= Helium-3 pair order parameter
{title2=$d_\mu(\widehat{\mathbf k})=\sum_i A_{\mu i}\widehat k_i$}
For a simple <spin-triplet p-wave pairing> description, let the $3\times3$ <matrix> $A$, with <complex number> entries, map an orbital direction $\widehat{\mathbf k}$ on the <Fermi surface> into a triplet gap vector $\mathbf d=A\widehat{\mathbf k}$. The indices refer to spin and orbital components. A scalar amplitude cannot retain all these relative orientations. <Anthony James Leggett>'s account connects that extra structure to the magnetic responses of the different phases.<Nobel Lecture: Superfluid 3He: the early days as seen by a theorist by Anthony James Leggett>[pp.1004–1008, broken spin-orbit symmetry and pairing-state response]{ref}
Here are two explicit idealizations, with a common positive scale $\Delta$ and an overall <quantum phase> omitted. They show how odd orbital pairing can have either nodes or an isotropic gap. They do not calculate which state has the lower <Helmholtz free energy>.
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