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Improved MeanField Theory: Bethe’s ApproximationTable of Contents:
We consider the case of Ising spins. The meanfield theoiy can be improved by using the partition function of an approximate Hamiltonian constructed as follows. We separate the whole Hamiltonian into two parts: a cluster containing a spin, say
where В is an applied magnetic field. The spin a_{0} and its z surrounding nearest neighbors
where a = J /k_{B}T, b = ц_{в}В/k_{B}T and с = ц_{в}Н / k_{B}T = [z.  1)/ < о > / k_{B} T. Summing on бт_{0} = ±1 and factorizing the other sums, one gets
The averaged <
Setting < (т_{0} >=< <т, >=< a >, one has Replacing Z± by (5.18), one obtains
This equation is used to determine selfconsistently c, namely < a >. In zero field (B = 0), one has b = 0 so that
It is observed that c = 0 is a solution of the last equation. However, there is a nonzero solution by making an expansion at small c (small < a >) to the third order:
from which one has
Since the lefthand side is positive, this equation admits a nonzero solution if [tanh a  > 0. This means that the nonzero solution exists if T < T_{c} where T_{c} is given by
It is interesting to note that for z — 1, namely a cluster of two spins, there is no solution for T_{c}, and for z = 2 (onedimensional chain) one has T_{c} = 0. This corresponds to the results from the exact solutions (see paragraphs 5.4.2 and 5.6, and Problems 1). We recall that the meanfield theory incorrectly yields T_{c} Ф 0 for one dimension [see Eq. (2.23)]. Landau–Ginzburg TheoryIt has been shown above that the meanfield theory suffers a serious flaw due to the fact that instantaneous local spin fluctuations have been neglected in this theory. For example, the meanfield theory can give rise to a phase transition where there is none in low dimensions. In addition, in the critical region neglecting fluctuations modifies the behavior of the phase transition as we will see when comparing the meanfield critical exponents with the exact ones. There exist several more efficient theories such as the high and low temperature series expansions [95], the LandauGinzburg theory and the renormalization group. In what follows, we present the LandauGinzburg theory and the concepts of the renormalization group. The LandauGinzburg theory is an extension of the meanfield theory which includes a great part of fluctuations so far neglected near the transition. The main idea is to start from an expansion of the free energy per spin / in the vicinity of T_{c} when the magnetization m is sufficiently small:
where С, A, В and D are constants, h is an applied magnetic field and the suffix MF denotes quantities coming from the meanfield theory. The form of this expansion and the sign of В reflect the system symmetry. In the case where h = 0 and В > 0, /_{M}r presents a minimum at m = 0 for T > T_{C}^{MF} and two symmetric minima at ±m_{0} for T < T_{C}^{MF} (see Fig. 5.2), indicating two degenerate ordered states. This degeneracy is removed when h ^ 0 as shown in Fig. 5.3. Figure 5.2 Meanfield free energy at Г > T_{c} (left) and at Г < T_{c} (right). Figure 5.3 Meanfield free energy at Г < T_{c} in an applied magnetic field. When В < 0, a firstorder transition is possible. At T = T_{c}, there are three equivalent minima of f_{M}F at 0, ±m_{0} (see Fig. 5.4) contrary to the case В > 0. This means that at the transition the three phases m = 0 (paramagnetic state) and ±m_{0} (ordered states) coexist. The energy distribution at T = T_{c} is thus bimodal, the peak at low Figure 5.4 Meanfield free energy at a firstorder transition. energy corresponds to the energy of the ordered phase while that at high energy corresponds to the energy of the disordered state. The distance between the two peaks is the latent heat which is observed at a firstorder transition. When an m^{3} term is present in the expansion (5.28), the transition is always of first order. MeanField Critical ExponentsThe critical exponents calculated by the meanfield theory are (see Chapter 2) = 1/2 [see (2.24)], у = 1 [see (2.38)]. One can find them again here by using (5.28). Putting t = (Г — T_{C}^{MF})/ T_{C}^{MF}, one has • When t < 0 and h = 0, /mf is minimum at mo oc (—£)?, so that P = 1/2.
Correlation FunctionThis subsection considers a system of Ising spins for simplicity. A simplified notation has been used in this paragraph: r instead of r. The correct notation will be recovered when necessary. The Hamiltonian is given by
The correlation function is calculated as follows:
where one has taken the spin at the origin 5(0) = 1. The trace was taken over all configurations. In the meanfield spirit, one replaces 5(r') on the righthand side of (5.30) by its average value < S(r') >, then one makes an expansion around T_{c} when < 5(r') > is small, one obtains using the notation m(r) =< 5(r) >. The Fourier transform of this equation gives
where C is a constant. For small k, one has where the sum on the term J (r)r of the second equality is zero because ] (r)r is an odd function of r [J (r) is an even function: J (r) = J (—r)]. / is defined by
and R^{2} is defined by
R^{2} is thus the order of the interaction range. One obtains from (5.32) and (5.33)
One recalls here that in the meanfield theoiy k_{B}T_{c} = /. One writes thus
where one has taken T ~ T_{c}. Putting
and using m(r) = G(r) of (5.30), one finally arrives at
The inverse Fourier transform of (5.39) gives
This form of G(r) justifies the fact that £ is called the "correlation length" in the meanfield theoiy. The expression (5.39) is called "OrnsteinZernike correlation function.” From (5.38), one sees that £ a t^{1/2}; therefore, v = 1/2 in this meanfield theory. In addition, at t = 0, £ tends to oc (see the following paragraph), G(k) of (5.39) is then proportional to 1/k^{2}. The inverse Fourier transform of this function is ^ for large r, indicating therefore that exponent i; of the meanfield theoiy is equal to 0 [see definition (5.14)]. Corrections to MeanField TheoryOne decomposes the following term [see (12.16)]:
where the average values of the linear terms in <5S_{r} and 6S,' are zero by symmetiy (rotating vectors in the xy plane). In the last equality, one has neglected, in the meanfield spirit, the following term:
This term, however, when taken into account, will improve the meanfield theory as seen below. Using the correlation function
one writes (5.42) as
One is interested now in a longdistance behavior, namely at small k. Using the Fourier transform of (5.39) for small k, one writes
where the integral is performed in the first Brillouin zone. One decomposes this integral as follows:
The first integral does not depend on £, namely independent of T. It contributes to a shift of the value of T_{c} calculated by the meanfield theory. The second integral depends on £ thus on T: It converges if к = ±n/a > oo, namely a *■ 0 (continuum limit), and if d (space dimension) < 4. By a simple dimension analysis, one sees that this integral is proportional to $^{2}~^{d} at the limit £ *■ oo. The second term is thus proportional to This term has been neglected before in the meanfield theory because it was wrongly considered as always small with respect to the term coming from the mean field. Let us examine the condition when it can be neglected: where t = ДД. One knows that£ = Rt~^{1/2} [see (5.38)], expression (5.46) can be thus rewritten as This condition for neglecting the second term of (5.45) is called "Ginzburg's criterion." One sees that when d < 4 this criterion is not satisfied near T_{c} where f > oo. By consequence, the meanfield theory which neglects fluctuations characterized by £^{4d} is not valid in the critical region for d < 4. The dimension d = 4 is called "upper critical dimension” for the Ising model with shortrange interaction. Renormalization GroupTransformation of the Renormalization Group: Fixed PointThe central idea of the renormalization group is to replace the set of parameters which define the system by another set of parameters while conserving the essential physical ingredients, in particular the system symmetry. This set of parameter is simpler to deal with. In the study of the phase transition, this transformation consists in dividing the system into blocks of spins and replacing each block by a single spin. This "new” spin interacts with the others by the renormalized interactions calculated while decimating the block. The distances between the new spins are measured with a new lattice constant. This procedure is repeated for the system of new spins to obtain the next generation of spins which is used to generate the following generation, and so on. At each iteration, also called decimation or transformation, one has a relation between the new interaction K' = (5]' and the previous one К = pj : The new correlation length £ which is a function of K' is equal to the previous correlation length divided by the factor called "dilatation” b defined as the ratio between the new and old lattice constants. One has
At the phase transition, £ becomes infinite, the measuring distance unit is no more important. The interaction constants K' and К become identical К' = К = К*. This point is called the “fixed point” in the renormalization group language. The fixed point is thus determined by
The onedimensional case is simple to proceed as seen in paragraph 5.4.2 below. However, in the case of a general dimension d > 1, relation (5.48) is rather complicated. It is often impossible to find a solution of (5.50). One then has to take into account physical considerations in order to find appropriate approximations. One considers a point К in the proximity of the fixed point K*. If the iteration process takes К away from the fixed point, K* is an unstable fixed point. This is a “run away” case. On the other hand, in the case where К tends to K* by iteration, К* is a stable fixed point (see Problem 5.4.2 below). The map of these trajectories near a fixed point with indicated moving directions is called a "flow diagram." An example is shown in Fig. 5.5. This figure corresponds to the case of a square lattice of Ising spins with interactions K = ^ in the x direction and K_{2} = in the у direction. P is the fixed point. For a given ratio K_{2}/Ki, namely one follows the discontinued line: Intersection Pi with the line separating regions of different flows is the critical point corresponding to that ratio K_{2}/K_{x}. The line of flow separation shown by the heavy solid line is the critical line. One sees that Pi runs toward P on the critical line; therefore, Pi and P belong to the same universality class: The universality class does not thus depend on the ratio K_{2}/K. One can calculate the critical exponents using the renormalization group if one knows how K' depends on К in the vicinity of a fixed point K*. One makes then an expansion of their relation (5.48) Figure 5.5 Flow diagram for a square lattice of Ising spins with nearest neighbor interactions K_{y} along the x axis and K_{2} along the у axis. See text for comments. around К*:
where one has replaced f(K*) by К” using (5.50), and one has used the following notation:
Now, near K* one knows that t
By identifying the two sides of the above equation, one gets
This example shows that the critical exponent v is a derivative of the renormalization group equation (5.48). The other critical exponents, defined in (5.9), (5.10), (5.11), (5.13) and (5.14), are calculated by the use of the free energy. Details are given elsewhere, for example, in Refs. [10, 54, 380]. They are connected by the following scaling relations: In addition, the two exponents concerning the spinspin correlation, v and )/, are connected via the following "hyperscaling relations”:
In summary, there are six critical exponents and four relations between them. It suffices to determine two among six exponents. The other four can be then calculated using the above four relations. To close this section, one emphasizes that, in addition to the results shown above, another important result of the renormalization group is the relations connecting the system size to the critical exponents: Physical quantities calculated at a finite system size are shown to depend on powers of the linear system size. These powers are simple functions of critical exponents. They are veiy useful for the determination of critical exponents by Monte Carlo simulation (see, for example, Refs. [33, 87,199]). Renormalization Group Applied to an IsingSpin ChainOne applies in the following the renormalization group to the case of a chain of Ising spins with a ferromagnetic interaction between nearest neighbors. One will show that there is no phase transition at finite temperature. One considers the following Hamiltonian:
where К = J /кдТ, J being a ferromagnetic interaction between nearest neighbors. The partition function is given by
To study this spin chain, one uses the decimation method. One divides the system into threespin blocks as shown in Fig. 5.6. One writes for the two blocks in the figure the corresponding factors in Z
Using the following equality for the case o„ = ±1: Figure 5.6 Blocks of three spins used for the decimation. where x = tanh K, one rewrites (5.60) as
Writing explicitly each term of the product and making the sum on
One can rewrite it as
where C is a constant. If one forgets C, the righthand side is similar to the initial Hamiltonian with a new interaction K' between the remaining spins a_{2} and 05. To calculate K' one writes
By identifying this with the lefthand side of (5.63), one obtains and
One renumbers the spins after the first decimation as follows: Figure 5.7 Flow diagram of a chain of Ising spins. where N/3 is the number of threespin blocks (IV: initial number of spins). The equation of the renormalization group is thus
From the above equation, one sees that К' = К if К = 0 and K = oo:
The flow diagram is shown in Fig. 5.7. Any point between К = 0 and К = oo moves to К = 0 after successive decimations. The nature of any point between these limits is therefore the same as that of К = 0 (Г = oo), namely it belongs to the paramagnetic phase. The onedimensional chain is thus disordered at any finite temperature. 
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