https://doi.org/10.1140/epjp/s13360-021-02013-7
Regular Article
Contact interactions II; Gross-Pitaewskii equation, Bose-Einstein condensate, Fermi sea
1
Department of Mathematics, University Sapienza Roma, Rome, Italy
2
Mathematics area, SISSA Trieste, Trieste, Italy
Received:
26
March
2021
Accepted:
30
September
2021
Published online:
19
November
2021
In Dell’Antonio (Eur Phys J Plus 13:1–20, 2021), we explored the possibility to analyse contact interaction in Quantum Mechanics using a variational tool, Gamma Convergence. Here, we extend the analysis in Dell’Antonio (Eur Phys J Plus 13:1–20, 2021) of joint weak contact of three particles to the non-relativistic case in which the free one particle Hamiltonian is 
. We derive the Gross–Pitaevskii equation for a system of three particles in joint weak contact. We then define and study strong contact and show that the Gross–Pitaevskii equation is also the variational equation for the energy of the Bose–Einstein condensate (strong contact in a four-particle system). We add some comments on Bogoliubov’s theory. In the second part, we use the non-relativistic Pauli equation and weak contact to derive the spectrum of the conduction electrons in an infinite crystal. We prove that the spectrum is pure point with multiplicity two and eigenvalues that scale as 
.
© The Author(s) 2021
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