Eduardo J. H. Lee, Xiaocheng Jiang, Manuel Houzet, Ramon Aguado, Charles M. Lieber, Silvano De Franceschi
The combination of superconductors and low-dimensional conductors embodies a rich, yet largely unexplored physics. In this hybrid system, macroscopic properties enforced by superconductivity can be controlled through electrically tunable microscopic degrees of freedom, inherent to a relatively small number of confined electrons. Here we consider the prototypical case of a quantum dot (QD) coupled strongly to a superconductor (S) and weakly to a normal-metal (N) tunnel probe. We investigate the magnetic properties of the lowest-energy, sub-gap states, which are governed by a competition between superconducting pairing and Coulomb repulsion. In a magnetic field, only when the ground state is a spin singlet, can the Zeeman splitting of the (excited) doublet be revealed by tunnel spectroscopy. The splitting is strongly influenced by a level-repulsion effect with the continuum of quasi-particle states; and it can induce a quantum phase transition (QPT) to a spin-polarized state. Our experimental results, supported by theory, hold relevance for current research on quantum-information devices and Majorana fermions in hybrid nanostructures.
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http://arxiv.org/abs/1302.2611
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