Explaining the Inclusion of Minus Sine in the Heisenberg Hamiltonian Definition

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SUMMARY

The inclusion of the minus sine in the Heisenberg Hamiltonian definition is crucial for accurately representing the physical behavior of spin systems. The Hamiltonian is defined as H=-∑_{i,j}J_{i,j}(S_{i}^+S_{j}^-+S_i^zS_j^z), where the sign of J determines the interaction type. For ferromagnetic interactions, J is positive, leading to alignment of spins, while for antiferromagnetic interactions, J is negative, resulting in anti-alignment. This sign convention is essential for correctly modeling the energy states of the system.

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matematikuvol
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Why is minus sine in definition of hamiltonian

[tex]H=-\sum_{i,j}J_{i,j}(S_{i}^+S_{j}^-+S_i^zS_j^z)[/tex]

Why not?

[tex]H=\sum_{i,j}J_{i,j}(S_{i}^+S_{j}^-+S_i^zS_j^z)[/tex]
 
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The choice of sign is determined by the physics. If the spins want to align (ferromagnetic) then you should have H = |J| S*S while if the spins want to anti-align (antiferromagnetic) then H=-|J|S*S is correct. Of course, you can always write H=-J S*S and then set J>0 for ferro and J<0 for anti-ferro. That's just a convention.
 

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