New phase of matter in lamellar phases of tethered, crystalline membranes

A qualitatively new, ``decoupled'' phase of matter occurs in lamellar phases of tethered, crystalline membranes at large membrane separations d. This decoupled phase has no first-order elastic couplings between successive membranes; as a result, translational order falls off algebraically, with a universal exponent transverse to the membranes. The universal d dependence of the elastic constants is calculated, as is the critical separation dc at which the decoupling phase transition occurs. Untethered, crystalline membranes melt, as d is increased, at some ddc.