Carsten Timm: Theory of superconductivity
Carsten Timm: Theory of superconductivity
Carsten Timm: Theory of superconductivity
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diagrams describing this situation. We define the scattering vertex Λ by<br />
−k<br />
−iω n<br />
−p −iΩ n<br />
−Λ ≡<br />
Λ<br />
k<br />
iω n<br />
p<br />
iΩ n<br />
−k<br />
−iω n −p −iΩ n<br />
−k<br />
−iω n<br />
−k<br />
1<br />
−i<br />
1<br />
ω n<br />
−p −iΩ n<br />
:= k − p, iω n − iΩ n<br />
+<br />
i<br />
ω n<br />
k −k<br />
1 ,<br />
1<br />
− i<br />
ω n<br />
k 1 − p,<br />
iω 1 + · · · (9.2)<br />
n − iΩ n<br />
k<br />
iω n<br />
p<br />
iΩ n<br />
k<br />
iω n<br />
1<br />
k 1 iω n<br />
p<br />
iΩ n<br />
This is a geometric series, which we can sum up,<br />
Λ = 1 + + + · · · =<br />
(9.3)<br />
1 −<br />
With our approximation<br />
we obtain<br />
−Λ(iω n ) ≈<br />
V RPA<br />
eff<br />
≈<br />
∑<br />
1<br />
1 − V 0 β iωn 1 , |iω1 n | < ω D<br />
{<br />
−V 0 for |iω n | < ω D ,<br />
0 otherwise,<br />
1<br />
V<br />
+V 0<br />
∑<br />
k 1<br />
G 0 k 1↑ (iω1 n) G 0 −k 1↓ (−iω1 n)<br />
for |iω n | < ω D and zero otherwise. Thus<br />
⎧<br />
⎪⎨<br />
−V 0<br />
∑<br />
∑<br />
1<br />
Λ(iω n ) ≈ 1 − V 0 β iωn ⎪⎩<br />
1 , |iω1 n | < ω 1<br />
D V k 1<br />
Gk 0 1↑ (iω1 n) G−k 0 for |iω n | < ω D ,<br />
1↓ (−iω1 n)<br />
0 otherwise.<br />
(9.4)<br />
(9.5)<br />
(9.6)<br />
We see that the scattering vertex Λ diverges if<br />
1 ∑ 1<br />
V 0<br />
β V<br />
iω 1 n<br />
|iωn| 1 < ω D<br />
This expression depends on temperature. We now evaluate it explicitly:<br />
∑<br />
∑ 1 ∑ 1 1<br />
∑<br />
V 0 k B T<br />
V iω<br />
iω 1 k n 1 − ξ k1 −iω 1 = V 0 k B T<br />
n − ξ k1<br />
n<br />
1 iω 1<br />
|iωn| 1 n<br />
< ω D |iωn| 1 < ω D<br />
k 1<br />
G 0 k 1 ↑(iω 1 n) G 0 −k 1 ↓(−iω 1 n) = 1. (9.7)<br />
∫∞<br />
−∞<br />
dξ D(µ + ξ)<br />
1<br />
(ω 1 n) 2 + ξ 2 (9.8)<br />
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