arXiv subjects

Michał Wierzbicki

Publications and source records attributed to Michał Wierzbicki.

7 recordsLinked to original sources

arXiv2026

Tuning Superconductivity by Isovalent Antimony Substitution in PrFeAs(O,F)

We investigate the effects of isovalent Sb substitution at the As site in fluorine-doped PrFeAs1-xSbxO0.7F0.3 (x = 0 to 1.0) through structural, Raman spectroscopy, density functional theory (DFT), transport, magnetotransport, and magnetic measurements. The superconducting transition temperature decreases gradually from ~48 K for the parent compound to ~44 K up to x = 0.3, followed by a rapid suppression at higher Sb concentrations due to increasing disorder and secondary phase formation. Raman spectroscopy and DFT reveal lattice expansion and pronounced softening of pnictogen related vibrational modes upon Sb substitution. Magnetotransport measurements up to 9 T show enhanced upper critical fields and increased vortex activation energy for moderate Sb doping, indicating stronger vortex pinning. However, the critical current density remains low because of poor intergranular connectivity. The results demonstrate a crossover from an electronically tuned superconducting state to a disorder-dominated regime in isovalently substituted iron pnictides.

cond-mat.supr-con↗

arXiv2026

Effect of Mn Substitution on Superconductivity in PrFeAs(O,F): Role of Magnetic Impurities

We investigate Mn substitution at the Fe site in PrFe1-xMnxAsO0.7F0.3 (0 to 0.1) using structural, Raman, density functional theory (DFT), transport, and magnetic measurements. X-ray diffraction and Raman analyses confirm preferential Mn incorporation into the FeAs planes, accompanied by lattice expansion and suppression of Fe-related vibrational modes. Electrical transport reveals a systematic decrease of the superconducting transition temperature from 48 K (x = 0) to complete suppression at x = 0.1, together with low-temperature resistivity upturns evolving toward insulating-like behavior. Magnetization and magnetotransport measurements show degradation of superconducting coherence, critical current density, upper critical field, and vortex activation energy with increasing Mn content. The results demonstrate that Mn acts as an efficient magnetic impurity, strongly perturbing the electronic and magnetic environment of the FeAs layers. Comparative analysis indicates relatively enhanced robustness of superconductivity in the Pr-based system, highlighting the role of rare-earth-dependent electronic correlations in impurity effects.

cond-mat.supr-con↗

arXiv2026

From many valleys to many topological phases - quantum anomalous Hall effect in IV-VI semiconductor quantum wells

Consistent with prior qualitative expectations for group IV-VI topological crystalline insulators, this work demonstrates, based on band structure and Chern number calculations, that Pb$_{1-x}$Sn$_x$Se/(PbSe)$_{1-y}$(EuS)$_y$ quantum wells constitute a promising and viable platform for realizing a variety of quantum anomalous Hall phases. The proposed basis transformation procedure for the multiband $\mathit{k} \cdot \mathit{p}$ Hamiltonian enables the treatment of wells grown along arbitrary crystallographic directions while explicitly accounting for the anisotropy of the material's isoenergetic surfaces. Numerical studies of $\langle 111\rangle$-, $\langle 110\rangle$- and $\langle 001\rangle$-oriented quantum wells predict attainable Chern numbers with magnitudes ranging from $1$ to $4$, depending on the quantum well width, Sn content, and relative orientation of the four projected $\mathrm{L}$ valleys with respect to the growth direction. The results further indicate that appropriate strain compensation is required to achieve high-quality quantization of the Hall conductance.

cond-mat.mes-hall↗

arXiv2025

Praseodymium doping effect on the superconducting properties of FeSe$_{0.5}$Te$_{0.5}$ bulks under ambient and high-pressure growth conditions

A series of Pr-doped FeSe$_{0.5}$Te$_{0.5}$ (Fe$_{1-x}$Pr$_x$Se$_{0.5}$Te$_{0.5}$; $x = 0$ to 0.3) bulks are prepared by conventional synthesis process at ambient pressure (CSP), and high gas pressure and high temperature synthesis (HP-HTS) methods. These bulks are well characterized by structural and microstructural analysis, Raman spectroscopy, transport, and magnetic measurements. The HP-HTS process of the parent bulks has enhanced the onset transition temperature ($T_c^{\rm onset}$) by 1.5 K and the critical current density ($J_c$) by two orders of magnitude compared to the CSP method. Pr-doped FeSe$_{0.5}$Te$_{0.5}$ up to 10% doping content prepared, either CSP or HP-HTS, slightly increases the unit cell volume, and high-pressure growth produces an almost pure superconducting phase, which confirms the successful Pr-doping at Fe sites. Raman spectroscopy measurements and DFT calculations suggest the substitution of Pr-atoms in the interlayer spacing of Fe(Se,Te) lattice. High-pressure growth of Fe$_{1-x}$Pr$_x$Se$_{0.5}$Te$_{0.5}$ also makes the sample less dense compared to the parent sample grown by HP-HTS. Transport and magnetic measurements depict that Tconset is almost unaffected by Pr-doping, whereas $J_c$ of Pr-doped FeSe$_{0.5}$Te$_{0.5}$ is enhanced by one order of magnitude relative to the parent sample developed by CSP but lower than that of the parent sample grown by HP-HTS. Hence, Pr-doping at Fe sites preserves $T_c^{\rm onset}$ and improves $J_c$ of FeSe$_{0.5}$Te$_{0.5}$ regardless of the doping contents and growth conditions. These results are promising for the practical application of iron-based superconductors to improve $J_c$ properties without affecting $T_c^{\rm onset}$ through CSP process and congruent with discoveries from other superconductors, like cuprates and MgB$_2$.

cond-mat.supr-con↗

arXiv2024

High-pressure growth effects on the superconducting properties of Sm-based oxypnictide superconductors

High-pressure synthesis can be an effective method for improving the sample quality of materials as well as their superconducting properties. In this paper, the synthesis process of F- doped SmFeAsO has been optimized by preparing a series of bulk SmFeAsO0.8F0.2 (Sm1111) using the high gas pressure and high-temperature synthesis (HP-HTS) method, considering various growth parameters like growth pressures (0-1 GPa) and heating time (0.5-2 h). Structural, microstructural, Raman spectroscopic, transport, and magnetic measurements are employed to comprehensively analyze these bulks and derive the conclusive findings. The parent SmFeAsO0.8F0.2 prepared by the conventional synthesis process at ambient pressure (CSP) has a transition temperature (Tc) of around 53-54 K, and the critical current density (Jc) of 103 A/cm2 at 5 K with a small amount of the impurity phases (SmOF and SmAs), consistent with previous reports. Interestingly, all bulks synthesized by HP-HTS have almost the same Tc and Jc as the parent sample. The optimal growth conditions are obtained as 900C, 1 hour, and 0.5 GPa with the sealed Ta-tube, which slightly improved the sample quality and the superconducting properties compared to other bulks grown by HP-HTS. Our study confirms that the existence of the impurity phases in the 1111 family is very robust and cannot be reduced by HP-HTS, leading to only a small variation in the observed superconducting properties of Sm1111 whether prepared by CSP or HP-HTS. This is the first comprehensive investigation of the high-pressure development of Sm1111, which shows distinct behaviour from other families of iron-based superconductors.

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↗

arXiv2008

Half-metallic ferromagnetism in binary compounds of alkali metals with nitrogen: Ab initio calculations

The first-principles full-potential linearized augmented plane-wave method based on density functional theory is used to investigate electronic structure and magnetic properties of hypothetical binary compounds of I$^{A}$ subgroup elements with nitrogen (LiN, NaN, KN and RbN) in assumed three types of cristalline structure (rock salt, wurtzite and zinc-blende). We find that, due to the spin polarized \textit{p} orbitas of N, all four compounds are half-metallic ferromagnets with wide energy bandgaps (up to 2.0 eV). The calculated total magnetic moment in all investigated compounds for all three types of crystal structure is exactly 2.00 $μ_{\text{B}}$ per formula unit. The predicted half-metallicity is robust with respect to lattice-constant contraction. In all the cases ferromagnetic phase is energetically favored with respect to the paramagnetic one. The mechanism leading to half-metallic ferromagnetism and synthesis possibilities are discussed.

cond-mat.mtrl-sci↗