Temperature-composition Phase Diagrams for Ba1-xSrxFe2As2 and Ba0.5Sr0.5(Fe1-yCoy)2As2

TL;DR

This study maps the temperature-composition phase diagrams of Ba1-xSrxFe2As2 and Ba0.5Sr0.5(Fe1-yCoy)2As2, revealing spin-density wave behavior, superconducting domes, and effects of doping and annealing on transition temperatures and electronic properties.

Contribution

It provides detailed phase diagrams and insights into doping effects, surface inhomogeneity, and transition behaviors in mixed alkaline earth metal iron arsenide superconductors.

Findings

Ba1-xSrxFe2As2 shows linear TN trend with composition.

Cobalt doping creates a superconducting dome with max TC = 19 K.

Annealing increases TC and reduces Sommerfeld coefficient.

Abstract

Single crystals of mixed alkaline earth metal iron arsenide materials of Ba1-xSrxFe2As2 and Ba0.5Sr0.5(Fe1-yCoy)2As2 are synthesized via the self-flux method. Ba1-xSrxFe2As2 display spin-density wave features (TN) at temperatures intermediate to the parent materials, x = 0 and 1, with TN(x) following an approximately linear trend. Cobalt doping of the 1 to 1 Ba:Sr mixture, Ba0.5Sr0.5(Fe1-yCoy)2As2, results in a superconducting dome with maximum transition temperature of TC = 19 K at y = 0.092, close to the maximum transition temperatures observed in unmixed A(Fe1-yCoy)2As2; however, an annealed crystal with y = 0.141 showed a TC increase from 11 to 16 K with a decrease in Sommerfeld coefficient from 2.58(2) to 0.63(2) mJ/(K2 mol atom). For the underdoped y = 0.053, neutron diffraction results give evidence that TN and structural transition (To) are linked at 78 K, with anomalies observed in magnetization, resistivity and heat capacity data, while a superconducting transition at TC ~ 6 K is seen in resistivity and heat capacity data. Scanning tunneling microscopy measurements for y = 0.073 give Dynes broadening factor of 1.15 and a superconducting gap of 2.37 meV with evidence of surface inhomogeneity.