  A Dirac electron system in solids mimics a relativistic quantum physics that
is compatible with Maxwell's equations, by which we anticipate unified
electromagnetic responses. We find a large orbital diamagnetism only along the
interplane direction and the nearly temperature-independent conductance of the
order of e2/h for the new 2D Dirac organic conductor, a-(BETS)2I3. Distinct
from conventional electrons in solids whose nonrelativistic effects bifurcate
electric and magnetic responses, the observed orbital diamagnetism scales the
electrical conductivity for a wide temperature range. This demonstrates that an
electromagnetic duality that is valid only within the relativistic framework is
revived in solids.
