Observation of sequential spin flips in quantum rings

We observe strong signatures of spin flips in quantum rings exposed to external magnetic fields in the Coulomb blockade regime. The signatures appear as a pattern of lines corresponding to local reduction of conductance, and they cover a large range of magnetic fields and number of electrons. The sequence of lines, as well as other features in the conductance, can be captured by many-electron calculations within density-functional theory. The calculations show that most lines in the pattern correspond to sequential spin flips between filling factors 2 and 1. We believe that the ability to probe individual spin flips provides an important step toward precise spin control in quantum ring devices.

Figure 1

(a) Image of the quantum ring device taken with an atomic force microscope. (b) Model potential used in the calculations within spin-density-functional theory.

Figure 2

Measured conductance in the quantum-ring device over a wide range of gate voltages and magnetic fields.

Figure 3

Addition energies calculated with spin-density-functional theory.

Figure 4

Experimental (upper panel) and theoretical (lower panel) cross sections of the electron addition spectra, respectively.

Figure 5

Calculated ground-state spins in quantum rings containing 18, 26, and 30 electrons, respectively, as a function of the magnetic field. The circles mark the regimes of local depolarization (see text).