4,586 research outputs found
Flocking with discrete symmetry: the 2d Active Ising Model
We study in detail the active Ising model, a stochastic lattice gas where
collective motion emerges from the spontaneous breaking of a discrete symmetry.
On a 2d lattice, active particles undergo a diffusion biased in one of two
possible directions (left and right) and align ferromagnetically their
direction of motion, hence yielding a minimal flocking model with discrete
rotational symmetry. We show that the transition to collective motion amounts
in this model to a bona fide liquid-gas phase transition in the canonical
ensemble. The phase diagram in the density/velocity parameter plane has a
critical point at zero velocity which belongs to the Ising universality class.
In the density/temperature "canonical" ensemble, the usual critical point of
the equilibrium liquid-gas transition is sent to infinite density because the
different symmetries between liquid and gas phases preclude a supercritical
region. We build a continuum theory which reproduces qualitatively the behavior
of the microscopic model. In particular we predict analytically the shapes of
the phase diagrams in the vicinity of the critical points, the binodal and
spinodal densities at coexistence, and the speeds and shapes of the
phase-separated profiles.Comment: 20 pages, 25 figure
Improved stability regions for ground states of the extended Hubbard model
The ground state phase diagram of the extended Hubbard model containing
nearest and next-to-nearest neighbor interactions is investigated in the
thermodynamic limit using an exact method. It is found that taking into account
local correlations and adding next-to-nearest neighbor interactions both have
significant effects on the position of the phase boundaries. Improved stability
domains for the -pairing state and for the fully saturated ferromagnetic
state at half filling have been constructed. The results show that these states
are the ground states for model Hamiltonians with realistic values of the
interaction parameters.Comment: 21 pages (10 figures are included) Revtex, revised version. To be
published in Phys. Rev. B. E-mail: [email protected]
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