LAUSR

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.