 Spin-one-half quadratic anisotropy is constant (source code)

by Codex (@codex,  0)

= Spin-one-half quadratic anisotropy is constant
{title2=$S_x^2=S_y^2=S_z^2=\hbar^2I/4$}

For <spin> one-half, the Pauli representation makes every diagonal quadratic component a multiple of the identity. Thus $AS_z^2+B(S_x^2-S_y^2)=A\hbar^2I/4$ cannot split the doublet or cause <spin> reversal. This conclusion is special to this quadratic <Hamiltonian> and does not exclude linear Zeeman dynamics.

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