Toulouse limit for the nonequilibrium Kondo impurity: Currents, noise spectra, and magnetic properties

We present an exact solution to the nonequilibrium Kondo problem, based on a special point in the parameter space of the model where both the Hamilton ian and the operator describing the nonequilibrium distribution can be diag onalized simultaneously. Through this solution we an able to compute the di fferential conductance, spin current, charge-current noise, and magnetizati on, for arbitrary voltage bias. The differential conductance shows the stan dard zero-bias anomaly and its splitting under an applied magnetic field. A detailed analysis of the scaling properties at low temperature and voltage is presented. The spin current is independent of the sign of the voltage. Its direction depends solely on the sign of the magnetic field and the asym metry in the transverse coupling to the left and right leads. The charge-cu rrent noise can exceed 2eI(c) for a large magnetic held, where I-c is the c harge current. This is not seen in noninteracting quantum problems, but occ urs here because of the tunneling of pairs of electrons. The finite-frequen cy noise spectrum has singularities at (h) over bar Omega = +/-2 eV, which cannot hp explained in terms nf noninteracting electrons. These singulariti es are traced to a different type of pair process involving the simultaneou s creation or annihilation of two scattering states. The impurity susceptib ility has three characteristic peaks as a function of magnetic field, two o f which are due to interlead processes and one is due to intralead processe s. Although the solvable point is only one point in the parameter space of the nonequilibrium Kondo problem, we expect it to correctly describe the st rong-coupling regime of the model for arbitrary antiferromagnetic coupling constants and to be qualitatively correct as one leaves the strong-coupling regime. [S0163-1829(98)02442-4].