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Events on Tuesday, July 12th, 2011

Condensed Matter Theory Group Seminar
Impurity effects in few-electron quantum dots
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Nga Nguyen, University of Maryland - College Park
Abstract: Doping semiconductor nanostructures, e.g. quantum dots (QDs), with magnetic impurities (Mn2+) offers a means to study spin manipulation through the strong spin exchange interaction between electrons (holes) and magnetic impurities. The magnetic properties of group II-VI semiconductor QDs doped with magnetic impurities have recently attracted considerable attention. Studies on those systems have led to fundamental insights in magnetism and resulted in e.g. different effective spin states. In the first part of my talk, I discuss the fundamental many-body effects in strongly interacting few-electron QDs doped with few magnetic impurities. We investigate ferromagnetic and antiferromagnetic phases as a consequence of dominant spin-spin electron-Mn2+ exchange interaction in the Cd(Mn)Te QDs. A non-trivial phase diagram, which exhibits substantial differences for systems with different number of electrons, is fully calculated. We found frustrated (spin-glass-like) regions, which only appear when the Mn-ions antiferromagnetically couple with each other and ferromagnetically couple with the electrons. We argue that Kohn's theorem no longer holds in Mn2+-doped parabolic few-electron QDs. The magneto-optical absorption (FIR) spectrum now depends strongly on the electron-electron interaction and varies with changing the number of electrons. Spin exchange interaction between electron and the Mn-ion results in different cyclotron resonance absorption lines quantitatively and qualitatively.

The second part of my talk is to discuss the effects of having unintentional charged impurities in laterally coupled two-dimensional double (GaAs, Si) QDs, where each dot contains one or two electrons and a single charged impurity. Using molecular orbital and configuration interaction methods, we calculate the effect of the impurity on the two-electron energy spectrum of each individual dot as well as on the spectrum of the coupled-double-dot two-electron system. We find that the singlet-triplet exchange splitting between the two lowest-energy states, both for the individual dots and the coupled-dot system, depends sensitively on the location of the impurity and its coupling strength (i.e. the effective charge). A strong electron-impurity coupling breaks down the equality of the two doubly occupied singlets in the left and the right dots, leading to a mixing between different spin singlets. As a result, the maximally entangled qubit states are no longer fully obtained in the zero-magnetic-field case.
Host: Robert Joynt
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