Symmetry and two electron wave function

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Discussion Overview

The discussion revolves around the implications of left-right symmetry on the eigenstates of a two-electron system with identical orbitals. Participants explore whether eigenstates can exist in a form that does not respect this symmetry, particularly in the context of fermionic operators and Hamiltonians.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions whether an eigenstate of the form ##|1,1,0,0>## can exist in a left-right symmetric system or if it must be expressed as a symmetric combination like ##\frac{|1,1,0,0>\pm|0,0,1,1>}{\sqrt(2)}##.
  • Another participant states that the only symmetry requirement for states is related to the interchange of identical particles, suggesting that general states do not need to adhere to specific symmetry forms unless they are eigenstates of certain operators.
  • A participant seeks clarification on whether left-right symmetry necessitates eigenstates of the Hamiltonian to be in a symmetric form or if non-symmetric forms are permissible.
  • There is a reiteration that for left-right symmetric systems, eigenstates can exist that do not exhibit this symmetry, particularly if there is degeneracy involved.
  • One participant draws an analogy to parity in one-particle systems, indicating that without degeneracy, eigenstates must have even or odd symmetry, while otherwise, symmetry is not required.

Areas of Agreement / Disagreement

Participants express differing views on the necessity of symmetry in eigenstates, with some arguing that symmetry is required under certain conditions, while others contend that it is not a strict requirement. The discussion remains unresolved regarding the implications of left-right symmetry on eigenstate forms.

Contextual Notes

Participants reference the role of degeneracy and the nature of Hamiltonians in determining the symmetry of eigenstates, but the specific conditions under which these factors apply remain unclear.

hokhani
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TL;DR
Understanding the relation between system symmetry and wave-function symmetry
In the picture below we have two identical orbitals A and B and the system has left-right symmetry. I use the notation ##|n_{A \uparrow}, n_{A \downarrow},n_{B \uparrow},n_{B \downarrow}>## which for example ##n_{A \uparrow}## indicates the number of spin-up electrons in the orbital A. I would like to know is it possible to have an eigenstate as ##|1,1,0,0>## in this left-right symmetric system or, because of the symmetry of system, we must only have symmetric wave-functions as ##\frac{|1,1,0,0>\pm|0,0,1,1>}{\sqrt(2)}##?
Any help is appreciated.
Picture3.png
 
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The only symmetry that states must follow is the one related to interchange of two identical particles.

(That said, if you need states to be eigenstates of some operators, like the Hamiltonian, then certain symmetries may need to be respected. But for a general state, there is no such requirement.)
 
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Thanks very much. In the second quantized form, the interchange of particles are included in the commutation of fermionic operators.
However, I would like to know whether the left-right symmetry of this system demands the eigenstates of the Hamiltonian be in the form ## \frac{1}{\sqrt(2)}(|1,1,0,0>\pm|0,0,1,1>)## or they can also have the form like ##|1,1,0,0>##?
 
What is the Hamiltonian?
 
I don't mean a specific Hamiltonian. I mean each left-right symmetric Hamiltonian for this system.
 
Sorry if I didn't convey myself. To explain in a better way; for a left-right symmetric system, can we have the eigenstates which doesn't have this symmetry?
 
hokhani said:
Sorry if I didn't convey myself. To explain in a better way; for a left-right symmetric system, can we have the eigenstates which doesn't have this symmetry?
If you have a degenerated eigenvalue with a left-right symmetric and a left-right anti-symmetric eigenstate, then take a superposition of them to get an eigenstate which is not left-right symmetric.
 
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gentzen said:
If you have a degenerated eigenvalue with a left-right symmetric and a left-right anti-symmetric eigenstate, then take a superposition of them to get an eigenstate which is not left-right symmetric.
Thanks, I got it. Like the parity in one-particle system, provided that there is no degeneracy, the eigenstates must have even or odd symmetry as ##\frac{1}{\sqrt(2)}(|1,1,0,0>\pm|0,0,1,1>)##. Otherwise, there is no demand for symmetry of the eigenstates.
 

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