Spin Blockade

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Self-sustaining dynamical nuclear polarization oscillations in quantum dots

M. S. Rudner1,2,3 and L. S. Levitov4 1 The Niels Bohr International Academy, Blegdamsvej 17, DK-2100 Copenhagen Ø, Denmark 2 Department of Physics, The Ohio State University, 191 W. Woodruff Ave., Columbus, OH 43210 3 Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences, A-6020 Innsbruck, Austria 4 Department of Physics, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139

Early experiments on spin-blockaded double quantum dots revealed surprising robust, large- amplitude current oscillations in the presence of a static (dc) source-drain bias [see e.g. K. Ono, S. Tarucha, Phys. Rev. Lett. 92, 256803 (2004)]. Experimental evidence strongly indicates that dynamical nuclear polarization plays a central role, but the mechanism has remained a mystery. Here we introduce a minimal albeit realistic model of coupled electron and nuclear spin dynamics which supports robust self-sustained oscillations. Our mechanism relies on a nuclear-spin analog of the tunneling magnetoresistance phenomenon (spin-dependent tunneling rates in the presence of an inhomogeneous Overhauser field) and nuclear spin diffusion, which governs dynamics of the spatial profile of nuclear polarization. The extremely long oscillation periods (up to hundreds of seconds) observed in experiments as well as the differences in phenomenology between vertical and lateral quantum dot structures are naturally explained in the proposed framework.
The coupling of electron and nuclear spin dynamics is responsible for a wide variety of intriguing transport phenomena in semiconductor devices. Spin exchange between electron and nuclear spins provides a mecha- nism for electron spin flips which can dramatically alter the behavior of systems such as spin-blockaded quantum dots, where transport is highly sensitive to spin selec- tion rules[1–7]. Furthermore, the nuclear spins produce a hyperfine (Overhauser) field that shifts the electronic Zeeman energy by an amount corresponding to an effec- tive field that may reach as high as a few Tesla when the nuclei are fully polarized. This Overhauser field can have dramatic consequences for transport in quantum dots, where discrete levels may be shifted in-to or out-of resonance[4, 8–10]. The combination of these two effects – electron-nuclear spin-exchange which polarizes nuclear spins, and subsequent back-action on energy-dependent spin flip rates – is responsible for a variety of interesting nonlinear dynamical effects such as multistability, hys- teresis, and intermittency[8–15].