Pressure Induced Unconventional Quantum Criticality in the Coupled Spin 1/2 Ladder Antiferromagnet with Frustrating Interactions*

Quantum spin-1/2 ladders with two legs have been severing as a model low-dimensional spin system for discovering and understanding new phenomena. Observation of Higgs amplitude mode and magnetic field-induced spontaneous quasiparticle decay and renormalization of quasiparticle dispersion were reported on a coupled spin-1/2 ladder antiferromagnetic compound C₉H₁₈N₂CuBr₄ using neutron scattering technique [1-3]. In contrast to most spin ladders reported to date, C₉H₁₈N₂CuBr₄ is close to a quantum critical point at ambient pressure and zero applied magnetic field. Very recently, by carrying out neutron scattering and AC heat capacity studies in the presence of hydrostatic pressure, we discovered an unconventional quantum criticality with the Néel-ordered state breaking down at a critical pressure of P_C ~1.0 GPa through a phase transition into a quantum-disordered state [4]. Estimates of the critical exponents suggest that this transition may fall outside the traditional Landau paradigm. Moreover, broad magnetic excitation continua were observed near the phase transition. We propose that the frustrating interlayer couplings may contribute to drive the system into an exotic gapped quantum paramagnetic regime, such as a quantum spin liquid phase. Our work offers an interesting experimental platform to investigate the non-trivial interplay between frustrated magnetism and quantum critical phenomena.

References:

[1] T. Hong et al., Phys. Rev. B 89, 174432 (2014).

[2] T. Hong et al., Nat. Commun. 8, 15148 (2017).

[3] T. Hong et al., Nat. Phys. 13, 638 (2017).

[4] T. Hong et al., Nat. Commun. 13, 3073 (2022).