Charge compensation effect inPr1−ySryFe1−xCoxAsO codoped with Sr and Co

Abstract

Superconductivity of polycrystalline Sr and Co codoped ${\mathrm{Pr}}_{1\ensuremath{-}y}$${\mathrm{Sr}}_{y}$${\mathrm{Fe}}_{1\ensuremath{-}x}$${\mathrm{Co}}_{x}$AsO samples was investigated by measuring resistivity, dc magnetic susceptibility, thermopower, and Hall effect. In the Pr${\mathrm{Fe}}_{1\ensuremath{-}x}$${\mathrm{Co}}_{x}$AsO samples with only Co doping, ${T}_{c}$ reaches a maximum of 16 K at $x=0.075--0.1$, and the variation of ${T}_{c}$ with Co content ($x$) is a dome-shaped curve. In the Co and Sr codoped ${\mathrm{Pr}}_{0.8}$${\mathrm{Sr}}_{0.2}$${\mathrm{Fe}}_{1\ensuremath{-}x}$${\mathrm{Co}}_{x}$AsO system, the phase diagram separates into two regions. The thermopower and Hall data indicate that the hole-type charge carrier dominates in the low Co doping region ($x$ $\ensuremath{\leqslant}$ 0.075) and the system becomes electron type in the high Co doping region ($x$ $\ensuremath{\geqslant}$ 0.075). ${T}_{c}$ first decreases with increasing Co content to a lowest ${T}_{c}$ of 3.5 K at $x=0.075$, then it increases and reaches a maximum of $~16$ K at $x=0.15$. The transition from hole-type to electron-type superconductivity in the nominal ${\mathrm{Pr}}_{0.8}$${\mathrm{Sr}}_{0.2}$${\mathrm{Fe}}_{1\ensuremath{-}x}$${\mathrm{Co}}_{x}$AsO system around $x=0.075$ implies that the charge-carrier density is one of the decisive factors to control superconductivity.