Spin effects in single-electron transistors

Abstract

Basic electron transport phenomena observed in single-electron transistors (SETs) are introduced, such as Coulomb-blockade diamonds, inelastic cotunneling thresholds, the spin-1/2 Kondo effect, and Fano interference. With a magnetic field parallel to the motion of the electrons, single-particle energy levels undergo Zeeman splitting according to their spin. The g-factor describing this splitting is extracted in the spin-flip inelastic cotunneling regime. The Kondo splitting is linear and slightly greater than the Zeeman splitting. At zero magnetic field, the spin triplet excited state energy and its dependence on gate voltage are measured via sharp Kondo peaks superimposed on inelastic cotunneling thresholds. Singlet-triplet transitions and an avoided crossing are analyzed with a simple two-level model, which provides information about the exchange energy and the orbital mixing. With four electrons on the quantum dot, the spin triplet state has two characteristic energy scales, consistent with a two-stage Kondo effect description. The low energy scale extracted from a nonequilibrium measurement is larger than those extracted in equilibrium.