Visualizing uniform lattice-scale pair density wave in single-layer FeSe/SrTiO3 films

TL;DR

This paper reports the first direct observation of a uniform lattice-scale pair density wave in single-layer FeSe/SrTiO3 films, revealing a novel modulation of superconducting properties at the atomic scale.

Contribution

It demonstrates the existence of a lattice-scale PDW in FeSe/SrTiO3, driven by interfacial symmetry breaking, with implications for understanding unconventional superconductivity.

Findings

Spatial modulation of Cooper-pairing gap within a single unit-cell

Visualization of superfluid density variation via Josephson current

Coexistence of lattice-scale modulation with larger PDW order

Abstract

Typical BCS superconductors are microscopically homogeneous in real space governed by the coherent Cooper pairs with high phase stiffness of superfluid density, which is characterized by a coherence length. However, a periodic oscillation of superconducting order parameter may develop driven by breaking the time-reversal or translational invariance. To date, such modulated orders were specific to each material systems, with a periodicity much larger than the lattice constant. Here we report the direct observation of a uniform lattice-scale pair density wave (PDW) in single-layer FeSe/SrTiO3 films, enforced by peculiar interfacial structure of crystal symmetries breaking. Our spectroscopic imaging scanning tunneling microscopy unravels a spatial modulation of Cooper-pairing gap within a single unit-cell, depending on inequivalent atomic sites. Prominent periodic variation of superfluid density is visualized via Josephson current by a superconducting tip, indicating a real-space oscillation of phase stiffness. Such a lattice-scale superconducting modulation, which coexists with a larger length scale of PDW order, indicates the lattice-scale variation of both pairing strength and phase stiffness. Our findings provide new insights into the intertwined density-wave orders of quasiparticle character in correlated electronic systems, and provoke future studies on the unconventional pairing interaction and phase stiffness in the two-dimensional limit.