Insulating and metallic spin glass in Ni-dopedKxFe2−ySe2single crystals

Abstract

We report electron doping effects by Ni in KxFe 2−δ−y NiySe2 (0.06 ≤ y ≤ 1.44) single crystal alloys. A rich ground state phase diagram is observed. Small amount of Ni (∼ 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.

Lattice dynamics of iron chalcogenides: Raman scattering study

“…60,63,64 Doping of K x Fe 2-y Se 2 with small Ni content also preserves the phase separation. 59 Similarly to the Co--doped case, in K x Fe 2-y-z Ni z Se 2 samples with z ≥ 0.73 low symmetry I4/m phase disappears. 59 Pure K x Ni 2-y Se 2 crystallizes in the tetragonal crystal structure with I4/mmm space group, without any indications of vacancy ordering, [65][66][67] as well as his Co-counterpart.…”

“…59 Similarly to the Co--doped case, in K x Fe 2-y-z Ni z Se 2 samples with z ≥ 0.73 low symmetry I4/m phase disappears. 59 Pure K x Ni 2-y Se 2 crystallizes in the tetragonal crystal structure with I4/mmm space group, without any indications of vacancy ordering, [65][66][67] as well as his Co-counterpart. K x Fe 2-y Se 2 samples doped with S on chalcogen site exhibit iron vacancy order for all doping levels.…”

“…Presence of both phases was also confirmed by Raman scattering measurements. 28,[59][60][61] On the other hand, Bao et al 62 reported phase separation between three phases (I4/m, I4/mmm and Pmna) in K x Fe 2-y Se 2 samples for 1.4 ≤ 2-y ≤ 1.6 and x > 0.85 (the samples have general formula K 2 Fe 3 Se 4 ) between T s and T * ≈ 350 K, whereas below T * only I4/m phase with ordered Fe vacancies persists. For Fe content 1.6 ≤ 2-y ≤ 1.7 the sample composition is described with K 2 Fe 4 Se 5 and below T s their space group is I4/m.…”

“…72 Namely, Cr-, Co-and Zn-doping rapidly suppresses SC in the K 0.8 Fe 2-y Se 2 , by inducing a large effective magnetic moment, thus destroying SC via a magnetic pair-breaking effect. 72 Recently reported study of Ni-doped K x Fe 2-y Se 2 single crystals 59 shows that small Ni content destroys SC, leading to the insulating behaviour, whereas with further Ni-doping metallic character of crystals gradually increases. It was also shown that these materials exhibit spin glass behaviour below critical temperature T f of 10-50 K, depending on the Ni concentration.…”

“…It was also shown that these materials exhibit spin glass behaviour below critical temperature T f of 10-50 K, depending on the Ni concentration. 59 K x Ni 2-y Se 2 have a local charge-density-wave state persisting up to 300 K. 65 Some authors 66 reported the metallic resistivity of K x Ni 2-y Se 2 single crystal up to the room temperature, without SC transition or bump in the ρ(T), unlike the K x Fe 2-y Se 2 . SC is suppressed probably due to the K and Ni deficiency, indicating the strong sensitivity to the sample composition.…”