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Nonzero expectation value of boson creation operator in ground state of a Bose-Einstein condensate

Since we are considering BEC, a ground state has a macroscopic occupation. It means that we can replace the field operators $a_{0}$ and $a^{\dagger}_{0}$ into c-numbers. Hence the ground state boson ...
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Composite fields and statistics

Disclaimer: I'm not a condensed matter theorist, and I find most statistical physics utterly impenetrable. However, this question also bothered me long enough to try to figure it out. Here's my ...
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How do we do experiments on gases at zero Kelvin?

For the technique of cooling and trapping, see https://www2.physics.ox.ac.uk/sites/default/files/2013-01-19/andrian_pdf_71380.pdf. By the way, trapping is not only needed to thermally isolate the ...
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How is a Bose-Einstein condensate produced from sodium atoms that do not have an integer spin?

The protons, neutrons, and electrons that make up any atom are all Fermions with spin $1/2$. You can't make a Bose-Einstein condensate out of just electrons, for instance. But a composite particle ...
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How the concept of "wave function of atom" in Bose-Einstein condensate should be interpreted from perspective of quantum field theory?

Such a theory would be an effective field theory. It is often the case that different degrees of freedom are separated by large differences in energy. For example when solving the wave equation for a ...
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What is the difference between absolute zero Kelvin and almost absolute zero?

In some thermodynamic contexts, the inverse temperature $\beta=1/T$ is a more meaningful quantity. This makes it more clear that 0 Kelvin, or $\beta = \infty$ is approachable but unattainable. And ...
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What is the difference between absolute zero Kelvin and almost absolute zero?

The difference is that $0$ is not the same as $10^{-9}$. The latter is nearly zero, but it's not zero. Why does it matter? Consider for example Charles's Law, which says that for gases, $V/T$ is a ...
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