Quantum criticality and entropy transfer in spin chains and planes—Pyridine oxide copper salts

We present magnetic field-dependent specific heat (C) data for [Cu(pyO)6](NO3)2 (pyO = pyridine oxide) (CPN), a molecular salt shown to be quasi-1D, and for a quasi-2D analogue, [Cu(pyO)6](BF4)2 (CPB). For CPN, a sharp feature indicating 3D ordering is observed at 0.16K in zero-field. As the field, H, is increased, the ordering temperature first increases, as expected for quasi-1D antiferromagnets, before decreasing rapidly for H above 3T. The field also transfers the entropy of short-range ordering (SRO) in the spin chains into the 3D ordering peak. At our lowest accessible temperature, T ~ 0.096K, C/T exhibits an enhanced peak at the critical field. Qualitatively similar behavior is found in CPB. These results demonstrate a potentially powerful new materials route to study quantum phase transitions.