  A superconductor in a magnetic field has surface currents that prevent the
magnetic field from penetrating its interior. These currents carry kinetic
energy and mechanical momentum. When the temperature is raised and the system
becomes normal the currents disappear. Where do the kinetic energy and
mechanical momentum of the currents go, and how? Here we propose that the
answer to this question reveals a key necessary condition for materials to be
superconductors, that is not part of conventional BCS-London theory:
superconducting materials need to have hole carriers.
