A non-simultaneous decoupling method for an exchange model of a ferromagnet

S. I. Kubarev, О. А. Пономарев

T. Kaneyoshi

Maria Teresa Mercaldo, Ileana Rabuffo, L. De Cesare +1 more

We analyze the effects induced by single-ion anisotropy on quantum criticality in a d -dimensional spin-3/2 planar ferromagnet. To tackle this problem we employ the two-time Green's function method, using the Tyablikov decoupling for exchange interactions and the Anderson-Callen decoupling for single-ion anisotropy. In our analysis the role of non-thermal control parameter which drives the quantum phase transition is played by a longitudinal external magnetic field. We find that the single-ion anisotropy has substantial effects on the structure of the phase diagram close to the quantum critical point.

Mason Protter

In this thesis we investigate how corrections originating from particle-hole interactions may be introduced into the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity using functional integral methods. The attractive Fermionic contact interaction is simultaneously decoupled in the Cooper and exchange channels, with the exchange-channel decoupling field treated as a small fluctuation about its zero mean-field value. Upon integrating out the exchange-channel fluctuation field, the system is described by a new effective action for the Cooper-channel decoupling field, which models the superconducting condensate. The saddle-point condition of this effective action generates the standard mean-field BCS equations with correction terms containing particle-hole polarization bubbles. The corrections obtained in this thesis are similar in spirit, albeit different in form, to those originally found in 1961 by Gor’kov and Melik-Barkhudarov (GMB). An advantage of the technique employed here, as opposed to the GMB approach, is the methodical manner in which fluctuations can be incorporated into superconductors.

Colin J. Thompson

Ger Vertogen, W.A. Smit