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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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Andreev Billiards 165<br />

form (30) – with effective parameters Neff and δeff. The determinant (22) for<br />

the excitation spect<strong>ru</strong>m then takes the form<br />

�<br />

Det 1 − α(E) 2 e 2iEτE/� S0(E)S0(−E) ∗�<br />

=0. (87)<br />

We can safely replace α(E) ≡ exp[−i arccos(E/∆)] →−i (since E ≪ ∆), but<br />

the energy dependence of the phase fac<strong>to</strong>r e2iEτE/� can not be omitt<strong>ed</strong>.<br />

In App. A we calculate the smallest positive E that solves (87), which is<br />

the excitation gap Egap of the effective RMT. The result is plott<strong>ed</strong> in Fig. 21<br />

(solid curve), as a function of τE/τdwell. The two asymp<strong>to</strong>tes (dott<strong>ed</strong> lines)<br />

are<br />

Egap = γ5/2 �<br />

�<br />

1 − (2γ − 1)<br />

τdwell<br />

τE<br />

�<br />

, τE≪τdwell , (88)<br />

τdwell<br />

Egap = π�<br />

�<br />

1 − (3 +<br />

2τE<br />

√ 8) τdwell<br />

�<br />

, τE≫τdwell , (89)<br />

τE<br />

with γ = 1<br />

2 (√5 − 1) the golden number.<br />

Fig. 21. Excitation gap of the Andreev billiard in the crossover from Thouless<br />

<strong>to</strong> Ehrenfest regimes. The solid curve is the solution of the effective RMT of [61],<br />

deriv<strong>ed</strong> in App. A. Th<strong>ed</strong>ott<strong>ed</strong> lines are the two asymp<strong>to</strong>tes (88) and(89). The<br />

dash<strong>ed</strong> curve is the result of the s<strong>to</strong>chastic model of [40], discuss<strong>ed</strong> in Sect. 8.3<br />

The τE time delay characteristic of the effective RMT was introduc<strong>ed</strong> in<br />

[61], but its effect on the excitation gap was not evaluat<strong>ed</strong> properly. 7 As a<br />

consequence the formula for the gap given in that paper,<br />

7 I am indebt<strong>ed</strong> <strong>to</strong> P. W. Brouwer for spotting the error.

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