Spin effects in a quantum ring

Abstract

Recent experiments are reviewed that explore the spin states of a ring-shaped manyelectron quantum dot. Coulomb-blockade spectroscopy is used to access the spin degree of freedom. The Zeeman effect observed for states with successive electron number allows to select possible sequences of spin ground states of the ring. Spinpaired orbital levels can be identified by probing their response to magnetic fields normal to the plane of the ring and electric fields caused by suitable gate voltages.This narrows down the choice of ground-state spin sequences. A gate-controlled singlet-triplet transition is identified and the size of the exchange interaction matrix element is determined.Key words: Quantum dots, quantum computing, exchange effects, Zeeman splitting In recent years, the vision of using quantum dots in the Coulomb blockade Email address: ihn@phys.ethz.ch (T. Ihn).

Addition energies and density dipole response of quantum rings under the influence of in-plane electric fields

“…Similar spin transitions induced by an applied magnetic field have been observed in Coulomb blockade spectroscopy experiments with vertical and lateral few-electron quantum dots 16 or by the application of a gate voltage at fixed B. 21 This technique has also been used in experiments with many-electron quantum rings, 22,23 where the gate voltage-induced singlet-triplet transitions have been related to the competition between the Hartree and the exchange interactions, which favor, respectively, the formation of singlet and triplet spin states. The second spin differences, S 2 ͑N͒ = S͑N +1͒ −2S͑N͒ + S͑N −1͒, have also been measured from the slopes of the Coulomb blockade peak spacings.…”

“…The second spin differences, S 2 ͑N͒ = S͑N +1͒ −2S͑N͒ + S͑N −1͒, have also been measured from the slopes of the Coulomb blockade peak spacings. 23 Our results at E = B = 0 are shown in Fig. 5.…”

“…Experimental and theoretical results involving, e.g., spin pairing, 1,2 Zeeman splitting 3-6 and spin states 7,8 due to in-plane or perpendicular magnetic fields, spin blockade effects, [8][9][10] and, of particular interest, spin transitions driven by magnetic fields [11][12][13][14][15][16][17][18][19] or gate voltages [20][21][22][23] have been reported.…”

“…24 Similar conclusions have been obtained from experiments with manyelectron quantum rings ͑QRs͒ fabricated with scanning force microscope nanolithographic techniques on an AlGaAsGaAs heterostructure. 22,23 In these ring structures, the effect of both magnetic and electric ͑E͒ fields on the gs spin has been investigated.…”

Isospin phases of vertically coupled double quantum rings under the influence of perpendicular magnetic fields

“…This property, together with the fact that in narrow enough QRs the electrons experiment a nearly onedimensional Coulomb repulsion, leads to the integer and fractional Aharonov-Bohm effects usually associated with the appearance of the so-called persistent currents in the ring. 9 These quantum-interference phenomena have been experimentally reported 10 and have motivated a series of theoretical works whose number is steadily increasing ͑see, e.g., Refs. 11-16 and references therein͒.…”