Effects of successive annealing on high-field electrical transport and the upper critical field in S-substituted FeTe

TL;DR

This study shows that successive annealing of $Fe_{1+y}Te_{1-x}S_x$ (O$_2$ annealing, HCl etching, vacuum annealing) reduces magnetic impurities from excess Fe, leading to enhanced superconducting transport under high magnetic fields. Using high-field MR and a dirty-limit WHH analysis, the authors find that the paramagnetic contribution to $H_{c2}$ decreases after processing (α drops from ~4.9 to ~4.4) while spin-orbit coupling remains fixed (λ$_{SO}$ ≈ 0.3), resulting in a modest increase of $H_{c2}(0)$ from ~23.5–24 T to ~25.8–26 T. The results imply that impurity reduction improves the robustness of superconductivity against magnetic fields and that further optimization could achieve larger upper critical fields approaching the orbital limit, with potential for magnet applications in S-substituted FeTe compounds.

Abstract

Since iron-based superconductors have been discovered, many scientists have focused on their characteristic properties, such as an unconventional mechanism and a high upper critical field. Sulphur-substituted FeTe compounds are one of the members of the iron-based superconductors; however, chemical processes, such as O$_2$ annealing, are needed to induce superconductivity because of the existence of excess iron in as-grown crystals. Thus, the removal of excess iron and the obtaining of clean sulphur-substituted FeTe can play a key role in the understanding of the superconducting properties and the application to the superconducting devices. In this study, we present the successive annealing effects on sulphur-substituted FeTe compounds to investigate the electrical transport properties under high magnetic fields. Our measurements show that successive annealing processes improve the electrical transport properties in the superconducting states under magnetic fields. The removal of excess iron acting as magnetic impurities is indicated by the improvement of the upper critical field and its analysis.

Figures (3)

• Figure 1: (a) Crystal structure of Fe$_{1+y}$Te with excess iron in the tetragonal phase produced by VESTA3VESTA. The dashed line shows the tetragonal unit cell. The arrows with labels $a$, $b$, and $c$ indicate the crystallographic orientations. (b) Schematic diagram of the successive annealing process. (c) Temperature dependence of the in-plane electrical resistivity $\rho_{ab}$ of the O$_2$- and vacuum-annealed Fe$_{1+y}$Te$_{1-x}$S$_x$ with $x = 0.088$ and $y = 0.096$ measured by a DC current of $I_\mathrm{DC} = 1.0$ mA. The dashed lines indicate the linear fit of $\rho_{ab}$. $T_\mathrm{c}^\mathrm{onset}$ and $T_\mathrm{c}^0$ are determined by fitting the slope of $\rho_{ab}$-$T$ and the temperature at which $\rho_{ab}$ shows a zero resistivity value in the experimental resolution, respectively. The inset in panel (c) indicates the temperature dependence of the superconducting shielding fraction (SSF) of the O$_2$- and vacuum-annealed samples. The SSF is estimated from the magnetisation under an external magnetic field strength of $H = 20.0$ Oe applied along the in-plane direction. The temperature dependence of the electronic specific heat divided by temperature, $C_\mathrm{e} / T$, of the as-grown sample and the O$_2$- and vacuum-annealed samples are also depicted as the inset in panel (c). Each figure and the data are reproduced from our previous studies Kurihara_PRM9.
• Figure 2: Temperature dependence of the in-plane electrical resistivity $\rho_\mathrm{ab}$ of the (a) O$_2$-annealed [(d) vacuum-annealed] Fe$_{1.052}$Te$_{0.908}$S$_{0.092}$ under several magnetic fields measured by the AC current with the frequency of 20 kHz and the root mean square value of $1.0$ mA. The external magnetic field $H_0$ is applied along the $ab$-plane of the sample. $\rho_{ab}$ data under finite magnetic fields in panels (a) and (d) are shifted vertically by 0.1 $\mathrm{m \Omega \cdot cm}$ for clarity. The inset in panel (a) shows a detailed view of the superconducting transition temperatures $T_\mathrm{c}^0$, $T_\mathrm{c}^\mathrm{offset}$, $T_\mathrm{c}^\mathrm{mid}$ and $T_\mathrm{c}^\mathrm{onset}$. The inset in panel (d) shows the temperature dependence of $\rho_{ab}$ normalized by $\rho_{ab}$ at 293 K of the O$_2$- and vacuum-annealed Fe$_{1.052}$Te$_{0.908}$S$_{0.092}$ at 0 T measured by a DC current of $I_\mathrm{DC} = 1.0$ mA. Magnetic-field dependence of $\rho_\mathrm{ab}$ of the (b) O$_2$-annealed [(e) vacuum-annealed] Fe$_{1.052}$Te$_{0.908}$S$_{0.092}$ at several temperatures measured by the AC current with the frequency of 20 kHz and the root mean square value of $3.0$ mA. In each measurement, the magnetic field direction is parallel to the AC current direction of the $ab$-plane. The red vertical arrows indicate the critical fields $H_\mathrm{c}^0$, $H_\mathrm{c}^\mathrm{offset}$, $H_\mathrm{c}^\mathrm{mid}$ and $H_\mathrm{c}^\mathrm{onset}$. $\rho_{ab}$ data above 3.0 K in panels (b) and (e) are shifted vertically by 0.1 $\mathrm{m \Omega \cdot cm}$ for clarity. Magnetic field-temperature phase diagram of (c) O$_2$-annealed [(f) vacuum-annealed] Fe$_{1.052}$Te$_{0.908}$S$_{0.092}$.
• Figure 3: Analytical results of the upper critical field $H_\mathrm{c2}$ of the O$_2$-annealed (red filled circles) and vacuum-annealed (green filled circles) Fe$_{1.052}$Te$_{0.908}$S$_{0.092}$ determined by $H_\mathrm{c}^\mathrm{offset}$ for the in-plane field direction, $H_0 // ab$. The red (green) solid line indicates the fit of the $H_\mathrm{c2}$ of the O$_2$ (vacuum) annealed sample using Eq. (\ref{['single band WHH']}) with $\alpha = 4.9$ and $\lambda_\mathrm{SO} = 0.3$ ($\alpha = 4.4$ and $\lambda_\mathrm{SO} = 0.3$). The red (grey) broken line shows the analytical result with $\alpha = 4.0$ and $\lambda_\mathrm{SO} = 0$ ($\alpha = 0$ and $\lambda_\mathrm{SO} = 0$) for the O$_2$-annealed sample. The inset shows the schematic view of the electrical resistivity measurements under magnetic fields.