Patrick Maletinsky
Eidgenössische Technische Hochschule Zürich
Tuesday, July 22, 2008
2:00 pm in SPL 52
Dynamics of Quantum Dot Nuclear Spin Polarization
Abstract: Electron spins trapped in solid-state systems exhibit strong hyperfine interactions with a nuclear spin reservoir, which is normally fluctuating and randomly oriented. As this represents a fundamental decoherence mechanism for the electron spin [1], several theoretical scenarios to suppress this effect in optically active semiconductor quantum dots (QDs) have been proposed [2,3]. However, implementation of these proposals requires a deeper understanding of the properties of the mesoscopic QD nuclear spin ensemble and of the possibilities of manipulating the nuclear spins.
For that purpose, we use optical preparation and detection of the spin and energy of QD electrons to manipulate and measure the average nuclear spin polarization (NSP) in a single, self-assembled QD [4]. Our experiments show that the transfer of spin information between the electron and the nuclei is strongly dependant on the degree of the nuclear spin polarization itself. This feedback of nuclear spin polarization on the electrons in form of an effective magnetic field makes the coupled electron-nuclear spin system behave in a highly nonlinear way [5]. I will present experimental evidence for these nonlinearities and discuss recent time resolved studies of NSP which show that a QD electron can also be very efficient in destroying an established QD NSP [6]. Moreover, due to the nonlinear behavior of the coupled electron-nuclear spin system, this nuclear spin decay can have interesting non-exponential characteristics in the presence of external magnetic fields. I will discuss how a systematic study of the dynamics of NSP in external magnetic fields could give a more detailed picture of the mechanisms causing QD nuclear spin depolarization and what role nuclear quadrupolar interactions might play for the QD nuclear spin ensemble. Ultimately, further understanding of these subtle interactions could enable us to improve the decoherence time of the electron spin.
[1] A. V. Khaetskii et al., Phys. Rev. Lett. 88, 186802 (2002).
[2] A. Imamoglu et al., Phys. Rev. Lett. 91, 17402 (2003).
[3] G. Giedke et al., Phys. Rev.A 74, 32316 (2006).
[4] C.W. Lai et al., Phys. Rev. Lett. 96, 167403 (2006).
[5] P. Maletinsky et al., Phys. Rev. B 75, 35409 (2007).
[6] P. Maletinsky et al., Phys. Rev. Lett. 99, 056804 (2007).