Comprehensive scenario for single-crystal growth and doping dependence of resistivity and anisotropic upper critical fields in (Ba1−xKx)Fe

Abstract

Large high-quality single crystals of hole-doped iron-based superconductor (Ba1-xKx)Fe2As2 were grown over a broad composition range 0.22 <= x <= 1 by inverted temperature gradient method. We found that high soaking temperature, fast cooling rate, and an adjusted temperature window of the growth are necessary to obtain single crystals of heavily K-doped crystals (0.65 <= x <= 0.92) with narrow compositional distributions as revealed by sharp superconducting transitions in magnetization measurements and close to 100% superconducting volume fraction. The crystals were extensively characterized by x-ray and compositional analysis, revealing monotonic evolution of the c-axis crystal lattice parameter with K substitution. Quantitative measurements of the temperature-dependent in-plane resistivity rho(T) found doping-independent, constant within error bars, resistivity at room temperature, rho(300 K), in sharp contrast with the significant doping dependence in electron and isovalent substituted BaFe2As2 based compositions. The shape of the temperature-dependent resistivity, rho(T), shows systematic dopingevolution, being close to T-2 in overdoped and revealing significant contribution of the T-linear component at optimum doping. The slope of the upper critical field, d H-c2/dT, scales linearly with T-c for both H parallel to c, H-c2,(c), and H parallel to ab, H-c2,H-ab. The anisotropy of the upper critical field. equivalent to Hc(2,ab)/H-c2,H-c determined near zero-field T-c increases from similar to 2 to 4-5 with increasing K doping level from optimal x similar to 0.4 to strongly overdoped x = 1. Large high-quality single crystals of hole-doped iron-based superconductor (Ba 1−x K x )Fe 2 As 2 were grown over a broad composition range 0.22 x 1 by inverted temperature gradient method. We found that high soaking temperature, fast cooling rate, and an adjusted temperature window of the growth are necessary to obtain single crystals of heavily K-doped crystals (0.65 x 0.92) with narrow compositional distributions as revealed by sharp superconducting transitions in magnetization measurements and close to 100% superconducting volume fraction. The crystals were extensively characterized by x-ray and compositional analysis, revealing monotonic evolution of the c-axis crystal lattice parameter with K substitution. Quantitative measurements of the temperature-dependent in-plane resistivity ρ(T ) found doping-independent, constant within error bars, resistivity at room temperature, ρ(300 K), in sharp contrast with the significant doping dependence in electron and isovalent substituted BaFe 2 As 2 based compositions. The shape of the temperature-dependent resistivity, ρ(T ), shows systematic doping-evolution, being close to T 2 in overdoped and revealing significant contribution of the T -linear component at optimum doping. The slope of the upper critical field, dH c2 /dT , scales linearly with T c for both H c, H c2,c , and H ab, H c2,ab . The anisotropy of the upper critical field γ ≡ H c2,ab /H c2,c determined near z...

Doping evolution of the superconducting gap structure in the underdoped iron arsenideBa1−xKxFe2As2revealed by thermal conducti

“…For the other dopings, we obtain H c2 (0), the value of H c2 (T ) at T → 0, by linear extrapolation. Note that the slope of the H c2 (T ) curves increases with increasing T c , as expected for superconductors in the clean limit, 41 which holds for K-Ba122 at all dopings. In the right panel of Fig.…”

“…If the gap is large everywhere on the Fermi surface, the coherence length will be small everywhere, and the Hc2 is determined from electrical resistivity measurements. 41 The black square shows the Hc2 value determined from thermal conductivity measurements on the Tc = 7 K sample. The lines are a linear fit of each data set, extrapolated to T = 0.…”

“…Right panel: Hc2(0) vs Tc from data in the left panel (red dots), plus one point at high doping from published data (black diamond). 41 The line is a polynomial fit of the data, used to extract the values of Hc2 for the samples with Tc ∈ [7, 25] K.…”

“…38, 41 We use the resistivity curves of each sample to determine T c and the residual resistivity ρ 0 , obtained from a smooth extrapolation of ρ(T ) to T = 0 (see Fig. 1).…”

Effects of isovalent substitution and pressure on the interplane resistivity of single-crystalBa(Fe1−xRux)2Tanatar1, Torikachvili2, Thaler3 et al. 2014Phys. Rev. BSelf Cite

“…Here dominant feature of ρ c (T ) is a broad cross-over maximum, which in the BaFe 2 As 2 parent compound takes place at T max ≈ 200 K. This maximum shifts to lower temperatures with Co-doping and does not change position with K-doping up to x=0.34 13 . The maximum in ρ c (T ) for the hole-doped materials correlates well with a slope-change feature in the in-plane transport 13,14 .…”