Quantum interference-induced stability of repulsively bound pairs of excitations

We study the dynamics of two types of pairs of excitations which are bound despite their strong repulsive interaction. One corresponds to doubly occupied sites in one-dimensional Bose-Hubbard systems, the so-called doublons. The other is pairs of neighboring excited spins in anisotropic Heisenberg spin-1/2 chains. We investigate the possibility of decay of the bound pairs due to resonant scattering by a defect or due to collisions of the pairs. We find that the amplitudes of the corresponding transitions are very small. This is a result of destructive quantum interference and explains the stability of the bound pairs.Comment: 12 pages, 3 figure

Mass distribution and structural parameters of Small Magellanic Cloud star clusters

In this work we estimate, for the first time, the total masses and mass function slopes of a sample of 29 young and intermediate-age SMC clusters from CCD Washington photometry. We also derive age, interstellar reddening and structural parameters for most of the studied clusters by employing a statistical method to remove the unavoidable field star contamination. Only these 29 clusters out of 68 originally analysed cluster candidates present stellar overdensities and coherent distribution in their colour-magnitude diagrams compatible with the existence of a genuine star cluster. We employed simple stellar population models to derive general equations for estimating the cluster mass based only on its age and integrated light in the B, V, I, C and T1 filter. These equations were tested against mass values computed from luminosity functions, showing an excellent agreement. The sample contains clusters with ages between 60 Myr and 3 Gyr and masses between 300 and 3000 Mo distributed between ~0.5 deg. and ~2 deg. from the SMC optical centre. We determined mass function slopes for 24 clusters, of which 19 have slopes compatible with that of Kroupa IMF (2.3 +/- 0.7), considering the uncertainties. The remaining clusters - H86-188, H86-190, K47, K63 and NGC242 - showed flatter MFs. Additionally, only clusters with masses lower than ~1000 Mo and flatter MF were found within ~0.6 deg. from the SMC rotational centre.Comment: 12 pages, 19 figures. Includes another 29 full-page figures of supplementary material. Accepted for publication in the MNRA