The fate of orbitons coupled to phonons

The key feature of an orbital wave or orbiton is a significant dispersion, which arises from exchange interactions between orbitals on distinct sites. We study the effect of a coupling between orbitons and phonons in one dimension using continuous unitary transformations (CUTs). Already for intermediate values of the coupling, the orbiton band width is strongly reduced and the spectral density is dominated by an orbiton-phonon continuum. However, we find sharp features within the continuum and an orbiton-phonon anti-bound state above. Both show a significant dispersion and should be observable experimentally.Comment: 7 pages, 7 figures; strongly enlarged, comprehensive revised version according to the referees' suggestions, in pres

Thermodynamics of Adiabatically Loaded Cold Bosons in the Mott Insulating Phase of One-Dimensional Optical Lattices

In this work we give a consistent picture of the thermodynamic properties of bosons in the Mott insulating phase when loaded adiabatically into one-dimensional optical lattices. We find a crucial dependence of the temperature in the optical lattice on the doping level of the Mott insulator. In the undoped case, the temperature is of the order of the large onsite Hubbard interaction. In contrast, at a finite doping level the temperature jumps almost immediately to the order of the small hopping parameter. These two situations are investigated on the one hand by considering limiting cases like the atomic limit and the case of free fermions. On the other hand, they are examined using a quasi-particle conserving continuous unitary transformation extended by an approximate thermodynamics for hardcore particles.Comment: 10 pages, 6 figure

High Order Perturbation Theory for Spectral Densities of Multi-Particle Excitations: S=1/2 Two-Leg Heisenberg Ladder

We present a high order perturbation approach to quantitatively calculate spectral densities in three distinct steps starting from the model Hamiltonian and the observables of interest. The approach is based on the perturbative continuous unitary transformation introduced previously. It is conceived to work particularly well in models allowing a clear identification of the elementary excitations above the ground state. These are then viewed as quasi-particles above the vacuum. The article focuses on the technical aspects and includes a discussion of series extrapolation schemes. The strength of the method is demonstrated for S=1/2 two-leg Heisenberg ladders, for which results are presented.Comment: 21 pages, 14 figures included; to appear in Eur. Phys. J. B All technical details for the computation of spectral densities by perturbative CUTs Minor misprints removed, references update

Excitations in one-dimensional S=1/2 quantum antiferromagnets

The transition from dimerized to uniform phases is studied in terms of spectral weights for spin chains using continuous unitary transformations (CUTs). The spectral weights in the S=1 channel are computed perturbatively around the limit of strong dimerization. We find that the spectral weight is concentrated mainly in the subspaces with a small number of elementary triplets (triplons), even for vanishing dimerization. So, besides spinons, triplons may be used as elementary excitations in spin chains. We conclude that there is no necessity to use fractional excitations in low-dimensional, undoped or doped quantum antiferromagnets.Comment: 4 pages, 1 figure include

Unifying Magnons and Triplons in Stripe-Ordered Cuprate Superconductors

Based on a two-dimensional model of coupled two-leg spin ladders, we derive a unified picture of recent neutron scattering data of stripe-ordered La_(15/8)Ba_(1/8)CuO_4, namely of the low-energy magnons around the superstructure satellites and of the triplon excitations at higher energies. The resonance peak at the antiferromagnetic wave vector Q_AF in the stripe-ordered phase corresponds to a saddle point in the dispersion of the magnetic excitations. Quantitative agreement with the neutron data is obtained for J= 130-160 meV and J_cyc/J = 0.2-0.25.Comment: 4 pages, 4 figures included updated version taking new data into account; factor in spectral weight corrected; Figs. 2 and 4 change

Multi-particle excitations and spectral densities in quantum spin-systems

The excitation spectrum of the 2-leg S=1/2 Heisenberg ladder is examined perturbatively. Using an optimally chosen continuous unitary transformation we expand the Hamiltonian and the Raman operator about the limit of isolated rungs leading to high order series expansions allowing to calculate spectral densities quantitatively. The 2-particle sector is examined for total momentum k=0. We show that triplet-triplet interaction gives rise to a band splitting.Comment: 2 pages, 1 figure; submitted to the proceedings of the SCES2001 conference (Physica B