Pressure-induced superconducting state of antiferromagneticCaFe2As2

Interface-induced superconductivity at ∼25 K at ambient pressure in undoped CaFe 2 As 2 single crystals

“…Note that the resistivity does not reach zero due to the incomplete formation of the mixing phase in the sample and the formation of microcracks in the crystals during the cT transition. The superconductivity is detected only in the t window where phase mixing occurs, consistent with previous reports of the existence of superconductivity in Ca122 at or near its O-cT phase boundary induced by pressure (22,23). However, detailed analyses of the XRD and superconductivity data reveal interesting additional subtleties and point to the important role of interfaces in the induction of superconductivity in Ca122.…”

“…Although the related superconducting volume fraction is much higher under nonhydrostatic pressure, the origin of the superconductivity remains unresolved. Both the coexistence of different phases (22,23) and a new tetragonal phase-T′, which appears only under uniaxial pressure--were proposed (24). The complications associated with the delicate pressure environment, however, seem to prevent a comprehensive experimental verification.…”

Thermal expansion of CaFe2As2: Effect of cobalt doping and postgrowth thermal treatment

“…In the AFe 2 As 2 family, and most probably throughout all ironarsenide based superconductors and their parent compounds, CaFe 2 As 2 (i) provides an extreme example of the strongly coupled structural and magnetic phase transitions 8,9 and (ii) demonstrates extreme sensitivity to the pressure or stress. [9][10][11][12][13][14][15][16] This extreme sensitivity to pressure allows a sample to be tuned from an antiferromagnetic, orthorhombic ground state to a nonmoment bearing, collapsed tetragonal one by applying just few kilobars of pressure. 9 Additionally, if a nonhydrostatic pressure component is present in the experiment, a (in many cases filamentary) superconducting phase can be detected, 10,[14][15][16][17] which is probably associated with a noncollapsed tetragonal phase being stabilized as part of a mixture of several crystallographic phases in a CaFe 2 As 2 sample at low temperatures.…”

“…[9][10][11][12][13][14][15][16] This extreme sensitivity to pressure allows a sample to be tuned from an antiferromagnetic, orthorhombic ground state to a nonmoment bearing, collapsed tetragonal one by applying just few kilobars of pressure. 9 Additionally, if a nonhydrostatic pressure component is present in the experiment, a (in many cases filamentary) superconducting phase can be detected, 10,[14][15][16][17] which is probably associated with a noncollapsed tetragonal phase being stabilized as part of a mixture of several crystallographic phases in a CaFe 2 As 2 sample at low temperatures. 16 Recently, the collapsed tetragonal phase and bulk superconductivity were induced in CaFe 2 As 2 at ambient pressure by manipulation of nanoscale precipitates through judicious annealing and quenching, combined with cobalt substitution in place of iron.…”

Superconductivity in Undoped CaFe 2 As 2 Single Crystals

“…Among these 122-type iron pnictides, CaFe 2 As 2 is very particular, which exhibits extreme sensitivity to external pressure on its crystal structure and magnetic properties at ground state [9][10][11][12][13][14][15], and the rare-earth doped Ca 1-x Pr x Fe 2 As 2 even reaches an unusual high-T c of 49 K with the mechanism still unclear [16,17]. For the undoped CaFe 2 As 2 compound, it generally crystallizes in the tetragonal ThCr 2 Si 2 -type crystal structure at room temperature, and undergoes a first-order phase transition into an orthorhombic phase at 170 K [18][19][20], which is also accompanied by a commensurate AFM transition [21].…”