Suppression of superconductivity by anisotropic strain near a nematic quantum critical point*

In most unconventional and high-temperature superconductors, superconductivity emerges as a nearby symmetry-breaking phase is suppressed by chemical doping or pressure, leading to the belief that the fluctuations associated with the symmetry-breaking phase are beneficial, if not responsible, for the superconducting pairing. A direct test to verify this hypothesis is to observe a decrease of the superconducting critical temperature (T_c) by applying the symmetry-breaking conjugate field that suppresses the dynamic fluctuations of the competing order. Here¹, using electrical transport, magnetic susceptibility, and x-ray diffraction measurements, we show that anisotropic strain, the conjugate field of nematicity, reduces the T_c of the iron pnictide superconductor Ba(Fe_1-xCo_x)₂As₂. For optimally doped samples we show a nearly fivefold reduction of T_c with less than one per cent of strain. This extreme sensitivity disappears as the doping is increased away from optimal. For underdoped samples, T_c becomes zero yielding a metallic ground state. In addition to providing direct evidence of the role played by the nematic fluctuations in the formation of the superconducting state, these results also demonstrate a superconductor to metal transition as a function of a clean tuning parameter in a three-dimensional system.

[1] Malinowski, P. et al. Nat. Phys. (2020).
https://doi.org/10.1038/s41567-020-0983-9