e e e z (slab) (10) where e e e z is a unit vector along the z-direction. The magnitude of a a a is zero in the (x, y, L z /2) plane and increases with the distance from this plane up to a maximum of 1/2. The direction of a a a is along -e e e z at the left and +e e e z at the right of the plane. In spherical geometry, the vector a a a is defined as