Direct visualization of phase separation between superconducting and nematic domains in Co-doped CaFe2As2 close to a first order phase transition

TL;DR

This study visualizes how biaxial strain induces alternating superconducting and nematic domains in Co-doped CaFe2As2, revealing phase separation near a first order phase transition with potential applications in quantum devices.

Contribution

It demonstrates strain-induced phase separation into superconducting and nematic domains, characterized by advanced microscopy, near a first order phase transition in Co-doped CaFe2As2.

Findings

Biaxial strain creates alternating tetragonal superconducting and orthorhombic nematic domains.

Superconducting domains are elongated, several tens of microns long, about 30 nm wide.

Superconducting domains retain the same critical temperature as unstrained samples.

Abstract

We show that biaxial strain induces alternating tetragonal superconducting and orthorhombic nematic domains in Co substituted CaFe2As2. We use Atomic Force, Magnetic Force and Scanning Tunneling Microscopy (AFM, MFM and STM) to identify the domains and characterize their properties, finding in particular that tetragonal superconducting domains are very elongated, more than several tens of micron long and about 30 nm wide, have the same Tc than unstrained samples and hold vortices in a magnetic field. Thus, biaxial strain produces a phase separated state, where each phase is equivalent to what is found at either side of the first order phase transition between antiferromagnetic orthorhombic and superconducting tetragonal phases found in unstrained samples when changing Co concentration. Having such alternating superconducting domains separated by normal conducting domains with sizes of order of the coherence length opens opportunities to build Josephson junction networks or vortex pinning arrays and suggests that first order quantum phase transitions lead to nanometric size phase separation under the influence of strain.