NEW BROKEN-PARITY STATE AND A TRANSITION TO ANOMALOUS LAMELLAE IN EUTECTIC GROWTH

We discover that a eutectic system exhibits pseudobicritical features corresponding to the simultaneous birth of two different broken-parity states. The first branch is the known tilted-growth mode, i.e, global parity breaking, it bifurcates from the usual lamellar symmetric stat e. At approximately the same criticial point, a second brach merges cl ose to the second fold we found previously. The two states thus bifurc ate from different basic states. This new branch owes its existence to the underlying degeneracy of the usual broken-parity state. It consis ts of a structure where one lamella assumes a right-traveling and the other a left-traveling state. As a consequence, the drift velocity (or equivalently the tilt angle) is smaller for the new branch. Close to the bicritical point, the bifurcation is described by a Landau theory with the tilt angle being the order parameter. From general considerat ions we can state that the new branch is locally stable in the kinetic sense, but less stable than the usual branch. This result is consiste nt with the conventional criterion based on comparison of undercooling , since the new branch has a higher undercooling. We propose a simple experimental protocol to have access to the new state. Finally, when t he two lamellae have equivalent properties (a symmetric case) the new state is not traveling while each lamella is asymmetric with respect t o its center and is a mirror image of the other lamella. This correspo nds to the so-called anomalous cells observed in noneutectic systems. We develop an analytic theory, in the same spirit as the one used for the parity breaking, to account for the transition to the anomalous st ate. The results are in qualitative agreement with the full numerical calculation.