Fig. 3: SHG in NiI2 under different pressures.

a Temperature-dependent SHG intensity in a NiI2 flake at seven fixed hydrostatic pressures, e.g., 2.7, 4.4, 6.0, 6.5, 7.0, 8.6, and 11.0 GPa. The inset shows the pressure dependence of the SHG intensity at approximately 15 K. b Angular-resolved SHG patterns for temperatures below and above the helimagnetic transition temperature at the six representative hydrostatic pressures. Here, the orange, green, and gray regions represent the helimagnetic state (HM), antiferromagnetic state (AFM) and intermediate state (IMS), respectively.