In most unconventional and high-temperature superconductors, superconductivity emerges as a nearby symmetry-breaking phase is suppressed by chemical doping or pressure^{1,2,3,4,5,6,7}. 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.

在大多数非常规高温超导体中，由于附近的对称破坏相受到化学掺杂或压力^{1,2,3,4,5,6,7的}抑制，因此出现了超导性。这导致人们相信，与对称破坏阶段有关的波动对超导对^8,9有利，如果不负责任的话。验证该假设的直接检验是观察超导临界温度（T _c）通过应用对称对称共轭场来抑制竞争顺序的动态波动。但是，非常规超导体中的大多数竞争相都破坏了平移对称性，因此需要很难通过实验实现的空间调制共轭场。在这里，我们证明了各向异性应变，即向列共轭场，降低了铁素铁的T _c。对于最佳掺杂的样品，我们显示出T _c降低了五倍，而应变却不到百分之一。对于掺杂不足的样品，T _c变为零，产生全金属基态。这些结果除了提供直接证据表明向列波动在超导状态形成中所起的作用外，还证明了高温超导体的可调机械控制，这是超导技术应用的重要一步。