Eigenvector form

Hello, I try to solve the Laplacian eigenvalue problem of fenics demos but with different domain. My bilinear form is Screenshot from 2021-02-17 12-58-31 and i have square domain (2D problem). The four largest eigenfunction will look as follows:

Can someone explain to me why they have this form with parallel lines. I expected to have a form like the one below:


Thank you !!!

The results you’ve posted at the bottom look like the eigenvectors normalised with respect to the mass matrix in a generalised eigenvalue problem. The results you’ve posted at the top look like the eigenvectors of the FE discretised diffusion operator.