Why Do Spin Flips at Fermi Level Occur?

[johnsmi]

Hi,

I have come across a sentance in an article which I do not quite understand.

flips of the spin due to H₁ occur only at the Fermi level

H₁ is just a perturbation Hamiltonian.

Why will spin flips only occur at the Fermi level?

Thank you

 

[torquil]

I'll take a wild guess that it is because the would-be flipped spin state is already occupied by another electron, for the states below the Fermi level. So the Pauli principle prevents it.