Date of Original Version
Exact results for the dynamic dimer and trimer structure factors of the one-dimensional s=1/2 XX model in a transverse magnetic field (∥z) are presented and discussed in relation to known exact results for the dynamic spin structure factors. In the framework of the Jordan-Wigner representation, the accessible spectrum of the dimer fluctuation operator is limited to two-fermion excitations, whereas that of the trimer fluctuation operator involves two- and four-fermion excitations. The spectral boundaries, soft modes, and singularity structure of the four-fermion excitation continuum as probed by the dynamic trimer structure factor are examined and compared to corresponding properties of the two-fermion excitation continuum, as probed by the dynamic dimer and transverse spin structure factors.
O. Derzhko, T. Krokhmalskii, J. Stolze, and G. Müller. Dimer and trimer fluctuations in the s=1/2 transverse XX chain. Physical Review B, 71(2005), 104432, pp. 1-12.
Available at: http://dx.doi.org/10.1103/PhysRevB.71.104432