Coulomb-Blockade-Induced Bound Quasiparticle States in a Double-Island Structure

We determine the low temperature shape of the Coulomb-blockade staircase in a superconducting double-island device. For an odd number of electrons, in the ground state the intrinsic quasiparticle is bound to the tunneling contact. For a single channel contact the gap between the ground state and the continuum of excited states is of the order of the Josephson energy E_J. The temperature dependence of the Coulomb-blockade step width is nonmonotonic, with the minimal width occurring at T_i∼E_J/ln⁡(√ΔE_J/δ), where Δ and δ are, respectively, the superconducting gap and mean level spacing in the island. For an even number of electrons, the Coulomb enhancement of the Josephson energy is shown to be significantly stronger than that for a single grain coupled to a lead. If the electrostatic energy favors a single broken Cooper pair, the resulting quasiparticles are bound to the contact at T=0.