Research Article | Open Access

B. Abdesselam, A. Chakrabarti, "Eigenstates and Eigenvalues of Chain Hamiltonians Based on Multiparameter Braid Matrices for All Dimensions", *International Scholarly Research Notices*, vol. 2013, Article ID 410859, 11 pages, 2013. https://doi.org/10.1155/2013/410859

# Eigenstates and Eigenvalues of Chain Hamiltonians Based on Multiparameter Braid Matrices for All Dimensions

**Academic Editor:**U. Kulshreshtha

#### Abstract

We study chain Hamiltonians derived from a class
of multidimensional, multiparameter braid matrices introduced and
explored in a series of previous papers. The *N*^{2} × *N*^{2} braid
matrices (for all *N*) have free parameters for even
*N* and for *N* odd. We present
systematic explicit constructions for eigenstates and eigenvalues
of chain Hamiltonians for and all chain lengths *r*. We
derive explicitly the constraints imposed on these states by
periodic (circular) boundary conditions. Our results thus cover
both open and closed chains. We then indicate how our formalism
can be extended for all . The dependence of the
eigenvalues on the free parameters is displayed explicitly,
showing how the energy levels and their differences vary in a
particular simple way with these parameters. Some perspectives
are discussed in conclusion.

#### 1. Introduction

In a series of previous paper [1–4], we have formulated and studied a class of braid matrices () with free parameters whose numbers increase as . Chain Hamiltonians corresponding to these matrices were also presented. Here, we undertake systematic study of chain Hamiltonians derived from these braid matrices presenting iterative and explicit constructions of eigenstates and eigenvalues for all dimensions and for all orders (chain lengths) .

We recapitulate briefly the constructions of the braid matrices satisfying (in standard notations used in [1–4])

Our class has a nested sequence of projectors as a basis which are defined as follows. For even (, ), where , , , , . Interchanging on the right, one obtains with (We use the notation () for a matrix with only one nonzero element, unity, on row and column .) On such an orthonormal complete basis of projectors, one defines (with domains defined below (2))

The crucial constraints on the free parameters give an braid matrix with free parameters. For odd (; ), using our definitions below (2), The braid matrix is now (, , , , ). The number of free parameters is now (An overall factor and redefinitions of the ’s, , etc. convert our previous convention for odd to the present one, which is more convenient for what follows.)

Defining the Hamiltonian acting on a chain of sites () is given by the standard where acts on the sites . For circular boundary conditions (or periodic), The Hamiltonians inherit the free parameters (see (6), (9)) of . This is the most striking features of our construction. The eigenvalues will be seen to depend linearly on these parameters (given by simple sums of the ’s). So, by varying them, one can vary the differences between the energy levels. We will first present, in the following sections, explicit results for the simplest cases. Then, an iterative formalism will be implemented to generalize them. Also, we will start with open chains and subsequently impose the constraints (12) for periodicity.

#### 2.

The braid matrix [3] is where Hence, where The chains Hamiltonian for sites are (adapting the notation of (11)). For circular boundary conditions (CBC), The base spaces (for different ’s) will be spanned by the tensor products of the fundamental states vectors Using the notations of our previous papers, and so on.

We now increase stepwise as follows.

##### 2.1.

Here,

From (15), (19), and (20) (and with ), The key to generalization is that, for the same , these two eigenstates have the same eigenvalue. This degeneracy opens the road to a simple iteration.

##### 2.2.

Now, Define In (23), acts on the first two factors of and acts on the last two. Hence, form (22) and (25), and from (26), Summing Since each can have values independently, we obtain a complete basis (for ) of eigenstates with the spectrum of eigenvalues with respective degeneracies (for the base space of 8 dimensions) The generalization is now evident.

##### 2.3. General Case

Iterating factors as in (24) one can define Now, . In this sum, acts on the factors giving Summing the full spectrum is given by The respective multiplicities are given by the coefficients of the expansion This provides the complete basis of eigenstates spanning the base space of dimension . Thus, we arrive (by iterating factors as in (32) at the complete solution for a chain of arbitrary length ( sites for any ). We have obtained explicitly all eigenstates and the respective eigenvalues.

##### 2.4. Cyclic Boundary Conditions (CBC)

So far, we have been dealing with open chains. Now, we impose periodic (cyclic) boundary conditions, namely, (18). We start again with the simplest cases which again indicate the general solution.

###### 2.4.1.

From (24), The part (P) satisfies the constraint (12) and the part (NP) does not. The part (NP) is eliminated in the following combination: The condition and the sum imposed above leave only two eigenstates with the common eigenvalue It will be shown below that for all the extreme eigenvalues are eliminated under (CBC) and the dimension of the base space is divided by a factor 4 (for ).

###### 2.4.2. General Case (All )

From (32), the (P) and (NP) parts are Thus, the direct generalization of (39) is which provides the eigenstates satisfying (CBC) with the spectrum of eigenvalues

#### 3.

We continue to study even dimensions. For , we had to deal with one pair of indices (, of Section 2). For , the states involve tensor products of . For , the basis involving will be studied using notations that facilitate the passage to all . For states involving only or , the results will be entirely analogous to those of Section 2 . But now, one has to include states involving both pairs. Changing the notations of [3] for convenience, we write (for ) the Hamiltonian as where As compared to of Section 2, there are now 8 independent parameters.

The fundamental basis states are (as compared to (19)) The tensor products, using our standard notations, are denoted as where, independently, , , and correspondingly, and similarly for .

The crucial constraint (always to be kept in mind) is incorporated in the blocks of (45) through (45), (47). We will write for and so on. A fundamental consequence is for and similarly for . Hence, where and assume the values independently. Hence, due to (50), also

##### 3.1.

Define exactly as in (25), (26) but with the domain for each index . (Changing any to amounts to the same with the corresponding as factor.) Consider

Hence, (analogously to (27)–(29)) The difference from (29) is that here the eigenvalues (taking (50) into account) have the domain For each case, independently, Each entry in (57) provides 8 possibilities (including multiplicities, easily counted) and thus in total spans the dimensional base space for .

##### 3.2. General Case

We generalized (24) to (55) in passing from to . Similarly, for all , we generalize (32) to with the domain for each . Now, and the evident generalization of (35) is Each term can contribute for the full range of the 's and the 's. But for a given value of in , one must have the same in and so on. This introduces constraints. Thus, for , for fixed 's, the 's provide possibilities (including multiplicities) and for each set of 's, the 's (each being or ) furnish possibilities. These together indeed span the base space of dimensions.

##### 3.3. CBC

Circular boundary conditions can be imposed now. As compared to the case , along with strong analogies, there are crucial differences. One evident difference is the larger domains of the indices. The constraints leave, for , the possibilities Now, one can have For , one can have But there is a somewhat more subtle difference. The fundamental constraint (5) and the symmetries of the projectors (see (3)) lead to for our free parameters. But we do not indent to impose, diminishing number of free parameters, which are an essential feature of our formalism, additional restrictions as , and so on. Let us explore the consequences for (CBC) for .

###### 3.3.1.

From (53), (54), one obtains periodic states by starting with as But for , these are not eigenstates of if . For only, one obtains eigenvalues

###### 3.3.2.

Displaying explicitly the indices in the generalizations of (69), (70), the periodic states are They are eigenstates only under constraints (independent of each other). The first and the second terms of (72) or (3.30) contribute to differing eigenvalues by which vanishes for (73) (along with (66)). Hence the results. For , there is one constraint less. The restrictions on the eigenvalues and on the dimensions of the base spaces can now be obtained in a straightforward fashion. Along with, as in Section 2, , there are now (71) and (73).

We have preferred to study the case (rather than ) directly after , since all even cases can be treated coherently together while all odd ones (, ) have features in common which are absent for . The prototype is provided by . We study this case in some detail in the following section. Then, we will only try to understand generalizations necessary for arbitrary dimensions .

#### 4.

The definition below (2), (), gives, for , While, for all other indices, . This is the central fact inducing special properties for odd (for even no ). We now study such features in some detail for . The braid matrices are (in notations analogous to (45)) where where As compared to our conventions for odd in previous papers [1, 3, 5], the above one differs by the introduction of an overall factor and redefinitions (for ) . This is convenient for our present purposes. Now, where (Compare this to (47), (48).) We have now 7 free parameters.

The chain Hamiltonian is now ( being a unit matrix) Again for periodicity for , the generalizations for are straight forward.

##### 4.1. Eigenstates and Eigenvalues

The base space is now spanned by tensor products of the fundamental states The eigenstates and eigenvalues of are (with ) Comparing this with (53), one notices how the 16-dimensional basis for is reduced (“contracted”) to a 9-dimensional one for the identification (for ) We will come back to this point.

###### 4.1.1.

We continue to elucidate the role of (84) by treating this case in detail. Here, For states involving only , one proceeds exactly as for (see (24)–(32)). One thus recovers formally the eight eigenstates not involving . But now, one also has the following possibilities in the iterative structure of the eigenstates: The total action of is evident form the left and the right factorizations displayed. The eigenvalues are, respectively, Counting the possible values of the 's, these provide eigenstates. Along with 8 states involving only , one thus obtains the full basis of 27 dimensions for (three sites with three possibilities for each). It is again instructive to note how the corresponding results for (54)–(56) reduce to those for under (84).

###### 4.1.2.

Before passing to the general case, it is helpful to display the role of the state for . Consider the state Now, , when are all on proceeds as in the foregoing sections. For , respectively, 2, the state reduces (due to ) to (with each ) The factors (2 for for each case) can be ignored. The eigenvalues are in order For multiple Index 2, one can have The corresponding states, eigenvalues, and multiplicities are easily written down factorizing out more and more 's. For example, for , one has with eigenvalue One ends with

###### 4.1.3.

The situation is analogous for all , with ever increasing possibilities and subcases as increases. We do not intend to track down such proliferations systematically. Let us, however, briefly indicate some essential features. Now, . One, correspondingly, starts with base states When each is , being absent, one proceeds as in Section 2. The multiplicity of can now be (when present) The number of possibilities (generalizing results as (92)) for the subsets is, respectively, One ends with

##### 4.2. CBC

The Index 2 again plays a special role concerning CBC for . For states involving only , one proceeds exactly as for (Section 2). But now leads to additional possibilities as shown below.

###### 4.2.1.

Apart from one now has also

###### 4.2.2.

Apart from periodic states (corresponding to (39)) one now has also