Investigation of the gas-liquid transition in the spinless fermion lattice gas with infinite-ranged attractive interaction is continued. Equation of state and phase diagram are calculated exactly at finite temperatures. Quantum shift of the critical temperature is also calculated. In some cases, e.g., in the case of the linear chain with nearest neighbor hopping, the obtained isotherms have two separate horizontal parts on suitable conditions showing double phase transitions and there exist two critical points in the phase diagram. As far as the density of states has a single peak, on the other hand, the system has only one critical point.
The selfconsistent high-density expansion for thermodynamic quantities of the quantum spin-polarized fermion lattice gas, derived in the preceding paper, is extended to higher orders. The (1/z)2 terms in the free energy are evaluated by summing two-loop diagrams and the corresponding equation for critical temperature in the classical limit is discussed. Ground state phase diagram of the system for arbitrary electron density is derived in the first and second order of the expansion. Properties of the half-filled, one-dimensional spinless fermion model at T = 0, obtained here in the first three orders in 1/z, are compared with the results of exact calculations and of other approximate methods. This provides a test for the unrenormalized high-density expansion method.
A model of spin-polarized fermions hopping on a two-dimensional lattice with a nearest-neighbor interaction V is studied. Random-phase-approximation calculations predict that the half-filled system undergoes a density-wave transition for positive values of V, an odd-angular-momentum pairing transition for small negative V, and a condensation-phase-separation transition for more negative values of V. The classical lattice-gas Ising limit matches onto the density-wave transition for V>0 and the condensation transition for V<0. A strong-coupling expansion in powers of the ratio of single-fermion transfer-matrix element t to the two-body interaction V provides the leading corrections to the Ising limit. In order to explore the intermediate-coupling regime, fermion Monte Carlo calculations were carried out and various Green's functions characterizing the quantum correlations evaluated. With use of finite-size scaling techniques, the density-wave and condensation phase boundaries were followed into the intermediate-coupling regime. At these transitions, measured quantities scaled well with the usual Ising indices. Unfortunately, the weak-coupling regime lies beyond the reach of these simulations, and we conclude that a method suitable for the weak-coupling regime is needed to complete the phase diagram.
We investigated possible superfluid phases at finite temperature in a two-component Fermi gas with density imbalance. In the frame of a general four-fermion interaction theory, we solved in the BCS region the gap equations for the pairing gap and pairing momentum under the restriction of fixed number densities and analyzed the stability of different phases by calculating the superfluid density and number susceptibilities. The homogeneous superfluid is stable only at high temperature and low number asymmetry, the inhomogeneous Larkin-Ovchinnikov-Fudde-Ferrell state survives at low temperature and high number asymmetry, and in between them there exists another possible inhomogeneous phase, that of phase separation. The critical temperatures and the orders of the phase transitions among the superfluid phases and normal phase are calculated analytically and numerically. The phase diagram we obtained in the temperature and number asymmetry plane is quite different from the one in the temperature and chemical potential difference plane for a system with fixed chemical potentials.
Using the Jordan-Wigner transformation we calculate exactly the ground state and low-temperature thermodynamic properties of the spin
S=1/2 isotropic
XX chain with four spin interaction. In terms of the equivalent spinless fermion (SF) representation the system is viewed as a lattice fermion gas with nearest-neighbor (
J) and next-next-next-neighbor (
J∗/4) hopping. It is shown that with the increase of four spin coupling, at
Jc∗=4J/3 the system experiences the Lifshitz type topological phase transition characterized by the tripling of Fermi points. The quantum phase transition (QPT) point marks transition from a gapless spin-liquid phase of standard
XX chain into again a gapless spin-liquid phase with different character of power-low decay of spin correlations. At the transition point the free fermion dispersion relation shows flattering at Fermi points, what determines singular character of density of states
ρ(ω)∼(ω/J)−2/3 and as a consequence unconventional temperature dependence of heat capacity of the system
C∼(T/J∗)1/3, and singular magnetic susceptibility of the system
χ(H)∼(H/J∗)−2/3. In the case of alternating magnetic field the system is characterized by the rich ground state phase diagram which contains fully polarized (ferromagnetic), gapped antiferromagnetic (AFM) and spin liquid phases. At the transition point from the gapped AFM phase into the gapless polarized spin liquid phase the system shows rapid increase of magnetization
m∼(H−Hc)1/6 and magnetic susceptibility a singular behavior as
χ(H)∼(H−Hc)−5/6.
According to the exact analytical Bethe-ansatz solution of the one-dimensional spin-1/2 XXZ model, we perform a numerical study of one-dimensional spinless fermions in an optical lattice in the presence of harmonic potential by using a local density approximation. We study the density profile, and obtain the -u phase diagram (here =NfV2/t is the characteristic dimensionless density and u=V/t is the interaction strength scaled in units of the hopping parameter t). With the increases of particle density and nearest-neighbour interaction, the system undergoes five different quantum phases. Through calculating the thermodynamic stiffness S, we find that it can be used as an order parameter. Its singular points can measure the quantum phase transitions in such a confined system.