Exact canonic eigenstates of the truncated Bogoliubov Hamiltonian in an interacting bosons gas

Abstract

In a gas of $N$ weakly interacting bosons \cite{Bogo1, Bogo2}, a truncated canonic Hamiltonian $\widetilde{h}_c$ follows from dropping all the interaction terms between free bosons with momentum $\hbar\mathbf{k}\ne\mathbf{0}$. Bogoliubov Canonic Approximation (BCA) is a further manipulation, replacing the number \emph{operator} $\widetilde{N}_{in}$ of free particles in $\mathbf{k}=\mathbf{0}$, with the total number $N$ of bosons. BCA transforms $\widetilde{h}_c$ into a different Hamiltonian $H_{BCA}=\sum_{\mathbf{k}\ne\mathbf{0}}\epsilon(k)B^\dagger_\mathbf{k}B_\mathbf{k}+const$, where $B^\dagger_\mathbf{k}$ and $B_\mathbf{k}$ create/annihilate non interacting pseudoparticles. The problem of the \emph{exact} eigenstates of the truncated Hamiltonian is completely solved in the thermodynamic limit (TL) for a special class of eigensolutions $|\:S,\:\mathbf{k}\:\rangle_{c}$, denoted as \textquoteleft s-pseudobosons\textquoteright, with energies $\mathcal{E}_{S}(k)$ and \emph{zero} total momentum. Some preliminary results are given for the exact eigenstates (denoted as \textquoteleft $\eta$-pseudobosons\textquoteright), carrying a total momentum $\eta\hbar\mathbf{k}$ ($\eta=\:1,\:2,\: \dots$). A comparison is done with $H_{BCA}$ and with the Gross-Pitaevskii theory (GPT), showing that some differences between exact and BCA/GPT results persist even in the TL. Finally, it is argued that the emission of $\eta$-pseudobosons, which is responsible for the dissipation $\acute{a}$ \emph{la} Landau \cite{L}, could be significantly different from the usual picture, based on BCA pseudobosons.