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Itinerant Spin Dynamics in Structures of ... - Jacobs University

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Chapter 3<br />

WL/WAL Crossover and <strong>Sp<strong>in</strong></strong><br />

Relaxation <strong>in</strong> Conf<strong>in</strong>ed Systems<br />

3.1 Introduction<br />

To build sp<strong>in</strong> based devices which rely on coherent sp<strong>in</strong> precession <strong>of</strong> conduction<br />

electrons[DD90, ZFD04], as presented <strong>in</strong> the <strong>in</strong>troduction <strong>of</strong> this work, it has to be analyzed<br />

under which conditions, such as wire geometry, type and <strong>in</strong>tensity <strong>of</strong> SOC, impurity density<br />

etc., sp<strong>in</strong> relaxation rate can be m<strong>in</strong>imized. We have shown <strong>in</strong> Sec.2.4 that if electron<br />

momentum is randomized due to disorder, SO <strong>in</strong>teraction is expected to result not only <strong>in</strong><br />

a sp<strong>in</strong> precession but <strong>in</strong> randomization <strong>of</strong> the electron sp<strong>in</strong> with rate 1/τ s .[DP72] In the<br />

follow<strong>in</strong>g we focus on the D’yakonov-Perel’ sp<strong>in</strong> relaxation mechanism.<br />

3.1.1 One-Dimensional Wires<br />

In one-dimensional wires, whose width W is <strong>of</strong> the order <strong>of</strong> the Fermi wave length<br />

λ F , impurities can only reverse the momentum p → −p. Therefore, the sp<strong>in</strong>-orbit field<br />

can only change its sign, when a scatter<strong>in</strong>g from impurities occurs. B SO (p) → B SO (−p) =<br />

−B SO (p). Therefore, the precession axis and the amplitude <strong>of</strong> the sp<strong>in</strong>-orbit field does not<br />

change, revers<strong>in</strong>g only the sp<strong>in</strong> precession, so that the D’yakonov-Perel’-sp<strong>in</strong> relaxation is<br />

absent <strong>in</strong> one-dimensional wires[KK00, MFA02]. In an external magnetic field, the precession<br />

around the magnetic field axis, due to the Zeeman-<strong>in</strong>teraction is compet<strong>in</strong>g with the<br />

sp<strong>in</strong>-orbit field, however. Then, as the electrons are scattered from impurities, both the<br />

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