Field-Induced Antiferromagnetism in the High-Temperature Superconductor La2-XSrXCuO4

There is strong evidence that magnetic interactions play a crucial role in the mechanism driving high-temperature superconductivity in cuprate superconductors. To investigate this we have done a series of neutron scattering measurements on La_2-xSr_xCuO₄ (LSCO) in an applied magnetic field. Below T_c the field penetrates the superconductor via an array of normal state metallic inclusions or vortices. Phase coherent superconductivity characterized by zero resistance sets in at the lower field-dependent irreversibility temperature (T_irr). We have measured optimally doped LSCO (x=0.16, T_c=38.5 K) and under-doped LSCO (x=0.10, T_c=29 K); both have an enhanced antiferromagnetic response in a field. Measurements of the optimally doped system at H=7.5 T show that sub-gap spin fluctuations first disappear with the loss of finite resistivity at T_irr, but then reappear at a lower temperature with increased lifetime and correlation length compared to the normal state. In the under-doped system elastic antiferromagnetism develops below T_c in zero field, and is significantly enhanced by application of a magnetic field. Phase coherent superconductivity is then established within the antiferromagnetic phase at T_irr; thus, the situation in under-doped LSCO is the reverse of that for the optimally doped LSCO where the zero-resistance state develops first before the onset of antiferromagnetism.