# Griffiths inequality

In statistical mechanics, the Griffiths inequality, sometimes also called Griffiths–Kelly–Sherman inequality or GKS inequality, named after Robert B. Griffiths, is a correlation inequality for ferromagnetic spin systems. Informally, it says that in ferromagnetic spin systems, if the 'a-priori distribution' of the spin is invariant under spin flipping, the correlation of any monomial of the spins is non-negative; and the two point correlation of two monomial of the spins is non-negative.

The inequality was proved by Griffiths for Ising ferromagnets with two-body interactions,[1] then generalised by Kelly and Sherman to interactions involving an arbitrary number of spins,[2] and then by Griffiths to systems with arbitrary spins.[3] A more general formulation was given by Ginibre,[4] and is now called the Ginibre inequality.

## Definitions

Let ${\displaystyle \textstyle \sigma =\{\sigma _{j}\}_{j\in \Lambda }}$ be a configuration of (continuous or discrete) spins on a lattice Λ. If AΛ is a list of lattice sites, possibly with duplicates, let ${\displaystyle \textstyle \sigma _{A}=\prod _{j\in A}\sigma _{j}}$ be the product of the spins in A.

Assign an a-priori measure dμ(σ) on the spins; let H be an energy functional of the form

${\displaystyle H(\sigma )=-\sum _{A}J_{A}\sigma _{A}~,}$

where the sum is over lists of sites A, and let

${\displaystyle Z=\int d\mu (\sigma )e^{-H(\sigma )}}$

be the partition function. As usual,

${\displaystyle \langle \cdot \rangle ={\frac {1}{Z}}\sum _{\sigma }\cdot (\sigma )e^{-H(\sigma )}}$

stands for the ensemble average.

The system is called ferromagnetic if, for any list of sites A, JA ≥ 0. The system is called invariant under spin flipping if, for any j in Λ, the measure μ is preserved under the sign flipping map σ → τ, where

${\displaystyle \tau _{k}={\begin{cases}\sigma _{k},&k\neq j,\\-\sigma _{k},&k=j.\end{cases}}}$

## Statement of inequalities

### First Griffiths inequality

In a ferromagnetic spin system which is invariant under spin flipping,

${\displaystyle \langle \sigma _{A}\rangle \geq 0}$

for any list of spins A.

### Second Griffiths inequality

In a ferromagnetic spin system which is invariant under spin flipping,

${\displaystyle \langle \sigma _{A}\sigma _{B}\rangle \geq \langle \sigma _{A}\rangle \langle \sigma _{B}\rangle }$

for any lists of spins A and B.

The first inequality is a special case of the second one, corresponding to B = ∅.

## Proof

Observe that the partition function is non-negative by definition.

Proof of first inequality: Expand

${\displaystyle e^{-H(\sigma )}=\prod _{B}\sum _{k\geq 0}{\frac {J_{B}^{k}\sigma _{B}^{k}}{k!}}=\sum _{\{k_{C}\}_{C}}\prod _{B}{\frac {J_{B}^{k_{B}}\sigma _{B}^{k_{B}}}{k_{B}!}}~,}$

then

{\displaystyle {\begin{aligned}Z\langle \sigma _{A}\rangle &=\int d\mu (\sigma )\sigma _{A}e^{-H(\sigma )}=\sum _{\{k_{C}\}_{C}}\prod _{B}{\frac {J_{B}^{k_{B}}}{k_{B}!}}\int d\mu (\sigma )\sigma _{A}\sigma _{B}^{k_{B}}\\&=\sum _{\{k_{C}\}_{C}}\prod _{B}{\frac {J_{B}^{k_{B}}}{k_{B}!}}\int d\mu (\sigma )\prod _{j\in \Lambda }\sigma _{j}^{n_{A}(j)+n_{B}(j)}~,\end{aligned}}}

where nA(j) stands for the number of times that j appears in A. Now, by invariance under spin flipping,

${\displaystyle \int d\mu (\sigma )\prod _{j}\sigma _{j}^{n(j)}=0}$

if at least one n(j) is odd, and the same expression is obviously non-negative for even values of n. Therefore, Z<σA>≥0, hence also <σA>≥0.

Proof of second inequality. For the second Griffiths inequality, double the random variable, i.e. consider a second copy of the spin, ${\displaystyle \sigma '}$, with the same distribution of ${\displaystyle \sigma }$. Then

${\displaystyle \langle \sigma _{A}\sigma _{B}\rangle -\langle \sigma _{A}\rangle \langle \sigma _{B}\rangle =\langle \langle \sigma _{A}(\sigma _{B}-\sigma '_{B})\rangle \rangle ~.}$

Introduce the new variables

${\displaystyle \sigma _{j}=\tau _{j}+\tau _{j}'~,\qquad \sigma '_{j}=\tau _{j}-\tau _{j}'~.}$

The doubled system ${\displaystyle \langle \langle \;\cdot \;\rangle \rangle }$ is ferromagnetic in ${\displaystyle \tau ,\tau '}$ because ${\displaystyle -H(\sigma )-H(\sigma ')}$ is a polynomial in ${\displaystyle \tau ,\tau '}$ with positive coefficients

{\displaystyle {\begin{aligned}\sum _{A}J_{A}(\sigma _{A}+\sigma '_{A})&=\sum _{A}J_{A}\sum _{X\subset A}\left[1+(-1)^{|X|}\right]\tau _{A\setminus X}\tau '_{X}\end{aligned}}}

Besides the measure on ${\displaystyle \tau ,\tau '}$ is invariant under spin flipping because ${\displaystyle d\mu (\sigma )d\mu (\sigma ')}$ is. Finally the monomials ${\displaystyle \sigma _{A}}$, ${\displaystyle \sigma _{B}-\sigma '_{B}}$ are polynomials in ${\displaystyle \tau ,\tau '}$ with positive coefficients

{\displaystyle {\begin{aligned}\sigma _{A}&=\sum _{X\subset A}\tau _{A\setminus X}\tau '_{X}~,\\\sigma _{B}-\sigma '_{B}&=\sum _{X\subset B}\left[1-(-1)^{|X|}\right]\tau _{B\setminus X}\tau '_{X}~.\end{aligned}}}

The first Griffiths inequality applied to ${\displaystyle \langle \langle \sigma _{A}(\sigma _{B}-\sigma '_{B})\rangle \rangle }$ gives the result.

More details are in.[5]

## Extension: Ginibre inequality

The Ginibre inequality is an extension, found by Jean Ginibre,[4] of the Griffiths inequality.

### Formulation

Let (Γ, μ) be a probability space. For functions fh on Γ, denote

${\displaystyle \langle f\rangle _{h}=\int f(x)e^{-h(x)}\,d\mu (x){\Big /}\int e^{-h(x)}\,d\mu (x).}$

Let A be a set of real functions on Γ such that. for every f1,f2,...,fn in A, and for any choice of signs ±,

${\displaystyle \iint d\mu (x)\,d\mu (y)\prod _{j=1}^{n}(f_{j}(x)\pm f_{j}(y))\geq 0.}$

Then, for any f,g,−h in the convex cone generated by A,

${\displaystyle \langle fg\rangle _{h}-\langle f\rangle _{h}\langle g\rangle _{h}\geq 0.}$

### Proof

Let

${\displaystyle Z_{h}=\int e^{-h(x)}\,d\mu (x).}$

Then

{\displaystyle {\begin{aligned}&Z_{h}^{2}\left(\langle fg\rangle _{h}-\langle f\rangle _{h}\langle g\rangle _{h}\right)\\&\qquad =\iint d\mu (x)\,d\mu (y)f(x)(g(x)-g(y))e^{-h(x)-h(y)}\\&\qquad =\sum _{k=0}^{\infty }\iint d\mu (x)\,d\mu (y)f(x)(g(x)-g(y)){\frac {(-h(x)-h(y))^{k}}{k!}}.\end{aligned}}}

Now the inequality follows from the assumption and from the identity

${\displaystyle f(x)={\frac {1}{2}}(f(x)+f(y))+{\frac {1}{2}}(f(x)-f(y)).}$

## Applications

• The thermodynamic limit of the correlations of the ferromagnetic Ising model (with non-negative external field h and free boundary conditions) exists.
This is because increasing the volume is the same as switching on new couplings JB for a certain subset B. By the second Griffiths inequality
${\displaystyle {\frac {\partial }{\partial J_{B}}}\langle \sigma _{A}\rangle =\langle \sigma _{A}\sigma _{B}\rangle -\langle \sigma _{A}\rangle \langle \sigma _{B}\rangle \geq 0}$
Hence ${\displaystyle \langle \sigma _{A}\rangle }$ is monotonically increasing with the volume; then it converges since it is bounded by 1.
• The one-dimensional, ferromagnetic Ising model with interactions ${\displaystyle J_{x,y}\sim |x-y|^{-\alpha }}$ displays a phase transition if ${\displaystyle 1<\alpha <2}$.
This property can be shown in a hierarchical approximation, that differs from the full model by the absence of some interactions: arguing as above with the second Griffiths inequality, the results carries over the full model.[6]
• The Ginibre inequality provides the existence of the thermodynamic limit for the free energy and spin correlations for the two-dimensional classical XY model.[4] Besides, through Ginibre inequality, Kunz and Pfister proved the presence of a phase transition for the ferromagnetic XY model with interaction ${\displaystyle J_{x,y}\sim |x-y|^{-\alpha }}$ if ${\displaystyle 2<\alpha <4}$.
• Aizenman and Simon[7] used the Ginibre inequality to prove that the two point spin correlation of the ferromagnetic classical XY model in dimension ${\displaystyle D}$, coupling ${\displaystyle J>0}$ and inverse temperature ${\displaystyle \beta }$ is dominated by (i.e. has upper bound given by) the two point correlation of the ferromagnetic Ising model in dimension ${\displaystyle D}$, coupling ${\displaystyle J>0}$, and inverse temperature ${\displaystyle \beta /2}$
${\displaystyle \langle \mathbf {s} _{i}\cdot \mathbf {s} _{j}\rangle _{J,2\beta }\leq \langle \sigma _{i}\sigma _{j}\rangle _{J,\beta }}$
Hence the critical ${\displaystyle \beta }$ of the XY model cannot be smaller than the double of the critical temperature of the Ising model
${\displaystyle \beta _{c}^{XY}\geq 2\beta _{c}^{\rm {Is}}~;}$
in dimension D = 2 and coupling J = 1, this gives
${\displaystyle \beta _{c}^{XY}\geq \ln(1+{\sqrt {2}})\approx 0.88~.}$
• There exists a version of the Ginibre inequality for the Coulomb gas that implies the existence of thermodynamic limit of correlations.[8]
• Other applications (phase transitions in spin systems, XY model, XYZ quantum chain) are reviewed in.[9]

## References

1. ^ Griffiths, R.B. (1967). "Correlations in Ising Ferromagnets. I". J. Math. Phys. 8 (3): 478–483. doi:10.1063/1.1705219.
2. ^ Kelly, D.J.; Sherman, S. (1968). "General Griffiths' inequalities on correlations in Ising ferromagnets". J. Math. Phys. 9: 466. doi:10.1063/1.1664600.
3. ^ Griffiths, R.B. (1969). "Rigorous Results for Ising Ferromagnets of Arbitrary Spin". J. Math. Phys. 10: 1559. doi:10.1063/1.1665005.
4. ^ a b c Ginibre, J. (1970). "General formulation of Griffiths' inequalities". Comm. Math. Phys. 16 (4): 310–328. doi:10.1007/BF01646537.
5. ^ Glimm, J.; Jaffe, A. (1987). Quantum Physics. A functional integral point of view. New York: Springer-Verlag. ISBN 0-387-96476-2.
6. ^ Dyson, F.J. (1969). "Existence of a phase-transition in a one-dimensional Ising ferromagnet". Comm. Math. Phys. 12: 91–107. doi:10.1007/BF01645907.
7. ^ Aizenman, M.; Simon, B. (1980). "A comparison of plane rotor and Ising models". Phys. Lett. A. 76. doi:10.1016/0375-9601(80)90493-4.
8. ^ Fröhlich, J.; Park, Y.M. (1978). "Correlation inequalities and the thermodynamic limit for classical and quantum continuous systems". Comm. Math. Phys. 59 (3): 235–266. doi:10.1007/BF01611505.
9. ^ Griffiths, R.B. (1972). "Rigorous results and theorems". In C. Domb and M.S.Green. Phase Transitions and Critical Phenomena. 1. New York: Academic Press. p. 7.