Exotic spin excitations in a two-dimensional quantum antiferromagnet near the quantum critical point

The spin-1/2 antiferromagnetic Heisenberg two-leg ladder is a prototype system for low-dimensional magnetism. Recently, a novel spin-1/2 molecular compound C₉H₁₈N₂CuBr₄ (DLCB for short) was synthesized [1]. In contrast to most spin ladders reported to date, DLCB is a rare example in which the interladder coupling is very near the critical value required to drive the system to a Néel-ordered phase below 2 K without the assistance of a magnetic field. The size of the ordered moment is ~0.4µ_B, much smaller than 1µ_B of S=1/2 free ions, due to strong quantum fluctuations [2]. Our neutron scattering studies on DLCB in conjunction with theoretical calculations show evidences of the field-induced spontaneous (T=0 K) magnon decay in an applied transverse magnetic field [3] and existence of Higgs amplitude mode, which is analygous to the Higgs boson in particle physics and characterized by fluctuation of amplitude of the order parameter, near the quantum critical point in two dimensions [4]. Our experimental work on new example of the spin-1/2 molecular magnet with low dimensionality not only helps reveal the exotic quantum many-body effects experimentally but also challenge and advance the related theoretical studies.

References:
[1] Awwadi et al., Inorg. Chem. 47, 9327 (2008).
[2] Hong et al., Phys. Rev. B 89, 174432 (2014).
[3] Hong et al., Nat. Commun. 8, 15148 (2017).
[4] Hong et al., Nat. Phys. 13, 638 (2017).