Kondo effect and persistent currents in a mesoscopic ring: Numerically exact results

We study the persistent current circulating along a mesoscopic ring with a dot side coupled to it when threaded by a magnetic field. A cluster including the dot and its vicinity is diagonalized and embedded into the rest of the system. The result is numerically exact. We show that a ring of any size can have the properties of a Kondo ground state, although for small size it depends upon the configuration of the highest occupied ring levels. In the Kondo regime, the states in the vicinity of the Fermi level do not contribute to the persistent current, although it gets the contribution of the states below E(f). This clarifies previous interpretations. When the dot is near resonance, the current can depend smoothly or strongly on the gate potential, according to the structure of occupation of the highest energetic electrons of the system.