Physical Review D
In a hard-wall model of holographic QCD, we find that nonlinear boundary dynamics are required in order to maintain the correct pattern of explicit and spontaneous chiral symmetry breaking beyond leading order in the pion fields. With the help of a field redefinition, we relate the requisite nonlinear boundary conditions to a standard Sturm-Liouville system. Observables insensitive to the chiral limit receive only small corrections in the improved description, and classical calculations in the hard-wall model remain surprisingly accurate.
Albrecht, Dylan; Erlich, Joshua; and Wilcox, Ronald J., Nonlinear boundary dynamics and chiral symmetry in holographic QCD (2012). Physical Review D, 85(11).