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Conditional Ising equilibrium phase predictions
Classical translation-invariant finite-range Ising pair models on Z^d, d>=1, under the stated sufficient uniqueness condition; finite-temperature nearest-neighbor chains; and zero-field two-dimensional nearest-neighbor ferromagnets with J>0 and beta J>=2 for coexistence and the pressure cusp. Conditional model predictions only; no measured critical point or universal phase diagram.
Statement
Assume Countable Choice and adopt Interacting classical Gibbs equilibrium adoption for the specified classical Ising model. In the SI interpretation of Classical Gibbs quantities and SI reference conventions, is in , is the coupling energy in joules, and is an energy bias per dimensionless spin in joules, not a magnetic-field strength; hence and are dimensionless. The conditional model predictions are: (i) for a one-dimensional nearest-neighbor chain with finite , , the DLR equilibrium state is unique; (ii) for a translation-invariant finite-range Ising pair interaction with symmetric zero-diagonal couplings , and constant finite field, the sufficient condition gives a unique DLR equilibrium state; and (iii) for the two-dimensional nearest-neighbor ferromagnet at and , there are distinct stationary DLR equilibrium states with opposite nonzero magnetization and the pressure is nondifferentiable in at zero. These deductions are restricted to the stated classical model; they do not report a measured transition or infer a phase diagram between the sufficient regimes.
Facts & Assumptions
Given: A specified classical Ising Hamiltonian, , finite-range couplings and fields as stated, Countable Choice, and the adopted Gibbs-equilibrium postulate.
The equilibrium interpretation is an adopted model assumption, not a theorem about equilibration or nature (Interacting classical Gibbs equilibrium adoption).
Countable Choice is used in the lattice probability construction (The Axiom of Countable Choice ()).
Under Countable Choice, the one-dimensional nearest-neighbor Ising chain with finite and finite has exactly one DLR state (All-positive-temperature one-dimensional Ising DLR uniqueness).
Under a strict Ising influence row-sum bound, the finite-range DLR state is unique (Unique interacting Ising state at sufficient high temperature).
Under Countable Choice, for the zero-field nearest-neighbor Ising ferromagnet on with and satisfying , there exist stationary DLR states and whose one-site expectations satisfy and at every site (Two-dimensional Ising Gibbs coexistence at low temperature).
Those coexistence states force nondifferentiability of the field-dependent pressure at (Coexistence implies an Ising pressure cusp).
Proof
Apply [F1] to the one-dimensional model to obtain uniqueness of its mathematical DLR equilibrium state; by the adoption [A1], this is the model's unique equilibrium-state prediction under the stated conditions.
Under [A2], apply [F2] whenever its strict influence sum is below one. This is a sufficient uniqueness regime and makes no assertion about parameters where the sum is at least one.
For the stated zero-field two-dimensional nearest-neighbor ferromagnet with and , [F3] supplies two stationary DLR states with magnetizations separated from zero.
By [F4], their variational inequalities force a nonzero gap between the left and right pressure secant slopes. Thus coexistence and the cusp follow conditionally from the adopted postulate and the stated parameters; no empirical accuracy or exact critical value is inferred.
Depends on
Interacting classical Gibbs equilibrium adoption
Classical Gibbs quantities and SI reference conventions
Unique interacting Ising state at sufficient high temperature
Two-dimensional Ising Gibbs coexistence at low temperature
Coexistence implies an Ising pressure cusp
All-positive-temperature one-dimensional Ising DLR uniqueness
The Axiom of Countable Choice ()