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arXiv:1001.0035v2 [hep-th] 25 Mar 2010Reconstruction of Baxter Q-operator fromSklyanin SOV |
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for cyclic representations ofintegrable quantum models |
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G. Niccoli |
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DESY,Notkestr. 85, 22603 Hamburg, GermanyDESY 09-227 |
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Abstract |
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In [1], the spectrum (eigenvaluesand eigenstates) of a latt ice regularizationsof the Sine- |
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Gordon model has been completely characterized in terms of p olynomial solutions with |
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certain propertiesof the Baxter equation. This characteri zation for cyclic representations |
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hasbeenderivedbytheuseofthe SeparationofVariables(SO V)methodofSklyaninand |
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bythedirectconstructionoftheBaxter Q-operatorfamily. Here,wereconstructtheBaxter |
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Q-operatorandthesamecharacterizationofthespectrumbyo nlyusingtheSOVmethod. |
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This analysis allows us to deduce the main features required for the extension to cyclic |
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representationsofotherintegrablequantummodelsofthis kindofspectrumcharacteriza- |
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tion. |
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Keywords: Integrable Quantum Systems;Separation of Variables;Baxt erQ-operator; PACScode 02.30.IK2 |
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1. Introduction |
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Theintegrabilityofa quantummodelisbydefinitionrelated to theexistenceofamutuallycommu- |
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tativefamily Qofself-adjointoperators Tsuchthat |
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(A) [T,T′] = 0, |
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(B) [T,U] = 0, |
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(C) if [ T,O] = 0,∀T,T′∈ Q, |
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∀T∈ Q, |
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∀T∈ Q,thenO=O(Q),(1.1) |
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whereUis the unitary operatordefining the time-evolutionin the mo del; note that the property(C) |
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stays for the completeness of the family Q. In the framework of the quantum inverse scattering |
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method [2, 3, 4] the Lax operator L(λ)is the mathematical tool which allows to define the transfer |
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matrix: |
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T(λ) = trC2M(λ),M(λ)≡/parenleftbiggA(λ)B(λ) |
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C(λ)D(λ)/parenrightbigg |
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≡LN(λ)...L1(λ),(1.2) |
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aoneparameterfamilyofmutualcommutativeself-adjointo perators. Theintegrabilityofthemodel |
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follows from T(λ)if the properties (B) and (C) of definition (1.1) can be proven for it. In some |
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quantummodeltheintegrabilityisderivedbyprovingtheex istenceofafurtherone-parameterfamily |
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ofself-adjointoperatorsthe Q-operatorwhichbydefinitionsatisfiesthefollowingproper ties: |
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[Q(λ),Q(µ)] = 0,[T(λ),Q(µ)] = 0,∀λ,µ∈C, (1.3) |
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plusthe Baxterequationwith thetransfermatrix: |
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T(λ)Q(λ) =a(λ)Q(q−1λ)+d(λ)Q(qλ). (1.4) |
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This is in particular the case for those models (like Sine-Go rdon [1]) for which the time-evolution |
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operatorUis expressedin terms of Q. A naturalquestionarises: Is the integrablestructure of t hese |
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quantummodelscompletelycharacterizedbythetransferma trixT(λ)? |
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Note that a standard procedure1to prove the existence of Q(λ)is by a direct construction of an |
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operatorsolutionoftheBaxterequation(1.4). Moreover,t hecoefficients a(λ)andd(λ)aswellasthe |
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analytic and asymptotics properties of Q(λ)are some model dependent features which are derived |
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bythe construction. Let usrecall thatthe generalstrategy [11, 12,13,14, 15] ofthisconstructionis |
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to findagaugetransformation2such that the action of each gaugetransformedLax matrixon Q(λ) |
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becomesupper-triangular. Thenthe Q-operatorassumesa factorized localformandthe problemof |
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its existence in such a form is reduced to the problem of the ex istence of some model dependent |
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specialfunction3. |
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1Itis worth recalling that there are also others constructio ns of theQ-operator. An interesting example is presented in the |
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series of works [5,6,7] by V.V. Bazhanov, S.L.Lukyanov and A .B.Zamolodchikov on the integrable structure of conformal |
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field theories. In [6,7] the Q-operator is obtained asatransfer-matrix byatrace proced ure ofafundamental L-operator with |
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q-oscillator representation for the auxiliary space (see al so [8, 9]). This construction can be extended to massive inte grable |
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quantum field theories as itwas argued by thesame authors in [ 10]. |
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2Itleaves unchanged thetransfer matrix while modifies the mo nodromy matrix M(λ)defined in (1.2) . |
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3Thequantumdilogarithm functions [16,17,18,19,20,21,22,23,24,25]forexample a ppear intheSinh-Gordon model |
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[26],in their non-compact form,and in the Sine-Gordon model[1],in their cyclicform.3 |
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It is worth pointing out that on the one hand the construction of these special functionsfor general |
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modelscanrepresenta concretetechnicalproblem4andthat ontheotherhandtheexistenceofsuch |
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functionsis onlya sufficientcriterionforthe existence of Q(λ). It is then a relevantquestionif it is |
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possibletobypassthiskindofconstructionprovidingadif ferentproofofthe existenceof Q(λ). |
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Given an integrable quantum model the first fundamental task to solve is the exact solution of its |
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spectral problem , i.e. the determination of the eigenvalues and the simultan eous eigenstates of the |
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operator family Q, defined in (1.1). There are several methods to analyze this s pectral problem as |
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thecoordinate Bethe ansatz [27, 28, 29], the TQmethod [28], the algebraic Bethe ansatz (ABA) |
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[2, 3, 4], the analyticBethe ansatz [30] and the separation of variables (SOV) meth od of Sklyanin |
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[31, 32, 33]; this last one seems to be more promising. Indeed , on the one hand it resolves the |
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problems related to the reduced applicability of other meth ods (like ABA) and on the other hand |
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it directly implies the completeness of the characterizati on of the spectrum which instead for other |
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methodshastobeproven. |
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For cyclic representations [34] of integrable quantum mode ls the SOV method should lead to the |
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characterizationoftheeigenvaluesandthesimultaneouse igenstatesofthetransfermatrix T(λ)bya |
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finite5systemofBaxter-likeequations. However,itisworthpoint ingoutthatsuchacharacterization |
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of the spectrum is not the most efficient; this is in particula r true in view of the analysis of the |
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continuum limit. Here the main question reads: Is it possibl e to define a set of conditions under |
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whichtheSOVcharacterizationofthespectrumcanbereform ulatedintermsofafunctionalBaxter |
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equation? In fact, this is equivalent to ask if we can reconst ruct theQ-operator from the finite |
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system of Baxter-like equations. In this case the solution o f the spectral problem is reduced to the |
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classification of the solutions of the Baxter equation which satisfy some analytic and asymptotic |
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propertiesfixedbythe operators TandQ. |
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The lattice Sine-Gordonmodelis used asa concreteexamplew herethese questionsaboutquantum |
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integrability find a complete and affirmative answer. Indeed , in section 3, we show that the SOV |
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characterization of the transfer matrix spectrum is exactl y equivalent to a functional equation of |
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the form detD(Λ) = 0, whereD(λ)(see (3.21)) is a one-parameter family of quasi-tridiagonal |
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matrices. In section 4, we show that this functional equatio n is indeed equivalent to the Baxter |
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functional equation and, in section 5, we use these results t o reconstruct the Baxter Q-operator |
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with the same level of accuracy obtained by the direct constr uction presented in [1]. It is worth |
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pointingoutthat these resultsallowusto provethat thetra nsfermatrix T(λ)(plustheΘ-chargefor |
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even chain) describes the family Qof complete commuting self-adjoint charges which implies t he |
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quantum integrabilityof the model accordingto definition ( 1.1). So that in the Sine-Gordonmodel |
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theBaxter Q-operatorplaysonlytheroleofa usefulauxiliaryobject. |
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Let us point out that one of the main advantages of the spectru m characterization derived for the |
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Sine-Gordonmodelisthe possibilityto proveanexactrefor mulationin termsof non-linearintegral |
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4TheSine-Gordon model at irrational values of the coupling β2is asimple case where this kind of problem emerges. |
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5Thenumber of equations in thesystem is finite and related to t hedimension of thecyclic representation.4 |
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equations6(NLIE).Thiswill bethe subjectof a futurepublicationwher ethe NLIEcharacterization |
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will lead us by the implementation of the continuum limit to t he description of the Sine-Gordon |
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spectrum in all the interesting regimes. These results will be shown to be consistent with those |
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obtained previously in the literature7[37, 38, 39, 40, 41, 42] (see [43, 44] for reviews). Note that |
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the methodbasedon thereformulationofthe spectralproble min termsofNLIEhasbeenalso used |
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recently [49] to derive the Sinh-Gordonspectrum in finite vo lume and to characterize the spectrum |
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in theinfraredandultravioletlimits. |
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The analysis of the Sine-Gordon model allows us to infer the m ain features required to extend this |
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kindofspectrumcharacterizationtocyclicrepresentatio nsofotherintegrablequantummodels. This |
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is particularly relevant for those models for which a direct construction of the Baxter Q-operator |
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encounterstechnicaldifficulties. |
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Acknowledgments. I would like to thank J. Teschner for stimulating discussion s and suggestions on a prelimi- |
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nary versionof this workand J.-M.Maillet for the interests hown. |
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I gratefullyacknowledge support from the ECbythe Marie Cur ie Excellence GrantMEXT-CT-2006-042695. |
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2. The Sine-Gordon model |
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Weusethissectiontorecallthemainresultsderivedin[1]o nthedescriptionintermsofSOVofthe |
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lattice Sine-Gordonmodel. This will be used as the starting point to introducea characterizationof |
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the spectrumof the transfermatrix T(λ)which will lead to the constructionof the Q-operatorfrom |
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SOV. |
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2.1 Definitions |
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Thelattice Sine-Gordonmodelcanbecharacterizedbythefo llowingLaxmatrix8: |
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LSG |
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n(λ) =κn |
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i/parenleftigg |
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iun(q−1 |
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2κnvn+q+1 |
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2κ−1 |
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nv−1 |
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n)λnvn−λ−1 |
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nv−1 |
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n |
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λnv−1 |
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n−λ−1 |
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nvniu−1 |
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n(q+1 |
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2κ−1 |
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nvn+q−1 |
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2κnv−1 |
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n)/parenrightigg |
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,(2.5) |
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whereλn≡λ/ξnfor anyn∈ {1,...,N}withξnandκnparameters of the model. For any n∈ |
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{1,...,N}thecoupleofoperators( un,vn)defineaWeyl algebra Wn: |
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unvm=qδnmvmun,whereq=e−πiβ2. (2.6) |
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We will restrictourattentiontothecase inwhich qisarootofunity, |
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β2=p′ |
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p, p,p′∈Z>0, (2.7) |
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6Thistype of equations werebefore introduced in adifferent framework in [35,36] |
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7See [45, 46] for a related model analyzed in the framework of A BA and [47, 48] for the corresponding finite volume |
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continuum limit. |
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8Thelattice regularization of the Sine-Gordon model that we consider here goes back to [4,50] and is related to formula- |
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tions which have morerecently been studied in [51,52,53].5 |
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withp≡2l+ 1odd andp′even so that qp= 1. In this case each Weyl algebra Wnadmits a |
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finite-dimensional representation of dimension p. In fact, we can represent the operators un,vnon |
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thespace ofcomplex-valuedfunctions ψ:SN |
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p→Cas |
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un·ψ(z1,...,z N) =unznψ(z1,...,z n,...,z N), |
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vn·ψ(z1,...,z N) =vnψ(z1,...,q−1zn,...,z N).(2.8) |
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whereSp={q2n;n= 0,...,2l}is a subset of the unit circle; note that Sp={qn;n= 0,...,2l} |
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sinceq2l+2=q. |
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Themonodromymatrix M(λ)definedin(1.2)intermsoftheLax-matrix(2.5)satisfiesthe quadratic |
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relations: |
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R(λ/µ)(M(λ)⊗1)(1⊗M(µ)) = (1⊗M(µ))(M(λ)⊗1)R(λ/µ), (2.9) |
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wheretheauxiliary R-matrixisgivenby |
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R(λ) = |
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qλ−q−1λ−1 |
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λ−λ−1q−q−1 |
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q−q−1λ−λ−1 |
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qλ−q−1λ−1 |
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. (2.10) |
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The elements of M(λ)generate a representation RNof the so-called Yang-Baxter algebra char- |
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acterized by the 4Nparametersκ= (κ1,...,κ N),ξ= (ξ1,...,ξ N),u= (u1,...,u N)and |
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v= (v1,...,v N); in the present paper we will restrict to the case un= 1,vn= 1,n= 1,...,N. |
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The commutation relations (2.9) are at the basis of the proof of the mutual commutativity of the |
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T-operators. |
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Inthecase ofa latticewith Nevenquantumsites, we havealso tointroducetheoperator: |
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Θ =N/productdisplay |
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n=1v(−1)1+n |
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n, (2.11) |
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whichplaystheroleofa gradingoperator inthe Yang-Baxteralgebra: |
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Proposition 6 of [1] Θcommuteswiththetransfermatrixandsatisfiesthefollowin gcommutation |
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relationswith theentriesofthemonodromymatrix: |
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ΘC(λ) =qC(λ)Θ,[A(λ),Θ] = 0, (2.12) |
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B(λ)Θ =qΘB(λ),[D(λ),Θ] = 0. (2.13) |
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Moreover,the Θ-chargeallowstoexpressthe asymptoticsofthetransferma trixas: |
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lim |
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logλ→∓∞λ±NT(λ) =/parenleftiggN/productdisplay |
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a=1κaξ±1 |
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a |
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i/parenrightigg |
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/parenleftbig |
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Θ+Θ−1/parenrightbig |
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. (2.14)6 |
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Let us denotewith ΣTthe spectrum(the set of the eigenvaluefunctions t(λ)) of the transfer matrix |
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T(λ). By the definitions(1.2) and (2.5), then ΣTis contained9inC[λ2,λ−2](N+eN−1)/2, where we |
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haveusedthenotatione N= 0forNoddand1forNeven. |
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Notethat inthecase of Neven,the Θ-chargenaturallyinducesthegrading ΣT=/uniontextl |
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k=0Σk |
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T,where: |
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Σk |
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T≡/braceleftigg |
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t(λ)∈ΣT: lim |
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logλ→∓∞λ±Nt(λ) =/parenleftiggN/productdisplay |
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a=1κaξ±1 |
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a |
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i/parenrightigg |
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(qk+q−k)/bracerightigg |
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.(2.15) |
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This simply follows by the asymptotics of T(λ)and by its commutativity with Θ. In particular, |
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anyt(λ)∈Σk |
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Tis aT-eigenvalue corresponding to simultaneous eigenstates of T(λ)andΘwith |
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Θ-eigenvalues q±k. |
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2.2 CyclicSOVrepresentations |
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TheseparationofvariablesmethodofSklyaninisbasedonth eobservationthatthespectralproblem |
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forT(λ)simplifies considerablyif one worksin an auxiliaryreprese ntationwherethe commutative |
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familyofoperators B(λ)isdiagonal. |
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InthecaseoftheSine-Gordonmodelthevectorspace10CpNunderlyingtheSOVrepresentationcan |
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beidentifiedwiththespaceoffunctions Ψ(η)definedforηtakenfromthediscreteset |
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BN≡/braceleftbig |
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(qk1ζ1,...,qkNζN); (k1,...,k N)∈ZN |
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p/bracerightbig |
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, (2.16) |
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onthesefunctions B(λ)actsasa multiplicationoperator, |
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BN(λ)Ψ(η) =ηeN |
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Nbη(λ)Ψ(η), b η(λ)≡N/productdisplay |
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n=1κn |
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i[N]/productdisplay |
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a=1(λ/ηa−ηa/λ) ; (2.17) |
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where[N]≡N−eNandη1,...,η[N]are the zerosof bη(λ). In the case of even Nit turns out that |
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we needa supplementaryvariable ηNinordertobeable toparameterizethe spectrumof B(λ). |
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In[1]wehaveproventhatforgeneralvaluesoftheparameter sκandξoftheoriginalrepresentation |
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it is possible to construct these SOV representationsand mo reoverwe have defined the map which |
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fixestheSOVparameter ηintermsoftheparameters κandξ. |
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In these SOV representations the spectral problem for T(λ)is reduced to the following discrete |
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system ofBaxter-likeequationsin thewave-function Ψt(η) =/a\}bracketle{tη|t/a\}bracketri}htofaT-eigenstate |t/a\}bracketri}ht: |
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t(ηr)Ψ(η) =a(ηr)T− |
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rΨ(η)+d(ηr)T+ |
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rΨ(η)∀r∈ {1,...,[N]},(2.18) |
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9Herewith C[x,x−1]Mwearedenoting the linear space ofthe Laurentpolynomials o f degreeMin thevariable x∈C. |
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10It is always possible to provide the structure of Hilbert spa ce to this finite-dimensional linear space. In particular, t he |
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scalar product in the SOVspace is naturally introduced by th e requirement that the transfer matrix is self-adjoint in th e SOV |
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representation. Appendix B addresses this issue.7 |
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whereT± |
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raretheoperatorsdefinedby |
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T± |
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rΨ(η1,...,η N) = Ψ(η1,...,q±1ηr,...,η N), |
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whilethe coefficients a(λ)andd(λ)are definedby: |
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a(λ) =N/productdisplay |
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n=1κn |
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iλn(1−iq−1/2λnκn)(1−iq−1/2λn |
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κn),d(λ) =qNa(−λq).(2.19) |
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Inthecase of Nevenwe haveto addto thesystem(2.18) thefollowingequatio ninthevariable ηN: |
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T+ |
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NΨ±k(η) =q±kΨ±k(η), (2.20) |
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fort(λ)∈Σk |
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Twithk∈ {0,...,l}.NotethatthecyclicityoftheseSOVrepresentationsisexpr essed |
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bytheidentificationof (T± |
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j)pwith theidentityforany j∈ {1,...,N}. |
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3. SOV characterization of T-eigenvalues |
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Let usintroducetheoneparameterfamily D(λ)ofp×pmatrix: |
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D(λ)≡ |
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t(λ)−d(λ) 0 ··· 0 −a(λ) |
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−a(qλ)t(qλ)−d(qλ) 0 ··· 0 |
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0...... |
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... ···... |
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... ···... |
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...... 0 |
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0... 0−a(q2l−1λ)t(q2l−1λ)−d(q2l−1λ) |
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−d(q2lλ) 0 ... 0−a(q2lλ)t(q2lλ) |
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(3.21) |
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wherefornow t(λ)isjust anevenLaurentpolynomialofdegree N+eN−1inλ. |
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Lemma 1. Thedeterminant detpDisanevenLaurentpolynomialofmaximaldegree N+eN−1in |
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Λ≡λp. |
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Proof.Let us start observingthat D(λq)is obtainedby D(λ)exchangingthe first and p-th column |
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andafterthefirst and p-throw,so that |
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det |
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pD(λq) = det |
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pD(λ)∀λ∈C, (3.22) |
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whichimpliesthat detpDisfunctionof Λ. Let usdevelopthedeterminant: |
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det |
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pD(Λ) =p/productdisplay |
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h=1a(λqh)+p/productdisplay |
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h=1a(−λqh)−qNa(λ)a(−λ) det |
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2l−1D(1,2l+1),(1,2l+1)(λ) |
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−qNa(λq)a(−λq) det |
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2l−1D(1,2),(1,2)(λ)+t(λ)det |
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2lD1,1(λ), (3.23)8 |
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whereD(h,k),(h,k)(λ)denotes the (2l−1)×(2l−1)sub-matrix of D(λ)obtained removing the |
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rowsandcolumns handkwhileDh,k(λ)denotesthe 2l×2lsub-matrixof D(λ)obtainedremoving |
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therowhandcolumn k. Theinteresttowardthisdecompositionof detpD(Λ)isduetothefact that |
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the matrices D(1,2),(1,2)(λ),D(1,2l+1),(1,2l+1)(λ)andD1,1(λ)aretridiagonal matrices. Following |
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thesamereasoningusedinLemma4toprovethat det2lD1,1(λ)isanevenfunctionof λwecanalso |
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showthatthisistruefor det2l−1D(1,2),(1,2)(λ)anddet2l−1D(1,2l+1),(1,2l+1)(λ). Fromtheparityof |
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these functionsthe parityof detpD(Λ)followsbyusing(3.23). |
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Beinga(λ),d(λ)andt(λ)Laurentpolynomialofdegree Ninλ,inthecaseof Neventhestatement |
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ofthelemmaisalreadyproven;so wehavejust toshowthat: |
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lim |
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logΛ→∓∞Λ±Ndet |
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pD(Λ) = 0 (3.24) |
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forNoddwhichfollowsobservingthat: |
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lim |
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logΛ→∓∞Λ±Ndet |
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pD(Λ) =i±pNN/productdisplay |
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n=1κp |
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nξ±p |
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ndet |
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p/vextenddouble/vextenddouble/vextenddoubleq−(1∓1)N/2δh,k+1−q(1∓1)N/2δh,k−1/vextenddouble/vextenddouble/vextenddouble. |
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(3.25) |
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The interesttowardthe function detpD(Λ)isdueto the fact thatit allowsthefollowingcharacteri- |
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zationofthe T-spectrum: |
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Lemma 2. ΣTis the set of all the functions t(λ)∈C[λ2,λ−2](N+eN−1)/2which satisfy the system |
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of equations: |
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det |
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pD(ηp |
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a) = 0∀a∈ {1,...,[N]}and(η1,...,η[N])∈BN, (3.26) |
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plusin thecaseof Neven: |
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lim |
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logΛ→∓∞Λ±Ndet |
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pD(Λ) = 0. (3.27) |
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Proof.The requirement that the system of equations (2.18) admits a non-zerosolution leads to the |
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equations(3.26),while theequation(3.27) foreven Nsimplyfollowsbyobservingthat: |
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lim |
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logΛ→∓∞Λ±Ndet |
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pD(Λ) = det |
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p/vextenddouble/vextenddouble/vextenddoubleq(1∓1)N/2δi,j−1+q−(1∓1)N/2δi,j+1−(qk+q−k)δi,j/vextenddouble/vextenddouble/vextenddouble |
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×(−1)N/productdisplay |
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n=1/parenleftbig |
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iκnξ± |
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n/parenrightbigp= 0. (3.28) |
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Note that the above characterization of the T-spectrum ΣTrequires as input the knowledge of BN, |
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i.e. the lattice of zeros of the operator B(λ). It is so interesting to notice that this characterization9 |
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has in fact a reformulation which is independent from the kno wledge of BN. To explain this let us |
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notethatLemma1allowsto introducethefollowingmap: |
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Dp,N:t(λ)∈C[λ2,λ−2](N+eN−1)/2→ Dp,N(t(λ))≡det |
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pD(Λ)∈C[Λ2,Λ−2](N+eN−1)/2. |
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(3.29) |
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Intermsofthismapwecanintroduceafurthercharacterizat ionofthespectrumofthetransfermatrix |
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T(λ). |
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Theorem 1. The spectrum ΣTof the transfer matrix T(λ)coincides with the kernel NDp,N⊂ |
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C[λ2,λ−2](N+eN−1)/2ofthe map Dp,N. |
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Proof.The inclusion NDp,N⊂ΣTis trivial thanks to Lemma 2, vice-versa if t(λ)∈ΣTthen |
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the function detpD(Λ)is zero in N+eNdifferent values of Λ2which thanks to Lemma 1 implies |
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detpD(Λ)≡0,i.e.ΣT⊂ NDp,N. |
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That is the set of eigenvalues of the transfer matrix T(λ)is exactly characterized as the subset of |
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C[λ2,λ−2](N+eN−1)/2whichcontainsallthesolutionsofthefunctionalequation detpD(Λ) = 0. In |
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thenextsectionwewill showthat thisfunctionalequationi s nothingelse thattheBaxterequation. |
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Remark 1. Let us note that the same kind of functional equation detD(Λ) = 0 also appears |
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in [54, 55, 56]. There it recasts, in a compact form, the funct ional relations which result from the |
|
truncatedfusionsoftransfermatrixeigenvalues. Itissor elevanttopointoutthatforthe BBS-model11 |
|
in the SOV representation the non-triviality condition of t he solutions of the system of Baxter-like |
|
equations has been shown [60] to be equivalent to the truncat ion identity in the fusion of transfer |
|
matrixeigenvalues. |
|
4. Baxterfunctional equation |
|
The main consequence of the previous analysis is that it natu rally leads to the complete character- |
|
ization of the transfer matrix spectrum in terms of polynomi al solutions of the Baxter functional |
|
equation. |
|
Theorem2. Lett(λ)∈ΣTthent(λ)definesuniquelyuptonormalizationapolynomial Qt(λ)that |
|
satisfiestheBaxterfunctionalequation: |
|
t(λ)Qt(λ) =a(λ)Qt(λq−1)+d(λ)Qt(λq)∀λ∈C. (4.30) |
|
Proof.The fact that given a t(λ)∈C[λ2,λ−2](N+eN−1)/2there exists up to normalizationat most |
|
one polynomial Qt(λ)that satisfies the Baxter functional equationhas been prove nin Lemma 2 of |
|
[1]. So we have to prove only the existence of Qt(λ)∈C[λ]. An interesting point about the proof |
|
givenhereisthatit isa constructiveproof. |
|
11TheBBS-model [12, 57,58,59] has been analyzed in the SOVapp roach in aseries of works [60,61,62].10 |
|
Let usnoticethatthe condition t(λ)∈ΣT≡ NDp,Nimpliesthatthe p×pmatrixD(λ)hasrank2l |
|
foranyλ∈C\{0}. Letusdenotewith |
|
Ci,j(λ) = (−1)i+jdet |
|
2lDi,j(λ) (4.31) |
|
the(i,j)cofactorof the matrix D(λ); then the matrix formedout of these cofactorshasrank 1, i.e. |
|
all thevectors: |
|
Vi(λ)≡(Ci,1(λ),Ci,2(λ),...,Ci,2l+1(λ))T∈Cp∀i∈ {1,...,2l+1}(4.32) |
|
areproportional: |
|
Vi(λ)/Ci,1(λ) =Vj(λ)/Cj,1(λ)∀i,j∈ {1,...,2l+1},∀λ∈C. (4.33) |
|
Theproportionality(4.33)oftheeigenvectorsV i(λ)implies: |
|
C2,2(λ)/C2,1(λ) =C1,2(λ)/C1,1(λ) (4.34) |
|
which,byusingtheproperty(A.69),canberewrittenas: |
|
C1,1(λq)/C1,2l+1(λq) =C1,2(λ)/C1,1(λ). (4.35) |
|
Moreover,thefirst elementinthe vectorialcondition D(λ)V1(λ) =0¯reads: |
|
t(λ)C1,1(λ) =a(λ)C1,2l+1(λ)+d(λ)C1,2(λ). (4.36) |
|
Let us note that from the form of a(λ),d(λ)andt(λ)∈ΣTit follows that all the cofactors are |
|
Laurentpolynomialofmaximaldegree122lNinλ: |
|
Ci,j(λ) = Ci,jλ−2lN+ai,j4lN−(ai,j+bi,j)/productdisplay |
|
h=1(λ(i,j) |
|
h−λ). (4.37) |
|
In Lemma 5, we show that the equations (4.35) and (4.36) imply that if C 1,1(λ)has a common |
|
zero with C 1,2(λ)then this is also a zero of C 1,2l+1(λ)and that the same statement holds ex- |
|
changing C 1,2(λ)with C 1,2l+1(λ). So we can denote with C1,1C1,1(λ),C1,2l+1C1,2l+1(λ)and |
|
C1,2C1,2(λ)the polynomials of maximal degree 4lNobtained simplifying the common factors in |
|
C1,1(λ), C1,2l+1(λ)and C1,2(λ). Then,byequation(4.35),theyhavetosatisfythe relation s: |
|
C1,2l+1(λ) =q¯N1,1C1,1(λq−1),C1,2(λ) =q−¯N1,1C1,1(λq)andC1,2l+1=ϕC1,1,(4.38) |
|
whereϕ≡C1,1/C1,2and¯N1,1is the degree of the polynomial C1,1(λ). So that equation (4.36) |
|
assumestheformofa Baxterequationin thepolynomial C1,1(λ): |
|
t(λ)C1,1(λ) = ¯a(λ)C1,1(λq−1)+¯d(λ)C1,1(λq), (4.39) |
|
12Theai,jandbi,jare non-negative integers and λ(i,j) |
|
h/ne}ationslash= 0for anyh∈ {1,...,4lN−(ai,j+bi,j)}.11 |
|
with coefficients ¯a(λ)≡q¯N1,1ϕa(λ)and¯d(λ)≡q−¯N1,1ϕ−1d(λ). Note that the consistence of the |
|
aboveequationimpliesthat ϕisap-rootoftheunity. Indeed,denotingwith ¯D(Λ)thematrixdefined |
|
asin(3.21) butwithcoefficients ¯a(λ)and¯d(λ), equation(4.39) implies: |
|
0 = det |
|
p¯D(Λ)≡(ϕp−1)/parenleftiggp/productdisplay |
|
h=1a(λqh)−ϕ−pp/productdisplay |
|
h=1a(−λqh)/parenrightigg |
|
. (4.40) |
|
The expansionfor detp¯D(Λ)in (4.40) is derivedby using the expansion(3.23) for detp¯D(Λ), the |
|
formulae13: |
|
det |
|
2lD1,1(λ) = det |
|
2lD1,1(λ), (4.41) |
|
det |
|
2l−1D(1,2),(1,2)(λ) = det |
|
2l−1D(1,2),(1,2)(λ), (4.42) |
|
det |
|
2l−1D(1,2l+1),(1,2l+1)(λ) = det |
|
2l−1D(1,2l+1),(1,2l+1)(λ), (4.43) |
|
andthecondition t(λ)∈ΣT. Finally,if wedefine: |
|
Qt(λ)≡λaC1,1(λ), (4.44) |
|
whereq−a=q¯N1,1ϕwitha∈ {0,..,2l},we getthestatementofthetheorem. |
|
Remark2. Theprevioustheoremimpliesthatforany t(λ)∈ΣTthepolynomialsolution Qt(λ)of |
|
theBaxterequationcanberelatedtothedeterminantofatri diagonalmatrixoffinitesize p−1. Note |
|
thatthe spectrumoftheSine-Gordonmodelinthecase ofirra tionalcoupling ¯β2shouldbededuced |
|
fromβ2=p′/prational in the limit β2→¯β2. In particular, this implies that underthis limit ( p→ |
|
+∞)thedimensionoftherepresentationdivergesaswellasthe sizeofthetridiagonalmatrixwhose |
|
determinant is associated to the solution Qt(λ)of the Baxter equation. It is then relevant to point |
|
out that in the case of the quantum periodic Toda chain the sol utions of the corresponding Baxter |
|
equationareexpressedintermsofdeterminantsofsemi-infi nitetridiagonalmatrices[63,13, 64]. |
|
It is worth noticing that the set of polynomials Qt(λ), introducedin the previoustheorem,admitsa |
|
moreprecisecharacterization: |
|
Theorem 3. Lett(λ)∈ΣTthent(λ)defines uniquely up to normalization a polynomial solution |
|
Qt(λ)oftheBaxterfunctionalequation(4.30) ofmaximaldegree 2lN. |
|
Inthecase Nodd,it results: |
|
Qt(0)≡Q0/\e}atio\slash= 0,andlim |
|
λ→∞λ−2lNQt(λ)≡Q2lN/\e}atio\slash= 0. (4.45) |
|
In the case Neven, the condition (4.45) selects t(λ)∈Σ0 |
|
Twhile fort(λ)∈Σk |
|
Twithk∈ {1,...,l} |
|
we havethecharacterization Q0=Q2lN= 0and: |
|
lim |
|
λ→0Qt(λq) |
|
Qt(λ)=q±k,lim |
|
λ→∞Qt(λq) |
|
Qt(λ)=q−(N±k). (4.46) |
|
13They follow from the tridiagonality of these matrices and by using Lemma3.12 |
|
Proof.Thankstoformula(A.74),thecofactor C 1,1(λ)∈C[λ,λ−1]2lNiseveninλandso it admits |
|
theexpansions: |
|
C1,1(λ) = C1,1λ−2lN+2˜a1,12lN−(˜a1,1+˜b1,1)/productdisplay |
|
i=1(λ(1,1) |
|
i−λ)(λ(1,1) |
|
i+λ).(4.47) |
|
Let us note now that by using the properties(A.69) and (A.74) , the relation (4.34) can be rewritten |
|
as: |
|
C1,1(λq)C1,1(λ) =qNC1,2(λ)C1,2(−λ). (4.48) |
|
Usingthat andthegeneralrepresentation(4.37)forthe cof actor C 1,2(λ), weget: |
|
a1,2= 2˜a1,1≡2a,b1,2= 2˜b1,1≡2b,C2 |
|
1,2=C2 |
|
1,1q−2(N+b)(4.49) |
|
and:/parenleftig |
|
λ(1,1) |
|
i/parenrightig2 |
|
=/parenleftig |
|
λ(1,2) |
|
i/parenrightig2 |
|
≡¯λ2 |
|
i,/parenleftig |
|
λ(1,2) |
|
i+2lN−(a+b)/parenrightig2 |
|
=/parenleftbig¯λi/q/parenrightbig2(4.50) |
|
with¯λi/\e}atio\slash= 0for anyi∈ {1,...,2lN−(a+b)}withaandb∈Z≥0. Note that the equation (4.49) |
|
andthefactthat ϕ≡C1,1/C1,2isap-rootofthe unityimply ϕ=qb+N. Thenwecanwrite: |
|
C1,1(λ) = Cλ−2lN+2a2lN−(a+b)/productdisplay |
|
i=1(¯λi+λ)(¯λi−λ), (4.51) |
|
C1,2(λ) =qaCλ−2lN+2a2lN−(a+b)/productdisplay |
|
i=1(¯λi+λ)((−1)H(x−i)¯λi−λq), (4.52) |
|
whereC≡C1,1andH(n)≡ {0forn <0,1forn≥0}is the Heaviside step function. Here, x |
|
isanon-negativeintegerwhichisfixedtozerothankstoform ula(4.38). Thenthesolution Qt(λ)of |
|
theBaxter equation(4.30) belongsto C[λ]2lNandhastheform: |
|
Qt(λ)≡λa2lN−(a+b)/productdisplay |
|
i=1(¯λi−λ). (4.53) |
|
Let usshow nowthe remainingstatementsof thetheoremconce rningthe asymptoticsof Qt(λ). To |
|
thisaimwe computethe limits: |
|
lim |
|
logλ→∓∞λ±2lNC1,1(λ) = det |
|
2l/vextenddouble/vextenddouble/vextenddoubleq−(1∓1)N/2δi,j+1+q(1∓1)N/2δi,j−1−(qk+q−k)δeN,1δi,j/vextenddouble/vextenddouble/vextenddouble |
|
i/ne}ationslash=1,j/ne}ationslash=1 |
|
×N/productdisplay |
|
h=1(κhξ±1 |
|
h |
|
i)2l= (δeN,1(1+(2l+1)δk,0)−1)N/productdisplay |
|
h=1(κhξ∓1 |
|
h |
|
i)2l,(4.54) |
|
whichimply: |
|
a=b= 0, (4.55)13 |
|
forNoddandNevenwitht(λ)∈Σ0 |
|
T,i.e. thecondition(4.45). Inthe remainingcases, Nevenand |
|
t(λ)/∈Σ0 |
|
T,the sameformulaimplies: |
|
a/\e}atio\slash= 0,b/\e}atio\slash= 0, (4.56) |
|
sothatQ0=Q2lN= 0,whiletheasymptoticsbehaviors(4.46)simplyfollowtaki ngtheasymptotics |
|
oftheBaxterequationsatisfied by Qt(λ). |
|
5.Q-operator: Existence andcharacterization |
|
Let us denote with Σtthe eigenspace of the transfer matrix T(λ)corresponding to the eigenvalue |
|
t(λ)∈ΣT,then: |
|
Definition 1. LetQ(λ)betheoperatorfamily definedby: |
|
Q(λ)|t/a\}bracketri}ht ≡Qt(λ)|t/a\}bracketri}ht ∀|t/a\}bracketri}ht ∈Σtand∀t(λ)∈ΣT, (5.57) |
|
withQt(λ)the element of C[λ]2lNcorresponding to t(λ)∈ΣTby the injection defined in the |
|
previoustheorem. |
|
Under the assumptions ξandκreal or imaginarynumbers, which assure the self-adjointne ssof the |
|
transfermatrix T(λ)forλ∈R,thefollowingtheoremholds: |
|
Theorem4. Theoperatorfamily Q(λ)isaBaxter Q-operator: |
|
(A)Q(λ)satisfieswith T(λ)thecommutationrelations: |
|
[Q(λ),T(µ)] = [Q(λ),Q(µ)] = 0∀λ,µ∈C, (5.58) |
|
plusthe Baxterequation: |
|
T(λ)Q(λ) =a(λ)Q(λq−1)+d(λ)Q(λq)∀λ∈C. (5.59) |
|
(B)Q(λ)isa polynomialofdegree 2lNinλ: |
|
Q(λ)≡2lN/summationdisplay |
|
n=0Qnλn, |
|
with coefficients Qnself-adjointoperators. |
|
(C)Inthecase Nodd,the operator Q2lN=idandQ0isaninvertibleoperator. |
|
(D)Inthecase Neven,Q(λ)commuteswiththe Θ-chargeandtheoperator Q2lNistheorthogonal |
|
projectionontothe Θ-eigenspacewith eigenvalue1. Q0hasnon-trivialkernel coincidingwith |
|
theorthogonalcomplementto the Θ-eigenspacewith eigenvalue1.14 |
|
Proof.Note that the self-adjointness of the transfer matrix T(λ)implies that Q(λ)is well defined, |
|
indeed its action is defined on a basis. The property (A) is a tr ivial consequence of Definition 1. |
|
Notethat theinjectivityofthemap t(λ)∈ΣT→Qt(λ)∈C[λ]2lNimplies: |
|
(Qt(λ))∗=Qt(λ∗)∀λ∈C (5.60) |
|
being(a(λ))∗=d(λ∗)and(t(λ))∗=t(λ∗). So we get the Hermitian conjugation property |
|
(Q(λ))†=Q(λ∗), i.e. the self-adjointness of the operators Qn. The properties (C) and (D) of |
|
the operators Q0andQ2lNdirectly follow from the asymptotics of the eigenfunction Qt(λ)while |
|
thecommutativityof Q(λ)andΘisa directconsequenceofthecommutativityof T(λ)andΘ. |
|
6. Conclusion |
|
Intheprevioussectionwehaveshownthatbyonlyusingthech aracterizationofthespectrumofthe |
|
transfer matrix obtained by the SOV method we were able to rec onstruct the Q-operator. It is also |
|
interestingto pointoutastheresultsderivedin [1]togeth erwiththoseofthepresentarticleyield: |
|
Theorem5. Thefamily Qwhichcharacterizesthequantumintegrabilityofthelatti ceSine-Gordon |
|
model(see definition(1.1)) isdescribedby thetransfermat rixT(λ)fora chainwith Noddnumber |
|
of siteswhile by T(λ)plustheΘ-chargefora chainwith Nevennumberof sites. |
|
Proof.LetusstartnoticingthatProposition3andTheorem4of[1]a rederivedonlyusingtheSOV |
|
method (i.e. without any assumption about the existence of t heQ-operator). So only using SOV |
|
analysis we have derived that for Nodd the transfer matrix T(λ)has simple spectrum while for |
|
Neven this is true for T(λ)plus theΘ-charge; i.e. they define a complete family of commuting |
|
observables and so satisfy the properties(A) and (C) of the d efinition (1.1). In this article we have |
|
moreover shown that the Q-operator is defined as a function of the transfer matrix whic h implies |
|
the property(B) of (1.1) recalling that in [1] the time-evol utionoperator Uhas been expressed as a |
|
functionofthe Q-operator. |
|
Let us shortly point out the main features required in abstra ct to extend to cyclic representationsof |
|
other integrable quantum models the same kind of spectrum ch aracterization derived here for the |
|
lattice Sine-Gordonmodel. |
|
R1.The model admits an SOV description and the spectrum of the tr ansfer matrix can be charac- |
|
terizedbyasystem ofBaxter-likeequationsin the T-wave-function Ψ(η) =/a\}bracketle{tη|t/a\}bracketri}ht: |
|
t(ηr)Ψ(η) =a(ηr)Ψ(η1,...,q−1ηr,...,η N)+d(ηr)Ψ(η1,...,qη r,...,η N),(6.61) |
|
where(η1,...,ηN)∈BNwithBNtheset ofzerosofthe B-operatorintheSOV representation. |
|
Here,theparameter qisa rootofunitydefinedasin (2.6) and(2.7).15 |
|
Note that for cyclic representationsof an integrable quant um model the set BNis a finite subset of |
|
CN. So the coefficients a(ηr)andd(ηr)are specified only in a finite number of points where they |
|
satisfy thefollowingaveragevaluerelations14: |
|
A(ηp |
|
r) =p/productdisplay |
|
k=1a(qkηr),D(ηp |
|
r) =p/productdisplay |
|
k=1d(qkηr). (6.62) |
|
HereA(Λ)andD(Λ)are the average values of the operator entries A(λ)andD(λ)of the mon- |
|
odromy matrix. Let us recall that the operator entries of the monodromymatrix are expected to be |
|
polynomials(orLaurentpolynomials)inthespectralparam eterλsothecorrespondingaverageval- |
|
uesarepolynomials(orLaurentpolynomials)in Λ≡λp. Itisthennaturaltointroducethefunctions |
|
a(λ)andd(λ)aspolynomial(orLaurentpolynomial)solutionsofthefoll owingaveragerelations: |
|
A(Λ)+γB(Λ) =p/productdisplay |
|
k=1a(qkλ),D(Λ)+δB(Λ) =p/productdisplay |
|
k=1d(qkλ), (6.63) |
|
whereB(Λ)istheaveragevalueoftheoperator B(λ)andγandδare constanttobe fixed. |
|
R2.Let usdenotewith Zf(λ)the set ofthezerosofthefunctions f(λ), then: |
|
∃λ0∈Za(λ):λ0/∈ ∪2l−1 |
|
h=0Zd(λqh). (6.64) |
|
R3.Theaveragevaluesofthefunctions aanddarenotcoincidinginallthezerosofthe B-operator: |
|
A(ηp |
|
a)/\e}atio\slash=D(ηp |
|
a)∀a∈ {1,...,[N]}and(η1,...,η[N])∈BN. (6.65) |
|
The requirement R1yields the introduction of the p×pmatrixD(λ), defined as in (3.21), by |
|
the functions a(λ)andd(λ)solutions of (6.63). This should allow us to reformulate the spectral |
|
problem for the transfer matrix as the problem to classify al l the solutions t(λ)to the functional |
|
equationdetpD(Λ) = 0ina modeldependentclassoffunctions. |
|
The requirement R2implies that the rank of the matrix D(λ)is almost everywhere 2l. Indeed, the |
|
condition (6.64) implies C 1,p(λ0)/\e}atio\slash= 0, independently from the function t(λ). Being the cofactor |
|
C1,p(λ)acontinuousfunctionofthespectralparametertheabovest atementontherankofthematrix |
|
D(λ)follows. Underthisconditionwecanfollowtheprocedurepr esentedinTheorem2toconstruct |
|
the solutionsof the Baxter equation. Then the self-adjoint nessof the transfer matrix Tallows us to |
|
proceedasinsection5to showthe existenceofthe Q-operatorasa functionof T. |
|
The requirement R3is a sufficient criterion15to show the simplicity of the spectrum of Twhich |
|
should imply that the full integrable structure of the quant um model should be described by the |
|
14Theequations in (6.62) are trivial consequences of the SOVr epresentation and of the cyclicity. |
|
15It is worth noticing that in the case of the Sine-Gordon model the criterion R3does not apply to the representations |
|
withun=vn= 1. Nevertheless, we have shown the simplicity of Tby using some model dependent properties of the |
|
coefficients a(λ)andd(λ), see section 5of [1].16 |
|
transfermatrixassoonastheproperty(B)indefinition(1.1 )isshownforthemodelunderconsider- |
|
ation. |
|
Following the schema here presented, in a future publicatio n we will address the analysis of the |
|
spectrumfortheso-called α-sectorsoftheSine-Gordonmodel(see[1]). Theuseofthisapproachi s |
|
in particularrelevantin these sectorsof theSine-Gordonm odelbecausea direct constructionof the |
|
Q-operatorleadstosometechnicaldifficulty. |
|
A. Properties ofthecofactors C i,j(λ) |
|
Let usconsideran M×Mtridiagonal matrix16O: |
|
O≡ |
|
z1y10··· 0 0 |
|
x1z2y20··· 0 |
|
0x2z3y3... |
|
......... |
|
...... 0 |
|
0...0xM−2zM−1yM−1 |
|
0 0...0xM−1zM |
|
(A.66) |
|
i.e. a matrix with non-zero entries only along the principal diagonal and the next upper and lower |
|
diagonals. |
|
Lemma 3. The determinantof atridiagonalmatrix is invariantundert he transformation ̺αwhich |
|
multiplies for αthe entries above the diagonal and for α−1the entries below the diagonal leaving |
|
theentriesonthediagonalunchanged. |
|
Proof.Letusnotethatthedeterminantofa tridiagonalmatrixadmi tsthefollowingexpansion: |
|
det |
|
MO=z1det |
|
M−1O1,1+x1y1det |
|
M−2O(1,2),(1,2), (A.67) |
|
wherewe haveused thesame notationsintroducedafterformu la(3.23). By usingit, we getthat the |
|
actionof̺αreads: |
|
det |
|
M̺α(O) =z1det |
|
M−1̺α(O)1,1+x1y1det |
|
M−2̺α(O)(1,2),(1,2). (A.68) |
|
Then the statement follows by induction noticing that the tr ansformation ̺αleaves always un- |
|
changedthedeterminantofa 2×2matrix. |
|
16An interesting analysis of the eigenvalue problem for tridi agonal matrices is presented in [65].17 |
|
Lemma 4. Thefollowingpropertieshold: |
|
Ch+i,k+i(λ) =Ch,k(λqi)∀i,h,k∈ {1,...,2l+1}, (A.69) |
|
and: |
|
C1,1(λ) =C1,1(−λ)andC2,1(λ) =qNC1,2(−λ). (A.70) |
|
Proof.Note that by the definition (4.31) of the cofactors C i,j(λ)the equations (A.69) are simple |
|
consequencesof qp= 1andareprovenexchangingrowsandcolumnsin thedeterminan ts. |
|
Let us provenow that the cofactor C 1,1(λ) = det 2lD1,1(λ)is an even function of λ. The tridiago- |
|
nalityofthematrix D1,1(λ)allowsusto usethepreviouslemma: |
|
C1,1(λ)≡det |
|
2l/vextenddouble/vextenddoublet(λqh)δh,k−a(λqh)δh,k+1−qNa(−λqh+1)δh,k−1/vextenddouble/vextenddouble |
|
h>1,k>1 |
|
= det |
|
2l/vextenddouble/vextenddoublet(λqh)δh,k−qNa(λqh)δh,k+1−a(−λqh+1)δh,k−1/vextenddouble/vextenddouble |
|
h>1,k>1 |
|
= det |
|
2l/vextenddouble/vextenddoublet(λqh)δh,k−d(−λqk)δk,h−1−a(−λqk)δk,h+1/vextenddouble/vextenddouble |
|
h>1,k>1 |
|
≡det |
|
2l(D1,1(−λ))T=C1,1(−λ). (A.71) |
|
To provenowthesecondrelationin (A.70) weexpandthe cofac tors: |
|
C2,1(λ) =2l+1/productdisplay |
|
h=2a(λqh)+d(λ) det |
|
2l−1D(1,2),(1,2)(λ), (A.72) |
|
C1,2(λ) =2l/productdisplay |
|
h=1d(λqh)+a(λq) det |
|
2l−1D(1,2),(1,2)(λ). (A.73) |
|
Byusingthesamestepsshownin(A.71),thetridiagonalityo fthematrixD (1,2),(1,2)(λ)impliesthat |
|
its determinant is an even function of λfrom which the statement C 2,1(λ) =qNC1,2(−λ)follows |
|
recallingthat d(λ) =qNa(−λq). |
|
Remark 3. Inthisarticlewe needonlytheproperties(A.70);however, it isworthpointingoutthat |
|
theyarespecialcasesofthefollowingpropertiesofthe cof actors: |
|
Ci,j(λ) =qN(i−j)Cj,i(−λ)∀i,j∈ {1,...,2l+1}. (A.74) |
|
Theproofof(A.74)canbedonesimilarlytothatof(A.70) but we omitit forsimplicity. |
|
Let ususe onceagainthenotation Zfforthe set ofthezerosofafunction f(λ), then: |
|
Lemma 5. Theequations(4.35)and(4.36) imply: |
|
ZC1,1∩ZC1,2≡ZC1,1∩ZC1,2l+1. (A.75)18 |
|
Proof.The inclusions/parenleftbig |
|
ZC1,1∩ZC1,2/parenrightbig |
|
\Za⊂ZC1,1∩ZC1,2l+1and/parenleftbig |
|
ZC1,1∩ZC1,2l+1/parenrightbig |
|
\Zd⊂ |
|
ZC1,1∩ZC1,2triviallyfollowbyequation(4.36). |
|
Let us observe now that C 1,2(λq−1)has no common zero with a(λ)and that C 1,2l+1(λq)has no |
|
common zero with d(λ). These statements simply follow from (A.73), (A.69)and(A. 72) when we |
|
recall that a(λ)has no common zero with/producttext2l−1 |
|
h=0d(λqh)and thatd(λ)has no common zero with/producttext2l+1 |
|
h=2a(λqh). So,if/parenleftbig |
|
ZC1,1∩ZC1,2/parenrightbig |
|
∩Zaisnotemptyand λ0∈/parenleftbig |
|
ZC1,1∩ZC1,2/parenrightbig |
|
∩Za,theequation |
|
(4.35) computed in λ=q−1λ0implies C 1,2l+1(λ0) = 0being C 1,2(λ0q−1)/\e}atio\slash= 0, i.e.λ0∈ |
|
ZC1,1∩ZC1,2l+1. Similarly,if/parenleftbig |
|
ZC1,1∩ZC1,2l+1/parenrightbig |
|
∩Zdisnotemptyand λ0∈/parenleftbig |
|
ZC1,1∩ZC1,2l+1/parenrightbig |
|
∩Zd, |
|
the equation (4.35) computed in λ=λ0implies C 1,2(λ0) = 0being C 1,2l+1(λ0q)/\e}atio\slash= 0, i.e.λ0∈ |
|
ZC1,1∩ZC1,2. So that (4.35) implies the inclusions/parenleftbig |
|
ZC1,1∩ZC1,2/parenrightbig |
|
∩Za⊂ZC1,1∩ZC1,2l+1and/parenleftbig |
|
ZC1,1∩ZC1,2l+1/parenrightbig |
|
∩Zd⊂ZC1,1∩ZC1,2inthiswaycompletingthe proofofthelemma. |
|
B. Scalarproduct inthe SOV space |
|
Here is described as a natural structure of Hilbert space can be provided to the linear space of the |
|
SOV representationbypreservingtheself-adjointnessoft hetransfermatrix. |
|
B.1 CyclicrepresentationsoftheWeylalgebra |
|
Here,we considerthecyclicrepresentationsoftheWeyl alg ebraW(n) |
|
qinthecase: |
|
up |
|
n=vp |
|
n= 1forβ2=p′/pwithp′evenandp= 2l+1odd. (B.76) |
|
At anysitenofthechain,weintroducethe quantumspace Rnwithvn-eigenbasis: |
|
vn|k,n/a\}bracketri}ht=qk|k,n/a\}bracketri}ht ∀|k,n/a\}bracketri}ht ∈Bn={|k,n/a\}bracketri}ht,∀k∈ {−l,...,l}}. (B.77) |
|
Note that the eigenvaluesof vndescribe the unit circle Sp={qk:k∈ {−l,...,l}},indeedql+1= |
|
q−l. OnRnisdefinedap-dimensionalrepresentationoftheWeyl algebrabysetting : |
|
un|k,n/a\}bracketri}ht=|k+1,n/a\}bracketri}ht ∀k∈ {−l,...,l} (B.78) |
|
with thecyclicitycondition: |
|
|k+p,n/a\}bracketri}ht=|k,n/a\}bracketri}ht. (B.79) |
|
B.2 Representationin the SOVbasis |
|
The analysis developed in [1] define recursively the eigenba sis{|¯η1qh1,...,¯ηNqhN/a\}bracketri}ht}of theB- |
|
operator in the original representation, i.e. as linear com binations of the elements of the basis |
|
{|h1,...,hN/a\}bracketri}ht ≡/circlemultiplytextN |
|
n=1|hn,n/a\}bracketri}ht}, where|hn,n/a\}bracketri}htare the elements of the vn-eigenbasis defined in |
|
(B.77). To writethischangeofbasisin amatrixformlet usin troducethe followingnotations: |
|
|yj/a\}bracketri}ht ≡ |¯η1qh1,...,¯ηNqhN/a\}bracketri}htand|xj/a\}bracketri}ht ≡ |h1,...,hN/a\}bracketri}ht (B.80)19 |
|
where: |
|
j:=h1+N/summationdisplay |
|
a=2(2l+1)(a−1)(ha−1)∈ {1,...,(2l+1)N}, (B.81) |
|
notethat thisdefinesa oneto onecorrespondencebetween N-tuples(h1,...,hN)∈ {1,...,2l+1}N |
|
and integers j∈ {1,...,(2l+1)N}, which just amountsto chose an orderingin the elementsof th e |
|
two basis. Underthisnotation,wehave: |
|
|yj/a\}bracketri}ht=W|xj/a\}bracketri}ht=(2l+1)N/summationdisplay |
|
i=1Wi,j|xi/a\}bracketri}ht, (B.82) |
|
where we are representing |xj/a\}bracketri}htas the vector |j/a\}bracketri}htin the natural basis in C(2l+1)NandW=||Wi,j|| |
|
is a(2l+1)N×(2l+1)Nmatrix. The matrix Wis defined by recursion in terms of the kernel K |
|
constructedinappendixCof[1], letususethenotation: |
|
K({h1,...,hN},k1,{k2,...,kN})≡KN(η|χ2;χ1), (B.83) |
|
whereweareconsideringthecase N−M = 1. Thenthe recursionreads: |
|
W(N) |
|
i,j=2l+1/summationdisplay |
|
k2,...,kN=1K({h1(j),...,hN(j)},h1(i),{k2,...,kN})W(N−1) |
|
¯h(i),a(k2,...,kN),(B.84) |
|
where we have introduced the index (N)and(N−1)in the matrices Wto make clear the step |
|
of the recursion. Here, (h1(j),...,hN(j))is the unique N-tuples corresponding to the integer j∈ |
|
{1,...,(2l+ 1)N}andh1(i)is the first entry in the unique N-tuples corresponding to the integer |
|
i∈ {1,...,(2l+1)N}. Moreover,wehavedefined: |
|
¯h(i) := 1+i−h1(i) |
|
2l+1∈ {1,...,(2l+1)(N−1)}anda(k2,...,kN) =k2+N/summationdisplay |
|
a=3(2l+1)(a−2)(ka−1), |
|
(B.85) |
|
Remarks: |
|
a)Underthechangeofbasis {|xj/a\}bracketri}ht} → {|yj/a\}bracketri}ht}thegenericoperatorX transformsforsimilarity: |
|
XSOV≡W−1XW, (B.86) |
|
so fromtheactionofthezerooperators ηaandtheshift operators T± |
|
aontheB-eigenbasis |yj/a\}bracketri}ht: |
|
ηa|yj/a\}bracketri}ht= ¯ηaqha(j)|yj/a\}bracketri}htandT± |
|
a|yj/a\}bracketri}ht=|yj±(2l+1)(a−1)/a\}bracketri}ht (B.87) |
|
we havethat: |
|
(ηa)SOV= ¯ηa||qha(j)δi,j||and/parenleftbig |
|
T± |
|
a/parenrightbig |
|
SOV=||δi,j±(2l+1)(a−1)||. (B.88)20 |
|
Fromtheaboveexpressionwe have17: |
|
(ηa)† |
|
SOV= (ηa)∗ |
|
SOVand/parenleftbig |
|
T± |
|
a/parenrightbig† |
|
SOV=/parenleftbig |
|
T∓ |
|
a/parenrightbig |
|
SOV. (B.89) |
|
b) The known transformation properties of the entries of the monodromy matrix in the original |
|
representationimply: |
|
/parenleftbiggDSOV(λ)CSOV(λ) |
|
BSOV(λ)ASOV(λ)/parenrightbigg |
|
=/parenleftigg |
|
G−1(ASOV(λ∗))†G−G−1(BSOV(λ∗))†G |
|
−G−1(CSOV(λ∗))†G G−1(DSOV(λ∗))†G/parenrightigg |
|
,(B.90) |
|
withGisapositiveself-adjointmatrixdefinedby G:=W†W. |
|
c)Thequantumdeterminantrelationis invariantundersimi laritytransformationsandso we have: |
|
a(λ)d(λq−1) =ASOV(λ)DSOV(λq−1)−BSOV(λ)CSOV(λq−1), (B.91) |
|
Lemma 6. Thebasis {|yj/a\}bracketri}ht}isnotanorthogonalbasisw.r.t. thenaturalscalarproduct on{|xj/a\}bracketri}ht}. |
|
Proof.Note that the condition {|yj/a\}bracketri}ht}is an orthogonal basis is equivalent to the statement Gis |
|
a diagonal matrix (with positive diagonal entries). Let us r ecall that the Hermitian conjugation |
|
propertyofB(λ)togetherwiththeYang-Baxtercommutationrelationsimply : |
|
[B†(λ),B(µ)] = [B(µ),C(λ∗)] =q−q−1 |
|
λ∗/µ−µ/λ∗(A(λ∗)D(µ)−A(µ)D(λ∗))/\e}atio\slash= 0 (B.92) |
|
that is the operator B(λ)is not a normal operator. Now let us show that the non-normali tyofB(λ) |
|
impliesthat Gisnotdiagonal. Indeed,wecanwrite: |
|
[B†(λ),B(µ)] =/parenleftbig |
|
W†/parenrightbig−1(BSOV(λ))†GBSOV(µ)W−1−WBSOV(µ)G−1(BSOV(λ))†W† |
|
=W(G−1(BSOV(λ))†GBSOV(µ)−BSOV(µ)G−1(BSOV(λ))†G)W−1.(B.93) |
|
Notenowthatifweassume Gdiagonal,then Gcommutesbothwith BSOV(λ)andwith(BSOV(λ))†, |
|
being all diagonal matrices in the SOV representation, whic h implies the absurd [B†(λ),B(µ)] = |
|
0. |
|
B.3 Scalarproductin theSOVspace |
|
The self-adjointness of the family T(λ)implies that the transfer matrix eigenstates are orthogona l |
|
undertheoriginalscalar product: |
|
δi,j= (|ti/a\}bracketri}ht,|tj/a\}bracketri}ht), (B.94) |
|
we have chosen the orthonormal ones. Note that the above equa tion naturally induces a scalar |
|
productintheSOV representationobtainedunderchangeofb asis: |
|
(|b/a\}bracketri}ht,|a/a\}bracketri}ht)SOV≡(G|b/a\}bracketri}ht,|a/a\}bracketri}ht) (B.95) |
|
17Here, weare using the standard notation for the adjoint X†≡(X∗)t.21 |
|
thatisascalarproductforwhichtheadjointofavector |a/a\}bracketri}htisthenaturaladjointtimesthematrix G: |
|
|b/a\}bracketri}ht†SOV≡ /a\}bracketle{tb|Gwith/a\}bracketle{tb|=/parenleftig |
|
(|b/a\}bracketri}ht)t/parenrightig∗ |
|
, (B.96) |
|
andsoforthegenericoperator Xwehave: |
|
X†SOV≡G−1X†G. (B.97) |
|
It istrivialtonoticethat: |
|
Lemma 7. The family of operators TSOV(λ)is self-adjoint w.r.t. †SOVand the eigenstates |
|
|tj/a\}bracketri}htSOV≡W−1|tj/a\}bracketri}htare orthonormal w.r.t. the scalar product defined in (B.95). Moreover, it |
|
results: |
|
/parenleftigg |
|
(ASOV(λ∗))†SOV(BSOV(λ∗))†SOV |
|
(CSOV(λ∗))†SOV(DSOV(λ∗))†SOV/parenrightigg |
|
=/parenleftbiggDSOV(λ)−CSOV(λ) |
|
−BSOV(λ)ASOV(λ)/parenrightbigg |
|
.(B.98) |
|
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