Imaging half-unit-cell Cooper-pair density waves in monolayer 1T$^{\prime}$-MoTe$_2$

TL;DR

This study visualizes half-unit-cell modulations of the superconducting gap in monolayer 1T'-MoTe2, revealing complex symmetry-breaking states linked to unconventional pairing mechanisms in a two-dimensional superconductor.

Contribution

First direct observation of half-unit-cell Cooper-pair density wave modulations in monolayer 1T'-MoTe2 using spectroscopic-imaging STM, highlighting their coexistence with larger-scale pair density waves.

Findings

Half-unit-cell modulations of the superconducting gap were identified.

Modulations are oriented perpendicular to zigzag Mo chains.

Superconductivity modulations are linked to unconventional electron pairing.

Abstract

Unconventional superconductors that spontaneously break space-group symmetries of their underlying crystal lattice are distinguished by spatial modulations of the superconducting order parameter. These states have recently captured significant attention in various strongly correlated materials, where the breaking of translational or intra-unit-cell symmetries results in the emergence of pair density waves with wavelengths extending across one or multiple unit cells. Here, we employ a spectroscopic-imaging scanning tunneling microscope to identify robust half-unit-cell modulations of the superconducting gap magnitude, coherence strength and subgap states in tunable 1T$^{\prime}$-MoTe$_2$ monolayer. The modulations are oriented perpendicular to the zigzag Mo chains and coexist with three-unit-cell primary pair density wave modulations along the Mo chains, both of which attenuate with increasing temperature and the application of external magnetic fields. Importantly, we find that the superconductivity modulations are strongly linked to unconventional electron pairing mechanisms, which significantly deviate from the Cooper pairing observed in traditional Bardeen-Cooper-Schrieffer superconductors. Our results advance the knowledge of Cooper-pair density modulations and the intricate interplay with other symmetry-breaking states in strongly correlated superconductors.