  A two-dimensional gas of massless Dirac fermions (MDFs) is a very useful
model to describe low-energy electrons in monolayer graphene. Because the MDF
current operator is directly proportional to the (sublattice) pseudospin
operator, the MDF current-current response function, which describes the
response to a vector potential, happens to coincide with the
pseudospin-pseudospin response function. In this work we present analytical
results for the wavevector- and frequency-dependent longitudinal and transverse
pseudospin-pseudospin response functions of noninteracting MDFs. The transverse
response in the static limit is then used to calculate the noninteracting
orbital magnetic susceptibility. These results are a starting point for the
construction of approximate pseudospin-pseudospin response functions that would
take into account electron-electron interactions (for example at the
random-phase-approximation level). They also constitute a very useful input for
future applications of current-density-functional theory to graphene sheets
subjected to time- and spatially-varying vector potentials.

Strongly Correlated Electrons;Mesoscale and Nanoscale Physics