Research: Pressure-induced ordering in a model gapped magnet

There is a class of materials which, despite the strong interactions between the magnetic ions, refuse to order magnetically down to zero temperature. This happens due to the energy gap, separating the singlet ground state from the 'magnetic' excited states. Such quantum paramagnets can be forced to order magnetically either by strong external magnetic field, or by applied pressure that would reduce the gap. The latter case is especially interesting, since it would correspond to an exotic type of z=1 quantum critical point in a three-dimensional system with linear excitation spectrum at low energies.

In our work, we demonstrate the pressure-induced magnetic ordering in a prototypical quantum paramagnet NiCl₂⋅4SC(NH₂)₂ (known as DTN). This material is a 'toy model' for quantum paramagnetism due to exceptional simplicity and high symmetry of its magnetic Hamiltonian. The high-frequency ac susceptibility and ultrasound propagation measurements under pressure directly show the closing of the gap and onset of spontaneous magnetic order in DTN near 4 kbar. At the same time, neutron diffraction measurements under pressure confirm that the observed quantum critical point is of purely magnetic nature and no structural distortions are involved. In turn, theoretical analysis allows us to quantify the pressure-induced changes in Hamiltonian parameters that drive the quantum criticality in DTN. These estimates are corroborated by the electron spin resonance spectroscopy under pressure.

While DTN was already known as one of the best model materials to study the phenomenon of field-induced magnetic ordering, our study reveals that it hosts a zero-field type of quantum critical point too. We hope that DTN would play an important role in understanding the exotic pressure-induced quantum criticality and associated spin dynamics in quantum paramagnets.

K. Y. Povarov, D. E. Graf, A. Hauspurg, S. Zherlitsyn, J. Wosnitza, T. Sakurai, H. Ohta, S. Kimura, H. Nojiri, V. O. Garlea, A. Zheludev, A. Paduan-Filho, M. Nicklas, S. A. Zvyagin,
Pressure-tuned quantum criticality in the large-D antiferromagnet DTN,
Nat. Commun. 15, 2295 (2024) (arXiv)