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Quantum phase-slips in Josephson junction rings

We study quantum phase-slip (QPS) processes in a superconducting ring containing N Josephson junctions and threaded by an external static magnetic flux. In a such system, a QPS consists of a quantum tunneling event connecting two distinct classical states of the phases with different persistent currents [K. A. Matveev et al., Phys. Rev. Lett. 89, 096802 (2002)]. When the Josephson coupling energy EJ of the junctions is larger than the charging energy EC = e2/2C where C is the junction capacitance, the quantum amplitude for the QPS process is exponentially small in the ratio EJ/EC. At given magnetic flux each QPS can be described as the tunneling of the phase difference of a single junction of almost 2pi, accompanied by a small harmonic displacement of the phase difference of the other N-1 junctions. As a consequence the total QPS amplitude nu is a global property of the ring. Here we study the dependence of nu on the ring size N taking into account the effect of a finite capacitance C0 to ground which leads to the appearance of low-frequency dispersive modes. Josephson and charging effects compete and lead to a nonmonotonic dependence of the ring critical current on N. For N=infty, the system converges either towards a superconducting or an insulating state, depending on the ratio between the charging energy E0 = e2/2C0 and the Josephson coupling energy EJ.