  The field-induced quantum phase transitions (QPT) of the spin ladder material
Bi(Cu(1-x)Znx)2PO6 have been investigated via 31P nuclear magnetic resonance
(NMR) on single-crystal samples with x = 0 and x = 0.01. Measurements at
temperatures between 0.25 K and 20 K in magnetic fields up to 31 T served to
establish the nature of the various phases. In BiCu2PO6, an incommensurate (IC)
magnetic order develops above a critical field mu0*H(c1) ~ 21 T; the field and
temperature dependences of the NMR lines and the resulting model for the spin
structure are discussed. Supported by results of Density-Matrix Renormalization
Group (DMRG) calculations it is argued that the observed field-induced IC order
involves the formation of a magnetic-soliton lattice. An additional QPT is
predicted to occur at H > H(c1). For x = 0.01, this IC order is found to be
stable against site disorder, although with a renormalized critical field.
