Superconductivity and magnetism in platinum-substituted ${\text{SrFe}}_2{\text{As}}_2$ single crystals

Single crystals of ${\text{SrFe}}_{2-x}{\text{Pt}}_x{\text{As}}_2$ $\left(0\le x\le 0.36\right)$ were grown using the self-flux solution method and characterized using x-ray crystallography, electrical transport, magnetic susceptibility, and specific heat measurements. The magnetic/structural transition is suppressed with increasing Pt concentration, with superconductivity seen over the range $0.08\le x\le 0.36$ with a maximum transition temperature $T_c$ of 16 K at $x=0.16$. The shape of the phase diagram and the changes to the lattice parameters are similar to the effects of other group VIII elements Ni and Pd, however the higher transition temperature and extended range of superconductivity suggest some complexity beyond the simple electron counting picture that has been discussed thus far.

Figure 1

(Color online) Variation in the $c$-axis lattice parameters with Pt concentration in single-crystal samples of ${\text{SrFe}}_{2-x}{\text{Pt}}_x{\text{As}}_2$ compared with that from powder data of ${\text{SrFe}}_{2-x}{\text{Ni}}_x{\text{As}}_2$ from Ref. 10. The $c$-axis length contracts with increasing Pt content, consistent with previous chemical substitution studies. Error bars were obtained by finding the lattice constant at the positions of the half-maximum points on the x-ray diffraction peaks for each sample.

Figure 2

(Color online) (a) Electrical resistivity of single-crystal samples of ${\text{SrFe}}_{2-x}{\text{Pt}}_x{\text{As}}_2$ normalized to 300 K (data are shifted vertically for clarity for all concentrations except $x=0.36$). The structural transition $T_0$, identified by the local mimina in $\rho \left(T\right)$, is suppressed with increasing Pt substitution, as indicated by the arrows. (b) Negative of the temperature derivative of the normalized resistivity data for characteristic samples from part (a). The antiferromagnetic transition $T_N$ is identified at the local maxima while the dashed line indicates the zero crossing of $-d\rho /dT$ corresponding to local minima in $\rho \left(T\right)$. At optimal doping, $x=0.16$, $T_0$ and $T_N$ are no longer discernible and the maximum $T_c$ for the system is found to be 16 K. Neither magnetic order nor superconductivity is observable at $x=0.36$, the highest concentration sample measured in this study.

Figure 3

(Color online) Magnetic field suppression of the resistive superconducting transition in optimally doped ${\text{SrFe}}_{1.84}{\text{Pt}}_{0.16}{\text{As}}_2$ for fields up to 14 T applied parallel to the $c$ axis. The resistivity is normalized to the room-temperature value. Inset: superconducting upper critical field $H_{c2}\left(\text{T}\right)$, defined at the 50% point of the normal-state resistivity for each field in the main panel, plotted with that in ${\text{SrFe}}_{2-x}{\text{Ni}}_x{\text{As}}_2$ from Ref. 29.

Figure 4

(Color online) Specific-heat capacity of optimally doped ${\text{SrFe}}_{1.84}{\text{Pt}}_{0.16}{\text{As}}_2$, with arrow indicating superconducting transition temperature. Upper inset: zoom of $C_p\left(T\right)/T$ near the superconducting transition, with lines indicating fits to the data above and below $T_c$ and arrows indicating the position where $\Delta C_p/T_c$ is determined. Lower inset: volume magnetic susceptibility of ${\text{SrFe}}_{1.84}{\text{Pt}}_{0.16}{\text{As}}_2$ measured in 1 mT field applied parallel to the crystallographic basal plane, following both ZFC and FC conditions. The sharpness of the transition, with onset near 16 K, is identified by the complete saturation of Meissner screening by 10 K. The bulk nature of the transition is shown by the achievement of nearly full screening fraction of $4\pi \chi$.

Figure 5

(Color online) Change in magnetic susceptibility $\left[\Delta \chi \equiv \chi \left(20\text{K}\right)-\chi \left(T\right)\right]$ of ${\text{SrFe}}_{2-x}{\text{Pt}}_x{\text{As}}_2$ normalized to the susceptibility value at 2 K. Samples were measured in 1 mT field applied parallel to the crystallographic basal plane for ZFC and FC conditions.

Figure 6

(Color online) Phase diagram of the ${\text{SrFe}}_{2-x}{\text{Pt}}_x{\text{As}}_2$ system. Magnetic (triangles), structural (diamonds), and superconducting (circles) transition temperatures are obtained from resistivity (solid symbols) and magnetic susceptibility (open symbols). Solid lines are added as guides for the structural and superconducting transitions. The error bars for the magnetic transition temperatures (triangles) represent the width of the peak in the $d\rho /dT$ data (see text).