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First-order phase transition in Onsager's model of hard rods in R3\mathbb R^3

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Abstract

We prove a first-order transition for hard rods in R3\mathbb R^3 whose orientations vary continuously over the sphere. For every sufficiently small positive ratio of diameter to length, the grand-canonical pressure has a nondifferentiable point, with a jump in number density at the intermediate Onsager scale. Following the phase-pressure strategy of Lebowitz, Mazel, and Presutti, we connect coexistence in the homogeneous Onsager functional to the microscopic pressure through an exact auxiliary-field representation, exponential decay of correlations in the rod gas at fixed auxiliary field, and estimates of the free-energy cost of interfaces between connected families of minimizing angular profiles.

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The pressure transition and the proof strategy

Let dωd\omega be probability area measure on S2S^2. A rod of axial length one and diameter ε>0\varepsilon> 0, with centre xx and axis uu, occupies

Rε(x,u)=x+[−1/2,1/2]u+B(0,ε/2).R_\varepsilon(x,u)=x+[-1/2,1/2]u+B(0,\varepsilon/2).

The orientation varies continuously over S2S^2, with uu and −u-u describing the same rod. For a bounded region Λ\Lambda, activity z>0z > 0, and admissible exterior configuration ξ\xi, define

ZΛ,ε(z∣ξ)=∑N≥0zNN!∫(Λ×S2)N1hc(X1,…,XN;ξ)∏j=1Ndxj dω(uj).(1)Z_{\Lambda,\varepsilon}(z\mid\xi)=\sum_{N\geq0}\frac{z^N}{N!}\int_{(\Lambda\times S^2)^N}\mathbf{1}_{\mathrm{hc}}(X_1,\ldots,X_N;\xi)\prod_{j=1}^N dx_j\,d\omega(u_j). \tag*{(1)}

The indicator requires disjoint rod interiors. Centres in Λ\Lambda are integrated; the exterior rods are fixed. For Λs=[−s/2,s/2]3\Lambda_s=[-s/2,s/2]^3, write

Pε(a)=lim⁡s→∞1∣Λs∣log⁡ZΛs,ε(ε−1ea∣∅).(2)P_\varepsilon(a)=\lim_{s\to\infty}\frac{1}{|\Lambda_s|}\log Z_{\Lambda_s,\varepsilon}(\varepsilon^{-1}e^a\mid\varnothing). \tag*{(2)}

Theorem 1.1 (Nondifferentiability of the pressure). The pressure PεP_\varepsilon exists, is finite, and is convex in aa. There are at∈Ra_t\in\mathbb{R}, ε0>0\varepsilon_0>0, and constants Δ,c,C>0\Delta,c,C>0 such that for every 0<ε<ε00<\varepsilon<\varepsilon_0 there is aεa_\varepsilon, with aε→ata_\varepsilon\to a_t as ε↓0\varepsilon\downarrow0, satisfying

Pε,+′(aε)−Pε,−′(aε)≥Δ2ε,cε≤Pε,±′(aε)≤Cε.(3)P'_{\varepsilon,+}(a_\varepsilon)-P'_{\varepsilon,-}(a_\varepsilon)\geq\frac{\Delta}{2\varepsilon},\qquad\frac{c}{\varepsilon}\leq P'_{\varepsilon,\pm}(a_\varepsilon)\leq\frac{C}{\varepsilon}. \tag*{(3)}

The one-sided derivatives are the limiting number densities from the corresponding sides of the transition.

Thus the transition occurs at number density of order ε−1\varepsilon^{-1} and occupied volume fraction of order ε\varepsilon. For axial length ℓ\ell and diameter dd, these scales become (ℓ2d)−1(\ell^2d)^{-1} and d/ℓd/\ell, respectively.

The strategy and its relation to LMP

Lebowitz, Mazel, and Presutti (LMP) [1, 2] transferred mean-field coexistence to a continuum system with a large but finite interaction range. Their method combines a variational description of the reference phases, estimates within each phase, control of interfaces, and an adjustment of the chemical potential that makes the corrected phase pressures equal. We follow this sequence. The model-specific work replaces their Kac interactions by hard exclusion and their scalar density minima by families of angular profiles. The contour step uses Pirogov–Sinai theory [3, 4], particularly Zahradník’s truncation and stability argument [5].

The conclusion here concerns the pressure. Once the physical pressure is identified with the maximum of phase pressures whose density responses are separated, their crossing proves the theorem. This avoids the further construction and analysis of distinct physical Gibbs measures carried out by LMP. The argument still needs locality of the reference gas and boundary estimates for restricted free energies; auxiliary Gibbs states are used to establish those estimates.

The following eleven steps are the subjects of the next eleven sections, in this order.

  1. Identify the homogeneous phases. The Onsager functional has an isolated isotropic minimizer and nonisotropic minimizers at one chemical potential. Their masses are separated. Analytic coordinates divide the complete minimizing set into finitely many connected components.

  1. Introduce an exact auxiliary field. Attach a normalized Gaussian variable to each spatial and angular cell. Integrating these variables restores the original partition function exactly. Conditional on the field, a quadratic count penalty provides the reference system used in the subsequent estimates.

  1. Derive the variational limit. The thin-rod overlap operator and a random graph representation give the finite-volume Onsager functional for the reference gas. This supplies the microscopic approximation that plays the role of coarse graining in LMP.

  1. Prove locality of the reference gas. The quadratic penalty makes the insertion generator coercive. Its inverse yields covariance decay uniformly in the field, the volume, and fixed exterior rods. This replaces LMP’s restricted reference-system uniqueness estimates.

  1. Construct a positive field action. Directional averaging splits the leading free energy into nonnegative terms. A transport rigidity theorem identifies their common zeros. Minimization over density profiles then gives a local field action and quantitative separation of its minimizing components.

  1. Restore all particle counts. A projection empties cells exceeding the reference count bound. A convergent expansion reconstructs their contribution and propagates the reference estimates to the exact rod integral.

  1. Recover the exact field interaction. Integrate the field response to express the exact interaction as the positive leading action plus a small summable error. A common additive normalization allows pressures with different reference profiles to be compared.

  1. Control boundary costs. Interpolate density profiles within a connected minimizing component and compare restricted field integrals. This treats the continuous families of angular minima. It gives surface bounds and a comparison of partition functions across a separating layer.

  1. Assign contours and their costs. Blocks where a phase cannot be continued form contours. Rigidity and isoperimetry supply their positive cost. The exact regional expansion retains the boundary terms needed to combine neighboring regions.

  1. Identify the physical pressure. Following LMP’s use of auxiliary phase pressures, truncate large contour weights and prove uniform surface estimates. An induction then shows that the physical pressure is the maximum of these phase pressures.

  1. Locate the crossing. Differentiate the original Gaussian representation. The homogeneous mass gap gives separated increments of the complete phase pressures, while their leading values agree. Adjusting the chemical potential forces a crossing and hence the asserted density jump.

Homogeneous reference phases

Let dωd\omega be probability area measure on S2S^2 and define

(Kf)(u)=∫S2∣u×v∣f(v) dω(v),Ωa(f)=∫S2f(log⁡f−1−a) dω+⟨f,Kf⟩.(Kf)(u) = \int_{S^2} |u \times v|f(v)\,d\omega(v), \qquad\Omega_a(f) = \int_{S^2} f(\log f - 1 - a)\,d\omega+ \langle f,Kf\rangle.

This is the grand-canonical form of Onsager’s second-virial functional [6]. Profiles are nonnegative integrable functions; the entropy may be +∞+\infty. Write ma=inf⁡Ωam_a = \inf\Omega_a and Wa={f:Ωa(f)=ma}W_a = \{f : \Omega_a(f) = m_a\}.

For bounds on fixed-mass Onsager minimizers, see Vollmer [7 Section 7.1]. The following lemma gives the bounds needed with variable mass.

Lemma 2.1 (Bounds and compactness of the minimizers). The set WaW_a is nonempty. Every minimizer is even and satisfies

log⁡f+2Kf=a,ea−2ea≤f≤ea.(4)\log f + 2Kf = a,\qquad e^{a-2e^a} \le f \le e^a. \tag*{(4)}

The minimizers are uniformly Lipschitz on the sphere. Their union over a compact parameter interval is relatively compact in the uniform norm, and limits at a convergent sequence of parameters are minimizers at the limiting parameter.

Proof. If ρ=∫f dω\rho= \int f\,d\omega, Jensen’s inequality and positivity of the kernel give

Ωa(f)≥ρlog⁡ρ−(1+a)ρ.\Omega_a(f) \ge\rho\log\rho- (1+a)\rho.

A minimizing sequence consequently has bounded mass and bounded positive entropy. It is uniformly integrable and has a weakly convergent subsequence in L1L^1. The entropy is lower semicontinuous. The interaction is continuous under this convergence: approximate its continuous kernel uniformly on S2×S2S^2 \times S^2 by finite sums of products of continuous functions, and use the uniform mass bound. This proves existence. Replacing f(u)f(u) by [f(u)+f(−u)]/2[f(u) + f(-u)]/2 preserves its interaction and mass and strictly lowers its entropy unless ff is even.

For positivity and the Euler equation, fix a minimizer ff, set V=2KfV = 2Kf, and consider L(g)=∫g(log⁡g−1−a+V) dωL(g) = \int g(\log g - 1 - a + V)\,d\omega. Its unique minimizer is g=ea−Vg = e^{a-V}. If L(g)<L(f)L(g) < L(f), convexity and the quadratic expansion give, for ft=(1−t)f+tgf_t = (1-t)f + tg,

Ωa(ft)−Ωa(f)≤t[L(g)−L(f)]+t2⟨g−f,K(g−f)⟩<0\Omega_a(f_t) - \Omega_a(f) \le t[L(g) - L(f)] + t^2\langle g-f,K(g-f)\rangle< 0

for small t>0t > 0, a contradiction. Thus f=ea−2Kff = e^{a-2Kf}. Since 0≤Kf≤∫f dω≤ea0 \le Kf \le\int f\,d\omega\le e^a, the bounds follow. The kernel is Lipschitz in its first argument, uniformly in the second. The Euler equation gives a uniform Lipschitz bound for ff. Arzelà–Ascoli and continuity of the functional on the resulting bounded positive profiles prove the compactness assertions. □\square

The spectrum below is the Onsager-kernel spectrum computed in [7 Corollary 10], divided by 4π4\pi because dωd\omega is probability measure.

Lemma 2.2 (Angular spectrum). On spherical harmonics of degree ll, the eigenvalue of KK is

κ0=π4,κ2n=−π2(2n−1)(2n+2)((2nn)4n)2(n≥1),κ2n+1=0.(5)\kappa_0 = \frac{\pi}{4}, \qquad\kappa_{2n} = -\frac{\pi}{2(2n-1)(2n+2)}\left(\frac{\binom{2n}{n}}{4^n}\right)^2 \quad(n \ge1), \qquad\kappa_{2n+1}=0. \tag*{(5)}

In particular inf⁡lκl=κ2=−π/32\inf_l \kappa_l=\kappa_2=-\pi/32, consecutive even eigenvalues are distinct, and ∣κ2n∣=O(n−3)|\kappa_{2n}|=O(n^{-3}).

Proof. The Funk–Hecke formula gives κl=12∫−111−t2Pl(t) dt\kappa_l=\frac{1}{2}\int_{-1}^{1}\sqrt{1-t^2}P_l(t)\,dt, where PlP_l is the Legendre polynomial. Put w(t)=1−t2w(t)=\sqrt{1-t^2} and denote the integrals of wPlwP_l and Pl/wP_l/w by IlI_l and JlJ_l. Two integrations by parts in Legendre’s equation give l(l+1)Il=2Il−Jll(l+1)I_l=2I_l-J_l; the boundary terms vanish and the remaining endpoint singularities are integrable. Odd integrals vanish. The generating function gives

∑l≥0Jlzl=∫0π(1−zeiθ)−1/2(1−ze−iθ)−1/2 dθ=π∑n≥0((2nn)4n)2z2n.\sum_{l\ge0}J_lz^l=\int_0^\pi(1-ze^{i\theta})^{-1/2}(1-ze^{-i\theta})^{-1/2}\,d\theta=\pi\sum_{n\ge0}\left(\frac{\binom{2n}{n}}{4^n}\right)^2z^{2n}.

Here conjugate off-diagonal terms combine into cosines with zero integral. This proves (5). Finally,

∣κ2n+2∣∣κ2n∣=4n2−14(n+1)(n+2)<1,\frac{|\kappa_{2n+2}|}{|\kappa_{2n}|}=\frac{4n^2-1}{4(n+1)(n+2)}<1,

and the central binomial coefficient bound gives the stated decay. □

The isotropic critical point is the constant r(a)>0r(a)>0 determined by log⁡r+2κ0r=a\log r+2\kappa_0r=a; it increases continuously with aa.

Onsager’s degree-two trial profiles show how global minimality can be lost before local stability [6 pp. 645–646]; see also Kayser and Raveché [8]. The following proof uses an explicit finite-amplitude comparison in the grand-canonical functional.

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