Nature Physics

ARPES measurements of the ‘failed’ superconductor LBCO-1/8 suggest that its pseudogap phase consists of two distinct components. The result could be an important clue into the nature of this phase in the copper oxide superconductors. A central issue in high-Tc superconductivity is the nature of the normal-state gap (pseudogap)1 in the underdoped regime and its relationship with superconductivity. Despite persistent efforts, theoretical ideas for the pseudogap evolve around fluctuating superconductivity2, competing order3,4,5,6,7,8 and spectral weight suppression due to many-body effects9. Recently, although some experiments in the superconducting state indicate a distinction between the superconducting gap and pseudogap 10,11,12,13,14, others in the normal state, either by extrapolation from high-temperature data15 or directly from La1.875Ba0.125CuO4 (LBCO-1/8) at low temperature16, suggest the ground-state pseudogap is a single gap of d-wave17 form. Here, we report angle-resolved photoemission data from LBCO-1/8, collected with improved experimental conditions, that reveal the ground-state pseudogap has a pronounced deviation from the simple d-wave form. It contains two distinct components: a d-wave component within an extended region around the node and the other abruptly enhanced close to the antinode, pointing to a dual nature of the pseudogap in this failed high-Tc superconductor that involves a possible precursor-pairing energy scale around the node and another of different but unknown origin near the antinode.

Superconducting phase fluctuations are often associated with the pseudogap phase of the copper-oxide superconductors. However, the same fluctuations exist in the overdoped part of the phase diagram where the pseudogap is absent, suggesting that phase fluctuations are independent of the pseudogap In underdoped cuprate superconductors, phase stiffness is low and long-range superconducting order is destroyed readily by thermally generated vortices (and anti-vortices), giving rise to a broad temperature regime above the zero-resistive state in which the superconducting phase is incoherent1,2,3,4. It has often been suggested that these vortex-like excitations are related to the normal-state pseudogap or some interaction between the pseudogap state and the superconducting state5,6,7,8,9,10. However, to elucidate the precise relationship between the pseudogap and superconductivity, it is important to establish whether this broad phase-fluctuation regime vanishes, along with the pseudogap11, in the slightly overdoped region of the phase diagram where the superfluid pair density and correlation energy are both maximal12. Here we show, by tracking the restoration of the normal-state magnetoresistance in overdoped La2−xSrxCuO4, that the phase-fluctuation regime remains broad across the entire superconducting composition range. The universal low phase stiffness is shown to be correlated with a low superfluid density1, a characteristic of both underdoped and overdoped cuprates12,13,14. The formation of the pseudogap, by inference, is therefore both independent of and distinct from superconductivity.