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(κ,ω) and S_xy(κ,ω) are obtained and compared with the known results for S_zz(κ,ω). While S_zz(κ,ω) is restricted to certain regions in the (κ,ω)-plane S_xx(κ,ω) and S_xy(κ,ω) are not. Nevertheless, the numerical results show that the latter structure factors are quite small outside the regions defined by S_zz(κ,ω) and moreover, that they may in many circumstances, especially at low temperature, be approximately described by a small number of broadened excitation branches.