177 research outputs found

    Superconductor-insulator duality for the array of Josephson wires

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    We propose novel model system for the studies of superconductor-insulator transitions, which is a regular lattice, whose each link consists of Josephson-junction chain of N≫1N \gg 1 junctions in sequence. The theory of such an array is developed for the case of semiclassical junctions with the Josephson energy EJE_J large compared to the junctions's Coulomb energy ECE_C. Exact duality transformation is derived, which transforms the Hamiltonian of the proposed model into a standard Hamiltonian of JJ array. The nature of the ground state is controlled (in the absence of random offset charges) by the parameter qβ‰ˆN2exp⁑(βˆ’8EJ/EC)q \approx N^2 \exp(-\sqrt{8E_J/E_C}), with superconductive state corresponding to small q<qcq < q_c . The values of qcq_c are calculated for magnetic frustrations f=0f= 0 and f=12f= \frac12. Temperature of superconductive transition Tc(q)T_c(q) and q<qcq < q_c is estimated for the same values of ff. In presence of strong random offset charges, the T=0 phase diagram is controlled by the parameter qΛ‰=q/N\bar{q} = q/\sqrt{N}; we estimated critical value qΛ‰c\bar{q}_c.Comment: 5 pages, 2 figure

    Coulomb effects in a ballistic one-channel S-S-S device

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    We develop a theory of Coulomb oscillations in superconducting devices in the limit of small charging energy ECβ‰ͺΞ”E_C \ll \Delta. We consider a small superconducting grain of finite capacity connected to two superconducting leads by nearly ballistic single-channel quantum point contacts. The temperature is supposed to be very low, so there are no single-particle excitations on the grain. Then the behavior of the system may be described as quantum mechanics of the superconducting phase on the island. The Josephson energy as a function of this phase has two minima which become degenerate at the phase difference on the leads equal to Ο€\pi, the tunneling amplitude between them being controlled by the gate voltage at the grain. We find the Josephson current and its low-frequency fluctuations and predict their periodic dependence on the induced charge Qx=CVgQ_x=C V_g with period 2e2e.Comment: 11 pages, REVTeX, 10 figures, uses eps

    Quantum spin metal state on a decorated honeycomb lattice

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    We present a modification of exactly solvable spin-(1/2) Kitaev model on the decorated honeycomb lattice, with a ground state of "spin metal" type. The model is diagonalized in terms of Majorana fermions; the latter form a 2D gapless state with a Fermi-circle those size depends on the ratio of exchange couplings. Low-temperature heat capacity C(T) and dynamic spin susceptibility \chi(\omega,T) are calculated in the case of small Fermi-circle. Whereas C(T)\sim T at low temperatures as it is expected for a Fermi-liquid, spin excitations are gapful and \chi(\omega,T) demonstrate unusual behaviour with a power-law peak near the resonance frequency. The corresponding exponent as well as the peak shape are calculated.Comment: 4 pages, 3 figure
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