Stability of superfluid phases in the 2D spin-polarized attractive Hubbard model

Solid State Theory Division, Faculty of Physics, Adam Mickiewicz University - Umultowska 85, 61-614 Poznań, Poland, EU

^a kujawa@amu.edu.pl

Received: 30 March 2011
Accepted: 15 June 2011

Abstract

We study the evolution from the weak coupling (BCS-like limit) to the strong coupling limit of tightly bound local pairs (LPs) with increasing attraction, in the presence of the Zeeman magnetic field (h) for d=2, within the spin-polarized attractive Hubbard model. The broken symmetry Hartree approximation as well as the strong coupling expansion are used. We also apply the Kosterlitz-Thouless (KT) scenario to determine the phase coherence temperatures. For spin-independent hopping integrals (t^↑=t^↓), we find no stable homogeneous polarized superfluid (SC_M) state in the ground state for the strong attraction and obtain that for a two-component Fermi system on a 2D lattice with population imbalance, phase separation (PS) is favoured for a fixed particle concentration, even on the LP (BEC) side. We also examine the influence of spin-dependent hopping integrals (mass imbalance) on the stability of the SC_M phase. We find a topological quantum phase transition (Lifshitz type) from the unpolarized superfluid phase (SC₀) to SC_M and tricritical points in the h-|U| and t^↑/t^↓-|U| ground-state phase diagrams. We also construct the finite temperature phase diagrams for both t^↑=t^↓ and t^↑≠t^↓ and analyze the possibility of occurrence of a spin-polarized KT superfluid.