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TT: Tiefe Temperaturen
TT 1: Supraleitung in mesoskopischen und nanoskopischen Strukturen
TT 1.2: Hauptvortrag
Montag, 22. März 1999, 10:00–10:30, F2
Reentrant spin susceptibility of ultrasmall superconducting grains — •F.W.J. Hekking1, A. di Lorenzo2, R. Fazio2, and G. Giaquinta2 — 1Theoretische Physik III, Ruhr-Universität Bochum, 44780 Bochum — 2Istituto di Fisica, Universitá di Catania & INFM, 95129 Catania, Italien
We study the thermodynamics of a small superconducting grain with a fixed number N of electrons, and an average level spacing δ comparable to the superconducting BCS gap Δ. The thermodynamic properties of such grains strongly depend on the parity of the number N of electrons. Moreover, for the smallest grains, superconducting fluctuations are so strong that a mean field treatment of the problem is insufficient. Both parity and fluctuation effects are conveniently taken into account using techniques based on functional integration.
Specifically, we calculate the spin susceptibility χ as a function of temperature T. For large grains with Δ > δ and N even, χ decreases exponentially with decreasing T. If N is odd, however, χ shows a nonmonotonic dependence on T, increasing as 1/T at the lowest temperatures (reentrance effect). As the grain size is decreased, a cross-over is found from superconducting to normal state behaviour. However, superconducting fluctuations become very strong and dominate the low-T behaviour of χ. In particular, the remarkable reentrance effect for odd N persists upon increasing δ up to values larger than Δ.