Specific Heat Studies of a 2D S $=$ \textonehalf Heisenberg Antiferromagnet

We report on the field-dependent specific heat of a highly two-dimensional Heisenberg, S $=$ 1/2 antiferromagnet (2D QHAF), [Cu(pz)$_{\mathrm{2}}$(2-OHpy)$_{\mathrm{2}}$](ClO$_{\mathrm{4}})_{\mathrm{2}}$, where pz $=$ pyrazine and 2-OHpy $=$ 2-pyridone. The copper atoms and pyrazine molecules form distorted rectangular layers of pyrazine-bridged copper(II) ions with the pyridone molecules normal to the layers, providing exceptional spacing between layers [1]. The zero-field specific heat of this compound (1.8 -- 35 K) is compared to the recent QMC simulations of the specific heat for the 2D QHAF. Under applied field, the temperature dependence of the specific heat varies smoothly, but no field-induced ordering is observed. This behavior differs from the field-induced ordering in the 2D QHAF Cu(pz)$_{\mathrm{2}}$(ClO$_{\mathrm{4}})_{\mathrm{2}}$ reported previously [2]. [1] V. Selmani, C. P. Landee, M. M. Turnbull, J. L. Wikaira, and F. Xiao. Inorg. Chem. Comm. 13, 1399-1401 (2010), doi: 10.1016/j.inoche.2010.07.045 [2] N. Tsyrulin, F. Xiao, A. P. Schneidewindm H. M. R\"{o}nnow, J. Gavilano, C. P. Landee, M. M. Turnbull, M. Kenzelmann. Phys. Rev. B, 81, 134409 (2010), doi: 10.1103/PhysRevB.81.134409.