Past exam of the quantum condensed matter course of the University of Cambridge / 2014 / 1 / c

The energy favoring spin polarization must compete with the increase in Fermi kinetic energy. Assume the usual three-dimensional zero-temperature parabolic conduction band at fixed $N$ and $V$. Write the spin polarization as $\zeta =(N_{\uparrow }-N_{\downarrow })/N$, so $N_{\uparrow ,\downarrow }=N(1\pm \zeta )/2$. The unpolarized Fermi energy isThe two spin populations have Fermi energies $E_F(1\pm \zeta {)}^{2/3}$. Summing their individual $3E_{F,\sigma }/5$ mean kinetic energies givesThe specified interaction is an energy per Electron, $U(1-{\zeta }^2)/4$. Thus the energy change per Electron near the unpolarized state isThe quadratic coefficient changes sign at the Stoner threshold for an interaction normalized per electron,The positive quartic term gives a continuous onset in this model. More globally, $f^{\prime \prime }(\zeta )=(E_F/3)[(1+\zeta {)}^{-1/3}+(1-\zeta {)}^{-1/3}]-U/2$. Convexity of $x^{-1/3}$ makes this nonnegative for $U\le 4E_F/3$, so no lower polarized minimum appears before that threshold.

At fixed number density $N/V$, the critical $U$ is independent of the total sample size. A comparison restricted to a fully polarized state gives a different numerical coefficient, $U>\frac{12}{5}{E}_F(2^{2/3}-1)$, but the same requested density scaling; it misses the partially polarized onset. One must also distinguish this $U$, with units of energy, from a contact coupling multiplying an interaction-energy density $g{n}_{\uparrow }{n}_{\downarrow }$: here $g=U/(N/V)$. Importing a differently normalized Stoner criterion would introduce an erroneous extra density factor.