We consider a system of two spins with degenerate Larmor frequencies under a STM bias and derive the dissipative Lindblad equations. We find that the tunneling elements to the environments (tip and substrate) generate an exchange interaction between the spins, as well as a Dzyaloshinskii-Moriya interaction in presence of spin-orbit coupling.
We show that when the spins have equal tunneling amplitudes (without spin-orbit) there is a dark state with vanishing decay rate. We also show that the resulting STM spectrum has additional transitions relative to those of conventional spin resonance experiments. The presence of dipole interaction is also considered, with similar conclusions.