Manipulating ground-state properties of NiCl₂·4SC(NH₂)₂ by pressure.

Magnetic order in gapped quantum antiferromagnets can be induced either by magnetic field or pressure. Such transitions are characterized by z = 2 or z = 1 dynamical critical exponents, determined by the quadratic or linear low-energy dispersion of spin excitations, respectively. While field-induced transitions (with the most common realization known as the Bose-Einstein magnon condensation) have been intensively studied, many details of pressure-induced phase transitions have remained an open question. Employing high-pressure tunnel-diode-oscillator (TDO) susceptibility, ultrasound, and high-field electron spin resonance (ESR) spectroscopy measurements, we revealed the pressure-induced magnetic ordering in the spin-1 chain system with strong planar anisotropy NiCl₂·4SC(NH₂)₂ (a.k.a. DTN), that we ascribe to the long-sought z = 1 criticality*. This transition occurs at an easily accessible pressure of about 4.2 kbar, allowing us to systematically study the low-temperature pressure-field phase diagram. Using RPA+DMRG approach, we calculated spin-Hamiltonian parameters of DTN as functions of the applied pressure, revealing excellent agreement with the Sakai-Takahashi phase diagram for a 1D spin-1 antiferromagnet. Our findings establish DTN as a perfect platform to investigate z = 1 quantum critical phenomena in strongly correlated spin systems.

*K. Yu. Povarov et al., Nature Commun. 15, 2295 (2024).