Finding eigenvalues of a Hamiltonian involving Sz, Sz^2 and Sx

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Discussion Overview

The discussion revolves around finding the eigenvalues of a Hamiltonian for a particle with spin S=5/2, specifically involving the operators Sz, Sz^2, and Sx. Participants explore methods for diagonalizing the Hamiltonian matrix, which is described as tridiagonal symmetric, and express challenges encountered in the process.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant presents the Hamiltonian and seeks help with diagonalization, noting that a is a variable while b and c can be assigned values.
  • Another suggests using the eigenvalue equation |H - E I| = 0 and mentions the utility of software tools like Mathematica for symbolic eigenvalues.
  • A participant reports that Mathematica does not solve the problem and expresses frustration with the complexity of the resulting equations.
  • There is a suggestion to use the Solve function to find roots in terms of a, but it is acknowledged that the process is cumbersome.
  • One participant emphasizes the difficulty of finding eigenvalues for a 6x6 matrix and notes that there should be 6 eigenvalues, not 7, which raises questions about the formulation of the problem.
  • Another participant mentions trying Maple for eigenvalue calculations but is uncertain about its effectiveness for a 6x6 matrix.

Areas of Agreement / Disagreement

Participants express a range of experiences and opinions regarding the methods for finding eigenvalues, with no consensus on a definitive solution or approach. Disagreement exists about the number of unknowns involved in the eigenvalue problem.

Contextual Notes

Participants note the complexity of the calculations and the potential for errors, as well as the limitations of software tools in handling the specific matrix structure.

Physicslad78
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I have the Hamiltonian for an S=5/2 particle given by:

H= a.Sz + b.Sz^2 +c.Sx where Sz and Sx are the spins in z and x directions respectively. The resulting matrix is tridiagonal symmetric but I can't find the eigenvalues..Any idea how to diagonalise it.

N.B: a is a variable and must be kept as a in matrix whereas b and c can be assigned values.


Thanks guys.
 
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I guess you'd just use the normal eigenvalue equation
\left|H - E I\right| = 0
where I represents the identity matrix... that, or Mathematica ;-) (seriously, if you have access to some software tool which can find symbolic eigenvalues, it's probably not worth the work to diagonalize a 6x6 matrix)

If you have trouble with that, post the details of the calculations you did. Sometimes it's just as simple as a little math error.
 
Mathematica doesn't solve it..Eigenvalues produces a root solution and solving a 7 equation system with 7 unknowns (if I want to find the eigenvectors) doesn't help either..Maybe I need a similarity transformation and I have tried some but in vain..dont know what to do..:(
 
You can use the Solve function to explicitly find the roots in the solution in terms of a, but it is rather ugly :/ One thing's for sure, I wouldn't want to have to come up with that by hand!
 
I would use some kind of program, it's not simple to find the eigenvalue of a 6x6 matrix unless there happens to be some real tricks...and I don't think there are for tridiagonal matrices (I don't know of any). There should be 6 eigenvalues (and eigen-vectors) though, not 7...o.O
 
Thanks guys..I have tried Solve as well but it just runs forever! MatterrWave, yeah I meant 7 unknowns (6 are the elements of spinor and the 7th is the eigenvalue)..I will see if I can meddle with it and get an answer
 
Uhm, Maple has a eigenvalue/eigenvector finder, though I'm not sure if it will be any good at a 6x6 matrix. I'm going to try and see. :p

EDIT: wow, even constructing these matrices takes a while...I may do it later <_<
 
Last edited:
Thanks a lot Matter wave...waiting for ur results if u do it or if any..lol..meanwhile will try sorting it out and have got a non separable differential equation waiting for me as well..:)
 

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