Evidence for Pressure Induced Unconventional Quantum Criticality in the Coupled Spin Ladder Antiferromagnet C₉H₁₈N₂CuBr₄

Quantum spin-1/2 ladders with two legs have been severing as a model low-dimensional spin system for discovery and understanding new phenomena. Here we present a comprehensive study of the effect of hydrostatic pressure on the magnetic structure and spin dynamics in a spin-1/2 coupled ladder antiferromagnet C₉H₁₈N₂CuBr₄ (DLCB for short). In DLCB, the inter-ladder coupling is sufficiently strong to drive the system to the long-range antiferromagnetic ordering phase below <i>T</i>_N=2.0 K. Analysis of the spin Hamiltionian suggests that DLCB is close to the quantum critical point in two dimensions at ambient pressure and zero field [1, 2]. The single-crystal heat capacity and neutron diffraction measurements suggets that the magnetic order breaks down above a critical pressure <i>P</i>_c ~1.0 GPa [3]. Estimates of the critical exponents suggest that this transition may fall outside the traditional Landau paradigm. By contrasting with quantum Monte Carlo calculations of the dynamic structure factor, the follow-up inelastic neutron scattering at 1.3 GPa further indicates an exotic quantum-disordered phase above <i>P</i>_c[3], characterized by two well-separated gapped modes, including one continuum-like and another resolution-limited excitation in distinct scattering channels.<br/><br/>References:<br/>[1] T. Hong <i>et al</i>., Phys. Rev. B <b>89</b>, 174432 (2014).<br/>[2] T. Hong <i>et al</i>., Nat. Phys. <b>13</b>, 638 (2017).<br/>[3] T. Hong <i>et al</i>., Nat. Commun. <b>13</b>, 3073 (2022).<br/><br/>Acknowledgements:<br/>This research used resources at High Flux Isotope Reactor and Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory (ORNL).