CONICET | Buscador de Institutos y Recursos Humanos

The critical behaviour of the Ising ferromagnet confined in pores of radius R and length L is studiedby means of Monte Carlo computer simulations. Quasi-cylindrical pores are obtained by replicatingn-times a triangular lattice disc of radius R, where L = na and a is the spacing between consecutivereplications. So, spins placed at the surface of the pores have less nearest-neighbours (NN) ascompared to 8 NN for spins in the bulk. These ?missing neighbour? effects undergone by surfacespins cause a strong suppression of surface ordering, leading to an ordinary surface transition. Also,the effect propagates into the bulk for small tubes (R ≤ 12) and the effective critical temperature ofthe pores is shifted towards lower values than in the bulk case. By applying the standard finite-sizescaling theory, subsequently supported by numerical data, we concluded that data collapse of relevantobservables, e.g., magnetization (m), susceptibility, specific heat, etc., can only be observed bycomparing simulation results obtained by keeping the aspect ratio C ≡ R/L constant. Also, by extrapolating?effective? R-dependent critical temperatures to the thermodynamic limit (R→∞, C fixed),we obtained TC(∞) = 6.208(4). As suggested by finite-size scaling arguments, the magnetization ismeasured at the critical point scales according to |m|TcRβν ∝RL12, where β and ν are the standardexponents for the order parameter and the correlation length, respectively. Furthermore, it is shownthat close to criticality the axial correlation length decreases exponentially with the distance. Thatresult is the signature of the formation of (randomly distributed) alternating domains of different magnetization,which can be directly observed by means of snapshot configurations, whose typical length(ξ ) is given by the characteristic length of the exponential decay of correlations. Moreover, we showthat at criticality ξ