  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.
