B 5. Consider a system of N spins that can take values o, € (-1,0,1). Denote each configuration by σ = (₁, ...,N), the m

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B 5. Consider a system of N spins that can take values o, € (-1,0,1). Denote each configuration by σ = (₁, ...,N), the magnetisation of o by M(o)= {i=10i and the alignment E() = 0. The MaxEnt distribution of spin configurations, given a constraint on the average magnetisation (M(o)) and the average alignment (E(o)) is P(o)= Z-¹ exp(hM(o) + JE(o)), where h and J are Lagrange multipliers and Z is the partition function. (a) [3 points] Show that the spin alignment can be written as N E(o) 2 [²(0)-20]. 2N i=1 (b) [17 points] Using the Gaussian identity 2п de e- dre-lab = -e6² a show that the partition function Z can be written for large N as Zx x / dre dre-Ny(zh,J) (2) > where the sub-leading proportionality constant is omitted, and p(x; h, J) = 2² 2J - log (1+2 cosh(h+z)). (c) [5 points] Apply the Laplace method to the integral in Eq. (2) and show that the free energy per spin f(h, J) in the large N limit is equal to p(x*; h, J). Provide explicitly the self-consistency equation satisfied by z*. (d) [5 points] Setting h = 0, determine the critical value Je of J above which the system displays collective behaviour, i.e. the value marking the transition between zero and non-zero typical magnetisation of the patterns in the absence of an external field. State the order of the phase transition.