UC Berkeley

The discovery of the iron-based superconductors (Fe-SCs) has been a great boon to the continuing development of our understanding of high-Tc superconductivity, providing a wealth of new information and a valuable theoretical testbed. One of the most interesting developments has been the observation of nematic quantum criticality near optimal doping in several Fe-SCs. This, together with evidence of electronic nematicity in the cuprates, has fueled speculation that high-Tc superconductivity may be related to nematic physics at a fundamental level.

Using ultrafast pump-probe reflectance as a photomodulation technique, we investigate superconducting and nematic order in the low-disorder BaFe2(As_{1−x}P_x)2 system, which contains a nematic quantum critical point. Our nonequilibrium approach is well-suited to the study of the onset and evolution of ordered phases, as the dominant effect of the pump pulse tends to be the suppression of the ordered phase. This yields a background-free probe of the order parameter, unlike measurements at equilibrium.

At and above optimal doping x = 0.31 we observe nematic order manifested in a change in sign of the pump-probe response under polarization rotation. This nematic phase is observed above the superconducting critical temperature and competes with the superconducting phase, and it typically onsets sharply with decreasing temperature in a manner inconsistent with a Curie-Weiss divergence.

We study the spatial variation of the nematic photomodulation response, and find that typical crystals have inhomogeneous nematicity on a length scale of 10−100 μm. In fact, the sign, magnitude, and onset temperature of the nematic response all vary significantly at optimal doping. The observation of this inhomogeneity helps resolve tension between localized probes that show evidence of nematicity, and bulk probes, which do not.

By combining maps of strain obtained via spatially resolved Laue diffraction with photomodulation maps of nematicity at optimal doping, we test the hypothesis that a strong nematic susceptibility amplifies strain anisotropies intrinsic to the crystal in order to produce the observed nematic behavior. We find that local strain anisotropies cannot account for the nematic order we observe, although applying a tensile uniaxial strain of 0.1% to an optimally doped sample in the Fe−Fe frame does bias the population of nematic domains.

We extend this multi-probe mapping analysis to the overdoped regime, and find that the nematic fraction of the samples studied decreased sharply for x > 0.39. However, each overdoped sample studied contains at least some regions of nematicity of both signs, with the same qualitative character as at optimal doping. Intriguingly, at the highest studied doping, x = 0.50, the apparent superconducting transition temperature in otherwise isotropic regions is found to change by 0.5 to 2.0 K under a π/2 rotation of the pump and probe polarization.