Crystal structure and superconducting properties of hole-doped Ca_0.89Na_0.11FFeAs single crystals

Abstract

We report for the first time on superconductivity achieved upon hole doping when Ca 2+ is partially substituted with Na + in CaFFeAs. Single crystals of Ca 0.89 Na 0.11 FFeAs up to 2 × 2 mm 2 in size with T c = 34.5 K were grown from NaCl flux. The superconducting transition temperature, with a narrow width ∼2 K, confirmed by both magnetic and resistivity measurements, indicates the high quality of our samples. Magnetic relaxation measurements show that the flux creep in this material is larger than in other iron-based superconductors. The existence of the wide magnetic reversible region and the pronounced flux-creep effect can be attributed to the higher anisotropy of this material.

Universal lower limit on vortex creep in superconductors

“…high [5][6][7][8]19 , which raised an important question: precisely what should we expect and how much lower can we go?…”

“…8,23 , where the data for FeSe was extracted from S(H) at different temperatures 9 and we measured Rb0.8Fe1.6SeS 41 . All previous studies of creep in Fe-SCs have been performed on single crystals [5][6][7][8]14,19,22,23,32,[42][43][44] , where self-fields (Hsf ∝ Jc × thickness) are higher than in our films. Consequently, we perform a direct comparison at !…”

Rare earth (RE – Ce, Gd) modified Nd1−xRExFeAsO0.7F0.3 superconductor with enhanced magneto-transport properties

“…The doped samples show competitively better J C (H) performance in comparison to the pristine sample Nd3. 36 In the present study, apart from uorine substitution, the major contribution to vortex pinning is from the substitutional defects due to rare earth doping at Nd site in Nd1111. Both NdCe1 and NdGd15 trail similar J C (H) behavior up to 2 T with NdCe1 excelling NdGd15 at higher elds up to 6 T, as expected from the M À H data.…”

Growth, magnetic and transport properties of Li‐doped SrFe2As2 single crystals

“…In this paper, we focus on crystal structure and physical properties of Li‐doped SrFe 2 As 2 . Partial substitution of Sr by Na and thus effectively hole doping was earlier shown to provoke superconductivity in Sr 1– x Na x Fe 2 As 2 polycrystalline (highest T c = 26 K at x = 0.4) and single crystalline ( T c = 36.5 K at x = 0.47) material. As a working hypothesis taken from the situation in rather simple BCS superconductors, one could expect to obtain even higher T c 's in metallic systems when the carrier densities at the Fermi level, N ( E F ), could be further enhanced.…”

“…So far, superconductivity in iron arsenides was found in systems with generally four different but similar crystal structures, of which the so‐called 122 phases are represented by A Fe 2 As 2 with A = Ca, Sr, Ba . Hole doping with alkali metals of the 122 and 1111 phases is found to be an effective tool to induce superconductivity in this class of high‐ T c materials . Superconductivity provoked by hole doping was first observed in Sr‐substituted LaFeAsO (La 1– x Sr x FeAsO) , shortly followed by reports of a superconducting transition as high as 38 K in potassium substituted BaFe 2 As 2 (Ba 1– x K x Fe 2 As 2 ) .…”