Dynamics of the spin-1/2 isotropic XY chain in a transverse field

Dynamic xx and xy spin pair correlation functions for the isotropic spin-1/2 XY chain are calculated numerically for long open chains in the presence of a transverse (z) magnetic field at finite temperature. From these data the first numerical results for the dynamic structure factors S-xx(kappa, omega) and S-xy(kappa, omega) are obtained and compared with the known results for S-zz(kappa, omega). While S-zz(kappa, omega) is restricted to certain regions in the (kappa, omega)-plane S-xx(kappa, omega) and S-xy(kappa, omega) are not. Nevertheless, the numerical results show that the latter structure factors are quite small outside the regions defined by S-zz(kappa, omega) and moreover, that they may in many circumstances, especially at low temperature, be approximately described by a small number of broadened excitation branches.