Insulating and metallic spin glass in K$_{x}$Fe$_{2-\delta-y}$Ni$_{y}$Se$_{2}$ (0.06 $\leq$ $y$ $\leq$ 1.44) single crystals

TL;DR

This study explores how nickel doping influences the electronic and magnetic phases of K$_{x}$Fe$_{2- ext{delta}-y}$Ni$_{y}$Se$_{2}$ single crystals, revealing a transition from superconducting to insulating spin glass and metallic states.

Contribution

It provides a detailed phase diagram showing the effects of Ni doping on structural, magnetic, and electronic properties of K$_{x}$Fe$_{2- ext{delta}-y}$Ni$_{y}$Se$_{2}$, highlighting the suppression of superconductivity and the emergence of spin glass behavior.

Findings

Ni doping suppresses superconductivity below 1.8 K.

Higher Ni content induces insulating spin glass states.

High Ni concentration restores metallic behavior and changes crystal symmetry.

Abstract

We report electron doping effects by Ni in K$_{x}$Fe$_{2-\delta-y}$Ni$_{y}$Se$_{2}$ (0.06 $\leq$ $y$ $\leq$ 1.44) single crystal alloys. A rich ground state phase diagram is observed. Small amount of Ni ($\sim$ 4\%) suppressed superconductivity below 1.8 K, inducing insulating spin glass magnetic ground state for higher Ni content. With further Ni substitution, metallic resistivity is restored. For high Ni concentration in the lattice the unit cell symmetry is high symmetry $I4/mmm$ with no phase separation whereas both $I4/m + I4/mmm$ space groups were detected in the phase separated crystals when concentration of Ni $<$ Fe. The absence of superconductivity coincides with the absence of crystalline Fe vacancy order.