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Statistical Mechanics - Physics at Oregon State University

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9.4. RELATION BETWEEN SUSCEPTIBILITY AND FLUCTUATIONS.203<br />

−0.5<br />

−0.6<br />

−0.7<br />

−0.8<br />

−0.9<br />

−1.0<br />

0.9<br />

0.95<br />

Figure 9.4: Finite sample case to find critical exponent.<br />

−1.6<br />

−1.7<br />

−1.8<br />

−1.9<br />

−2.0<br />

0.5<br />

0.75<br />

Figure 9.5: Cluster results to find critical exponent.<br />

9.4 Rel<strong>at</strong>ion between susceptibility and fluctu<strong>at</strong>ions.<br />

As we have remarked a few times, response functions are rel<strong>at</strong>ed to fluctu<strong>at</strong>ions.<br />

This can easily be shown for the susceptibility. By definition we have<br />

x<br />

1.0<br />

x<br />

1.0<br />

1.05<br />

1.25<br />

χ = ∂ Tr S0e<br />

∂h<br />

−β(H−hM)<br />

Tr e−β(H−hM) 1.1<br />

1.5<br />

(9.59)<br />

This deriv<strong>at</strong>ive can be performed explicitly, since H and M = <br />

i Si commute.<br />

We find

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