Visualizing the Energy-gap Modulations of the Cuprate Pair Density Wave State

Abstract

When Cooper pairs are formed with finite center-of-mass momentum, the defining characteristic is a spatially modulating superconducting energy gap $\Delta (r)$. Recently, this concept has been generalized to the pair density wave (PDW) state predicted to exist in a variety of strongly correlated electronic materials such as the cuprates. Although the signature of a cuprate PDW has been detected in Cooper-pair tunnelling, the distinctive signature in single-electron tunneling of a periodic $\Delta (r)$ modulation has never been observed. Here, using a new approach, we discover strong $\Delta (r)$ modulations in Bi${}_2$Sr${}_2$CaCu${}_2$O${}_8$+$\delta$ that have eight-unit-cell periodicity or wavevectors $Q=2\pi /a_0(1/8,0)$; $2\pi /a_0(0,1/8)$. This constitutes the first energy-resolved spectroscopic evidence for the cuprate PDW state. An analysis of spatial arrangements of $\Delta (r)$ modulations then reveals that this PDW is predominantly unidirectional, but with an arrangement of nanoscale domains indicative of a vestigial PDW. Simultaneous imaging of the local-density-of-states $N(r,E)$ reveals electronic modulations with wavevectors $Q$ and $2Q$, as anticipated when the PDW coexists with superconductivity. Finally, by visualizing the topological defects in these $N(r,E)$ density waves at $2Q$, we discover them to be concentrated in areas where the PDW spatial phase changes by $\pi$, as predicted by the theory of half-vortices in a PDW state. Overall, this is a compelling demonstration, from multiple single-electron signatures, of a PDW state coexisting with superconductivity at zero magnetic field, in the canonical cuprate Bi${}_2$Sr${}_2$CaCu${}_2$O${}_8$+$\delta$.

Final submitted version. arXiv admin note: text overlap with arXiv:2109.14031

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