semimetal

Semimetal is a term used in solid-state physics to describe a material whose electronic structure places it between a metal and a semiconductor. In a conventional metal, the valence and conduction bands overlap widely so that many states are available at the Fermi level. In a typical semiconductor, a finite energy gap separates the valence and conduction bands. A semimetal has only a small overlap between bands or a vanishing band gap, resulting in a small number of charge carriers at the Fermi level. Consequently, both electrons and holes can contribute to electrical conduction, with carrier densities much smaller than in metals.

Because of the small carrier density, semimetals often show unusual transport properties, including highly anisotropic conduction and large magnetoresistance in some materials. Their Fermi surfaces feature pockets corresponding to electrons and holes.

Traditional examples include bismuth, arsenic, and antimony, which were among the earliest recognized semimetals. Mercury telluride in certain compositions and conditions is also considered a semimetal. In two dimensions, graphene can behave as a semi-metal with a zero band gap and linear energy-momentum relation near the Dirac point.

In modern condensed matter physics, the term semimetal is also used to describe materials hosting Dirac or Weyl fermions, known as topological semimetals, which display protected surface or bulk electronic states and unusual transport phenomena.