Violation of Cluster Property in Heisenberg Antiferromagnet

There are concepts that are accepted in our daily life, but are not trivial in physics. One of them is the cluster property that means there exist no relations between two events which are sufficiently separated. In a paper recently published by the author, it has been pointed out that this cluster property violates in the correlation function of the spin operator for the spin 1/2 XXZ antiferromagnet on the square lattice. In this paper, we investigate the spin 1/2 Heisenberg antiferromagnet on the square lattice, which has SU(2) symmetry. In order to study the cluster property, we need to calculate the ground state accurately. For this purpose, we employ the effective model based on the magnetization of the sub-lattices. Then we can define the quasi-degenerate states to calculate the ground state. Including two kinds of interactions which break SU(2) symmetry into the Hamiltonian, we obtain the ground state quantitatively. We find that two kinds of spin correlation functions due to degenerate states are not zero when the lattice size is large but finite. The magnitude of one of them is the same as the one previously found in the XXZ antiferromagnet, while another one is much larger when the additional interaction is strong. We then conclude that in Heisenberg antiferromagnet correlation functions violate the cluster property and the magnitude of the violation qualitatively differs from the one in the XXZ antiferromagnet.

Entanglement found in many-body systems is a quite interesting subject [11] [12] in terms of the quantum phase transition [13] [14] [15] [16], because the correlation must be found even at the long distance where the whole system changes entirely.The entangled correlation at infinity distance implies the violation of the cluster decomposition [17] or the cluster property [18], which is a fundamental concept of physics that there is no relation between two events that occur infinitely apart from each other.Therefore, we find active studies on the cluster property in the many-body systems [19] [20] and in quantum field theory [21] [22], including QCD [23] [24].
In the previous paper [25], we have investigated the cluster property of spin 1/2 XXZ antiferromagnet on the square lattice.For this antiferromagnet, the ground state realizes semi-classical Neel order [26], in other words, spontaneous symmetry breaking (SSB) [17] [27] of U(1) symmetry [28] [29] [30] [31] [32].In the previous study, we supposed that the lattice size is very large but finite.On this condition, we clarified that the essential property of SSB is the existence of the quasi-degenerate states between which the expectation value of the local operator is not zero.Introducing an additional interaction that explicitly breaks the symmetry, we determined the ground state definitely using the quasi-degenerate states.Then we found that the correlation function in the ground state violated the cluster property and the magnitude of the violation depended on the strength of the symmetry breaking interaction.
In this paper, we investigate the violation of cluster property for Heisenberg quantum spin on the square lattice, whose continuous symmetry is SU (2).SSB in this system can be explained by the effective model which realizes the magnetization of the sub-lattices [26].By this model, we can present a definite description of the quasi-degenerate states.Here we should note that the quasi-degenerate states are the essential ingredients of SSB, as stressed in the previous work [25].
On the square lattice of size N, we can divide the whole lattice into two sub-lattices, which are called A sub-lattice and B sub-lattice.The semi-classical order of this spin system implies that the magnitude of the total spin on each sub-lattice is the order of N. Therefore, we can denote the state of spin system on a sub-lattice by its magnitude and its z-component.
On the finite lattice, the researchers studying this system have shown that the energy depends only on the spin magnitude of the whole lattice.Therefore the ground state as well as the energy can be determined by the effective Hamiltonian [26] that contains the squared total spin.By this effective Hamiltonian ˆeff H and the set of the quasi-degenerate state, we can make a complete description for the ground state even if we include the additional interactions.
In order to obtain the ground state uniquely on the finite lattice we introduce two additional interactions 1 V and 2 V .The former interaction eliminates the degeneracy on the z-component of the total spin, and the latter induces the Contents of this paper are as follows.In the next section we introduce the model studied in this paper, which is the spin 1/2 Heisenberg antiferromagnet on the square lattice, and define two sub-lattices.In Section 3 we give the effective Hamiltonian ˆeff H by which we calculate the ground state in terms of states that are eigen states of the sub-lattice magnetizations.In order to confirm the validity of ˆeff H we present numerical results which are calculated by the exact diagonalization on the quite small lattices.Using ˆeff H we show that the ground state is described by the associated Legendre polynomial when the lattice size is large [33].In Section 4 we define the additional interactions 1 V and 2 V which explicitly break SU(2) symmetry.Then we calculate the matrix elements of these interactions by the eigen states of the sub-lattice magnetizations.
In Section 5, we calculate the ground state when the Hamiltonian contains the symmetry breaking interactions in addition to the effective Hamiltonian.We give the Hamiltonian in the continuous approximation which is reliable when the lattice size is large.Then obtaining the two-dimensional partial differential equation and solving it, we find the ground state.Also we discuss the conditions for our method to be reasonable.In Section 6, we calculate the correlation functions due to the degenerate states.Here one should notice that we have two kinds of the correlation functions because there are two kinds of Nambu-Goldstone mode in SU(2) symmetry.In Section 7, we employ linear spin wave theory [28] to calculate the correlation functions due to Nambu-Goldstone mode, which keep the cluster property.The violation of the cluster property due to the degenerate states, therefore, could be observable only when the former is smaller than the latter.
In Section 8, we discuss, using the correlation functions presented in Sections 6 and 7, the violation of the cluster property in the correlation functions due to the degenerate states.Then we numerically estimate the violation when the lattice size N is 10 20 = (10 10 ) 2 .This size is determined by considering that the molecular distance is the order of 10 −10 m and the length of the macroscopic material is the order of 1 m.From our results we conclude that in Heisenberg antiferromagnet correlation functions due to the degenerate states violate the cluster property, and that the magnitude of the violation qualitatively differs from that in XXZ antiferromagnet.The final section is devoted to summary and discussion.

Heisenberg Antiferromagnet on Square Lattice
The model we study here is the spin The lattice size N is given by The Hamiltonian Ĥ in our study is given by Here ˆi S  is the spin operator at the site i and ( ) , i j denotes the nearest neighbor pair on the square lattice.In order to make ˆz i S a diagonal matrix, the basis state is defined by β γ , the spin operators with x-, yand z-components are given by the Pauli matrix l σ , ˆˆ2 , 2, 2.
The reason for this assignment is that in the analysis by linear spin wave theory [28] we would like to use the real matrix element on ˆy i S [25].
Since the Hamiltonian (2) has the SU(2) symmetry, the total spin operators commute with Ĥ , ˆˆ, Therefore the energy eigen state of Ĥ with the energy eigen value , J M E has also the quantum numbers J and M of SU(2) symmetry for the whole lattice.
For this antiferromagnet on the square lattice we divide the whole lattice into two kinds of sub-lattices, which are called A sub-lattice and B sub-lattice.In order to give a definition of A sub-lattice (B sub-lattice), we use integers x i and y i for the site i to introduce a symbol i P ( ) Using i P we introduce the spin operators on each sub-lattice, ( ) The eigen state of ( ) ˆU S  and ˆz U S will be used in the next section to construct , J M ψ in (5).World Journal of Condensed Matter Physics

Effective Hamiltonian
The model we study is based on the one where the spin system on each sub-lattice is ferromagnet [26] and is formulated by .
We can insist that A J is the order of the size N, because the semi-classical order is found in the Heisenberg antiferromagnet.Note that we have B A J J = by the translational symmetry of Ĥ .
The effective Hamiltonian [26] is Here 0 E is the lowest energy with 0 J = and 0 χ is the uniform suscepti- bility.The energy eigen values of the state , let us make numerical calculation by the exact diagonalization on the quite small lattices, so that we can confirm the assumptions for ˆeff H . First, we calculate the energy ( ) J E N on lattices of size N = 20, 26, 32 and 36 to examine the energy postulate in (10).The results are shown in Figure 1, where we find the remarkable agreement between our data and the postulate.Next, we calculate the squared magnitude of the ferromagnet on A sub-lattice with the lattice size N, which is given by , .
when the large ferromagnet is realized on A sub-lattice ( )  (8).Sandvik [34] made the extensive study on this dependence to obtain the value on the infinitely large lattice accurately.
In order to construct   Here U ε is defined in (7).Note that ( ) ( ) ( ) Then the eigen state of ˆeff H should be the solution of the following equation , , ˆˆˆˆˆˆˆ2 1 .
In general the eigen state can be represented by a linear combination of products of , .
As described in textbooks on quantum mechanics, ( ) is the Clebsch-Gordan coefficient and it is obtained by algebraic calculations.
In our study ( ) O N .In addition we assume that A J J  because there exists the suppression factor ( ) ( ) in the Hamiltonian.We do not impose, however, any restriction on the relation of 2  J and 2 M .In this case O J , which will be understood in the following discussion.Since M J is quite small in our assumption we approximate the equation of the Clebsh-Gordan coefficients by neglecting the terms of M J .The calculation of these coefficients in this approximation has been extensively studied by Sprung, van Dijk, Martorell and Criger [33], which we follow here.For a fixed M we introduce a variable Z defined by 2, 2, 2.
Here Z is an integer when M is an even integer, while Z is a half-integer when M is an odd integer.For a fixed ( )
Here Z ψ denotes the coefficient for the state Z .The non-zero off-diagonal matrix element in the calculation is .
Since the matrix element is real we also have For ˆẑ z A B S S , we obtain the diagonal matrix element Then the eigen equation for This equation yields an equation of Z From ( 18) we obtain, based on the approximation to neglect the term of Here we introduce a variable x , ( ) Applying ( 20) and ( 23) to ( 22) the equation for Z Let us replace the variable Z by x in (24).In addition we introduce φ ψ ≡ ∆ in order to obtain the smooth function of x .Then we have Expanding ( ) when ( ) , we can neglect the term with ( ) x ∆ .Finally we obtain the equation The solution of this equation is the associated Legendre function ( ) Taking the normalization into account we thus obtain Then the state .

Additional Interactions
In this section we introduce two symmetry breaking interactions which are necessary to obtain the unique ground state.In SU(2) symmetry we have two independent generators among three generators, ( ) , Ŝα being defined in (4).We adopt ˆy Q and ˆz Q as the independent generators.The additional interactions 1 V and 2 V have to break the symmetry related to these operators.Since we impose that 1 V breaks the symmetry on ˆy Q , we assume The parameter 1 g should be positive because states with large M should be suppressed.For its magnitude we do not impose any severe constraint because 1 V commutes with the Hamiltonian Ĥ .The reason to choose the squared operator is that this interaction gives a non-trivial effect to the correlation function, which will be described in Section 6.Another interaction 2 V , which has been already introduced in the study of XXZ antiferromagnet [25], is given by This interaction breaks the symmetry on ˆz Q .The positivity of 2 g is con- ventional because it changes when the phase of states changes.The reason why 2 g should be small is that, since , this interaction modifies the quasi-degenerate states by inducing the magnon states.
The Hamiltonian ˆV H is the sum of the Hamiltonian (2) and these interactions, which is given by Note that, because and z, we completely break SU(2) symmetry by these interactions so that we can obtain the unique ground state with parameters ˆˆˆˆ.
Here we can express the whole Hamiltonian only by the spin operators ˆU S α on U sub-lattice ( U=A,B ).Therefore the eigen states of V to the state is easily calculated since, from (5), this interaction does not change the state The interaction 2 V , on the other hand, changes the state By the algebraic argument on the spin-one operator ˆA S + , we find that Let us calculate these coefficients A c ± and 0 .
For the associated Legendre function we have the following relation, Then from (30) we have Assuming  , which we can realize through the parameters 1 g and 2 g , the above relation is replaced by Thus we obtain 2 . Applying the same discussion to 0 A c and A c − , we finally obtain that 0 , 0, . 2 2 Note that we adopt the convention in (12) for the phase on the states J M .From ( 43) and (44) we have As for ˆA S − , we use the relation ˆˆˆˆ.
Then we obtain ˆˆ. 2 From this expression we see that the interaction 2 V does not change the even-odd property of J M − .

Ground State
In the previous section we presented the matrix elements of the effective Hamiltonian with additional interactions.Using these matrix elements we now solve the eigen equation to find the ground state of .
Here V E Ψ is the energy eigen value and gives the diagonal element ( ) ( ) .
From ( 48) and (49) we obtain the equation for .
Since the even-odd property of J M − is kept in this equation, we examine the equation only for even values of J M − .We introduce .
Then the eigen equation becomes . The difference values s ∆ and t ∆ will be fixed later.Also using the stag- gered or the order parameter m + [26], A J is estimated by . 2 Then we obtain the equation Making the continuous approximation to expand ( ) , s s t t φ ± ∆ ± ∆ by the partial differential, we have the equation In this equation we set As a result the eigen equation becomes , .

V g m s s t m g g N t s t s t E g m N s t
The eigen solution of (58) is l should be an odd integer.The energy eigen value in (58) is then given by, with 1,3,5, s l =  , and 0,1, 2,3, Note that the values of the variables s and t are ( ) because the solution (59) does not contain the parameters N, 1 g and 2 g .The requirement is then satisfied by the condition Thus the conditions we impose are ( ) ( ) Here the second condition is necessary because of the continuous condition.
The ground state V G and the ground energy

Correlation Functions Due to Quasi-Degenerate States
In this and the next sections we calculate the spin correlation function ( ) , which is defined by 1 .
Here V G is given by (63).As described in the previous work [25], we can divide the correlation function into the contributions ( ) ( ) ( ) ( ) By this replacement we obtain 6.1.

F i j , ∆
Let us consider the first term of ( ) Notifying the convention (12) for the phase on the state , J M and the state , J M , we have Here the function ( ) is defined in (59).Then we obtain ( ) ( ) For the second term in (67) with x α = , we notice that We have By this expression we obtain This brings us From ( 71) and (76) we conclude that ( ) 0.
For the second term in (68) we have ( 1) 2 . 2 Therefore we obtain in the ground state is limited to the state with even J M − , and ˆx S changes the even state to the odd state while ( ) change this even-odd property, the third term in the above Equation (68) vanishes.Thus we have, using (54),

( ) zz DS F i j , ∆
In calculations (67) and (68) with z α = , we used the state Z introduced in Section 3. Then we have is expressed by Z 's as shown in (31), we see by the algebraic We can calculate the coefficients by The expectation value for z c + is, when Here we used the relation on the associated Legendre polynomial ( ) J Finally we obtain ( ) ( ) We replace the spin operators by the annihilation operators ˆi b and the creation operators † ˆi b of the magnon, ( ) 1 .
Instead of ˆi b we use ˆk b  , which is Fourier transform of ˆi b , ( ) Here the wave vector is defined by using integers x n and y n .One should note that in LSWT the additional interaction ( ) induces only the constant contribution to the ground state energy, and no effect to the excitation energy.In order to show it, we express 1 V Since 1 V is expressed by the magnon operator with the zero-wave vector, this interaction gives only the constant to the ground state energy.Therefore it is safe to neglect the contributions from 1 V in the following study.
For the magnon excitation with the non-zero wave vector we diagonalize the World Journal of Condensed Matter Physics Hamiltonian by † ˆˆĉosh sinh .
Here the parameter k θ  is determined by ( ) Then the energy k ω  of the magnon is given by ( ) We would like to obtain the spin correlation function at the large distance on the quite large lattice.Therefore we are interested in the magnon with the small energy.In the case 1 x k  and 1 y k  , the energy is given by ( ) ( ) In the spin system with SU(2) symmetry on the square lattice, we have another kind of the magnon with the small energy, whose wave vector k  is near ( ) , π π .In the case of Note that k γ  is negative in this case.We see that the gap energy ( ) is proportional to 2 g when 2 g is small.By this gap energy we expect that the spin correlation function decreases exponentially at the large distance.
Let us explicitly calculate the spin correlation function of the spin operator ( ) . We express the spin operator using the operators ˆk α  and † ˆk Here we introduce notation k ω  defined by ( ) Also we define that ( ) From the expression (108) we can calculate the spin correlation function due to Nambu-Goldston mode.Since Here note that we can neglect the contribution by the magnon with ( ) ( )   , , x y k k π π  , because in the second expression the first term cancels with the second term due to ( ) For the correlation function of the spin operator ˆz i S , Nambu-Goldstone mode with the wave vector near ( ) , π π gives the large contribution.This oper- ator is given by Then we have ( In addition we obtain  When the distance between i and j is large, the summation on k  can be dominated by the contributions by the low energy.Then we can replace the energy by ( ) ( ) Replacing the summation by the integral we obtain the correlation function at the large distance r [25], ( ) ( )

Cluster Property
In this section we discuss the violation of the cluster property in the spin correlation function for the antiferromagnet on the square lattice with SU(2) symmetry.
Each spin correlation function ( ) is the sum of the function due to the quasi-degenerate states and the one due to Nambu-Goldstone mode, which have been calculated in the previous sections.Let us collect our results.First we consider ( ) . Using expressions of s ∆ and t ∆ in (57) the results for ( ) given in (81) and for ( ) Since the lattice size N is fixed to be 10 20 , the parameters in (119) are the strength 1 g and the strength 2 g of the additional interactions 1 V and 2 V .In addition to the condition 2 1 g  , we impose the following conditions on 1 g and 2 g , so that the conditions (62) are also satisfied, With (120) it is easy to see that we can neglect the second term in both expressions of (119).As a result ( ) We see that these correlation functions do not depend on the distance and these values are non-zero as far as N is finite.
As for the contribution from Nambu-Goldstone mode we conclude that, based on discussions in Section 7, Here we obtain  in (118).Also we have From the final results of (121) and (122) we conclude that the cluster property violates in the correlation functions due to the quasi-degenerate states, and that the functions due to Nambu-Goldstone mode exponentially decrease with distance.Although this conclusion is the same as that for U(1) symmetry [25], there are several characteristic properties newly found for SU(2) symmetry.resemble each other's in shape.As 2 g increases the value of c r on each curve increases monotonously until it reaches its maximum value, which we denote max c r , and then decreases monotonously.The location of the maximum point shifts depending on α and, also on the values of 1 g for x α = .Since ( ) holds in the area above these curves, where c r r > , we have a possibility to observe the violation of the cluster property there.We see that the value of 2 g which maximizes c r , say max 2 g , is larger when 1 g is larger.We also see that max c r is smaller for larger 1 g .Hereafter we focus our attention to curves of xx DS F in order to make our discussion simple.In the area to the left of the maximum point, namely for  F would be observable in a wide range of r and 1 g provided that 2 g is moderately small.

Summary and Discussions
In this paper, we investigated the violation of the cluster property for the spin 1/2  Spontaneous symmetry breaking for the system can be explained by the model where the ferromagnets of A and B sub-lattices couple to form the effective Hamiltonian [26].In this model, we could clearly define the quasi-degenerate states and could calculate the ground state including the interactions that explicitly and completely break SU(2) symmetry.Note that, in the concrete calculation of the ground state, we have kept the lattice size large but finite.
In order to find the violation of the cluster property, we calculated the spin correlation functions.We examined two kinds of correlation functions which consist of the function due to the degenerate states and the one due to Nambu-Goldstone mode.We calculated the former functions DS F αα using the quasi-degenerate states.As for the latter functions F becomes large since it is proportional to 1 g .By these enhancements of the correlation functions, we find the wide region in the parameter space where it would be possible to observe the violation of cluster property.This fact encourages us to search for the violation in experiments.

Figure 1
Figure 1.The energy

Figure 2
Figure 2. The expectation value the Hermite polynomial and l N is the normalization factor.We should notice that there are some constraints on the wave function and the parameters.The constraint on the wave function comes from the positivity of J s s = ∆ .By this constraint we impose that ( )

.
Goldstone mode is realized in the operators of ˆx i S and ˆz i S we calculate the correlation functions due to the quasi-degenerate states, which are examine the contribution due to the quasi-degenerate states, we assume that we can replace the contribution of the local operator ˆi S α by the uniform contribution of Therefore we have the following replacement.
67) with x α = .Repeating the discussion in Section 4, we have calculate the first term of ( ) , xx DS F i j in (68) with x α = .Following the calculation for (70) we obtain World Journal of Condensed Matter Physics

Figure 3 .and 1 .
Figure 3.The correlation functions xx DS F and zz DS F defined in (121) with the lattice size N = 10 20 .They are plotted as a function of 2 g which is the strength of the symmetry breaking interaction 2V defined by(33).The solid lines are the results of xx DS F for

Figure 4 . 1 .
Figure 4.The distance c r where the magnitude of

F r for several values of 1 g
other hand, values of c r scarcely depend on 1 g .The results in Figure4suggest in what region the observation can be expected.In Figure5we plot ( ) NG F r versus r with two typical values of 2 g , which we choose to make it easy to understand the measurable area.Then we compare values of xx .The value of c r in Figure4is now indicated by the cross point where the horizontal line of xx DS F and the curve of ( ) NG F r meet.Our results show that xx DS

F
violate the cluster property in Heisenberg spin systems with SU(2) symmetry.Also when there is the interaction 111) World Journal of Condensed Matter Physics