L-1

Spin glass Transition

Experiments

Parlare dei campioni di rame dopati con il magnesio, marino o no: trovare due figure una di suscettivita e una di calore specifico, prova della transizione termodinamica.

Edwards Anderson model

We consider for simplicity the Ising version of this model.

Ising spins takes two values $\sigma =\pm 1$ and live on a lattice of $N$ sitees $i=1,2,\dots ,N$. The enregy is writteen as a sum between the nearest neighbours <i,j>:

$E=\sum _{<i,j>}{J}_{ij}{\sigma }_i{\sigma }_j$

The key point of the model proposed by Edwards and Anderson is that the couplings $J_{ij}$ are i.i.d. random variables with zero mean. We set $\pi (J_{ij})$ the coupling distribution indicate the avergage over the couplings called disorder average, with an overline:

$\overline{J_{ij}}\equiv \int d{J}_{ij}{J}_{ij}\pi (J_{ij})=0$

It is crucial to assume $\overline{J_{ij}}=0$, otherwise the model displays ferro/antiferro order. We sill discuss two distributions:

• Gaussian couplings

$\pi (J)=\frac{e^{-J^2/2}}{\sqrt{2\pi }}$

• bimodal couplings with $J_{ij}=1$ or $J_{ij}=-1$ with probability $1/2$.

Bibliography

• Theory of spin glasses, S. F. Edwards and P. W. Anderson, J. Phys. F: Met. Phys. 5 965, 1975