Experimental data collected for the preparation of the manuscript: "Copper doping effects on the superconducting properties of Sm-based oxypnictides"

Journal of the American Ceramic Society 107, 6806-6820 (2024)

Abstract:

A systematic investigation has been performed by synthesis and comprehensive characterization of a series of SmFe1−xCuxAsO0.8F0.2 bulks (x = 0–0.2). These samples are well characterized by structural, Raman spectroscopy, microstructural, transport, magnetic measurements, and supplementary calculations within density functional theory (DFT). The parent compound, SmFeAsO0.8F0.2 (Sm1111), exhibits a superconducting transition temperature (Tc) of approximately 54 K. The lattice volume (V) is increased with Cu substitution (x) without observing any impurity phase related to copper, which confirms the successful incorporation of Cu at Fe sites in the superconducting FeAs layers. These analyses are also well in agreement with Raman spectroscopy measurements and relevant DFT results. The superconducting transition is decreased systematically with copper doping and completely suppressed for 7% Cu-doped Sm1111 (x = 0.07). A large amount of Cu substitution (x ≥ 0.07) has demonstrated the metal to insulate transition in the low-temperature range, and no impurity phase was observed even at high Cu doping levels (x = 0.2). The calculated critical current density of the parent sample is suppressed with copper substitution, suggesting the reduced pinning centers, sample density, and grain connections, as confirmed by the microstructural analysis. Our studies suggest that the substitution of Cu in the superconducting FeAs layer, resulting the enlargement of the lattice volume, is a source of strong disorder scattering, leading to the suppression of Tc and the emergence of metal-to-insulator, unlike the more successful carrier doping by nickel (Ni) or cobalt (Co), as previously reported.

[In the published article, Figs. 3 and 4 are the image diagrams.]

• FIGURE 1: (A) X-ray diffraction patterns (XRD) of powdered SmFe_1−xCu_xAsO_0.8F_0.2 (x = 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.07, 0.1, and 0.2) samples at room temperature. (B) An enlarged view of the main peak (102) position of the parent SmFeAsO_0.8F_0.2 with respect to various Cu substitutions (x). The variation of (C) lattice parameter (a), (D) lattice parameter (c), and (E) lattice volume (V) with the nominal values of Cu dopant.

• FIGURE 2: Raman spectra of Cu-doped SmFeAsO_0.8F_0.2 bulks

• FIGURE 3: Elemental mapping for the constituent elements of SmFe_1−xCu_xAsO_0.8F_0.2 polycrystalline samples.

• FIGURE 4: Backscattered electron image (BSE; AsB) of SmFe_1−xCu_xAsO_0.8F_0.2.

• FIGURE 5: (A) The variation of resistivity (ρ) with the temperature up to the room temperature for SmFe_1−xCu_xAsO_0.8F_0.2. (B) Low-temperature variation of the resistivity up to 60 K for various Cu-doped SmFeAsO_0.8F_0.2.

• FIGURE 6: (A) The temperature dependence of the normalized magnetic moment for SmFe_1−xCu_xAsO_0.8F_0.2 (x = 0, 0.01, 0.02, 0.03, 0.05) in ZFC and FC modes under a magnetic field of 20 Oe. (B) The variation of the critical current density (J_c) with the applied magnetic field for x = 0 and 0.01 at a temperature of 5 K and a magnetic field of up to 9 T.

• FIGURE 7: The variation of (A) the onset transition temperature (T_c ^onset), (B) the transition width (ΔT = T_c^onset−T_c^offset), and (C) Residual resistivity ratio (RRR) (= ρ_{300 K}/ρ_{60 K}) of SmFe_1−xCu_xAsO_0.8F_0.2 with the nominal doping contents (x). Following that, the calculated error bars for T_c^onset, ΔT, and RRR are around 3%, 6%, and 4%, which are included in (A–C), respectively, for our Cu-doped SmFeAsO_0.8F_0.2.