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Symmetry 2014, 6(1), 103-110; doi:10.3390/sym6010103
Published: 17 March 2014
Abstract: We consider a (2 + 1)-dimensional massless Dirac equation in the presence of complex vector potentials. It is shown that such vector potentials (leading to complex magnetic fields) can produce bound states, and the Dirac Hamiltonians are η-pseudo Hermitian. Some examples have been explicitly worked out.
In recent years, the massless Dirac equation in (2 + 1) dimensions has drawn a lot of attention, primarily because of its similarity to the equation governing the motion of charge carriers in graphene [1,2]. In view of the fact that electrostatic fields alone cannot provide confinement of the electrons, there have been quite a number of works on exact solutions of the relevant Dirac equation with different magnetic field configurations, for example, square well magnetic barriers [3–5], non-zero magnetic fields in dots , decaying magnetic fields , solvable magnetic field configurations , etc. On the other hand, at the same time, there have been some investigations into the possible role of non-Hermiticity and symmetry  in graphene [10–12], optical analogues of relativistic quantum mechanics  and relativistic non-Hermitian quantum mechanics , photonic honeycomb lattice , etc. Furthermore, the (2 + 1)-dimensional Dirac equation with non-Hermitian Rashba and scalar interaction was studied . Here, our objective is to widen the scope of incorporating non-Hermitian interactions in the (2 + 1)-dimensional Dirac equation. We shall introduce η pseudo Hermitian interactions by using imaginary vector potentials. It may be noted that imaginary vector potentials have been studied previously in connection with the localization/delocalization problem [17,18], as well as phase transition in higher dimensions . Furthermore, in the case of the Dirac equation, there are the possibilities of transforming real electric fields to complex magnetic fields and vice versa by the application of a complex Lorentz boost . To be more specific, we shall consider η-pseudo Hermitian interactions  within the framework of the (2 + 1)-dimensional massless Dirac equation. In particular, we shall examine the exact bound state solutions in the presence of imaginary magnetic fields arising out of imaginary vector potentials. We shall also obtain the η operator, and it will be shown that the Dirac Hamiltonians are η-pseudo Hermitian.
2. The Model
The (2 + 1)-dimensional massless Dirac equation is given by:
Then, from (3), the equations for the components are found to be (in units of ℏ = 1):
2.1. Complex Decaying Magnetic Field
It is now necessary to choose the function, f(x). Our first choice for this function is:
Now, from the second of Equation (7), we obtain:
Let us now examine the upper component, ϕ1. Since ϕ2 is known, one can always use the intertwining relation:
Therefore, we find that it is indeed possible to create bound states with an imaginary vector potential. We shall now demonstrate the above results for a second example.
2.2. Complex Hyperbolic Magnetic Field
Here, we choose f(x), which leads to an effective potential of the complex hyperbolic Rosen–Morse type:
3. η-Pseudo Hermiticity
Let us recall that a Hamiltonian is η-pseudo Hermitian if :
We recall that in both the cases considered here, the Hamiltonian is of the form:
Here, we have studied the (2 + 1)-dimensional massless Dirac equation (we note that if a massive particle of mass m is considered, the energy spectrum in the first example would become . Similar changes will occur in the second example, too). in the presence of complex magnetic fields, and it has been shown that such magnetic fields can create bound states. It has also been shown that Dirac Hamiltonians in the presence of such magnetic fields are η-pseudo Hermitian. We feel it would be of interest to study the generation of bound states using other types of magnetic fields, e.g., periodic magnetic fields.
One of us (P. R.) wishes to thank INFN Sezione di Perugia for supporting a visit during which part of this work was carried out. He would also like to thank the Physics Department of the University of Perugia for its hospitality.
Conflicts of Interest
The authors declare no conflict of interest.
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