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I am trying to solve the eigenvalues of a hamiltonian enter image description here

The code I used is typed below.

t = 0.1; 
h = {{0, (-t)*(1 + Exp[(-I)*ky]), 0}, {0, 0, (-t)*(1 + Exp[I*kx])}, {0, 0, 0}}; 
ham = h + Assuming[{Element[kx, Reals], Element[ky, Reals]}, Refine[ConjugateTranspose[h]]]; 
FullSimplify[Eigenvalues[ham], Assumptions -> Element[{kx, ky}, Reals]]

The Eigenvalues gives an undesired complex number (which should be elminated inside and outside the square root)

enter image description here

How can I elminate the undesired complex number?

Thank you in advance!

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  • $\begingroup$ Try PowerExpand to simplify your last result. $\endgroup$ Apr 20, 2020 at 9:30
  • $\begingroup$ Thanks! It works for me. $\endgroup$
    – Rosetta
    Apr 20, 2020 at 9:31
  • $\begingroup$ Fine, you're welcome. $\endgroup$ Apr 20, 2020 at 9:32

2 Answers 2

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Since symbolic calculations are better done with exact, rather than floating-point, numbers, the following was done with t = 1/10:

Eigenvalues[ham // ExpToTrig]
(*
  { 0,
   -Sqrt[2 + Cos[kx] + Cos[ky]]/(5 Sqrt[2]),
    Sqrt[2 + Cos[kx] + Cos[ky]]/(5 Sqrt[2])}
*)

Note: It will "work" with t = 0.1 but the factor in the denominator gets incorporated as Real coefficients in the square root:

(*
  { 0,
   -Sqrt[0.04 + 0.02 Cos[kx] + 0.02 Cos[ky]],
    Sqrt[0.04 + 0.02 Cos[kx] + 0.02 Cos[ky]]}
*)

Thus rationalizing 0.1 is not necessary, but one should be aware that round-off error often leads to problems in symbolic manipulation.

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At least for this case, one can take an indirect route through CharacteristicPolynomial[]:

ham = Simplify[(# + ConjugateTranspose[#]) &[{{0, (-t)*(1 + Exp[(-I)*ky]), 0},
                                              {0, 0, (-t)*(1 + Exp[I*kx])},
                                              {0, 0, 0}}], {t, kx, ky} ∈ Reals]
   {{0, -(1 + E^(-I ky)) t, 0}, {-(1 + E^(I ky)) t, 0, -(1 + E^(I kx)) t},
    {0, -(1 + E^(-I kx)) t, 0}}

cp = FullSimplify[CharacteristicPolynomial[ham, λ], {t, kx, ky} ∈ Reals]
   λ (4 t^2 - λ^2 + 2 t^2 (Cos[kx] + Cos[ky]))

λ /. Solve[cp == 0, λ]
   {0, -Sqrt[2] t Sqrt[2 + Cos[kx] + Cos[ky]], Sqrt[2] t Sqrt[2 + Cos[kx] + Cos[ky]]}
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