool to characterizespectral gaps. ARPES has been instrumental in establishing the anisotropicd-wave structure of the superconducting gap in high-transition temperature (Tc)cuprates, which is different from the conventional isotropic s-wavesuperconducting gap. Shortly afterwards, ARPES demonstrated that an anomalousgap above Tc, often termed the pseudogap, follows a similar anisotropy. Thenature of this poorly understood pseudogap and its relationship withsuperconductivity has since become the focal point of research in the field. Toaddress this issue, the momentum, temperature, doping, and materials dependenceof spectral gaps have been extensively examined with significantly improvedinstrumentation and carefully matched experiments in recent years. This articleoverviews the current understanding and unresolved issues of the basicphenomenology of gap hierarchy. We show how ARPES has been sensitive to phasetransitions, has distinguished between orders having distinct broken electronicsymmetries, and has uncovered rich momentum and temperature dependentfingerprints reflecting an intertwined & competing relationship between theordered states and superconductivity that results in multiplephenomenologically-distinct ground states inside the superconducting dome.These results provide us with microscopic insights into the cuprate phasediagram.