Spintronics is a relatively new field in which an electron’s spin, not just its charge, is exploited in devices and circuits. Physicists at the Institute for Microstructural Science in Ottawa, Canada, have connected a quantum dot to spin-polarized leads in an external magnetic field. They emptied the dot of conduction electrons and then added them back one at a time. The researchers found that the total spin of the electrons depended both on the number of electrons in the dot and on the applied magnetic field. With fewer than about 20 electrons in the dot, an even number of spins paired off in singlet states with zero net spin, whereas an odd number had a net spin corresponding to the unpaired electron. Above the critical number, however, the additional electrons all had the same spin polarization. Furthermore, the single-spin, singlet, and polarized phases of the dot each allowed different currents...

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