Excitation spectra of structurally dimerized and spin-Peierls chains in a magnetic field

The dynamical spin structure factor and the Raman response are calculated f or structurally dimerized and spin-Peierls chains in a magnetic field, usin g exact diagonalization techniques. In both cases there is a spin liquid ph ase composed of interacting singlet dimers at small fields h<h(c1), an inco mmensurate regime (h(c1) <h<h(c2)) in which the modulation of the tripler e xcitation spectra adapts to the applied held, and a fully spin-polarized ph ase above an upper critical field h(c2). For structurally dimerized chains, the spin gap closes in the incommensurate phase, whereas spin-Peierls chai ns remain gapped. In the spin liquid regimes, the dominant feature of the t riplet spectra is a one-magnon bound state, separated from a continuum of s tates at higher energies. There are also indications of a singlet bound sta te above the one-magnon triplet.