Freezing of a soft-core fluid in a one-dimensional potential: Appearance of a locked smectic phase

We investigate the phase behaviour of a two-dimensional colloidal model system of ultra-soft particles on a substrate which varies periodically along one spatial direction. Our calculations are based on mean-field density functional theory for a system of particles interacting via an ultra-soft potential, that is, the generalized exponential model with index four (\mbox{GEM-4}). For suitable substrate periodicities (with commensurability parameter $p=2$), we find a succession of phase transitions from a modulated liquid to a locked smectic and then to a locked floating solid phase. The appearance of a locked smectic phase is consistent with earlier theoretical predictions and experiments for freezing of more repulsive systems on structured surfaces (with $p=2$). However, the present ultra-soft system does not display re-entrant melting. We here investigate the details of the density distributions of the different phases, thereby supplementing earlier work on GEM-4 systems with $p=1$ [Phys. Rev. E \textbf{101}, 012609 (2020)]. Interestingly, the observed succession of phase transitions can be triggered through different paths along which physical control parameters are changed.