Directly visualizing nematic superconductivity driven by the pair density wave in NbSe$_2$

TL;DR

This study uses scanning tunneling microscopy to reveal how pair density waves in NbSe2 interact with charge density waves, leading to nematic superconductivity characterized by broken rotational symmetry.

Contribution

It uncovers the correlation between PDW states and distinct CDW phases in NbSe2, demonstrating the emergence of nematic superconductivity driven by PDW.

Findings

PDWs coexist with CDWs in NbSe2 with a commensurate structure.

Superconducting gap size is in phase with charge density in both CDW regions.

Nematic superconductivity arises due to spontaneous rotational symmetry breaking in the AC-CDW region.

Abstract

Pair density wave (PDW) is a distinct superconducting state characterized by a periodic modulation of its order parameter in real space. Its intricate interplay with the charge density wave (CDW) state is a continuing topic of interest in condensed matter physics. While PDW states have been discovered in cuprates and other unconventional superconductors, the understanding of diverse PDWs and their interactions with different types of CDWs remains limited. Here, utilizing scanning tunneling microscopy, we unveil the subtle correlations between PDW ground states and two distinct CDW phases -- namely, anion-centered-CDW (AC-CDW) and hollow-centered-CDW (HC-CDW) -- in 2H-NbSe$_2$. In both CDW regions, we observe coexisting PDWs with a commensurate structure that aligns with the underlying CDW phase. The superconducting gap size, $\Delta(r)$, related to the pairing order parameter is in phase with the charge density in both CDW regions. Meanwhile, the coherence peak height, $H(r)$, qualitatively reflecting the electron-pair density, exhibits a phase difference of approximately $2\pi/3$ relative to the CDW. The three-fold rotational symmetry is preserved in the HC-CDW region but is spontaneously broken in the AC-CDW region due to the PDW state, leading to the emergence of nematic superconductivity.