Persistent charge and spin currents in the long-wavelength regime for graphene rings

We address the problem of persistent charge and spin currents on a Corbino disk built from a graphene sheet. We consistently derive the Hamiltonian including kinetic, intrinsic (ISO), and Rashba spin-orbit interactions in cylindrical coordinates. The Hamiltonian is carefully considered to reflect hermiticity and covariance. We compute the energy spectrum and the corresponding eigenfunctions separately for the intrinsic and Rashba spin-orbit interactions. In order to determine the charge persistent currents, we use the spectrum equilibrium linear response definition. We also determine the spin and pseudospin polarizations associated with such equilibrium currents. For the intrinsic case, one can also compute the correct currents by applying the bare velocity operator to the ISO wave functions or alternatively the ISO group velocity operator to the free wave functions. Charge currents for both SO couplings are maximal in the vicinity of half-integer flux quanta. Such maximal currents are protected from thermal effects because the contributing levels plunge (∼1 K) into the Fermi sea at half-integer flux values. Such a mechanism, makes them observable at readily accessible temperatures. Spin currents only arise for the Rashba coupling, due to the spin symmetry of the ISO spectrum. For the Rashba coupling, spin currents are canceled at half-integer fluxes but they remain finite in the vicinity, and the same scenario as above protects spin currents.