  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.
