Advances in Mathematical Physics

Volume 2015, Article ID 304093, 5 pages

http://dx.doi.org/10.1155/2015/304093

## A Novel Approach to Solve Quasiexactly Solvable Pauli Equation

Department of Engineering Physics, Gaziantep University, 27310 Gaziantep, Turkey

Received 9 October 2014; Accepted 6 December 2014

Academic Editor: Nikos Mastorakis

Copyright © 2015 Ramazan Koç et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

The spectra for some specific forms of external magnetic fields in Pauli equation are obtained in the framework of the asymptotic iteration method (AIM). AIM is applied to find the solution of Pauli equation. When the method is applied to quasiexactly solvable systems, it not only easily gives the corresponding spectrum, but also produces accurate results for the eigenvalues of the system having *sl*(2) symmetry.

#### 1. Introduction

It is well known that motion of the charged particles in electromagnetic fields described by Pauli equation is one of the fascinating phenomena in modern physics, providing a general approach to understanding the properties of electrons. The Hamiltonian for this system when is where , are the momentum operators, is the gyromagnetic ratio, and and are the vector potential of the electromagnetic field and the Pauli matrix, respectively. The Pauli equation has been solved exactly for uniform magnetic field , , leading the Landau levels. Moreover, it was proved in [1] that for any general magnetic field perpendicular to the plane, the ground state is solved exactly as determined in [2–4]. The general result of [1] can be viewed as a very special case of the recently discovered QES systems that only a finite spectrum to be solved with algebraic methods [5–9]. The Pauli equation has been solved in a quasiexactly solvable model [10] for possible (nonuniform) magnetic fields [11]. If not impossible, it seems difficult to find other forms of the magnetic field such that the Pauli equation could be exactly solved. Therefore, the forms of magnetic fields, for which Pauli equation could be exactly solved, are indeed rather rare.

Therefore, a natural question arising at this point is can the asymptotic iteration method be applied to these systems? The answer to this question is the main topic of this paper. An asymptotic iteration technique which improves both analytical and numerical solutions of the problems has been introduced by Çiftçi et al. [12]. Afterwards, it has been developed for many physical systems in order to obtain the whole spectra [13, 14]. It has been also suggested to solve the Schrödinger equation and developed for some quantum optical systems [15]. Later, a considerable attention has been paid to AIM [12, 16–19]. The results obtained by this method fit with the solutions of many exactly solvable differential equations in the form of the Schrödinger equation. It also gives accurate result for the nonexactly solvable Schrödinger equation such as sextic oscillator, cubic oscillator, and deformed coulomb potential, which are important in applications to many problems in physics. Although AIM has been applied to solve the Schrödinger equation, its application to the solution of the Pauli equation is still missing. It is well known that the AIM is very powerful in describing many physical problems. It improves both analytical and numerical solutions and also it is useful for understanding the nature of physical structures.

One can show that the termination relation in the corresponding method reflects effectively the peculiar properties of the functions and . After appropriate change of the variable, their hidden polynomial nature can be revealed. On these polynomials of finite order, the finite-dimensional representations of the hidden algebraic structure underlying the quasiexact solvability are realized. Also with this hidden polynomial structure, the nonlinear supersymmetry discussed in [10] can be related.

#### 2. Formalism of the Asymptotic Iteration Method

The AIM is proposed to solve the second-order differential equations and details can be found in [12]. In this section we systematically apply the method to obtain eigenvalues of the Pauli equation for various nonuniform magnetic fields.

Consider the following second-order differential equation: where and , are functions defined in and they have sufficiently many continuous derivatives. and denote derivatives of with respect to . It is easy to show that th derivative of the function can be written as where and are given by the recurrence relations If we have, for sufficiently large , then the solution of (2) can be written as follows: where and are the integration constants. In calculating the parameters in (4), when , we take the initial conditions as and [14] and the termination condition is for

Before going further we note that, in this paper, we will determine magnetic fields that lead to QES solvable potential forms in Pauli equation. We consider the magnetic fields only in asymmetric gauge. Solution of the Pauli equation including nonuniform magnetic fields has recently been studied in the framework of the QES systems [10, 11]. Ho and Roy presented a general procedure for determining possible (nonuniform) magnetic fields such that the Pauli equation becomes QES with an underlying symmetry. But our paper includes a part of the motivation those QES systems which can be solved by using AIM and it gives the whole spectrum with accurate results.

The following section includes the solution of Pauli Equation within the framework of the AIM.

#### 3. Magnetic Field

In this paper, we deal with magnetic field in asymmetric gauge with vector potentials as in [11] In this case components of the magnetic field with asymmetric gauge are given by where is an arbitrary function of and . The Pauli Hamiltonian then takes the form The variables and can be separated by introducing the wavefunction Here () are the eigenvalues of , and is a two-component function of . Then the Hamiltonian, , can be written as

It is obvious that the Hamiltonian has been written in the one-dimensional form of the Schrödinger equation with the potential. This final form is the SUSY form of the Pauli equation with the superpotential .

Now, our aim is to determine the magnetic fields that lead to the QES potential forms. By defining the magnetic fields in appropriate forms that construct QES potentials, we will try to apply AIM to the resultant Hamiltonian.

##### 3.1. Class I

In order to obtain the Class I potential, the corresponding magnetic field can be defined as In this case we can write the following eigenvalue problem form (12) for part that satisfies the Schrödinger equation under the normalizable zero energy state condition where is the Class I potential according to Turbiner’s classification in [6]. After redefining the variable as and using in (14), we get the following differential equation: where , , , , and . Before applying AIM to (16), we have to obtain the asymptotic form for . It is easy to see that when and , , and after substituting into (16), we obtain where , , and . Now, we can apply AIM to this final equation with and and we will show that (17) has exact solutions under some constraints on the parameters , , , and .

If we apply AIM quantization condition, that is, from (4) by substituting and , in order to obtain polynomial solutions we see that the parameters must be equal to , and values are given in Table 1; we will show the results for type 1 and for simplicity. Note that there are two types of solutions that give us the polynomials, for example, for , (as given above), and . If we use the first value of we get the results given in Table 1, but one can use the second value of and get different results which give us the polynomials. We will not give this second situation here because of complication on the calculations. In this paper, we use only type solution. If we use , , , , and , we have and . If we go back to definitions of , , , and , we find the following expression for the energy as follows: If the parameters in (18) are mapped to the parameters in energy values in [10, 11], the energy values can be in accord. In Table 1, we will show the results for type 1 and for simplicity. Similarly, in Table 2, we will show the results of different values of for for type 2 solution.