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Non-Fermi liquid behaviour in non-equilibrium transport through Co doped Au chains connected to four-fold symmetric leads

We calculate the differential conductance as a function of temperature and bias voltage, $G(T,V)$, through Au monoatomic chains with a substitutional Co atom as a magnetic impurity, connected to a four-fold symmetric lead. The system was recently proposed as a possible scenario for observation of the overscreened Kondo physics. Stretching the chain, the system could be tuned through a quantum critical point (QCP) with three different regimes, overscreened, underscreened and non Kondo phases. We present calculations of the impurity spectral function by using the numerical renormalization group (NRG) for the three different regimes characterizing the QCP. Non trivial behaviour of the spectral function is reported near the QCP. Comparison with results using the non crossing approximation (NCA), shows that the latter is reliable in the overscreened regime, when the anisotropy is larger than the Kondo temperature. For these parameters, which correspond to realistic previous estimates, $G(T,V)$ calculated within NCA exhibits clear signatures of the non-Fermi liquid behaviour within the overscreened regime.