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Quasi-one- and quasi-two-dimensional perfect Bose gas: the second critical density and generalised condensation

In this letter we discuss a relevance of the 3D Perfect Bose gas (PBG) condensation in extremely elongated vessels for the study of anisotropic condensate coherence and the "quasi-condensate". To this end we analyze the case of exponentially anisotropic (van den Berg) boxes, when there are two critical densities $ρ_c < ρ_m$ for a generalised Bose-Einstein Condensation (BEC). Here $ρ_c$ is the standard critical density for the PBG. We consider three examples of anisotropic geometry: slabs, squared beams and "cigars" to demonstrate that the "quasi-condensate" which exists in domain $ρ_c < ρ< ρ_m$ is in fact the van den Berg-Lewis-Pulé generalised condensation (vdBLP-GC) of the type III with no macroscopic occupation of any mode. We show that for the slab geometry the second critical density $ρ_m$ is a threshold between quasi- two-dimensional (quasi-2D) condensate and the three dimensional (3D) regime when there is a coexistence of the "quasi-condensate" with the standard one-mode BEC. On the other hand, in the case of squared beams and "cigars" geometries critical density $ρ_m$ separates quasi-1D and 3D regimes. We calculate the value of difference between $ρ_c, ρ_m$ (and between corresponding critical temperatures $T_m, T_c$) to show that observed space anisotropy of the condensate coherence can be described by a critical exponent $γ(T)$ related to the anisotropic ODLRO. We compare our calculations with physical results for extremely elongated traps that manifest "quasi-condensate".