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It has been recently argued [1,2 ] that 2d melting of hard and soft repulsive disks occurs in two steps, with
a continuous BKT transition between the solid and hexatic phases, and a first order transition between the
hexatic and liquid phases, when density or packing fraction are decreased at constant temperature. The hexatic
phase has no positional order but quasi long-range orientational order, while the solid phase has quasi
long-range positional and proper long-range orientational order.
We show, for passive particles (disks and dumbbells),
that liquid and quasi long-range orientationally
ordered zones co-exist close to the liquid phase, within a narrow interval of packing fractions. When self-propulsion is switched on, our brownian dynamic simulations suggest that the above co-existence region continuously extend to the strong activity region
where cluster aggregation had already been observed. There is therefore no discontinuity in the phase diagram between the passive and active
regions with phase separation.
1. E. Bernard and W. Krauth, Phys. Rev. Lett. 107, 155704 (2011).
2. G. Gonnella, L. Cugliandolo, A. Suma, P. Di Gregorio, in preparation
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