The one-dimensional frustrated Heisenberg antiferromagnet

The one-dimensional S=1/2 antiferromagnetic Heisenberg model with nearest- (J1) and next-nearest-neighbor (J2) interactions is investigated, over the whole temperature region, by employing the double-time Green function method and performing a decoupling proposed by Kondo and Yamaji for cases without long-range order. The susceptibility is calculated and compared to other numerical calculations existing in the literature. We also evaluate the critical value for the J2/J1 ratio and find good agreement with other theoretical estimates