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A quantum critical point (QCP) marks a zero-temperature phase transition which separates different ground states of a system. Generally, quantum criticality can be probed by varying an external control parameter such as pressure or magnetic field. This is not the case, however, for the spin S = 1/2 antiferromagnetic Heisenberg chain (AFHC) which is inherently quantum critical as any kind of changes in the parameters of the underlying Hamiltonian gives rise to a different ground state. The AFHC also remains quantum critical in an external magnetic field up to the saturation field Bs, which marks the endpoint of a quantum critical line in the T-B plane. At B = Bs for the S = ½ AFHC, as for any other B-induced QCP, the competition between the two different ground states on both sides of the QCP causes unusual behaviour in the thermodynamic properties, even at finite temperatures. For example, the magnetocaloric effect (MCE) is expected to diverge at B = Bs upon cooling. We present the first experimental study of the MCE of an S = ½ AFHC in the vicinity of the B-induced QCP. For these experiments we used a metalorganic Cu(II)-coordination polymer, which is not only an excellent representation of an S = ½ AFHC, but also excels in having a moderate saturation field Bs of only about 4 T, small enough for laboratory fields to access the QCP. Work has been performed in collaboration with D. Jaiswal-Nagar, Y. Tsui, A. Honecker, U. Tutsch, K. Removic-Langer, M. Lang |
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