- •General aspects
- •Introduction
- •Single particle
- •General aspects
- •Traps
- •Many particles
- •Basics of second quantization
- •Bosons
- •Fermions
- •Single particle operator
- •Two particle operator
- •Bosons
- •Free Bose gas
- •General properties
- •BEC in lower dimensions
- •Trapped Bose gas
- •Parabolic trap
- •Weakly interacting Bose gas
- •BEC in an isotr. harmonic trap at T=0
- •Comparison of terms in GP
- •Thomas-Fermi-Regime
- •Fermions
- •Free Fermions
- •General properties
- •Pressure of degenerated Fermi gas
- •Excitations of Fermions at T=0
- •Trapped non-interacting Fermi gas at T=0
- •Weakly interacting Fermi gas
- •Ground state
- •Decay of excitations
- •Landau-Fermi-Liquid
- •Zero Sound
- •Bardeen-Cooper-Shieffer-Theory
- •General treatment
- •BCS Hamiltonian
- •General energy-momentum relation
- •Calculation for section 3.3.1
- •Lifetime and Fermis Golden Rule
- •Bibliography
22 |
CHAPTER 2. BOSONS |
2.2Trapped Bose gas
2.2.1 Box
In a box with V = L3, N particles and infinite potential at the walls the wave function of one particle is of the form
y(~r) = |
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sin( L nxx)sin( L nyy)sin( L nzz) |
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~n |
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T N 32 |
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e0 TCN |
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N |
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N = å |
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(2.37) |
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T |
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~n exp |
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e~n |
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Since N has to remain finite even for T ! 0, en m T has to hold (at least for low T ). To look at this we assume that
e0 m e1 e0
en m = en e0 + e0 m en e0:
|{z}
0
Using this assumption we get |
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exp |
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d3n |
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e0 |
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e0 |
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(2.38)
(2.39)
(2.40)
(2.41)
(2.42)
(2.43)
2.2. TRAPPED BOSE GAS |
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23 |
In (2.41) we substituted 4nx = 1 ) 4px = |
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Z dnx dny dnz = |
p~ |
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Zp>0 d3 p = (2p~)3 |
Zp d3 p: |
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In (2.42) we note, that only terms with p e0 are important. From (2.43) we get
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m = T ln |
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which is macroscopically small for T TC. This justifies our assumption above 2. For the number of particles in the first excited state we get at e1 e0 T TC
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Therefore the occupation of the ground state is macroscopically larger than that of any other state.
2.2.2Parabolic trap
In an isotropic parabolic trap with N particles and oscillator frequency w the energy is
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2actually it only shows that our assumption is self consistent
24 |
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CHAPTER 2. BOSONS |
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The same arguments as in 2.2.1 hold true that m ! e0 for T & 0. Redefining |
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nx = |
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Solving this for the critical temperature we get
1 |
1 |
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TC = ~wN 3 x (3) |
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(2.55)
(2.56)
(2.57)
(2.58)
(2.59)
(2.60)
Quantitatively the critical temperature for a trapped gas can be obtained from the same type of arguments as in the homogeneous case:
~ |
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lD pmT n¯ |
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Since for a classical oscillator potential and kinetic energy have the same magnitude and the latter is related via (1.2) to temperature we can estimate the size of the cloud as
mw2R2 T
2 2
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r
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1 T
w m
1
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