Anomalous Suppression of the Orthorhombic Lattice Distortion in Superconducting $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2$ Single Crystals

High-resolution x-ray diffraction measurements reveal an unusually strong response of the lattice to superconductivity in $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2$. The orthorhombic distortion of the lattice is suppressed and, for Co doping near $x=0.063$, the orthorhombic structure evolves smoothly back to a tetragonal structure. We propose that the coupling between orthorhombicity and superconductivity is indirect and arises due to the magnetoelastic coupling, in the form of emergent nematic order, and the strong competition between magnetism and superconductivity.

Figure 1

(a) Tetragonal-to-orthorhombic transition temperature $T_S$ (filled circles) and superconducting transition temperature $T_c$ (open circles) from the present measurements as a function of the average value of Co doping as determined from WDS. The filled triangles and squares represent $T_{\delta }{}^{\max }$ and $\delta (0)$ as described in the text. Dashed lines serve as guides to the eye. (b) The zero–field cooled and field cooled magnetization for all samples in this study.

Figure 2

Temperature evolution of the (1 1 10) Bragg peak in $\mathrm{Ba}({\mathrm{Fe}}_{0.938}{\mathrm{Co}}_{0.062}{)}_2{\mathrm{As}}_2$ at several temperatures. The red lines represent fits to the data using either one (for $T=38\mathrm{K}$ and 8.3 K) or two (for $T=28\mathrm{K}$, 22 K and 18.5 K) Lorentzian squared peaks. For this sample, $T_S=32\mathrm{K}$ and $T_c=23\mathrm{K}$.

Figure 3

The measured orthorhombic distortion $\delta$ as a function of temperature. Filled symbols represent the distortion determined from the positions of two peak fits to the data. The open symbols represent an upper limit on the distortion extracted from the line broadening of a single peak fit to the data relative to the peak width well above $T_S$. Labeled arrows denote the measured $T_N$ for several samples.

Figure 4

The $T\mathrm{\text{−}}x$ phase diagram for $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2$ compiled from data in Refs. 22 (open symbols), 24 (filled symbols for $T_N$), and the present study (filled symbols for $T_S$ and $T_c$). The extension of the tetragonal-to-orthorhombic phase line into the superconducting dome is represented by the dashed line.