# Eigenvalues of a non-Hermitian complex periodic potential

I have an eigenvalue problem:

$$-\frac{d^2}{dx^2} \psi(x) +V(x)\psi(x) = E \psi(x)$$

where $$V(x)$$ is a complex periodic potential: $$V(x) = 4[\cos^2(x) + i 0.3 \sin(2x)]$$

It has been claimed that the eigenvalues of this problem are all real (this is always the case if the coefficient of $$\sin$$ is less than $$0.5$$, which in this case is $$0.3$$), although it's not Hermitian.

To verfy this, I have used the code provided here by @Jens periodic boundary conditions and NDEigensystem. But, Mathematica doesn't return anything.

The code in the above link is (with an additional 1/2 for the kinetic term):

spectrum[n_, dim_: 10000][potential_, {var_, varMin_, varMax_},  kBloch_: 0] :=
Module[
{e, v, vRange, dx, grid, potentialGrid, eKin, ePot, min, interpolate},
vRange = varMax - varMin;
interpolate =
ListInterpolation[Append[#, First[#]], {{varMin, varMax}},
PeriodicInterpolation -> True] &;
dx = N[vRange/dim];
grid = Range[varMin, varMax, dx];
eKin = -(1/2)
NDSolveFiniteDifferenceDerivative[2, grid,
PeriodicInterpolation -> True]["DifferentiationMatrix"] -
I kBloch NDSolveFiniteDifferenceDerivative[1, grid,
PeriodicInterpolation -> True]["DifferentiationMatrix"];
potentialGrid = Table[potential + kBloch^2/2, {var, Most[grid]}];
(* eKin is periodically interpolated,
so its last element is internally dropped by  FiniteDifferenceDerivative, as redundant. Therefore,
I also have to drop the last grid element in potentialGrid. *)
min = Min[potentialGrid];
(* Matrix for the potential is shifted so its minimum entry is  zero, guaranteeing that eigenvalues will be sorted in descending order: *)
ePot = DiagonalMatrix[SparseArray[potentialGrid - min]];
{e, v} = Eigensystem[eKin + ePot, -n];
(* Final step: turn vectors on spatial grid back into functions of x by interpolation: *)
Append[
Reverse /@ {e + min, Map[interpolate[#/Max[Abs[#]]] &, v]},
(* In the eigenvalues,
potential offset min was added back to get original energy scale. *)
interpolate[potentialGrid]
]]


And the potential is defined (with an extra 1/2):

potential[x_] = 1/2 (4((Cos[x])^2 + I 0.3 Sin[2x]));
{eigenvals, eigenvecs, pot} = spectrum[potential[x], {x, -Pi/2, Pi/2}];
2 eigenvals

• Welcome to Mathematica Stack Exchange. It would be good if you could provide the code that you used. You can use functions from the Q&A you linked to, but still you ought to provide the code that is specific to this question. – C. E. Dec 2 '18 at 2:06
• Thanks for your comment. I will add the code. – user61688 Dec 2 '18 at 2:07
• Have you tried NDEigensystem for this? – user21 Dec 2 '18 at 9:33
• @user21 I have tried the methods in the mentioned post, I don't get any errors, but Mathematica just tries to calculate, and nothing happens. – user61688 Dec 2 '18 at 11:58
• @user61688 the code you indicated is intended for real potentials. The code that you indicated is intended for real potentials. I can rewrite it in case of complex potential. But it makes no sense. – Alex Trounev Dec 2 '18 at 14:01

I'm not sure that the numerical model will answer the question. See for yourself

eq = -psi''[x] + V[x]*psi[x];
V[x_] := 4 (Cos[x]^2 + I*3/10*Sin[2*x])
bc = PeriodicBoundaryCondition[psi[x], x == 2*Pi,
Function[x, x - 2*Pi]];

{vals, funs} = NDEigensystem[{eq, bc}, psi[x], {x, 0, 2*Pi}, 15];

vals

(*Out[]= {1.69967 + 4.31281*10^-15 I, 2.12775 + 2.21988*10^-14 I,
3.71198 - 9.12658*10^-15 I, 5.94789 - 2.42772*10^-15 I,
6.24787 + 5.6383*10^-15 I, 11.0421 - 3.01808*10^-14 I,
11.0579 + 2.3364*10^-14 I, 18.073 + 3.75656*10^-14 I,
18.0734 - 2.29703*10^-14 I, 27.2021 - 7.97291*10^-6 I,
27.2021 + 7.97291*10^-6 I, 38.546 + 6.01283*10^-14 I,
38.5461 - 8.84619*10^-14 I, 52.2869 - 1.18226*10^-13 I,
52.2871 + 1.2985*10^-13 I}*)
Table[Plot[Abs[funs[[i]]], {x, 0, Pi}, PlotLabel -> vals[[i]]], {i,
10}] Partially managed to reproduce Fig 1 from the article. The authors apparently did not take into account that when k -> 0 an anomaly occurs. Or in their method, this anomaly does not appear.

eq = y''[x] + 2*I*k*y'[x] - k^2*y[x] + V[x]*y[x];
V[x_] := 4*(Cos[x]^2 + I*n/10*Sin[2*x])
bc = PeriodicBoundaryCondition[y[x], x == Pi, Function[x, x - Pi]];

vals102 =
Table[Flatten[{k,
Re[NDEigensystem[{eq /. n -> 2, bc}, y[x], {x, 0, Pi}, 2][[1,
1]]]}], {k, -1, 1, .01}];
vals202 =
Table[Flatten[{k,
Re[NDEigensystem[{eq /. n -> 2, bc}, y[x], {x, 0, Pi}, 2][[1,
2]]]}], {k, -1, 1, .01}];

P1 = ListLinePlot[{vals102, vals202}, Frame -> True,
FrameLabel -> {"Bloch wave number", "Eigenvalue \[Beta]"}]
vals105 =
Table[Flatten[{k,
Re[NDEigensystem[{eq /. n -> 5, bc}, y[x], {x, 0, Pi}, 2][[1,
1]]]}], {k, -1, 1, .01}];
vals205 =
Table[Flatten[{k,
Re[NDEigensystem[{eq /. n -> 5, bc}, y[x], {x, 0, Pi}, 2][[1,
2]]]}], {k, -1, 1, .01}];
P2 = ListLinePlot[{vals105, vals205}, Frame -> True,
FrameLabel -> {"Bloch wave number", "Eigenvalue \[Beta]"}]
Show[P1, P2] • Thanks. However, these eigenvalue, although real, are not the same results as published in the paper. – user61688 Dec 2 '18 at 14:01
• ah! I think because you have chosen the period $2 \pi$. But, $V(x)$ has period of $\pi$. – user61688 Dec 2 '18 at 14:03
• @user61688 What is the name of the article? – Alex Trounev Dec 2 '18 at 14:18
• Beam Dynamics in PT symmetric Optical Lattices, Physical Review Letters – user61688 Dec 2 '18 at 14:22
• @user61688 thank you, I read this article, but I do not understand what they represent in FIG. 1 – Alex Trounev Dec 2 '18 at 15:50