Critical Properties of gapped spin-1/2 chains and ladders in a magnetic   field
  An interesting feature of spin-1/2 chains with a gap is that they undergo a commensurate-incommensurate transition in the presence of an external magnetic field $H$. The system is in a gapless incommensurate regime for all values of the magnetic field between the lower critical field $H_{c1}$ and an upper critical field $H_{c2}$, where it is gapless and has power law correlations. We calculate the critical exponents for such a generic gapped system in the incommensurate regime at the critical field $H_{c1}$ and in its vicinity. Our analysis also applies to the spin-1/2 ladder. We compute the full dynamical susceptibilities at finite temperature. We use the same to discuss the thermal broadening of various modes and obtain the low temperature behaviour of the nuclear spin relaxation rate $T_1^{-1}$. We discuss the results obtained here for the special cases of the dimerised chain, frustrated chain and the spin-1/2 ladder. 
R. Chitra and T. Giamarchi