  In order to characterize nuclear matter under extreme conditions, we
calculate all diffusion transport coefficients related to baryon, electric and
strangeness charge for both a hadron resonance gas and a simplified kinetic
model of the quark-gluon plasma. We demonstrate that the diffusion currents do
not depend only on gradients of their corresponding charge density. Instead, we
show that there exists coupling between the different charge currents, in such
a way that it is possible for density gradients of a given charge to generate
dissipative currents of another charge. Within this scheme, the charge
diffusion coefficient is best viewed as a matrix, in which the diagonal terms
correspond to the usual charge diffusion coefficients, while the off-diagonal
terms describe the coupling between the different currents. In this letter, we
calculate for the first time the complete diffusion matrix including the three
charges listed above. We find that the baryon diffusion current is strongly
affected by baryon charge gradients, but also by its coupling to gradients in
strangeness. The electric charge diffusion current is found to be strongly
affected by electric and strangeness gradients, whereas strangeness currents
depend mostly on strange and baryon gradients.
