Transition-Metal Substitutions in Iron Chalcogenides

TL;DR

This study investigates how substituting transition metals Co and Ni into FeTeSe affects its electrical properties and superconductivity, revealing different doping effects and localization phenomena impacting the critical temperature.

Contribution

It provides detailed experimental insights into the effects of Co and Ni doping on transport properties and superconductivity in FeTeSe, highlighting the roles of electron doping and localization.

Findings

Superconductivity is suppressed at different doping levels for Co and Ni.

Hall coefficient sign changes indicate hole localization at low temperatures.

Ni doping causes stronger electron localization than Co.

Abstract

The $ab$-plane resistivity and Hall effect are studied in Fe$_{1-y}$M$_y$Te$_{0.65}$Se$_{0.35}$ single crystals doped with two transition metal elements, M = Co or Ni, over a wide doping range, $0 \leq y \leq 0.2$. The superconducting transition temperature, $T_{c}$, reaches zero for Co at $y \simeq 0.14$ and for Ni at $y \simeq 0.032$, while the resistivity at the $T_{c}$ onset increases weakly with Co doping, and strongly with Ni doping. The Hall coefficient $R_H$, positive for $y$ = 0, remains so at high temperatures for all $y$, while it changes sign to negative at low $T$ for $y > 0.135$ (Co) and $y > 0.06$ (Ni). The analysis based on a two band model suggests that at high $T$ residual hole pockets survive the doping, but holes get localized upon the lowering of $T$, so that the effect of the electron doping on the transport becomes evident. The suppression of the $T_c$ by Co impurity is related to electron doping, while in case of the Ni impurity strong electron localization most likely contributes to fast decrease of the $T_c$.