Low-temperature microstructural studies on superconducting CaFe2As2

Abstract

Undoped CaFe 2 As 2 (Ca122) can be stabilized in two slightly different non-superconducting tetragonal phases, PI and PII, through thermal treatments. Upon proper annealing, superconductivity with a T c up to 25 K emerges in the samples with an admixture of PI and PII phases. Systematic low-temperature X-ray diffraction studies were conducted on undoped Ca122 samples annealed at 350 °C over different time periods. In addition to the diffraction peaks associated with the single-phase aggregation of PI and PII, a broad intermediate peak that shifts with annealing time was observed in the superconducting samples only. Our simulation of phase distribution suggests that the extra peak is associated with the admixture of PI and PII on the nanometer scale. High-resolution transmission electron microscopy confirms the existence of these nano-scale phase admixtures in the superconducting samples. These experimental results and simulation analyses lend further support for our conclusion that interfacial inducement is the most reasonable explanation for the emergence of superconductivity in undoped Ca122 single crystals.

Interfacial Superconductivity Achieved in Parent AEFe2As2 (AE = Ca, Sr, Ba) by a Simple and Realistic Annealing Route

“…The as-grown Ca122 sample has a tetragonal phase at room temperature and ambient pressure (blue curve in Figure a). Low-temperature XRD spectra of as-grown Ca122 focused on the (008) peak (Figure S2a) clearly show that the sample undergoes a tetragonal-to-orthorhombic phase transition between 150 and 170 K, which is characteristic of the PII phase according to previous reports. ,,− After measurements of the as-grown Ca122 samples were completed, they were annealed at 350 °C under an As atmosphere for different amounts of time. The XRD profile of the 144 h Ca122–As sample (black curves in Figure a and its inset) shows extra peaks when compared with that of the as-grown sample, which is attributable to gray arsenic.…”

“…With the advancement of experimental techniques, interfacial superconductivity in artificially assembled materials has attracted increasing interest in recent years. Superconductivity has been induced or enhanced in hybrid heterostructures at the interface between two materials, for example, ∼3 K superconductivity in the mixture of Ru and Sr 2 RuO 4 , , ∼0.2 K superconductivity in the LaAlO 3 /SrTiO 3 heterostructure, , ∼50 K superconductivity in the La 1.55 Sr 0.45 CuO 4 /La 2 CuO 4 heterostructure, 65 K or even 109 K superconductivity in the monolayer FeSe films on SrTiO 3 substrates, , ∼25 K superconductivity in the two-phase admixture of CaFe 2 As 2 , , and up to 1.7 K superconductivity between two graphene layers twisted at a particular “magic” angle . Such emergent superconductivity at interfaces resembles that of the reduced-dimension layered superconductors, such as cuprates, iron-based superconductors, and transition-metal dichalcogenide superconductors , with very similar characteristics, thus providing ample opportunities to decode the pairing mechanisms of unconventional superconductors.…”