One-dimensional spin-1/2 Heisenberg antiferromagnet in a weak external magnetic field

The one-dimensional spin-1/2 Heisenberg antiferromagnet in a weak magnetic field h is studied using the bosonization method. We derive a set of renorm alization-group equations. The fixed point is reached when the field is sca led to the value at which the system is quarter filled. As the magnetic fie ld varies, a continuum line of fixed points is formed. We compute the unifo rm longitudinal susceptibility chi(z)(h). The singular behavior of chi(z)(h ) as h-->0 is found to be contained in l/ln(h(0)/h) with h(0) a nonuniversa l constant. The spin-spin correlations in the magnetic field are calculated .