Cezariusz Jastrzebski

arXiv2026

Fluorine-substitution-dependent phase diagram and superconducting properties of Sm-based oxypnictides synthesized by a high-pressure growth technique

A series of SmFeAsO1-xFx (Sm1111) bulk samples (x = 0.05 to 0.40) are synthesized by an in-situ cubic-anvil high-pressure technique at 4 GPa and systematically characterized through structural, microstructural, Raman, transport, and magnetic measurements. Structural analysis confirms that the tetragonal Sm1111 phase remains dominant across the entire substitution range, with lattice parameters decreasing smoothly as fluorine content increases, demonstrating effective incorporation of F even in the overdoped regime (x = 0.4). Raman spectroscopy provides complementary, local-phase-sensitive evidence that supports the structural analysis and confirms fluorine substitution in the main Sm1111 phase. In the underdoped region, the superconducting transition temperature (Tc) is enhanced by 10-17 K and the critical current density (Jc) is increased by up to an order of magnitude compared with conventionally synthesized samples at the ambient pressure. The upper critical field, estimated using the WHH model, reaches values approaching 200 T, reflecting strong paramagnetic limitation and multiband effects. TAFF analysis reveals a power-law field dependence of the activation energy, consistent with collective vortex pinning in polycrystalline iron-based superconductors. The superconducting phase diagram constructed from Tc and Jc versus fluorine content reveals a dome-like trend, with a maximum Tc of 57 K and Jc of 10^4 A cm^-2 at the optimal doped region. Comparison with conventionally synthesized Sm1111 samples demonstrates that high-pressure growth significantly extends the effective fluorine substitution range and enhances the superconducting performance, particularly in the underdoped regime. These results establish high-pressure fluorine substitution as an effective materials-engineering approach for tuning the phase diagram and optimizing the superconducting properties of Sm1111.

cond-mat.supr-con↗

arXiv2024

Copper doping effects on the superconducting properties of Sm-based oxypnictides

A systematic investigation has been performed by synthesis and comprehensive characterization of a series of SmFe1-xCuxAsO0.8F0.2 bulks (x = 0 to 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 greater than 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.

cond-mat.supr-con↗