5
votes
2answers
256 views

Naive questions on Goldstone modes and a possible duality relation?

For example, let's consider a 1D spin-1/2 ferromagnetic (FM) Heisenberg chain $H=-J\sum_{i=1}^{N}\mathbf{S}_i\cdot\mathbf{S}_{i+1}$ with periodic boundary conditions. Now we want to study its low ...
1
vote
0answers
41 views

Fluctuation-Dissipation theorems in an infinite quantum system

So for a quantum spin chain, one can easily prove via the partition function that you have a fluctuation-dissipation type relation between the magnetic susceptibility and the variance of the ...
3
votes
1answer
654 views

What is a free fermion model?

Title says it all really.. Why is the XX spin chain a free fermion (non-interacting) model, and the XXZ chain not? Is it right that $\sum_l a_l^\dagger a_{l+1}$ isn't an interaction between fermions ...
0
votes
1answer
272 views

Seeking a specific quantum spin system of interacting spin 1/2 particles

Is there a system of interacting quantum spin 1/2 particles (of any topology) whose the states where all spins are up or down are eigenstates of its Hamiltonian and yet does not conserve the total ...
9
votes
3answers
1k views

To calculate the correlation functions of an XX spin chain, Wick's theorem is used. But is it valid for a chain of any size?

The correlation functions found in Barouch and McCoy's paper (PRA 3, 2137 (1971)) for the XX spin chain use a method which uses Wick's theorem. For the zz correlation function, this gives $\langle ...