Exotic configuration

However, most impurities and defects are Jalm-Teller unstable at high-symmetry sites or/and react covalently with the host crystal much more strongly than interstitial copper. The latter is obviously the case for substitutional impurities, but also for interstitials such as O (which sits at a relaxed, puckered bond-centred site in Si), H (which bridges a host atom-host atom bond in many semiconductors) or the self-interstitial (which often fonns more exotic stmctures such as the split-(l lO) configuration). Such point defects migrate by breaking and re-fonning bonds with their host, and phonons play an important role in such processes. 

The quasi-spin label is perhaps the most exotic quantum characteristic of these states. It is in any case a true shell characteristic since it extends over several configurations. Most importantly it gives rise to very strong selection rules as we will demonstrate in the next section. 

Molecules belonging to C, have only a center of inversion, their group consisting of E, i]. An example is the staggered configuration of an exotic substituted ethane ... 

In both charm and noncharm sectors, new problems have been raised in hadron spectroscopy. One question concerns the existence of localized diquark clusters inside baryons [11]. Another hot topic concerns the possible hybrid states qqqg with a valence gluon and, possibly, exotic quantum numbers. We find it important to analyse carefully the dynamics of ordinary qqq baryons before concluding the need for new configurations. 

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