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A small mesoscopic metallic grain with a large Thouless conductance is described by the so-called universal Hamiltonian, which includes pairing and ferromagnetic exchange correlations. We investigate the competition between superconductivity and ferromagnetism in the ground state of the grain [1] and at finite temperature [2]. Pairing correlations favor minimal ground-state spin while the exchange interaction favors maximal spin polarization. We find an intermediate regime in which superconducting and ferromagnetic correlations coexist [1]. This conclusion is based on an exact solution of the universal Hamiltonian while a mean-field theory cannot describe pairing correlations in this intermediate regime. We also study the thermodynamic properties of the grain [2] and determine how the thermodynamic signatures of pairing correlations [3] are affected by the spin exchange interaction. These exchange interaction effects differ qualitatively between the BCS regime and fluctuation-dominated regime.
[1] S. Schmidt, Y. Alhassid and K. van Houcke, Europhys. Lett. 80, 47004 (2007). [2] K. van Houcke, Y. Alhassid, S. Schmidt and S. Rombouts, in preparation (2007). [3] Y. Alhassid, L. Fang and S. Schmidt, arXiv:cond-mat/0702304 (2007). |
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