Title: Face centered cubic and hexagonal close packed skyrmion crystals in centro-symmetric magnets,
Abstract: Skyrmions are disk-like objects that typically form triangular crystals in
two dimensional systems. This situation is analogous to the so-called "pancake
vortices" of quasi-two dimensional superconductors. The way in which skyrmion
disks or pancake skyrmions pile up in layered centro-symmetric materials is
dictated by the inter-layer exchange. Unbiased Monte Carlo simulations and
simple stabilization arguments reveal face centered cubic and hexagonal close
packed skyrmion crystals for different choices of the inter-layer exchange, in
addition to the conventional triangular crystal of skyrmion lines. Moreover, an
inhomogeneous current induces sliding motion of pancake skyrmions, indicating
that they behave as effective mesoscale particles.