The possible charge, spin and orbital currents that can be created in a material. The charge can give rise through its motion to a conventional current (the charge current), but spin and orbital dipoles or quadrupoles can also generate more-complex types of current. Indeed, the spin (S) and orbital (L) angular momentum are pseudovectors that change sign where there is time-reversal symmetry, and then the spin and orbital currents carrying spin or orbital dipoles are time-reversal conserving. Instead, the currents carrying orbital or spinorbital quadrupoles break time-reversal symmetry and yield non-vanishing amplitudes for the dipole and quadrupole observables at a given momentum.
