Latest revision |
Your text |
Line 1: |
Line 1: |
| The '''1-dimensional Ising model''' is an [[Ising Models| Ising model]] that consists of a system with <math> N </math> spins in a row. The energy of the system is given by
| | Model: |
| | Consider a system with <math> N </math> spins in a row. |
|
| |
|
| :<math> U = -J \sum_{i=1}^{N-1} S_{i} S_{i+1} </math>,
| | The energy of the system will be given by |
| | |
| | <math> U = -K \sum_{i=1}^{N-1} S_{i} S_{i+1} </math>, |
|
| |
|
| where each variable <math> S_j </math> can be either -1 or +1. | | where each variable <math> S_j </math> can be either -1 or +1. |
|
| |
|
| The [[partition function]] of the system will be: | | The partition function of the system will be: |
| | |
| :<math> Q_N = \sum_{\Omega^N } \exp \left[ K \sum_{i=1}^{N-1} S_i S_{i+1} \right]</math>,
| |
| | |
| | |
| where <math> \Omega^N </math> represents the possible configuration of the N ''spins'' of the system,
| |
| and <math> K = J/k_B T </math>
| |
| | |
| :<math> Q_{N} = \sum_{S_1} \sum_{S_2} e^{K S_1S_2} \sum_{S_3} e^{K S_2 S_3} \cdots \sum_{S_{N-1}} e^{K S_{N-2} S_{N-1}}\sum_{S_{N}} e^{K S_{N-1} S_{N} }
| |
| </math>
| |
| | |
| Performing the sum of the possible values of <math> S_{N} </math> we get:
| |
| | |
| :<math> Q_{N} = \sum_{S_1} \sum_{S_2} e^{K S_1S_2} \sum_{S_3} e^{K S_2 S_3} \cdots \sum_{S_{N-1}} e^{K S_{N-2} S_{N-1}} \left[ 2 \cosh ( K S_{N-1} ) \right]
| |
| </math>
| |
| | |
| Taking into account that <math> \cosh(K) = \cosh(-K) </math>
| |
| | |
| :<math> Q_{N} = \sum_{S_1} \sum_{S_2} e^{K S_1S_2} \sum_{S_3} e^{K S_2 S_3} \cdots \sum_{S_{N-1}} e^{K S_{N-2} S_{N-1}} \left[ 2 \cosh ( K ) \right]
| |
| </math>
| |
| | |
| Therefore:
| |
| | |
| :<math> Q_{N} = \left( 2 \cosh K \right) Q_{N-1} </math>
| |
|
| |
|
| :<math> Q_N = 2^{N} \left( \cosh K \right)^{N-1} \approx ( 2 \cosh K )^N </math>
| | <math> Q_N = \sum_{\Omega^N } \exp \left[ - K \sum_{i=1}^{N-1} S_i S_{i+1} \right]</math>, |
|
| |
|
| The [[Helmholtz energy function]] in the [[thermodynamic limit]] will be
| | where <math> \Omega^N </math> represents the possible configuration of the N ''spins'' of the system. |
|
| |
|
| :<math> A = - N k_B T \log \left( 2 \cosh K \right) </math>
| | to be continued ... |
| ==References==
| |
| # Rodney J. Baxter "Exactly Solved Models in Statistical Mechanics", Academic Press (1982) ISBN 0120831821 Chapter 2 (freely available [http://tpsrv.anu.edu.au/Members/baxter/book/Exactly.pdf pdf])
| |
| [[Category: Models]]
| |