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arXiv:cond-mat/0206054 · Physical review. B, Condensed matter

Quantum-dot lithium in the strong-interaction regime: Depolarization of electron spins by a magnetic field

S. A. Mikhaǐlov, Nadja A. Savostianova

Published Jul 17, 2002• 29 citations
arXivHTMLPDF

Abstract

A magnetic field usually leads to polarization of electron spins. It is shown that in a system of strongly interacting particles applying a magnetic field may lead to the opposite effect: depolarization of electron spins. Results of the work are based on an exact-diagonalization study of quantum-dot lithium---a system of three Coulomb interacting two-dimensional electrons in a parabolic confinement potential.

DOI
10.1103/physrevb.66.033307

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A magnetic field usually leads to polarization of electron spins. It is shown that in a system of strongly interacting particles applying a magnetic field may lead to the opposite effect: depolarization of electron spins. Results of the work are based on an exact-diagonalization study of quantum-dot lithium---a system of three Coulomb interacting two-dimensional electrons in a parabolic confinement potential.
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The interplay of confinement and Coulomb interactions in quantum dots can lead to strongly correlated phases differing qualitatively from the Fermi liquid behavior. We explore how the presence of magnetic impurities in quantum dots can provide additional opportunities to study correlation effects and the resulting ordering in carrier and impurity spins. By employing exact diagonalization we reveal that seemingly simple two-carrier quantum dots lead to a rich phase diagram. We propose experiments to verify our predictions; in particular, we discuss interband optical transitions as a function of temperature and magnetic field.
We study the pumping of spin-polarized electrons in a double quantum dot system with up to two electrons per dot, via an applied AC field and a constant magnetic field. The behaviour of the current through the double-dot system is studied as a function of the AC field and coupling to the leads, using a Markov master equation approach for the time evolution of the reduced density matrix. For up to two electrons in the system, we find that the formation of a spin-triplet state blocks the current through the device, and analyse possible solutions. When we incorporate three-and four-particle states, with up to two opposite spin electrons per dot, we find a regime where the pumping of spin-polarized electrons is realized through double occupancy states in each dot. This property is robust against spin-relaxation and decoherence processes which are taken into account phenomenologically. Finally we study the effects of applying a pulsed AC field and the possibility of the resolution of Rabi oscillations.
This thesis investigates the use of wave-function methods for the study of quantum-dot systems. It investigates single dots, using quantum Monte Carlo techniques in a wide range of magnetic field values, and a double-dot system, using the exact-diagonalization method.\n\nThe thesis proposes simple yet accurate many-particle wave functions for various angular-momentum and spin states, for both weak and strong magnetic fields. Using these trial wave functions, it evaluates various properties of dots and studies Wigner crystallization and spin polarization for the weak-field limit. For strong magnetic fields, the thesis investigates ground states of different spin polarizations as a function of the magnetic field and tests the commonly used lowest-Landau-level approximation. The results are compared to calculations from the density-functional theory.\n\nFinally, the thesis presents a method that combines the accuracy of the exact-diagonalization method and the scalability of Monte Carlo methods. The Monte Carlo-based diagonalization is a promising tool for use in situations that cannot be handled with a simple trial wave function, and have too many particles for an exact-diagonalization treatment. For quantum dots, methods for efficiently evaluating the high-magnetic-field basis functions and their gradients are demonstrated.
In moderately strong magnetic fields, the difference in Land\'e ggg factors in each of the dots of a two-electron double quantum dot (DQD) may induce oscillations between singlet and triplet states of an entangled pair and lead to a nonequilibrium electron spin polarization in the system. The calculated bipolar polarization reflects the strong spin correlation in the spatially separated pair of electrons that occupy almost degenerate states in a coupled DQD. We will show that this polarization may partially survive the rapid inhomogeneous decoherence caused by random nuclear magnetic fields.