Tunneling Enhancement of the Superconducting Transition Temperature



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The observed \tc enhancement in heterostructures of high-temperature superconductors is investigated. The model assumes a two-layer system; a metallic layer providing large carrier concentration, (a charge-transfer layer), and a pairing layer, providing the order parameter, with a single-particle tunneling term. The model is solved within the mean-field approximation for the order parameter, but exactly by means of the functional integral formalism for the effect of the interlayer hopping term, t.. We are able to show, analytically, the dependence of major quantities on the interlayer hopping term. Pre-existing fermion pairs, which develop at TMF, develop phase coherence and condense into a superfluid phase at a lower temperature, determined by the effective Kosterlitz-Thoulless hamiltonian, whose form we derive, and calculate numerically, the value of the transition temperature as a funtion of the interlayer hopping parameter t. There is enhancement of the superconducting temperature for all values of the pairing layer Coulomb coupling, U, but this enhancement is non-monotonic with t for large and intermediate coupling values, while it exhibits monotonicity in the low (Coulomb) coupling regime. This is found to be true for a wide range of fermionic densities. The \emph{BCS} ratio, that is the ratio of (twice) the gap in the energy spectrum at T=0, or ground state, to the mean field transition temperature, shows a monotonic departure from the BCS value of 3.5, with increasing t.



High temperature superconductivity, Interlayer coupling, Negative-U coupling