On the Degenerate Multiplicity of the sl₂ Loop Algebra for the 6V Transfer Matrix at Roots of Unity
We review the main result of cond-mat/0503564. The Hamiltonian of the XXZ spin chain and the transfer matrix of the six-vertex model has the sl₂ loop algebra symmetry if the q parameter is given by a root of unity, q₀2N = 1, for an integer N. We discuss the dimensions of the degenerate eigenspace ge...
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irk-123456789-1464462019-02-11T01:23:28Z On the Degenerate Multiplicity of the sl₂ Loop Algebra for the 6V Transfer Matrix at Roots of Unity Deguchi, T. We review the main result of cond-mat/0503564. The Hamiltonian of the XXZ spin chain and the transfer matrix of the six-vertex model has the sl₂ loop algebra symmetry if the q parameter is given by a root of unity, q₀2N = 1, for an integer N. We discuss the dimensions of the degenerate eigenspace generated by a regular Bethe state in some sectors, rigorously as follows: We show that every regular Bethe ansatz eigenvector in the sectors is a highest weight vector and derive the highest weight dk±, which leads to evaluation parameters aj. If the evaluation parameters are distinct, we obtain the dimensions of the highest weight representation generated by the regular Bethe state. 2006 Article On the Degenerate Multiplicity of the sl₂ Loop Algebra for the 6V Transfer Matrix at Roots of Unity / T. Deguchi // Symmetry, Integrability and Geometry: Methods and Applications. — 2006. — Т. 2. — Бібліогр.: 30 назв. — англ. 1815-0659 2000 Mathematics Subject Classification: 81R10; 81R12; 81R50; 81V70 http://dspace.nbuv.gov.ua/handle/123456789/146446 en Symmetry, Integrability and Geometry: Methods and Applications Інститут математики НАН України |
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We review the main result of cond-mat/0503564. The Hamiltonian of the XXZ spin chain and the transfer matrix of the six-vertex model has the sl₂ loop algebra symmetry if the q parameter is given by a root of unity, q₀2N = 1, for an integer N. We discuss the dimensions of the degenerate eigenspace generated by a regular Bethe state in some sectors, rigorously as follows: We show that every regular Bethe ansatz eigenvector in the sectors is a highest weight vector and derive the highest weight dk±, which leads to evaluation parameters aj. If the evaluation parameters are distinct, we obtain the dimensions of the highest weight representation generated by the regular Bethe state. |
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Article |
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Deguchi, T. |
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Deguchi, T. On the Degenerate Multiplicity of the sl₂ Loop Algebra for the 6V Transfer Matrix at Roots of Unity Symmetry, Integrability and Geometry: Methods and Applications |
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Deguchi, T. |
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Deguchi, T. |
| title |
On the Degenerate Multiplicity of the sl₂ Loop Algebra for the 6V Transfer Matrix at Roots of Unity |
| title_short |
On the Degenerate Multiplicity of the sl₂ Loop Algebra for the 6V Transfer Matrix at Roots of Unity |
| title_full |
On the Degenerate Multiplicity of the sl₂ Loop Algebra for the 6V Transfer Matrix at Roots of Unity |
| title_fullStr |
On the Degenerate Multiplicity of the sl₂ Loop Algebra for the 6V Transfer Matrix at Roots of Unity |
| title_full_unstemmed |
On the Degenerate Multiplicity of the sl₂ Loop Algebra for the 6V Transfer Matrix at Roots of Unity |
| title_sort |
on the degenerate multiplicity of the sl₂ loop algebra for the 6v transfer matrix at roots of unity |
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Інститут математики НАН України |
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2006 |
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On the Degenerate Multiplicity of the sl₂ Loop Algebra for the 6V Transfer Matrix at Roots of Unity / T. Deguchi // Symmetry, Integrability and Geometry: Methods and Applications. — 2006. — Т. 2. — Бібліогр.: 30 назв. — англ. |
| series |
Symmetry, Integrability and Geometry: Methods and Applications |
| work_keys_str_mv |
AT deguchit onthedegeneratemultiplicityofthesl2loopalgebraforthe6vtransfermatrixatrootsofunity |
| first_indexed |
2025-07-10T23:35:09Z |
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2025-07-10T23:35:09Z |
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1837304929151090688 |
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Symmetry, Integrability and Geometry: Methods and Applications Vol. 2 (2006), Paper 021, 10 pages
On the Degenerate Multiplicity of the sl2 Loop
Algebra for the 6V Transfer Matrix at Roots of Unity
Tetsuo DEGUCHI
Department of Physics, Faculty of Science, Ochanomizu University,
2-1-1 Ohtsuka, Bunkyo-Ku, Tokyo 112-8610, Japan
E-mail: deguchi@phys.ocha.ac.jp
Received October 31, 2005, in final form February 06, 2006; Published online February 17, 2006
Original article is available at http://www.emis.de/journals/SIGMA/2006/Paper021/
Abstract. We review the main result of cond-mat/0503564. The Hamiltonian of the XXZ
spin chain and the transfer matrix of the six-vertex model has the sl2 loop algebra symmetry
if the q parameter is given by a root of unity, q2N
0 = 1, for an integer N . We discuss the
dimensions of the degenerate eigenspace generated by a regular Bethe state in some sectors,
rigorously as follows: We show that every regular Bethe ansatz eigenvector in the sectors
is a highest weight vector and derive the highest weight d̄±k , which leads to evaluation
parameters aj . If the evaluation parameters are distinct, we obtain the dimensions of the
highest weight representation generated by the regular Bethe state.
Key words: loop algebra; the six-vertex model; roots of unity representations of quantum
groups; Drinfeld polynomial
2000 Mathematics Subject Classification: 81R10; 81R12; 81R50; 81V70
1 Introduction
The XXZ spin chain is one of the most important exactly solvable quantum systems. The
Hamiltonian under the periodic boundary conditions is given by
HXXZ =
1
2
L∑
j=1
(
σX
j σX
j+1 + σY
j σY
j+1 + ∆σZ
j σZ
j+1
)
. (1)
Here the XXZ anisotropy ∆ is related to the q parameter by ∆ = (q + q−1)/2. The XXZ
Hamiltonian (1) is derived from the logarithmic derivative of the transfer matrix of the six-
vertex model, and hence they have the same set of eigenvectors.
Recently, it was explicitly shown that when q is a root of unity the XXZ Hamiltonian com-
mutes with the generators of the sl2 loop algebra [13]. Let q0 be a primitive root of unity
satisfying q2N
0 = 1 for an integer N . We introduce operators S±(N) as follows
S±(N) =
∑
1≤j1<···<jN≤L
q
N
2
σZ
0 ⊗ · · · ⊗ q
N
2
σZ
0 ⊗ σ±j1 ⊗ q
(N−2)
2
σZ
0 ⊗ · · · ⊗ q
(N−2)
2
σZ
0
⊗ σ±j2 ⊗ q
(N−4)
2
σZ
0 ⊗ · · · ⊗ σ±jN
⊗ q
−N
2
σZ
0 ⊗ · · · ⊗ q
−N
2
σZ
0 .
They are derived from the Nth power of the generators S± of the quantum group Uq(sl2)
or Uq(ŝl2). We also define T (±) by the complex conjugates of S±(N), i.e. T±(N) =
(
S±(N)
)∗
.
The operators, S±(N) and T±(N), generate the sl2 loop algebra, U(L(sl2)), in the sector
SZ ≡ 0 (modN). (2)
mailto:deguchi@phys.ocha.ac.jp
http://www.emis.de/journals/SIGMA/2006/Paper021/
2 T. Deguchi
Here the value of the total spin SZ is given by an integral multiple of N . In the sector (2), the ope-
rators S±(N) and T±(N) (anti-)commute with the transfer matrix of the six-vertex model τ6V (v),
and they commute with the Hamiltonian of the XXZ spin chain [13]:
[S±(N),HXXZ ] = [T±(N),HXXZ ] = 0.
One of the most important physical questions is to obtain the degenerate multiplicity of
the sl2 loop algebra. Let us denote by |R〉 a regular Bethe state with R down spins. Here
we define regular Bethe states by such Bethe ansatz eigenvectors that are constructed from
finite and distinct solutions of the Bethe ansatz equations, whose precise definition will be given
in Section 2. For any given regular Bethe state in the sector (2), we may have the following
degenerate eigenvectors of the XXZ Hamiltonian
S−(N)|R〉, T−(N)|R〉,
(
S−(N)
)2|R〉, T−(N)S+(N)T−(N)|R〉, . . . .
However, it is nontrivial how many of them are linearly independent. The number should
explain the degree of the spectral degeneracy. We thus want to know the dimensions of the
degenerate eigenspace generated by the Bethe state |R〉. Here we note that some of the spectral
degeneracies of the XYZ spin chain (the eight-vertex model) at roots of unity were first discussed
by Baxter [2] (see also [3, 4]). In fact, there are such spectral degeneracies of the XYZ spin
chain at roots of unity that are closely related to the sl2 loop algebra symmetry of the XXZ spin
chain [9, 10, 17, 18]. Quite interestingly, various aspects of the spectral degeneracies of the XXZ
spin chain have been discussed by several authors from different viewpoints [1, 21, 22, 27, 29].
Recently, there has been some progress on the degenerate multiplicity as far as regular Bethe
states in some sectors such as (2) are concerned [11]. For the XXZ spin chain at roots of unity,
Fabricius and McCoy had made important observations on degenerate multiplicities of the sl2
loop algebra, and conjectured the ‘Drinfeld polynomials of Bethe ansatz eigenvectors’ [14, 15, 16].
However, it was not clear whether the representation generated by a Bethe state is irreducible or
not. Here we remark that a Drinfeld polynomial is defined for an irreducible finite-dimensional
representation. Motivated by the previous results [14, 15, 16], an algorithm for determining the
dimensions of the representation generated by a given regular Bethe state has been rigorously
formulated in some sectors such as (2) and some restricted cases of evaluation parameters aj ,
which will be defined in Section 5. We show rigorously [11] that every regular Bethe state in the
sectors is highest weight, assuming a conjecture that the Bethe roots are continuous with respect
to the q parameter at roots of unity. We evaluate the highest weight d̄±k for the Bethe state,
which leads to evaluation parameters aj . Here d̄±k will be defined in Section 4. Independently, it
has been shown [8] that if the evaluation parameters of a highest weight representation of the sl2
loop algebra are distinct, then it is irreducible. It thus follows that the ‘Drinfeld polynomial of
a regular Bethe state’ corresponds to the standard Drinfeld polynomial defined for the irreducible
representation generated by the regular Bethe state, if the evaluation parameters are distinct.
The conjecture of Fabricius and McCoy has been proved at least in the sectors such as (2) and
in the case of distinct evaluation parameters. Thus, the purpose of the paper is to review the
main points of the complete formulation of [11] briefly with some illustrative examples. Some
other aspects of the sl2 loop algebra symmetry have also been discussed [5, 24, 30].
The contents of the paper is given as follows. In Section 2, we introduce Bethe ansatz
equations and regular solutions of the Bethe ansatz equations. We define regular Bethe states.
In Section 3, we discuss briefly the sl2 loop algebra symmetry of the XXZ spin chain at roots of
unity. Here, the conditions of roots of unity are specified precisely. In Section 4, we explain the
Drinfeld realization of the sl2 loop algebra, i.e. the classical analogues of the Drinfeld realization
of the quantum affine group Uq(ŝl2). In Section 5, we review the algorithm for determining the
degenerate multiplicity of a regular Bethe state in the sectors. Here we note that Theorem 2
Degenerate Multiplicity of the sl2 Loop Algebra for the 6V Model at Roots of Unity 3
generalizes the su(2) symmetry of the XXX spin chain shown by Takhtajan and Faddeev [28].
In Section 6 we discuss some examples of the Drinfeld polynomials explicitly.
2 Bethe ansatz equations and the transfer matrix
2.1 Regular solutions of the Bethe ansatz equations
Let us assume that a set of complex numbers, t̃1, t̃2, . . . , t̃R satisfy the Bethe ansatz equations
at a root of unity:(
sinh(t̃j + η0)
sinh(t̃j − η0)
)L
=
M∏
k=1;k 6=j
sinh(t̃j − t̃k + 2η0)
sinh(t̃j − t̃k − 2η0)
, for j = 1, 2, . . . , R. (3)
Here the parameter η is defined by the relation q = exp(2η), and η0 is given by q0 = exp(2η0).
If a given set of solutions of the Bethe ansatz equations are finite and distinct, we call them
regular. We call a set of solutions of the Bethe ansatz equations Bethe roots.
A set of regular solutions of the Bethe ansatz equations leads to an eigenvector of the XXZ
Hamiltonian. We call it a regular Bethe state of the XXZ spin chain or a regular XXZ Bethe
state, briefly. Explicit expressions of regular Bethe states are derived through the algebraic
Bethe ansatz method [23].
2.2 Transfer matrix of the six-vertex model
We now introduce L operators for the XXZ spin chain. Let Vn be two-dimensional vector spaces
for n = 0, 1, . . . , L. We define an operator-valued matrix Ln(z) by
Ln(z) =
(
Ln(z)11 Ln(z)12
Ln(z)21 Ln(z)22
)
=
(
sinh (z In + ησz
n) sinh 2η σ−n
sinh 2η σ+
n sinh (z In − ησz
n)
)
.
Here Ln(z) is a matrix acting on the auxiliary vector space V0, where In and σa
n (a = z,±)
are operators acting on the nth vector space Vn. The symbol I denotes the two-by-two identity
matrix, σ± denote σ+ = E12 and σ− = E21, and σx, σy, σz are the Pauli matrices.
We define the monodromy matrix T by the product:
T (z) = LL(z) · · ·L2(z)L1(z).
Here the matrix elements of T (z) are given by
T (z) =
(
A(z) B(z)
C(z) D(z)
)
.
We define the transfer matrix of the six vertex model τ6V (z) by the following trace:
τ6V (z) = trT (z) = A(z) + D(z).
We call the transfer matrix homogeneous. It is invariant under lattice translation.
3 The sl2 loop algebra symmetry at roots of unity
We shall show the sl2 loop algebra symmetry of the XXZ spin chain at roots of unity in some
sectors. We shall discuss two cases with even L and odd L.
4 T. Deguchi
3.1 Roots of unity conditions
Let us explicitly formulate roots of unity conditions as follows.
Definition 1 (Roots of unity conditions). We say that q0 is a root of unity with q2N
0 = 1,
if one of the three conditions hold: (i) q0 is a primitive Nth root of unity with N odd (qN
0 = 1);
(ii) q0 is a primitive 2Nth root of unity with N odd (qN
0 = −1); (iii) q0 is a primitive 2Nth root
of unity with N even (qN
0 = −1). We call the cases (i) and (iii) type I, the case (ii) type II.
In the case of SZ ≡ 0 (mod N) we consider all the three conditions of roots of unity. However,
in the case of SZ ≡ N/2 (mod N) with N odd, we consider only the condition (i) of roots of
unity, i.e. q0 is a primitive Nth root of unity with N odd (qN
0 = 1).
3.2 (Anti-)commutation relations at roots of unity
We show the sl2 loop algebra symmetry of the XXZ spin chain in the following two sectors:
(a) in the sector SZ ≡ 0 (mod N) where q0 is a root of unity with q2N
0 = 1, as specified in
Definition 1; (b) in the sector SZ ≡ N/2 (mod N) with N odd where q0 is a primitive Nth root
of unity.
Let us assume that there exists a set of regular solutions of Bethe ansatz equations (3) with
R down-spins, i.e. R regular Bethe roots. We also assume that the lattice size L, the number of
regular Bethe roots R and the integer N satisfy the following relation:
L− 2R = nN, n ∈ Z. (4)
If n is even, the regular Bethe state |R〉 is in the sector SZ ≡ 0 (mod N), while if n is odd and
N is also odd, then it is in the sector SZ ≡ N/2 (mod N). Here we recall that the symbol |R〉
denotes the regular Bethe state constructed from the given R regular Bethe roots.
It has been shown [13] that operators S±(N) and T±(N) (anti-)commute with the transfer
matrix of the six-vertex model τ6V (v) in the sector SZ ≡ 0 (mod N) at q0 with q2N
0 = 1
S±(N) τ6V (z) = qN
0 τ6V (z) S±(N), T±(N) τ6V (z) = qN
0 τ6V (z) T±(N). (5)
Furthermore, it is also shown [11] that in the sector SZ ≡ N/2 (mod N) when N is odd and
q0 satisfies qN
0 = 1, operators S±(N) and T±(N) (anti-)commute with the transfer matrix of the
six-vertex model τ6V (v).
From the (anti-)commutation relations (5) it follows that the operators S±(N) and T±(N)
commute with the XXZ Hamiltonian in the sector SZ ≡ 0 (mod N) when q0 satisfies q2N
0 = 1,
and in the sector SZ ≡ N/2 when N is odd and qN
0 = 1. Here we recall that the XXZ Hamiltonian
HXXZ is given by the logarithmic derivative of the (homogeneous) transfer matrix τ6V (v).
3.3 The algebra generated by S±(N) and T ±(N)
Let us discuss the algebra generated by the operators [13], S±(N) and T±(N). When q0 is of
type I, we have the following identification [13]:
E+
0 = T−(N), E−
0 = T+(N), E+
1 = S+(N), E−
1 = S−(N),
−H0 = H1 =
2
N
SZ . (6)
When q is of type II, we have the following [9]:
E+
0 =
√
−1 T−(N), E−
0 =
√
−1 T+(N), E+
1 =
√
−1 S+(N),
Degenerate Multiplicity of the sl2 Loop Algebra for the 6V Model at Roots of Unity 5
E−
1 =
√
−1 S−(N), −H0 = H1 =
2
N
SZ . (7)
Here
√
−1 denotes the square root of −1 (cf. (A.13) of [9]; see also [24]). The operators E±
j
and Hj for j = 0, 1, are the Chevalley generators of the affine Lie algebra ŝl2. In fact, the
operators E±
j , Hj for j = 0, 1, satisfy the defining relations [20] of the sl2 loop algebra [13]:
H0 + H1 = 0, [Hi, E
±
j ] = ±aijE
±
j , i, j = 0, 1, (8)
[E+
i , E−
j ] = δijHj , i, j = 0, 1, (9)
[E±
i , [E±
i , [E±
i , E±
j ]]] = 0, i, j = 0, 1, i 6= j. (10)
Here, the Cartan matrix (aij) of A
(1)
1 is defined by(
a00 a01
a10 a11
)
=
(
2 −2
−2 2
)
.
The Serre relations (10) hold if q0 is a primitive 2Nth root of unity, or a primitive Nth root
of unity with N odd [13]. We derive it through the higher order quantum Serre relations due
to Lusztig [26]. The Cartan relations (8) hold for generic q. The relation (9) holds for the
identification (6) when q0 is a root of unity of type I, and for the identification (7) when q0 is
a root if unity of type II.
In the sector SZ ≡ 0 (mod N) we have the commutation relation [13]:[
S+(N), S−(N)
]
= (−1)N−1qN 2
N
SZ .
Here the sign factor (−1)N−1qN is given by 1 or −1 when q is a root of unity of type I or II,
respectively. In the case of the sector SZ ≡ N/2 (mod N) with N odd and q0 a primitive Nth
root of unity, we have the following commutation relation:[
S+(N), S−(N)
]
=
2
N
SZ .
3.4 Some remarks on quantum groups at roots of unity
Let the symbol U res
q (g) denote the algebra generated by the q-divided powers of the Cheval-
ley generators of a Lie algebra g [7]. The correspondence of the algebra U res
q0
(g) at a root of
unity, q0, to the Lie algebra U(g) was obtained essentially through the machinery introduced by
Lusztig [25, 26] both for finite-dimensional simple Lie algebras and infinite-dimensional affine
Lie algebras. In fact, by using the higher order quantum Serre relations [26], it has been shown
that the affine Lie algebra U(ŝl2) is generated by the operators such as S±(N) at roots of unity.
However, in the case of the affine Lie algebras ĝ, the highest weight conditions for the Drinfeld
generators are different from those for the Chevalley generators. Through the highest weight
vectors of the Drinfeld generators, finite-dimensional representations were discussed by Chari
and Pressley for U res
q0
(ĝ) [7].
4 The Drinfeld realization of the sl2 loop algebra
Finite-dimensional representations of the sl2 loop algebra, U(L(sl2)), are derived by taking the
classical analogues of the Drinfeld realization of the quantum sl2 loop algebra, Uq(L(sl2)) [6, 7].
The classical analogues of the Drinfeld generators, x̄±k and h̄k (k ∈ Z), satisfy the defining
relations in the following:
[h̄j , x̄
±
k ] = ±2x̄±j+k, [x̄+
j , x̄−k ] = h̄j+k, for j, k ∈ Z.
Here [h̄j , h̄k] = 0 and [x̄±j , x̄±k ] = 0 for j, k ∈ Z.
6 T. Deguchi
Let us now define highest weight vectors. In a representation of U(L(sl2)), a vector Ω is called
a highest weight vector if Ω is annihilated by generators x̄+
k for all integers k and such that Ω is
a simultaneous eigenvector of every generator of the Cartan subalgebra, h̄k (k ∈ Z) [6, 7]:
x̄+
k Ω = 0, for k ∈ Z, (11)
h̄kΩ = d̄+
k Ω, h̄−kΩ = d̄−−kΩ, for k ∈ Z≥0. (12)
We call a representation of U(L(sl2)) highest weight if it is generated by a highest weight vector.
The set of the complex numbers d̄±k given in (12) is called the highest weight. It is shown [6] that
every finite-dimensional irreducible representation is highest weight. To a finite-dimensional
irreducible representation V we associate a unique polynomial through the highest weight d̄±k [6].
We call it the Drinfeld polynomial. Here the degree r is given by the weight d̄±0 .
It is easy to see that the highest weight vector of a finite-dimensional irreducible representa-
tion V is a simultaneous eigenvector of operators (x̄+
0 )k(x̄−1 )k/(k!)2 for k > 0, and the Drinfeld
polynomial of the representation V has another expression as follows [11]
P (u) =
r∑
k=0
λk(−u)k, (13)
where λk denote the eigenvalues of operators (x̄+
0 )k(x̄−1 )k/(k!)2. It is noted that the author
learned the expression of the Drinfeld polynomial (13) from Jimbo [19] (see also [14, 16]).
5 Algorithm for evaluating the degenerate multiplicity
5.1 A useful theorem on the sl2 loop algebra
Let Ω be a highest weight vector and V the representation generated by Ω. Here V is not
necessarily irreducible. Suppose that V is finite-dimensional and h̄0Ω = r Ω. We define a poly-
nomial PΩ(u) by the relation (13) with λk. We show that the roots of the polynomial PΩ(u) are
nonzero and finite, and the degree of PΩ(u) is given by r [11]. Let us factorize PΩ(u) as
PΩ(u) =
s∏
k=1
(1− aku)mk ,
where a1, a2, . . . , as are distinct, and their multiplicities are given by m1,m2, . . . ,ms, respec-
tively. Then, we call aj the evaluation parameters of Ω. Here we note that r is given by the
sum: r = m1 + · · · + ms. If all the multiplicities are given by 1, i.e. mj = 1 for j = 1, 2, . . . , s,
we say that evaluation parameters a1, a2, . . . , ar are distinct. In the case of distinct evaluation
parameters (i.e. mj = 1 for all j), we have the following theorem [8]:
Theorem 1. Every finite-dimensional highest weight representation of the sl2 loop algebra with
distinct evaluation parameters a1, a2, . . . , ar is irreducible. Furthermore, it has dimensions 2r.
Theorem 1 plays an important role in connecting the polynomial PΩ(u) with an irreducible
finite-dimensional representation. In fact, if V has distinct evaluation parameters, then it is
irreducible, and the polynomial PΩ(u) is equivalent to the Drinfeld polynomial of V . Moreover,
we rigorously obtain the dimensions of V .
Theorem 1 has been shown in [8] by using the fact that the specialized irreducible modules
for the quantum algebra are quotients of the Weyl module. Here we note that theorem 1 is
also derived by constructing explicitly a basis of a highest weight representation with distinct
evaluation parameters [12].
Degenerate Multiplicity of the sl2 Loop Algebra for the 6V Model at Roots of Unity 7
5.2 Regular Bethe states as highest weight vectors
For the XXZ spin chain at roots of unity, Fabricius and McCoy made important observations
on the highest weight conjecture [14, 15, 16]. Motivated by them, we discuss the following:
Theorem 2. (i) Every regular Bethe state |R〉 in the sector SZ ≡ 0 (modN) at q0 is a highest
weight vector of the sl2 loop algebra. Here q0 is a root of unity with q2N
0 = 1, as specified
in Definition 1. (ii) Every regular Bethe state |R〉 in the sector SZ ≡ N/2 (modN) at q0 is
a highest weight vector of the sl2 loop algebra. Here N is odd and q0 is a primitive N th root of
unity.
The Theorem 2 is proved in [11] by assuming the conjecture that for a given regular Bethe
state in the sector SZ ≡ 0 (mod N) (or SZ ≡ N/2 (mod N)) the set of solutions of the Bethe
ansatz equations are continuous with respect to the parameter q at the root of unity q0. In some
cases such as R = 0 or 1, the conjecture is trivial.
By the method of the algebraic Bethe ansatz, we derive the following relations [11]:
S+(N) |R〉 = T+(N) |R〉 = 0,(
S+(N)
)k(
T−(N)
)k
/(k!)2 |R〉 = Z+
k |R〉 for k ∈ Z≥0,(
T+(N)
)k(
S−(N)
)k
/(k!)2 |R〉 = Z−k |R〉 for k ∈ Z≥0. (14)
Here, the operators S±(N), T+(N), T−(N) and 2SZ/N satisfy the same defining relations of the
sl2 loop algebra as generators x̄±0 , x̄+
−1, x̄−1 and h̄0, respectively, and hence the relations (14)
correspond to (11) and (12).
In equations (14) eigenvalues Z±k are explicitly evaluated as follows
Z+
k = (−1)kN χ̃+
kN , Z−k = (−1)kN χ̃−kN .
Here the χ̃±m have been defined by the coefficients of the following expansion with respect to
small x:
φ(x)
F̃±(xq0)F̃±(xq−1
0 )
=
∞∑
j=0
χ̃±j xj ,
where φ(x) = (1− x)L and F̃±(x) =
R∏
j=1
(1− x exp(±2t̃j)).
5.3 Drinfeld polynomials of regular Bethe states
and the degenerate multiplicity
Let |R〉 be a regular Bethe state at q0 in one of the sectors specified in Theorem 2. The Drinfeld
polynomial of the regular Bethe state |R〉 is explicitly derived by putting λk = (−1)kN χ̃+
kN into
equation (13). Here, the coefficients χ̃±kN are explicitly evaluated as [11]
χ̃±kN =
min(L,kN)∑
n=0
(−1)n
(
L
n
) ∑
n1+···+nR=kN−n
e
±
R∑
j=1
2nj t̃j
R∏
j=1
[nj + 1]q0 . (15)
Here [n]q = (qn−q−n)/(q−q−1) and the sum is taken over all nonnegative integers n1, n2, . . . , nR
satisfying n1 + · · ·+ nR = kN − n: when R = 0, n is given by n = kN .
For the case of distinct evaluation parameters, we obtain the algorithm for the degeneracy of
a regular Bethe state as follows.
Corollary 1. Let |R〉 be a regular Bethe state such as specified in Theorem 2. If the Drin-
feld polynomial of the representation V generated by |R〉 gives evaluation parameters aj with
multiplicities mj = 1 for j = 1, 2, . . . , r, then we have dim V = 2r, where r = (L− 2R)/N .
8 T. Deguchi
6 Examples of Drinfeld polynomials of Bethe states
6.1 The vacuum state with even L
We now calculate the Drinfeld polynomial P (u) for the the vacuum state |0〉 where L = 6 and
N = 3 with q3
0 = 1. When N is odd and qN
0 = 1, we have λ+
k = Z+
k = (−1)kχ̃+
kN . From the
formula (15) we have
χ̃+
3 = (−1)3
(
6
3
)
= −20, χ̃+
6 = (−1)6
(
6
6
)
= 1.
Thus we have λ+
1 = 20 and λ+
2 = 1, and the Drinfeld polynomial is given by
P (u) = 1− 20u + u2.
Here, the evaluation parameters are given by
a1, a2 = 10± 3
√
11.
We note that the two evaluation parameters are distinct, and the degree of P is two, i.e. r = 2,
m1 = m2 = 1. Therefore, the degenerate multiplicity is given by 22 = 4.
6.2 The vacuum state with odd L
Let us calculate the Drinfeld polynomial P (u) for the odd L case. We consider the the vacuum
state |0〉 where L = 9 and N = 3 with q3
0 = 1. The vacuum state |0〉 is in the sector SZ ≡ 3/2
(mod 3), since SZ = 9/2 = 3/2 + 3.
From the formula (15) we have
χ̃+
3 = (−1)3
(
9
3
)
= −84, χ̃+
6 = (−1)6
(
9
6
)
= 84, χ̃+
9 = (−1)9
(
9
9
)
= −1.
Thus we have λ+
1 = 84, λ+
2 = 84, and λ+
3 = 1. The Drinfeld polynomial is given by
P (u) = 1− 84u + 84u2 − u3.
Here, the evaluation parameters are given by
a1, a2 =
1
2
(
83± 9
√
85
)
.
We note that the three evaluation parameters are distinct, and the degree of P is three, i.e.
r = 3, m1 = m2 = m3 = 1. Therefore, the degenerate multiplicity is given by 23 = 8.
6.3 The regular Bethe state with one down-spin (R = 1)
For the case of R = 1, the Bethe ansatz equations at generic q are given by(
sinh(tj + η)
sinh(tj − η)
)L
= 1 for j = 0, 1, . . . , L− 1.
We solve the Bethe ansatz equations in terms of variable exp(2tj) as follows
exp(2tj) =
1− ωjq
q − ωj
for j = 0, 1, . . . , L− 1, (16)
where ωj denotes an Lth root of unity: ωj = exp
(
2π
√
−1j/L
)
, for j = 0, 1, . . . , L− 1.
Degenerate Multiplicity of the sl2 Loop Algebra for the 6V Model at Roots of Unity 9
Let us assume that the regular Bethe state with one down-spin is in the sector SZ ≡ 0
(modN) and q0 be a root of unity with q2N
0 = 1, or in the sector SZ ≡ N/2 (modN) where q0
is a primitive Nth root of unity with N odd. Then we have from (15)
χ̃+
kN =
min(nN+2,kN)∑
`=0
(−1)j [kN + 1− `]q0
(
nN + 2
`
) (
1− ωjq0
q0 − ωj
)kN−`
.
Here we have from (4) that L = nN + 2 when R = 1.
Let us consider the case of N = 3, and q0 = exp(±2π
√
−1/3). Here L = 8. The regular
Bethe state with rapidity t̃2 (the case of j = 2 in equation (16)) has the Drinfeld polynomial in
the following:
P (u) = 1− 13
(
2−
√
3
)
u +
(
7− 4
√
3
)
u2,
where the evaluation parameters a1 and a2 are given by
a1, a2 =
1
2
(
13±
√
165
) (
2−
√
3
)
.
The dimensions of the highest weight representation generated by the Bethe state are therefore
given by 22 = 4.
Acknowledgements
The author would like to thank K. Fabricius and B.M. McCoy for useful discussions. He would
also like to thank the organizers for their kind invitation to the Sixth International Conference
“Symmetry in Nonlinear Mathematical Physics”, June 20–26, 2005, Institute of Mathematics,
Kyiv, Ukraine. He is grateful to many participants of the conference for useful comments and
discussions. This work is partially supported by the Grant-in-Aid (No. 17540351).
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1 Introduction
2 Bethe ansatz equations and the transfer matrix
2.1 Regular solutions of the Bethe ansatz equations
2.2 Transfer matrix of the six-vertex model
3 The sl2 loop algebra symmetry at roots of unity
3.1 Roots of unity conditions
3.2 (Anti-)commutation relations at roots of unity
3.3 The algebra generated by S(N) and T(N)
3.4 Some remarks on quantum groups at roots of unity
4 The Drinfeld realization of the sl2 loop algebra
5 Algorithm for evaluating the degenerate multiplicity
5.1 A useful theorem on the sl2 loop algebra
5.2 Regular Bethe states as highest weight vectors
5.3 Drinfeld polynomials of regular Bethe states and the degenerate multiplicity
6 Examples of Drinfeld polynomials of Bethe states
6.1 The vacuum state with even L
6.2 The vacuum state with odd L
6.3 The regular Bethe state with one down-spin (R=1)
|