We study the effect of magnetic field and temperature on the amount of coherence of two arbitrary spins in the Heisenberg chain. While increasing temperature always leads to lowering the value of this coherence, in case of magnetic field, it increases the coherence up to a certain value and ultimately it decreases its value to zero. Moreover we find that in a Heisenberg chain, at any temperature the coherence between spins vanishes with distance exponentially with a characteristic distance given by ∼ J k T $\sim \frac {J}{kT}$ . The effect of magnetic field is only to set the magnitude of this coherence.

中文翻译：

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海森堡自旋链中的热相干性

我们研究了磁场和温度对海森堡链中两个任意自旋的相干量的影响。虽然升高温度总是会导致该相干值的降低，但在磁场的情况下，它会将相干性增加到某个值，并最终将其值降低到零。此外，我们发现在海森堡链中，在任何温度下，自旋之间的相干性随距离呈指数消失，特征距离由 ∼ J k T $\sim \frac {J}{kT}$ 给出。磁场的作用只是设置这种相干性的大小。