Anomalous conductance of a spin-1 quantum dot.

We interpret the recent observation of a zero-bias anomaly in spin-1 quantum dots in terms of an underscreened Kondo effect. Although spin-1 quantum dots are expected to undergo a two-stage quenching effect, in practice the log-normal distribution of Kondo temperatures leads to a broad temperature region dominated by underscreened Kondo physics. General arguments, based on the asymptotic decoupling between the partially screened moment and the leads, predict a singular temperature and voltage dependence of the conductance G and differential conductance g, resulting in dg/dT approximately 1/T and dG/dV approximately 1/V. Using a Schwinger boson approach, we show how these qualitative expectations are borne out in a detailed many body calculation.