Spins arriving, on the dot
At the interface between two dissimilar semiconductors, it is possible to form a quantum dot—a pool of electrons confined to the nanometer scale—by applying a potential with metallic gates. The quantum dot has discrete energy levels, much like an atom, and the electron tunneling current through the dot from external leads can depend on the number of electrons (Coulomb blockade) or the net spin (Kondo effect) on the dot.
In a Rapid Communication appearing in Physical Review B, Sami Amasha, now at Stanford University, and colleagues at MIT, the University of Basel, and the University of California, Santa Barbara, report puzzling behavior in the spin-dependent tunneling of electrons into a quantum dot. A magnetic field applied parallel to the dot splits the energy levels (Zeeman splitting) on the dot. Ignoring the spin-orbit interaction, one would expect the tunneling rates for electrons with spin-up and spin-down to be the same, since the energy levels in the leads are similarly shifted by the field. Instead, the group finds that with increasing magnetic field, the tunneling rate for the spin-down state is less than that of the spin-up state, and is completely suppressed by . But, by adjusting the metallic gates to make the quantum confinement potential more symmetric, they can make the tunneling rates for the spin-up and spin-down states identical.
Since Amasha and colleagues can rule out obvious explanations invoking misalignment of the magnetic field or spin-orbit effects in the dot or the leads, their results pose an open challenge to fully explaining spin-dependent tunneling in quantum dots. - Sarma Kancharla