Main field of study: 011200 Physics
Area of specialization: Statistical Theory of Inhomogeneous Media
Department of Statistical Physics 
Scientific advisor: professor M.Yu. Nalimov
Reviewer: associate professor M .V. Komarova

On a phase transition in large spin Fermi systems 
G.A. Kalagov

We have studied a Fermi model with attractive U(N)-symmetric interaction at the finite temperatures by the quantum field renormalization group (RG) method.  This model can be realized in Fermi systems with N spin degrees of freedom. The phase transitions and Cooper pairing have been considered. We have performed the instanton analysis to study analytical properties of perturbative series in the model. Also the Borel resummation procedure was used to obtain infrared behavior of the RG equations. It has been found that for N>2 the RG flux leaves the system's stability region – the system undergoes a first order phase transition. To estimate the temperature of the transition to superconducting or superfluid phase the RG analysis for composite operators has been performed using one-loop approximation. As the result this analysis shows that for 3d-systems estimated phase transition temperature is higher than well known theoretical estimations based on continuous phase transition formalism.

The list of the publications
1. G.A. Kalagov and M.Yu. Nalimov. Higher-order asymptotics and critical indexes in the theory// Nuclear Physics B. V.884, P.672-683, 2014
2. G.A. Kalagov, M.V. Kompaniets and M.Y. Nalimov. Renormalization-group study of a superconducting phase transition: Asymptotic behavior of higher expansion orders and results of three-loop calculations// Theoretical and Mathematical Physics, November 2014, Volume 181, Issue 2, pp 1448-1458
3. G.A. Kalagov, M.V. Kompaniets and M.Y. Nalimov. Renormalization-group investigation of a superconducting U(N)-phase transition using five loops calculations// Nuclear Physics B. (in print)