ULTRASOFT, CLUSTER-FORMING SYSTEMS UNDER COMPRESSION

Martes 4 de marzo a las 12:00 en el aula 300

Marta Montes Saralegui

Insitut für Theoretische Physik and Center for Material Science,

Technische Universität Wien

We study the response of two-dimensional ultra-soft cluster crystals
[1] under the influence of an external pressure- and
temperature-reservoir. Using molecular dynamics simulations, the
system is in contact with a combined baro- and thermostat which is
realized via a an ensemble of ideal gas particles [2]: pressure and
temperature are triggered via the number of the particles and their
velocities. Starting from a fluid state the system is compressed until
it solidifies in an ordered hexagonal cluster lattice. Upon further
increasing the pressure, the volume decreases while the lattice
constant remains (essentially) constant. This is achieved by a merging
process of neighbouring clusters. The mutual repulsion of the
particles within these clusters leads, in turn, to activated hopping
processes of individual particles which start to migrate through the
system. After some time, the processes of cluster merging and particle
hopping lead to an equilibrated state [3]. By tracing the particle
positions we study this complex interplay of the two processes at
different temperature and compression rates.

Our setup also allows to extract the equation-of-state of the system
(i.e. the density as a function of pressure at some given temperature)
via a combined compression-annealing experiment [4]. Due to the finite
compression rate of our barostat we first obtain metastable states of
our system along the compression line; launching expansion processes
at selected pressure values leads to hysteresis loops. Now we take at
given pressure value states along these hystereses and heat the
corresponding configurations until they melt. Cooling these states
subsequently down to the desired temperature leads via an annealing
process to an equilibrium state at this particular pressure value,
specified by an esentially ideal hexagonal arrangment of the clusters.
Performing this annealing procedure for different pressure values
leads to the equation of state.


[1] B. Mladek et al, PRL 96, 045701 (2006)
[2] M. Grünwald and C. Dellago, Mol. Phys. 104,3709 (2006)
[3] M. Montes-Saralegui et al, submitted to JPCM
[4] M. Montes-Saralegui et al, submitted to Soft Matter