Entanglement in dipolar coupling spin system in equilibrium state

We study the appearance of the entangled states in a one-dimensional finite chain of dipolar-coupling nuclear spins of 1/2 in the conditions of thermodynamic equilibrium. It is shown that entanglement is achieved by the application of a low external magnetic field in which the Zeeman interaction energy is the order of or even less than the dipolar interaction one. When these energies are equal, the critical temperature, i. e. the temperature of the entanglement appearance, coincides with the temperature at which the heat capacity of the spin chain achieves its maximum. The obtained relationship between the critical temperature and the magnetic field can be considered as an entanglement witness. The dependences of the heat capacity on temperature and magnetic field have different character for entangled and separable states and can be served for experimental detection of entangled states.