  The nucleation of a droplet of stable cylinder phase from a metastable
lamellar phase is examined within the single-mode approximation to the
Brazovskii model for diblock copolymer melts. By employing a variational ansatz
for the droplet interfacial profile, an analytic expression for the interfacial
free-energy of an interface of arbitrary orientation between cylinders and
lamellae is found. The interfacial free-energy is anisotropic, and is lower
when the cylinder axis is perpendicular to the interface than when the
cylinders lie along the interface. Consequently, the droplet shape computed via
the Wulff construction is lens-like, being flattened along the axis of the
cylinders. The size of the critical droplet and the nucleation barrier are
determined within classical nucleation theory. Near the lamellar/cylinder phase
boundary, where classical nucleation theory is applicable, critical droplets of
size 30--400 cylinders across with aspect ratios of 4--10 and nucleation
barriers of 30--40 k_B T are typically found. The general trend is to larger
critical droplets, higher aspect ratios and smaller nucleation barriers as the
mean-field critical point is approached.
