Quantum delocalization and correlation effects in one-dimensional chains of adsorbed hydrogen atoms

Quantum delocalization and correlation effects in one-dimensional chains of bosons are treated using a Bose-Hubbard Hamiltonian including on-site and nearest-neighbor repulsion terms. The parameters were chosen in such a way that the calculations are appropriate for hydrogen atoms adsorbed in the troughs of fcc(110) surfaces. Employing direct diagonalization of the Hamilton matrix for small periodic systems, we find that the hydrogen atoms are always delocalized except for half-filling corresponding to a coverage of rho = 1/2, where an ordered structure results for small tunnel parameters and sufficiently large nearest-neighbor repulsion, in accordance with experimental findings. For this coverage, a phase diagram as a function of the tunnel parameter and the nearest-neighbor repulsion is determined. Only if the translational invariance of the chain is perturbed, ordered structures for other coverages can be created. Larger systems are studied using the density matrix renormalization group (DMRG) algorithm. Using the finite length version of the DMRG algorithm, we find ordered states also for coverages of rho = 1/3 and 1/4 which are obviously a consequence of the perturbation caused by the termination of the finite chains.