Structural supermodulation enables resonant Raman determination of crystalline orientation in Bi2Sr2CaCu2O8+x flakes

Bi2Sr2CaCu2O8+x (Bi-2212) is a d-wave superconductor with unconventional properties intrinsic to the material itself. These properties are of growing interest for quantum technologies in electronics and photonics, and twisted Josephson junctions can make direct use of them. However, Bi-2212 is strongly anisotropic, and it is challenging to find its crystal axes in flakes in advance, which limits device engineering. Here, we show that polarized resonant Raman spectroscopy reveals the orientation of both the a and b axes of Bi-2212 optically and non-destructively. By correlating Laue diffraction, HAADF-STEM and polarized Raman scattering on the same bulk crystal, we assign the b axis, along which the incommensurate structural supermodulation propagates, to the intensity maximum of the 27 cm-1 supermodulation mode, which varies in counter-phase with the 118 cm-1 Bi/Sr mode, along the a axis. Multi-wavelength measurements (458, 532, 660 nm) show that resulting in-plane contrast peaks at 532 nm and nearly vanishes at 660 nm, indicating that the anisotropy originates from an electronic resonance induced by the supermodulation. Using this resonance, we determine the crystallographic axes of exfoliated flakes and access their symmetry channels, providing a rapid orientation tool for cuprate van der Waals heterostructures.