Smectic Pair Density Wave Order in EuRbFe4As4

TL;DR

This paper provides evidence for a zero-field smectic pair density wave superconducting state in EuRbFe4As4, characterized by unidirectional modulations that vanish above the magnetic transition, distinct from other density wave orders.

Contribution

It demonstrates the existence of a primary, zero-field PDW state in EuRbFe4As4, identified through SI-STM measurements showing long-range unidirectional modulations without other density wave orders.

Findings

Superconducting gap exhibits incommensurate unidirectional modulations.

Modulations disappear above the magnetic transition temperature.

No evidence of other density wave orders in the material.

Abstract

The pair density wave (PDW) is a novel superconducting state in which Cooper pairs carry center-of-mass momentum in equilibrium, leading to the breaking of translational symmetry. Experimental evidence for such a state exists in high magnetic field and in some materials that feature density wave orders that explicitly break translational symmetry. However, evidence for a zero-field PDW state that exists independent of other spatially ordered states has so far been elusive. Here, we show that such a state exists in the iron pnictide superconductor EuRbFe4As4 (Eu-1144), a material that features coexisting superconductivity (Tc ~ 37K) and magnetism (Tm ~ 15 K). We show from the Spectroscopic Imaging Scanning Tunneling Microscopy (SI-STM) measurements that the superconducting gap at low temperature has long-range, unidirectional spatial modulations with an incommensurate period of ~8 unit cells. Upon raising the temperature above Tm, the modulated superconductor disappears, but a uniform superconducting gap survives to Tc. When an external magnetic field is applied, gap modulations disappear inside the vortex halo. The SI-STM and bulk measurements show the absence of other density wave orders, showing that the PDW state is a primary, zero-field superconducting state in this compound. Both four-fold rotational symmetry and translation symmetry are recovered above Tm, indicating that the PDW is a smectic order.