Unusual Magnetic-Pressure Response of an $S=1$ Antiferromagnetic Linear-Chain near the $D/J \approx 1$ Critical Point $^{\ast}$

An $S = 1$ chain, [Ni(HF$_2$)(3-Clpy)$_4$]BF$_4$ (py = pyridine), has been identified to have nearest-neighbor antiferromagnetic interaction $J/k_B = 4.86$~K and single-ion anisotropy $D/k_B = 4.3$~K, while avoiding long-range order to 25 mK.$^{\dagger}$ With $D/J = 0.88$, this system is close to the $D/J \approx 1$ gapless quantum critical point between the Haldane and Large-$D$ phases. The magnetization was studied at 50~mK $\leq T \leq~1$~K and with $B \leq 10$~T.$^{\ddagger}$ Using a magnetometer equipped with a pressure cell, the low-field (0.1~T), high temperature ($T \geq 2$~K) magnetic susceptibility was studied to 1.47~GPa. These data suggest the response at ambient pressure$\,^{\dagger}$ changes between 0.24~GPa and 0.35~GPa. These studies have been or are being extended by $^1$H NMR experiments capable of varying the pressure, inelastic neutron scattering investigations down to 300~mK, and X-ray diffraction at 300~K and with pressures up to nominally 1.5~GPa. The status of the analysis of all of the data sets will be provided.\\ $\dagger$~J.L.~Manson \emph{et al.}, Inorg.~Chem. 51 (2012) 7520. \\ $\ddagger$~J.-S.~Xia \emph{et al.}, arxiv.1409.5971 (2014).\\ $^{\ast}$~This work is a collaboration involving M.K.~Peprah, P.A.~Quintero, J.S.~Xia, Y.~Tao, X.~Zuo, J.M.~P\'erez, A.~Garcia, S.E.~Brown, S.~Lapidus, and J.L.~Manson.