= Ernst Ising
{c}
{tag=Physicist}
{title2=1900-1998}
{wiki}
= E. Ising
{c}
{synonym}
= Ernest Ising
{c}
{synonym}
<Ernst Ising> solved the one-dimensional nearest-neighbor version of the <spin> model now called the <Ising model>. For ferromagnetic coupling $J$ and zero external field, its correlations decay with separation as $\langle s_0s_r\rangle=[\tanh(J/(k_BT))]^r$. At every positive <temperature> the factor has magnitude below one, so correlations vanish at arbitrarily large distance and there is no finite-temperature long-range ferromagnetic order in this model. The result makes the role of dimensionality and fluctuations concrete, rather than assuming that an ordering interaction automatically creates macroscopic order.<Beitrag zur Theorie des Ferromagnetismus by Ernst Ising>[1925 paper, linear-chain calculation and absence of positive-temperature magnetic ordering]{ref}
https://www.physik.tu-dresden.de/itp/members/kobe/isingconf.html
\Image[https://www.physik.tu-dresden.de/itp/members/kobe/isingphbl/ising1.jpg]
{title=Portrait of <Ernst Ising>}
{width=240}
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