Gyroid Phase in Nuclear Pasta

Nuclear matter is considered to be inhomogeneous at subnuclear densities that are realized in supernova cores and neutron star crusts, and the structures of nuclear matter change from spheres to cylinders, slabs, cylindrical holes, and spherical holes as the density increases. In this Letter, we discuss other possible structures, that is, gyroid and double-diamond morphologies, which are periodic bicontinuous structures discovered in a block copolymer. Utilizing the compressible liquid drop model, we show that there is a chance of gyroid appearance near the transition point from a cylinder to a slab and the volume fraction at this point is also similar for nuclear and polymer systems. Although the five shapes listed initially have been long thought to be the only major constituents of so-called nuclear pasta at subnuclear densities, our findings imply that this belief needs to be reconsidered.