Effect of Co doping and hydrostatic pressure on SrFe₂ As₂

Abstract

We report a pressure study on electron doped SrFe$_{2-x}$Co$_x$As$_2$ by electrical-resistivity ($\rho$) and magnetic-susceptibility ($\chi$) experiments. Application of either external pressure or Co substitution rapidly suppresses the spin-density wave ordering of the Fe moments and induces superconductivity in SrFe$_2$As$_2$. At $x=0.2$ the broad superconducting (SC) dome in the $T-p$ phase diagram exhibits its maximum $T_{c,{\rm max}}=20$ K at a pressure of only $p_{\rm max}\approx 0.75$ GPa. In SrFe$_{1.5}$Co$_{0.5}$As$_2$ no superconductivity is observed anymore up to 2.8 GPa. Upon increasing the Co concentration the maximum of the SC dome shifts toward lower pressure accompanied by a decrease in the value of $T_{c,{\rm max}}$. Even though, superconductivity is induced by both tuning methods, Co substitution leads to a much more robust SC state. Our study evidences that in SrFe$_{2-x}$Co$_x$As$_2$ both, the effect of pressure and Co-substitution, have to be considered in order to understand the SC phase-diagram and further attests the close relationship of SrFe$_2$As$_2$ and its sister compound BaFe$_2$As$_2$.Comment: 6 pages, 6 figure

Qualitative model of high-$T_c$ superconductivity

“…Indeed, experiment [20] shows similarities between structural distortions under and chemical doping in superconducting BaFe 2 As 2 . Similar effects are found in SrFe 2 As 2 doped by Co [21] and CaFe 2 As 2 doped by Sr [22].…”

Evidence of precursor orthorhombic domains well above the electronic nematic transition temperature in Sr(Fe1−x Co x )2As2

“…The reported Co content is an average of the values obtained on one side of the sample at several spots of the crystal surface as suggested by the error in x. Such relatively large value of x might seem at first sight inconsistent with the presence of magnetic ordering and structural phase transitions (see below), considering previous studies in this family [42][43][44]. However, it is well known that the detailed phase diagrams of Fe pnictides may depend sensibly on the detailed growth procedure [5].…”

“…Undoped 122-compounds show nearly coincident T AF and T o [5,15], whereas well separated transition temperatures are observed in other series, such as in Ba(Fe 1−x Co x ) 2 As 2 [16][17][18], Sr(Fe 1−x Co x ) 2 As 2 [6,42,43,49], NaFe 1−x T x As (T = Co, Rh) [19,20] and even in parent oxypnictides such as LnFeAsO (Ln = La,Ce,Pr,Nd) [21][22][23][24][25][26]. So far, the origin of this large separation in some cases and its possible relation with lattice disorder has not been fully understood.…”