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## Dynamics of generalized PT-symmetric dimers with time periodic gain-loss

F. Battelli, J. Diblik, M. Feckan, J. Pickton, M. Pospisil, H. Susanto

A Parity-Time (PT)-symmetric system with periodically varying-in-time gain and loss modeled by two coupled Schrodinger equations (dimer) is studied. It is shown that the problem can be reduced to a perturbed pendulum-like equation. This is done by finding two constants of motion. Firstly, a generalized problem using Melnikov type analysis and topological degree arguments is studied for showing the existence of periodic (libration), shift periodic (rotation), and chaotic solutions. Then these general results are applied to the PT-symmetric dimer. It is interestingly shown that if a sufficient condition is satisfied, then rotation modes, which do not exist in the dimer with constant gain-loss, will persist. An approximate threshold for PT-broken phase corresponding to the disappearance of bounded solutions is also presented. Numerical study is presented accompanying the analytical results.

http://arxiv.org/abs/1412.0164
Pattern Formation and Solitons (nlin.PS); Quantum Gases (cond-mat.quant-gas); Classical Analysis and ODEs (math.CA); Optics (physics.optics)

## PT-Symmetric Optomechanically-Induced Transparency

H. Jing, Z. Geng, S. K. Özdemir, J. Zhang, X.-Y. Lü, B. Peng, L. Yang, F. Nori

Optomechanically-induced transparency (OMIT) and the associated slow-light propagation provide the basis for storing photons in nanofabricated phononic devices. Here we study OMIT in parity-time (PT)-symmetric microresonators with a tunable gain-to-loss ratio. This system features a reversed, non-amplifying transparency: inverted-OMIT. When the gain-to-loss ratio is steered, the system exhibits a transition from the PT-symmetric phase to the broken-PT-symmetric phase. We show that by tuning the pump power at fixed gain-to-loss ratio or the gain-to-loss ratio at fixed pump power, one can switch from slow to fast light and vice versa. Moreover, the presence of PT-phase transition results in the reversal of the pump and gain dependence of transmission rates. These features provide new tools for controlling light propagation using optomechanical devices.

http://arxiv.org/abs/1411.7115
Quantum Physics (quant-ph); Optics (physics.optics)

## Singular Mapping for a PT-Symmetric Sinusoidal Optical Lattice at the Symmetry-Breaking Threshold

H. F. Jones

A popular PT-symmetric optical potential (variation of the refractive index) that supports a variety of interesting and unusual phenomena is the imaginary exponential, the limiting case of the potential $$V_0[\cos(2\pi x/a)+i\lambda\sin(2\pi x/a)]$$ as $$\lambda\to1$$, the symmetry-breaking point. For $$\lambda<1$$, when the spectrum is entirely real, there is a well-known mapping by a similarity transformation to an equivalent Hermitian potential. However, as $$\lambda\to1$$, the spectrum, while remaining real, contains Jordan blocks in which eigenvalues and the corresponding eigenfunctions coincide. In this limit the similarity transformation becomes singular. Nonetheless, we show that the mapping from the original potential to its Hermitian counterpart can still be implemented; however, the inverse mapping breaks down. We also illuminate the role of Jordan associated functions in the original problem, showing that they map onto eigenfunctions in the associated Hermitian problem.

http://arxiv.org/abs/1411.6451
Optics (physics.optics); Mathematical Physics (math-ph); Quantum Physics (quant-ph)

## Kink scattering from a parity-time-symmetric defect in the $$\phi^4$$ model

Danial Saadatmand, Sergey V. Dmitriev, Denis I. Borisov, Panayotis G. Kevrekidis, Minnekhan A. Fatykhov, Kurosh Javidan

In this paper, we study the $$\phi^4$$ kink scattering from a spatially localized PT-symmetric defect and the effect of the kink’s internal mode (IM) is discussed. It is demonstrated that if a kink hits the defect from the gain side, a noticeable IM is excited, while for the kink coming from the opposite direction the mode excitation is much weaker. This asymmetry is a principal finding of the present work. Similar to the case of the sine-Gordon kink studied earlier, it is found that the $$\\phi^4$$ kink approaching the defect from the gain side always passes through the defect, while in the opposite case it must have sufficiently large initial velocity, otherwise it is trapped by the loss region. It is found that for the kink with IM the critical velocity is smaller, meaning that the kink bearing IM can pass more easily through the loss region. This feature, namely the “increased transparency” of the defect as regards the motion of the kink in the presence of IM is the second key finding of the present work. A two degree of freedom collective variable model offered recently by one of the co-authors is shown to be capable of reproducing both principal findings of the present work. A simpler, analytically tractable single degree of freedom collective variable method is used to calculate analytically the kink phase shift and the kink critical velocity sufficient to pass through the defect. Comparison with the numerical results suggests that the collective variable method is able to predict these parameters with a high accuracy.

http://arxiv.org/abs/1411.5857
Pattern Formation and Solitons (nlin.PS)

## $$E_2$$-quasi-exact solvability for non-Hermitian models

Andreas Fring

We propose the notion of $$E_2$$-quasi-exact solvability and apply this idea to find explicit solutions to the eigenvalue problem for a non-Hermitian Hamiltonian system depending on two parameters. The model considered reduces to the complex Mathieu Hamiltonian in a double scaling limit, which enables us to compute the exceptional points in the energy spectrum of the latter as a limiting process of the zeros for some algebraic equations. The coefficient functions in the quasi-exact eigenfunctions are univariate polynomials in the energy obeying a three-term recurrence relation. The latter property guarantees the existence of a linear functional such that the polynomials become orthogonal. The polynomials are shown to factorize for all levels above the quantization condition leading to vanishing norms rendering them to be weakly orthogonal. In two concrete examples we compute the explicit expressions for the Stieltjes measure.

http://arxiv.org/abs/1411.4300
Quantum Physics (quant-ph); Mathematical Physics (math-ph)

## Quantum star-graph analogues of PT-symmetric square wells. II: Spectra

Miloslav Znojil

For non-Hermitian equilateral q-pointed star-shaped quantum graphs of paper I [Can. J. Phys. 90, 1287 (2012), arXiv 1205.5211] we show that due to certain dynamical aspects of the model as controlled by the external, rotation-symmetric complex Robin boundary conditions, the spectrum is obtainable in a closed asymptotic-expansion form, in principle at least. Explicit formulae up to the second order are derived for illustration, and a few comments on their consequences are added.

http://arxiv.org/abs/1411.3828
Quantum Physics (quant-ph); Spectral Theory (math.SP)

## Discrete solitons and vortices on two-dimensional lattices of PT-symmetric couplers

Zhaopin Chen, Jingfeng Liu, Shenhe Fu, Yongyao Li, Boris A. Malomed

We introduce a 2D network built of PT-symmetric dimers with on-site cubic nonlinearity, the gain and loss elements of the dimers being linked by parallel square-shaped lattices. The system may be realized as a set of PT-symmetric dual-core waveguides embedded into a photonic crystal. The system supports PT-symmetric and antisymmetric fundamental solitons (FSs) and on-site-centered solitary vortices (OnVs). Stability of these discrete solitons is the central topic of the consideration. Their stability regions in the underlying parameter space are identified through the computation of stability eigenvalues, and verified by direct simulations. Symmetric FSs represent the system’s ground state, being stable at lowest values of the power, while anti-symmetric FSs and OnVs are stable at higher powers. Symmetric OnVs, which are also stable at lower powers, are remarkably robust modes: on the contrary to other PT-symmetric states, unstable OnVs do not blow up, but spontaneously rebuild themselves into stable FSs.

http://arxiv.org/abs/1411.3943
Optics (physics.optics); Pattern Formation and Solitons (nlin.PS)

## Pseudo PT-symmetric lattice

C. Yuce

We study pseudo PT symmetry for a tight binding lattice with a general form of the modulation. Using high-frequency Floquet method, we show that the critical non-Hermitian degree for the reality of the spectrum can be manipulated by varying the parameters of the modulation. We study the effect of periodical and quasi-periodical nature of the modulation on the pseudo PT symmetry.

http://arxiv.org/abs/1411.3459
Quantum Physics (quant-ph); Mathematical Physics (math-ph)

## Real discrete spectrum in complex non-PT-symmetric Scarf II potential

Zafar Ahmed, Amal Nathan Joseph

Hitherto, it is well known that complex PT-symmetric Scarf II has real discrete spectrum in the parametric domain of unbroken PT-symmetry. We reveal new interesting complex, non-PT-symmetric parametric domains of this versatile potential, $$V(x)$$, where the spectrum is again discrete and real. Showing that the Hamiltonian, $$p^2/2m+V(x)$$, is pseudo-Hermitian could be challenging, if possible.

http://arxiv.org/abs/1411.3231
Quantum Physics (quant-ph); Mathematical Physics (math-ph)

## Global existence of solutions to coupled PT-symmetric nonlinear Schrödinger equations

Dmitry E. Pelinovsky, Dmitry A. Zezyulin, Vladimir V. Konotop

We study a system of two coupled nonlinear Schrodinger equations, where one equation includes gain and the other one includes losses. Strengths of the gain and the loss are equal, i.e., the resulting system is parity-time (PT) symmetric. The model includes both linear and nonlinear couplings, such that when all nonlinear coefficients are equal, the system represents the PT-generalization of the Manakov model. In the one-dimensional case, we prove the existence of a global solution to the Cauchy problem in energy space $$H_1$$, such that the $$H_1$$-norm of the global solution may grow in time. In the Manakov case, we show analytically that the $$L_2$$-norm of the global solution is bounded for all times and numerically that the $$H_1$$-norm is also bounded. In the two-dimensional case, we obtain a constraint on the $$L_2$$-norm of the initial data that ensures the existence of a global solution in the energy space $$H_1$$.

http://arxiv.org/abs/1411.2895
Analysis of PDEs (math.AP)