How to load a Scalars Overlay for a Model Node via Python

[Aping]

Hello,

I want to overlay some .mgh file on model (lh.pial),
the model can be loaded by:
model = slicer.util.loadModel(‘lh.pial’)
but it did not work by:
scalar = slicer.util.loadScalarOverlay(‘surf.mgh’)
Is there a function to overlay the .mgh on model?

Thanks so much.