Density Matrix Renormalization Group Applied to the Ground State of the   XY-Spin-Peierls System
  We use the density matrix renormalization group (DMRG) to map out the ground state of a XY-spin chain coupled to dispersionless phonons of frequency $% \omega $. We confirm the existence of a critical spin-phonon coupling $% \alpha _c\propto \omega ^{0.7}$ for the onset of the spin gap bearing the signature of a Kosterlitz-Thouless transition. We also observe a classical-quantum crossover when the spin-Peierls gap $\Delta $ is of order $% \omega $. In the classical regime, $\Delta >\omega $, the mean-field parameters are strongly renormalized by non-adiabatic corrections. This is the first application of the DMRG to phonons. 
L.G. Caron and S. Moukouri (Universit\'e de Sherbrooke, QC, Canada )