Unusual superconducting state at 49 K in electron-doped CaFe ₂ As ₂ at ambient pressure

Abstract

We report the detection of unusual superconductivity up to 49 K in single crystalline CaFe 2 As 2 via electron-doping by partial replacement of Ca by rare-earth. The superconducting transition observed suggests the possible existence of two phases: one starting at 49 K, which has a low critical field < 4 Oe, and the other at 21 K, with a much higher critical field > 5 T. Our observations are in strong contrast to previous reports of doping or pressurizing layered compounds AeFe 2 As 2 (or Ae122), where Ae = Ca, Sr, or Ba. In Ae122, hole-doping has been previously observed to generate superconductivity with a transition temperature ( T c ) only up to 38 K and pressurization has been reported to produce superconductivity with a T c up to 30 K. The unusual 49 K phase detected will be discussed.

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

“…The observations offer an avenue to higher T c. this family seems to be rather unusual. One intriguing observation has been the nonbulk superconductivity up to 49 K induced in Ca122 at ambient pressure by slight Ca replacement with a rareearth element, La, Ce, Pr, or Nd (17)(18)(19). A systematic study on the slightly rare-earth-doped Ca122 suggests that the nonbulk superconductivity detected may be associated with the naturally occurring interfaces associated with defects in the samples (20).…”

Local Inhomogeneity and Filamentary Superconductivity in Pr-DopedCaFe2As2

“…The issues of doping and inhomogeneity are well exemplified in Ca 1−x Pr x Fe 2 As 2 superconductor [7,8]. This electron doped system can show T c ≈ 45 K, the highest among the pnictides with ThCr 2 Si 2 crystal structure [9][10][11].…”

Interplay of chemical disorder and electronic inhomogeneity in unconventional superconductors

“…It was pointed out that T c in electron-doped A122 Fe-SCs could potentially be even higher, comparable to the T c,max = 38 K found in hole-doped A122 materials, if the damaging dopant disorder were taken out of the FeAs layer and moved into the buffer layer between the two FeAs planes. Indeed, this was found to be the case in the rare-earth-doped Ca122 family, 134,135 with T c reaching 49 K in Pr x Ca 1Àx Fe 2 As 2 (Pr-Ca122). However, the high T c appeared in only B10% of the volume, while the bulk of the material showed T c B 10-20 K. Extensive tests have so far failed to determine the origin of this mysterious low volume fraction high-T c phase, as they appear to have ruled out surface superconductivity, contaminant phases, and strain mechanics, 135 leaving dopant clustering as one of the few remaining hypotheses.…”