Dirac semimetal

In most materials, an electron is sluggish, like a marble rolling in a bowl — it speeds up gradually as you push it, because it carries a definite weight. But in a few special materials, electrons behave instead like beams of light: they have no effective mass and always move at a fixed, brisk speed. A Dirac semimetal is a material where the electrons act like these massless, light-like particles.

More precisely, a Dirac semimetal is a material whose energy bands meet at isolated points where the valence and conduction bands touch in a cone shape, like two ice-cream cones tip to tip. Near such a point, an electron's energy rises in straight-line proportion to its momentum, which is exactly the rule obeyed by massless relativistic particles — the kind described by the Dirac equation. So inside the crystal, electrons mimic massless particles that race along at a single fixed velocity, far below light speed but constant in the same way.

This matters because these light-like electrons give such materials remarkable properties — very high mobility, unusual responses to magnetic fields — and they serve as a parent state from which other topological phases can be born by gently breaking a symmetry. A common confusion is to think the electrons literally travel at the speed of light; they do not. They merely obey the same massless equation, but with a velocity hundreds of times slower. And the cones occur only at special points, not everywhere in the material.