### Note for projected coherent state approximation of

●### bosonic FQHE in a spherical geometry

### Takahiro Mizusaki

Abstract

Recently a bosonic counterpart of the Fhctional Quantum Hall Effect (FQHE) has been theoretically pointed out in fast rotating Bose-Einstein Condensates (BEC). In this study, exact diagonalization has played an important role but it can not be applied to a large bosom system. Tb overcome this restriction, a variational method

by the spln projected bosom coherent state is promlSlng. In this Note, this method is presented in detail.

### 1 1ntroduction

### The Tsui's paper of the FQHE was published in 1982 and rcportcd that the clcctrons in

two-dimensional plane with strong magnetic丘eld behave in quite a strange way ln addition to### Integer QHE (IQHE)･ During this quarter century since this discovery, to clarify this strange

### but an interesting phenomenon, many concepts (for instance, Laughlin's wave function,

### composite fermion theory) have been appeared l2]･ In the FQHE, electrons in completely

degenerate Landau levels feel a strong repulsive force, which breaks this degeneracy inquite a non-trivial way. This problem can never be handled by mean-field method. This

is genuine non-perturbative and is very di侃cult to understand. Its basic nature has been, however, clariBed by a notion of composite fermion in a very unique way.

Recently experiments of Bose-Einstein condensates (BEC) by bosonic atoms have made a rapid progress. In addition to molecule, metallic cluster and nuclei, we can find a new kind of quantum system withfinitc number of particles and can control its interaction strength.

### Moreover, for such a BBC, a bosonic counterpart of the FQHE is theoretically anticipated

to be appeared l3] if we can rotate it very fast, because, in a mathematical sense, structureof magneticfield and rotation is the same. As this problem is also non-perturbative, to

investigate this physics, exact diagonalization method for small number of bosons is quite

### useful. Several bosorlic FQHE phases have beerl foundand composite ferITlion picture has

been again confirmed l3]･Inany-Particle system grows exponentially as a function of the number of involved particles. Tb ovcrcomc this restriction, a variational method by the spln projected bosom coherent state is promislng. In this Note, this method is presentedindetail.

2 Basic algebra for coherent state

A creation (allnihilation) operator c£1 (cm) creates (annihilates) a boson with spin S and

its I-projection S2 - m′. A usual bosonic commutation relation for c and cI'S,

### lcml, CLJ - 6m1,m2

=)is satisfied.

Next we consider a new coherent bosom operator bl by combining cL(m - -S, …S)

lincarly as

blx - ∑xmcrln (2)

where I/s are coefncients and subscript X is attached for bl to clarify its parameter

depen-dence. As a commutation relation for

lbx,bk] - ∑結xklcm,Ct] - ∑X㌫Jrn,

bl is normalized if回2 - 1.

A boson condensate state is defined, by this operator, as

### 榊)) -義(bTx) IO)

(3)

(4)

### where N is a boson number and lxL2 - 1 and lO) is a vaccum state.

In order to evaluate various matrix clcmcnts by this condcnsatc state, wc consider scvcral

commutation relations. First we consider a commutation relation as

lbxフ(bL)"] - NZ (bL)"-1 (5)

where Z - X* ･ y･ This relation is easily derived by recursively applying a followlng relation,

lbx, (bL)"] - lbx,bL] (bL)"Ll +舶X, (bL)"~1]

- Z(bL)"~1 +舶X, (bTy)"11]･

By eq･ (2), we carl easily evaluate an overlap between the same or different coherellt States

aS?

Here we use a followlng recursive relations as,

### Next we consider energy expectation values between coherent states. The bosonic FQHE

Hamiltonian H can be expressed in the general two-body form as

H - ∑ vijklCfc,TckCl (7)

where γ's are interaction matrix elements･ The energy expectation values are generally

expressed as

(Mx)lHlO(y)) -去(ol(bx)H(bL)lo)･ (8)

慧:eo r.ei ｡:.edVyat: ra:Oinn ≡ : n(eor i l',bxw'e"dCefr望cek fCi r(fut"e

0

### )

is necessary. As Hamiltonian canof expectatioll Values of olle-body alld two-body terms.

The expectation value of one-body term c,fcl between coherent states is shown as,

## (o I(bx)"clcl (bL)"Lo) - Nx-!Z"-1

This formula is easily derived by following relations,

### cl (bL)" lO) - [cl7 (bL)"]LO) - Nyl (bL)"Ll lO)

and

### (bx)" ctTcl (bL)"

_{[(bx)",cf] lcl, (bL)"]Lo) ･}

There丘)re one-body energy lS expressed as

(*(I) Ecfcll拍)) (4,(I) L4,(I)) -.＼十II･I (9) (10) (ll) (12) whcrc Z= 1.

Next we consider an expectation vallle Of two-body terms in the same way. In this

derivation, a followlng relation is useful,

lAlB,C]日0) - ABC

when AIO) - BLO) - 0･ This is because

### lAlB,C]] - ABCIACB-BCA+CBA.

(13)

Now we derive a formula for two-body terms as

(拍) 1clc,TckCIL拍))

(4,(I)悼(I))

- N(N - 1)xi*肴7:kXl _{(15)}

whcrc Z= 1.

### From the eq･ (13) , ckCl(bL)"EO) - [cklcl,(bL)"]]lO) holds, then -c can obtain a

relation,

ck･Cl (bL)" lO) - N(N - 1)ylyk匪)"~2 lO)･ (16) By applying the above equation to bra alld ket states,

(oI(bx)"cfc,TckCl (bL)"lo) - N2(N- 1)2弼ykyl(ol(bx)"｢2 (bL)"L2lo) (17) can be obtained.

3 Spin projected energy of coherent state

### In the bosonic FQHE, a repulsive 61force between boson pair coupled with maximum spin

is illtrOduccd. This force is cxtrcmcly difBcult to handle by mean-field theory. For instance,

the expectation energy by the coherent state is far from the true energy. In general, a

colleI･ent State Often breaks symmetry properties which the Hamiltonian conscrvcs because

of its restricted form. If we recover such symITletry properties, approximation of wave

function becomes better l4, 5]･ Therefore we improve coherent state as a trial wave function

by spln Projection.

As each bosoII Carries the spin S and the Hamiltonian conserves total spin, N boson system has elgenStateS With definite total spin. To realize this condition, Spln Projection scelnS tO be useful because such a projectioII method has been found to be quite useful in illteraCting shell model [6] and interacting bosoll model [7]･ Fllrthermore I found that sllCh

a quantum number projection is quite powerful for lattice spin system l8] and fermionic

### FQHE[9]･

Spin projection operator is defined as

where α,β,7 are Euler's angles and Sy, Sz are y- and I- components of spin operator S･

The DgTl{(α,0,7) is a Wigner7s D-function. Spin projected coherent state回(S,M,.77)),

### which has a definite spin S and its I-projection M, is dcfincd as7

S

14,(S,M,I)) - ∑ gKPNSi,Kl,4)(I,L･))･

K=-S

(21)

where gK are additional parameters, which can be determined by diagonalization of S x S

matrix. After some spin algebra, Spln Projected energy lS Shown as,

### (4,(S,M,I)lH14,(S,M,I)) (4,(I)LHPS14,(i/.))

(4,(S, M, I)14,(S, M, I)) (4,(I)lPS14,(I/I)) PS ≡ ∑ gKg*K′ PB,K′･ Jヽ~八~′ where (22) (23)In actual computations, 3-fold integration of eq･ (22) concernillg Euler's allgles is nu-merically carried out. Numerator and denominator of eq･ (22) can be cvalllatCd by the

weighted sum of (4,(I)IHlO(y)), (4,(I)悼(y)) where y staIlds for spin rotation of t77, i･e･,

y - R(α,0,7)I. These matrix elemelltS Can be obtained by

(79(I)悼(y)) - ZN (24)

all(1

(棉) lcfc,TckCll 4,(y)) - N(N - 1)招,TykylZ("~2) (25) Compared to fermionic formulae, structure of matrix elements is rather simple and they have a colnmOn dependence of power of Z. This nature can bc useful to rcducc the mesh points of Eulcr7s angles cspccially in the large bosom number.

In this way, We can obtain a spin projected energy for arbitrary coherentJ States. A rest

problem is how to determine the parametersこr'S, Whichgivc a lowest spln projected energy,

i. C., optimization probleln.

4 0ptimization of spin projected coherent state

To prcscnt an optimization method, we cxtcnd boson coherent states. Coherent bosoll

operat()rs砿(k - 1,2S+ 1) are defined by combining cL(m - -S, …S) linearly as

where D′s are coefBcients. We asslgn the coherent state defined in previous sections to

k - 1, that is,砿=1 - bk, bk=1 - bx and D1,m - Xm. As a commutation relation for bt

becomes

[bi, b,T] - ∑ D芸,,,Pj,klcm, Cb - ∑D嵩Am,

this relation becomes canonical, i･e･, lbi, b,t] - 6i,i if DDI - Ⅰ･

Spin projected energy is dcfincd as

### ES=

### (4,(I) LHPSI4,(I))

### (4,(I)lPSI4,(I))

(27)

(28)

Due to a bosonic extension of the Thouless's theorem, arbitrary states which are not

orthog-onal to I4,(I)) can be expressed by e∑-,1 Zγ湖bl I4,(I)) where Z,s are parameters. Then

gradient l′ [10] of spin projected energy surface is given by

7m-∇mES-(*(I)LPS(H - ES)札bl14,(I)) (*(I)IPS悼(I))

(29)

As the evaluation of spin-projected cncrgy, the calculation of this gradient needs (4,(I) 14,(y)) ,

(4,(I)鶴,,blL4,(y)), (,O(I)LH14,(y)) and (4,(I)lH札b114,(y)), where y is the spin rotation of I.

Thercforc formulae for the second and fourth matrix elements are newly necessary. Thc second term can be rewritten as

(4,(I)砿b114,(y)) - Dm,jD;,i(4,(I)Jc,tcII4,(y)),

and the fourth term is given by

(30)

(4,(I)lcfc,TckClbL/bl My)) - Dm,pDf,q(*(I)lc%Tc,TckCICh14,(y))･ (31)

Bccausc of cfc,TckCICLcq - (弼C去ckCICq + cfc,1･ckCq6l,p + cIc,tcICq6k,p,

a matrix clement for normal ordcrcd form is nccdcd.

Here we consider the most general matrix elements of n-creation and n-annihilation

type operators, i･e･, (4,(I)lcfl ･ ･ ･ CfncJんCJ114,(y))･ From a relation as

### cjn Cj. (bL)" LO) - [C,h･ lcjl, (bL)"] -･]IO),

an extension of eq･ (16) is

### cjn Cjl (bL)" IO) -

Ⅳ!### (N-n)

### yj"･･･yjl (bL)"｢o).

Thcrcfore, such a matrix elcmcnt is glVen in a followlng COmPaCt form,

Thus, We can evaluate a gradient vector in spin projected energy surface and improve the

coherent state along its steepest descent direction･ The improved coherent state回(X′)) is

glVen by its gradient as

14,(X′)) - e-mbLjbl困(I))

where 77 lS a parameter. Due to followlng two relations as

er (症)" LO) - (erbke-r)"erlO) - (bk,)" IO)

whcrc Il - rnmbrl,Lbl and岐′ - erblxe-r, and

岐′ -er岐e-1-bi+[r,bll]+･･･ where Dl,i - Xj, Parameters Of improved coherent state change as

D'li - Dlj -り∑ TmDmj. m>1 (35) (36) (37) (38)

Then by applying this process rccllrSively, we can obtain the (local) minimum of spin-projected energy. This is a bosonic courlterpart Of spin-projected Hartree-Fock method in fermion

systems [10]･

In this paper, I summarized a detailed formulation of projected cohererlt State

Calcula-tion･ By this formalism, a reglOn, Whcrc this approximation becomes well, is found in the

N-2S chart. This will be reported elsewllere.

### Acknowledgement

This study was mainly carried out when I took my sabbatical leave at Orsay, Universite

### Paris-Sud (UPS) iII Marcll. 2007 - March. 2008. There7 Prof. Schuck and Prof. Jolicoellr

### suggested me a study of bosonic FQHE, for which subject my previous rcscarches (cspccially,

Interacting Bosom Model, projection techniqlleS and fermionic FQHE) may be useful i一l a

diffcrcnt way･ I greatly apprcciatc thcIn and their collaborations. Finally I would like to express my greatest acknowledge to my urliversity in glVlng me an Opportunity of olle-year sabbatical leave.

### References

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### [4] R. E. Peierls and JI Yoccoz, Proc･ Roy･ Soc･ A70 381-387 (1957)･

[5] R. E. Peierls and D. J. Thouless, Nucl･ Phys･ 38 154-176 (1962)･