Matter in strong magnetic fields

The properties of matter are drastically modified by strong magnetic fields, B much greater thanm(e)(2)e(3)c/h(3)=2.35x10(9)G (1G=10(-4)T), as are typically found on the surfaces of neutron stars. In such strong magnetic fields, the Coulomb force on an electron acts as a small perturbation compared to the magnetic force, The strong-field condition can also be mimicked in laboratory semiconductors. Because of the strong magnetic confinement of electrons perpendicular to the field, atoms attain a much greater binding energy compared to the zero-field case, and various other bound states become possible, including molecular chains and three-dimensional condensed matter. This article reviews the electronic structure of atoms, molecules, and bulk matter, as well as the thermodynamic properties of dense plasma, in strong magnetic fields, 10(9)G much less thanB less than or similar to 10(16)G. The focus is on the basic physical pictures and approximate scaling relations, although various theoretical approaches and numerical results are also discussed. For a neutron star surface composed of light elements such as hydrogen or helium, the outermost layer constitutes a nondegenerate, partially ionized Coulomb plasma if B less than or similar to 10(15)G (at temperature T greater than or similar to 10(6)K), and may be in the form of a condensed liquid if the magnetic field is stronger (and T greater than or similar to 10(6)K). For an iron surface, the outermost layer of the neutron star can be in a gaseous or a condensed phase, depending on the cohesive property of the iron condensate.