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Bose-Einstein Condensates in Rotating Traps and Optical ... - BEC

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90 Bogoliubov excitations of Bloch state condensates<br />

Bogoliubov amplitudes<br />

The functions ũjq(z) <strong>and</strong> ˜vjq(z) <strong>in</strong> (7.11,7.12) are periodic with period d. Their Fourier<br />

expansion reads<br />

The normalization condition requires<br />

d <br />

l<br />

ũjq(z) = <br />

˜vjq(z) = <br />

<br />

b ∗ jqlbj ′ ql − c ∗ jqlcj ′ ql<br />

l<br />

l<br />

2πz<br />

il<br />

bjql e d , (7.17)<br />

2πz<br />

il<br />

cjql e d . (7.18)<br />

<br />

= δjj ′ . (7.19)<br />

With q satisfy<strong>in</strong>g periodic boundary conditions this is sufficient to ensure the orthonormalization<br />

conditions (7.8,7.9) where the <strong>in</strong>dex σ is replaced by the b<strong>and</strong> <strong>in</strong>dex j <strong>and</strong> the quasi-momentum<br />

q of the excitations.<br />

In Figs. 7.5 <strong>and</strong> 7.6 we plot the square modulus of the Fourier coefficients bjql <strong>and</strong> cjql at<br />

different values of q <strong>in</strong> the first <strong>and</strong> the second Bogoliubov b<strong>and</strong> for gn =0.5ER at a lattice<br />

depth of s =1, s =5<strong>and</strong> s =10respectively.<br />

At all considered lattice depth, the relative importance of the Bogoliubov vjq-amplitude<br />

with respect to the ujq-amplitude dim<strong>in</strong>ishes <strong>in</strong> the transition from the lowest to the first<br />

excited b<strong>and</strong>. In fact, the contribution of the vjq-amplitude is negligible <strong>in</strong> all considered cases<br />

<strong>in</strong> the second b<strong>and</strong>. Hence, for gn =0.5ER, apart from an energy offset, essentially only the<br />

lowest Bogoliubov b<strong>and</strong> differs from the Bloch b<strong>and</strong>s of a s<strong>in</strong>gle particle. This can be expla<strong>in</strong>ed<br />

by the fact that for this choice of the parameter gn/ER, the heal<strong>in</strong>g length of the system is<br />

comparable to the lattice period d, a sett<strong>in</strong>g which is typical of current experiments. As a<br />

consequence, when the lattice is off the vjq-amplitude is relevant only at momenta ¯hq

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