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Startpage > News > Calendar of Events > Bipolar Spin Blockade


Bipolar Spin Blockade and Dark States in Triple Quantum Dots

Festkörperkolloquium, 15.11.12


Spin blockade[1] is commonly employed in double quantum dot devices as the critical element in the spin to charge detection process in spin qubit manipulation experiments. In Pauli blockade transport measurements in double quantum dots current flows freely in one direction, but is blockaded in the other due to the Pauli exclusion principle. The blockade is found to be not perfect at zero magnetic field due to a mixing of singlet and triplet states in a field gradient resulting from the hyperfine interactions. These leakage currents are dramatically suppressed when a small external field splits off triplet states from the singlet one. In this talk equivalent process in triple quantum dots in series will be discussed. We will show novel leakage mechanisms due to coherent transport in triple quantum dot states. In triple dots it has been predicted [2,3] current only flows when four configurations are degenerate, i.e. at quadruple points. When a bias is applied in a magnetic field spin blockade is observed where current is suppressed independent of the bias direction [5]. As the field is reduced to zero spin blockade leakage resonances appear within the transport region. We focus on two very sharp resonances which correspond to a configuration when states in the left and right dot but not the center dot align. The resonances are found to result from a purely quantum coherent effect: electrons occupy states that involve their transference from one extreme to the other of the triple quantum dot without ever visiting the center. Such states have been invoked theoretically for possible applications such as spin bussing or quantum rectification [4]. Our calculations, based on the density matrix formalism, accurately reproduce the experimental transport features for the different configurations that unambiguously show bipolar spin blockade. The inspection of the off diagonal elements of the stationary density matrix proves that in fact, the left-right resonances stem from a purely coherent process between the left and right dots, that almost entirely avoids the passage of an electron tunneling through the center dot. We will describe these coherent superpositions, often known as Dark states, in different triple quantum dots configurations[6].

[1] K. Ono , D. G. Austing ,Y. Tokura and S. Tarucha, Science 297, 5585, (2002).
[2] M. C. Rogge and R. J. Haug, Phys. Rev. B, 77, 193306 (2008).
[3] G. Granger, L. Gaudreau, A. Kam , M. Pioro-Ladrière, S.A. Studenikin Z. R.
Wasilewski, P. Zawadzki and A. S. Sachrajda, Phys. Rev. B, 82, 075304(2010).
[4] D. Greentree, J. H. Cole, A. R. Hamilton and LL. C. L. Hollenberg, Phys. Rev. B


Prof. Gloria Platero,Instituto de Ciencia de Materiales de Madrid


Institut f√ľr Festk√∂rperphysik


Donnerstag, 15. November 2012, 16:15 bis 18:00 Uhr


Appelstraße 2 (268)
30167 Hannover

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