Pressure-induced gap closing in the highly symmetric quantum paramagnet DTN

We experimentally demonstrate the pressure-induced closing of the spin gap in the model tetragonal S = 1 quantum paramagnet NiCl₂·4SC(NH₂)₂ (abbreviated as DTN) [1]. Employing a TDO technique, we show that the spin gap vanishes at a critical pressure P_c~4.3 kbar. This is below the structural phase transition at P_irr~6 kbar, revealed by powder neutron diffraction. Thus the high, nearly axial spin symmetry of DTN is retained. We describe the experimentally measured critical fields (separating the gapped state, antiferromagnetically ordered state, and field-polarized state) employing linear spin-wave theory for H_c2 and a quasi-1D numeric approximation for H_c1. The studies are complemented by high-pressure electron spin resonance measurements confirming the proposed scenario. The undistorted tetragonal symmetry and full control over the magnetic Hamiltonian render DTN as an ideal platform for further pursuit of the exotic physics near a z = 1 quantum criticality in 3D [2-4].

[1] K. Povarov et al., arXiv 2306.15450 (2023)

[2] P. Merchant et al., Nat. Physics 10, 373 (2008)

[3] H. Scammell et al., Phys. Rev B 95, 024420 (2017)

[4] H. Scammell et al., Phys. Rev. B 96, 174414 (2017)