Skyrme term (${\mathcal{L}}_4=\frac{1}{32e^2}\mathrm{tr}([L_{\mu },L_{\nu }][L^{\mu },L^{\nu }])$)

The particular quartic-derivative interaction in the Skyrme model, with $L_{\mu }=U^{†}{\partial }_{\mu }U$. Its static energy is positive and scales inversely with soliton size, opposing collapse under Derrick scaling. It is one choice of four-derivative mesonic interaction, not the whole general fourth-order chiral perturbation theory Lagrangian.