Title: THERMODYNAMICS OF THE BOSE-SYSTEM WITH A SMALL NUMBER OF PARTICLES
Author(s): I.O.Vakarchuk, A.A.Rovenchak (Department for Theoretical Physics of Lviv National University, 12 Drahomanov Str., 79005, Lviv, Ukraine)

A theoretical description of the interacting Bose-system is proposed. It is based on the extrapolation of the results obtained for the systems with a small number of particles $N=2, 3, 4$, etc. to the bulk case of $N=\infty$. It is shown that already the system with $N=12, 13$ behaves almost as a bulk in a wide temperature range. Special attention is paid to the phase transition in these systems. The hard sphere potential is used in calculations. The sequence of heat capacity maxima is approximated as $C^{\rm max}_N/N\simeq13.6-aN^{-\varepsilon}$ with $\varepsilon=0.0608$ giving the value of bulk heat capacity as 13.6 while experimental value is close to 16. The temperature of $\lambda$-transition is estimated as 2.1-2.3 K versus experimental 2.17 K. Quite good qualitative and satisfactory quantitative agreement with the experimental data has been achieved.

Key words: Boson systems, finite systems, lambda-transition, specific heat
PACS: 05.30.Jp, 74.25.Bt