Multiple charge density waves and superconductivity nucleation at antiphase domain walls in the nematic pnictide Ba$_{1-x}$Sr$_{x}$Ni$_{2}$As$_{2}$

TL;DR

This study reveals multiple charge density waves and their interaction with nematic fluctuations and superconductivity in Ba$_{1-x}$Sr$_{x}$Ni$_{2}$As$_{2}$, highlighting complex order coexistence and domain wall effects.

Contribution

It uncovers a new commensurate charge density wave and clarifies the symmetry of an incommensurate CDW, demonstrating the coexistence of three distinct CDWs in this material.

Findings

Discovery of a new commensurate CDW with period two lattice parameters.

Identification of the incommensurate CDW as a rotationally symmetric '4Q' state.

Heterogeneous superconductivity nucleates at antiphase domain walls in the CDW.

Abstract

How superconductivity interacts with charge or nematic order is one of the great unresolved issues at the center of research in quantum materials. Ba$_{1-x}$Sr$_{x}$Ni$_{2}$As$_{2}$ (BSNA) is a charge ordered pnictide superconductor recently shown to exhibit a six-fold enhancement of superconductivity due to nematic fluctuations near a quantum phase transition (at $x_c=0.7$). The superconductivity is, however, anomalous, with the resistive transition for $0.4 < x< x_c$ occurring at a higher temperature than the specific heat anomaly. Using x-ray scattering, we discovered a new charge density wave (CDW) in BSNA in this composition range. The CDW is commensurate with a period of two lattice parameters, and is distinct from the two CDWs previously reported in this material. We argue that the anomalous transport behavior arises from heterogeneous superconductivity nucleating at antiphase domain walls in this CDW. We also present new data on the incommensurate CDW, previously identified as being unidirectional, showing that is a rotationally symmetric, "4$Q$" state with $C_4$ symmetry. Our study establishes BSNA as a rare material containing three distinct CDWs, and an exciting testbed for studying coupling between CDW, nematic, and SC orders.