Stripes of increased diamagnetic susceptibility in underdoped superconducting $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$ single crystals: Evidence for an enhanced superfluid density at twin boundaries

Superconducting quantum interference device microscopy shows stripes of increased diamagnetic susceptibility in the superconducting state of twinned, orthorhombic, underdoped crystals of $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$, but not in tetragonal overdoped crystals. These stripes are consistent with enhanced superfluid density on twin boundaries.

Figure 1

(Color online) Local susceptibility image in underdoped $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$, indicating increased diamagnetic shielding on twin boundaries. (a) Local diamagnetic susceptibility, at $T=17\text{K}$, of the $ab$ face of sample UD1 ($x=0.051$ and $T_c=18.25\text{K}$), showing stripes of enhanced diamagnetic response (white). In addition there is a mottled background associated with local $T_c$ variations that becomes more pronounced as $T\to T_c$. Overlay: sketch of the scanning SQUID’s sensor. The size of the pickup loop sets the spatial resolution of the susceptibility images. [(b) and (c)] Images of the same region at (b) $T=17.5\text{K}$ and (c) at $T=18.5\text{K}$ show that the stripes disappear above $T_c$. A topographic feature (scratch) appears in (b) and (c).

Figure 2

(Color online) Sketches of (a) ${\text{BaFe}}_2{\text{As}}_2$ unit cells in the (b) tetragonal and (c) orthorhombic phases (orthorhombic distortion exaggerated for clarity and three-dimensional projection view used to convey the out-of-plane locations of the arsenides). (d) Possible twin boundary configuration. Spins on the Fe sites are drawn in the configuration that they would have in the absence of the twin boundary. (e) The phase diagram of the electron-doped superconductor $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$ modified from Ref. 1. The samples measured in this work are marked by circles and the orange shaded area below $x=0.051$ marks the region where stripes or twin boundaries were observed in these samples.

Figure 3

(Color online) Effect of thermal cycling on the locations of the stripes. [(a)–(c)] Local susceptibility images at 17 K (a) before and (b) after thermal cycling to $T=20\text{K}$, above $T_c$ but below $T_{S/SDW}$, show (c) unchanged stripe locations for all stripes. [(d)–(f)] Images (d) before and (e) after thermal cycling to 90 K, above $T_{S/SDW}$, show (f) changed stripe locations for most stripes. This figure shows overlapping regions representative of much larger regions imaged under similar conditions.

Figure 4

(Color online) (a) Temperature dependence of the local diamagnetism. (Left axis) Typical susceptibility signal $\chi \left(T\right)$ in sample UD3 vs $T/T_c$, measured as the difference between the susceptibility high above the sample and in contact at a specific location (method 1) or averaged over a region of the sample (method 2). (Right axis) Amplitude for stripes $\Delta \chi \left(T\right)$ in UD3 [stripes shown in (b) and (c)]. $T$ is scaled by the local $T_c=18\text{K}$. [(b) and (c)] Susceptometry images of UD3 at (b) 5 K and (c) 15 K. Dark disks at the vortex locations (b) are artifacts related to nonlinearity in the SQUID feedback loop at this set point.