 In 2014 researchers discovered that sodium <unk> can exist as a form of matter called a “ three @-@ dimensional topological Dirac semi @-@ metal ” ( 3DTDS ) that possess 3D Dirac fermions in bulk . It is a natural , three @-@ dimensional counterpart to graphene with similar electron mobility and velocity . <unk> and topological insulators ( such as those in 3DTDS ) are both crystalline materials that are electrically insulating inside but conducting on the surface , allowing them to function as transistors and other electronic devices . While sodium <unk> ( Na 
