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Heiss W.D. (ed.) Quantum dots.. a doorway to - tiera.ru

Heiss W.D. (ed.) Quantum dots.. a doorway to - tiera.ru

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158 C.W.J. Beenakker<br />

its momentum). The Poincaré map would then consist of a single point. At<br />

non-zero excitation energy E the retroreflection occurs with a slight change in<br />

py, because of the difference 2E in the kinetic energy of electrons (at energy<br />

EF + E) and holes (at energy EF − E).<br />

The resulting slow drift of the periodic trajec<strong>to</strong>ry traces out a con<strong>to</strong>ur in<br />

the surface of section. These are isochronous con<strong>to</strong>urs [61], meaning that the<br />

time T between Andreev reflections is the same for each point x, px on the<br />

con<strong>to</strong>ur. The adiabatic invariance of T follows from the adiabatic invariance<br />

of the action integral I over the nearly periodic motion from electron <strong>to</strong> hole<br />

and back <strong>to</strong> electron:<br />

�<br />

I = pdq =2ET . (75)<br />

Since E is a constant of the motion, adiabatic invariance of I implies adiabatic<br />

invariance of the time T between Andreev reflections.<br />

Adiabatic invariance is defin<strong>ed</strong> in the limit E → 0 and is therefore distinct<br />

from invariance in the sense of Kolmogorov-Arnold-Moser (KAM) [7], which<br />

would require a critical E ∗ such that a con<strong>to</strong>ur is exactly invariant for E

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