We study weakly repulsive Bose-Bose mixtures in two and three dimensions at zero temperature using the functional renormalization group (FRG). We examine the RG flows and the role of density and spin fluctuations. We study the condition for phase separation and find that this occurs at the mean-field point within the range of parameters explored. Finally, we examine the energy per particle and condensation depletion. We obtain that our FRG calculations compare favorably with known results from perturbative approaches for macroscopic properties.
We derive exact renormalization-group equations for the
n-point vertices (
n=0,1,2,⋯) of interacting single-component Bose-Einstein condensates based on the vertex expansion of the effective action. They have a notable feature of automatically satisfying Goldstone's theorem (I), which yields the Hugenholtz-Pines relation
Σ(0)−μ=Δ(0) as the lowest-order identity. Using them, it is found that the anomalous self-energy
Δ(0) vanishes below
dc=4 (
dc=3) dimensions at finite temperatures (zero temperature), contrary to the Bogoliubov theory predicting a finite "sound-wave" velocity
vs∝[Δ(0)]1/2>0. It is also argued that the one-particle density matrix
ρ(r)≡⟨ψ^†(r1)ψ^(r1+r)⟩ for
d0. The anomalous dimension
η at finite temperatures is predicted to behave for
d=4−ε dimensions ($0