Spin excitations in a quantum dot with magnetic field and spin orbit coupling

Arturo Tagliacozzo

Department of Physics, Universita di Napoli "Frederico II", Italy

A.Tagliacozzo (1), B. Jouault (2), P. Lucignano (1,3) and D. Giuliano(4)

(1) Universita di Napoli "Federico II" and "Coherentia-INFM", Dipartimento di Scienze Fisiche, Monte S.Angelo, 80126 Napoli (Italy)
(2) Groupe d'Etude des Semiconducteurs, Universite Montpellier II, 34095 Montpellier (France)
(3) International School for Advanced Studies and "Democritos-INFM", Via Beirut 2-4, 34014 Trieste (Italy)
(4) Dipartimento di Fisica, Universita' della Calabria, Arcavacata di Rende (CS) (Italy)

Correlations in a quantum dot are enhanced when strong electron-electron interactions make the spacing between energy levels narrower. By applying a magnetic field orthogonal to the dot, the total spin can be tuned. Electron correlations drive the dot through level crossings to higher angular momenta, as well as to higher total spin,S, states. By increasing the magnetic field, the dot reaches the state of full spin polarization, as shown by exact diagonalization results for few (N< 7) electrons[1]. However, the crossings of the levels can be turned into anticrossings by means of a gate voltage that enhances the spin\x{2013}orbit interaction (Rashba coupling).The z component of the total spin S is no longer a good quantum number and a spin texture arises in the low-energy excited states[2]. The first of these is mostly a collective spin excitation, with a spin density having a spin flipped at the origin with respect to the ground state. The piling up of a reversed spin can be squeezed at the center of the dot by acting on the gate voltage. By increasing N, the anticrossings become less pronounced till a crossover takes place, eventually resembling the dot to a quantum Hall ferromagnet (QHF) at filling close to one[3]. The first excited state recalls the skyrmion, the Goldstone mode of the QHF. In the dot the gap is finite and is tuned by the Rashba coupling.The crossover can be monitored by Far Infrared Radiation (FIR) and is quite sharp already at N= 5[2]. This opens up many exciting possibilities of manipulating the electron spin density in a quantum dot and of adding a Berry phase to the manybody wavefunction[4].

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