Bose-Einstein Condensates in Rotating Traps and Optical ... - BEC
Bose-Einstein Condensates in Rotating Traps and Optical ... - BEC
Bose-Einstein Condensates in Rotating Traps and Optical ... - BEC
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8.2 Static structure factor <strong>and</strong> sum rules 113<br />
The behaviour of S(p) at small momenta can be described exactly us<strong>in</strong>g sum-rule arguments.<br />
Insert<strong>in</strong>g (8.8) <strong>in</strong>to (8.19) <strong>and</strong> (8.20) we can write<br />
<br />
p<br />
Zj(p)¯hωj(p) =Ntot<br />
j<br />
2<br />
, (8.26)<br />
2m<br />
<br />
<br />
<br />
Zj(p) <br />
<br />
κ<br />
¯hωj(p) = Ntot . (8.27)<br />
j 2<br />
p→0<br />
In lowest order <strong>in</strong> p the Bogoliubov b<strong>and</strong>s ¯hωj(p) behave like<br />
Hence, the f-sum rule (8.26) can only be ensured at small p if<br />
¯hω1(p)| p→0 = cp , (8.28)<br />
¯hωj>1(p)| p→0 = const . (8.29)<br />
Z1(p) ∼ p, (8.30)<br />
Zj>1(p) ∼ p 2 . (8.31)<br />
Insert<strong>in</strong>g this result <strong>and</strong> (8.28,8.29) <strong>in</strong>to (8.18,8.27) we f<strong>in</strong>d that both the non-energy weighted<br />
sum rule (8.18) <strong>and</strong> the compressibility sum rule (8.20) are exhausted by the contribution from<br />
thefirstb<strong>and</strong>whenp → 0, high energy b<strong>and</strong>s giv<strong>in</strong>g rise to contributions of O(p 2 ). Thus, we<br />
can rewrite (8.18,8.20) <strong>in</strong> the form<br />
S(p)| p→0 = 1<br />
<br />
Z1(p) <br />
<br />
¯hω1(p) <br />
p→0<br />
Ntot<br />
= Ntot<br />
Comb<strong>in</strong><strong>in</strong>g these two equations <strong>and</strong> us<strong>in</strong>g κ =1/m ∗ c 2 we obta<strong>in</strong><br />
S(p) −→<br />
p → 0<br />
Z1(p) ∼ p, (8.32)<br />
κ<br />
. (8.33)<br />
2<br />
|p|<br />
2m ∗ c<br />
(8.34)<br />
This shows that <strong>in</strong> the presence of 2-body <strong>in</strong>teractions the low-p behaviour of the static<br />
structure factor is entirely determ<strong>in</strong>ed by phonon correlations. This result holds for any value<br />
of s as long as gn = 0. In the absence of the optical lattice m ∗ = m <strong>and</strong> c co<strong>in</strong>cides with<br />
the Bogoliubov sound velocity cuni = gn/m of the uniform system. S<strong>in</strong>ce we can write<br />
m ∗ c = √ m ∗ κ −1 <strong>and</strong> both m ∗ <strong>and</strong> κ −1 <strong>in</strong>crease with s, we f<strong>in</strong>d that the presence of the lattice<br />
results <strong>in</strong> an enhanced suppression of the static structure factor at low values of p, as clearly<br />
shown <strong>in</strong> Figs.8.7 <strong>and</strong> 8.8.