LAUSR

In most unconventional and high-temperature superconductors, superconductivity emerges as a nearby symmetry-breaking phase is suppressed by chemical doping or pressure 1 – 7 . This has led to the belief that the fluctuations associated with the symmetry-breaking phase are beneficial, if not responsible, for the superconducting pairing 8 , 9 . 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. However, most of the competing phases in unconventional superconductors break translational symmetry, requiring a spatially modulated conjugate field that is difficult to realize experimentally. Here, we show that anisotropic strain, the conjugate field of nematicity, reduces the T c of an iron pnictide. For optimally doped samples we show a fivefold reduction of T c with less than one per cent of strain. For underdoped samples, T c becomes zero yielding a fully 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 demonstrate tunable mechanical control of a high-temperature superconductor, an important step forward for technological applications of superconductivity. Using doped BaFe 2 As 2 , the authors test whether nematicity is linked to superconductivity in the iron pnictides by applying the conjugate field to nematicity—a specific form of strain—and observe that the critical temperature decreases.