Dimer and trimer fluctuations in the s=1/2 transverse XX chain
  Exact results for the dynamic dimer and trimer structure factors of the one-dimensional s=1/2 XX model in a transverse magnetic field ($\parallel z$) 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-fermion 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 with corresponding properties of the two-fermion excitation continuum, as probed by the dynamic dimer and transverse spin structure factors. 
Oleg Derzhko, Taras Krokhmalskii, Joachim Stolze and Gerhard M\"uller