Higgs Amplitude Mode in a Two-Dimensional Quantum Antiferromagnet near the Quantum Critical Point0
Tao Hong, Masashige Matsumoto, Yiming Qiu, WangChun Chen, Thomas R. Gentile, Shannon Watson, Firas F. Awwadi, Mark M. Turnbull, Sachith E. Dissanayake, Harish Agrawal, Rasmus Toft-Petersen, Bastian Klemke, Kris Coester, Kai P. Schmidt, David A Tennant
Spontaneous symmetry-breaking quantum phase transitions play an essential role in current condensed matter physics1-3. The collective excitations in the broken-symmetry phase near the quantum critical point can be characterized by fluctuations of phase and amplitude of the order parameter. The phase oscillations correspond to the massless Nambu-Goldstone modes whereas the massive amplitude mode, analogous to the Higgs mode in the particle physics4-6, is prone to decay into a pair of low-energy Nambu-Goldstone modes in low dimensions2,7,8. The observation of a Higgs amplitude mode in low dimensions is an outstanding experimental challenge. Here, using the inelastic neutron scattering and application of the bond-operator theory, we directly and unambiguously identify the Higgs amplitude mode in a two-dimensional quantum S=1/2 coupled spin ladder compound C9H18N2CuBr4, which is located close to a quantum critical point. Owing to an anisotropic spin gap, it kinematically prevents such decay and the Higgs amplitude mode acquires an infinite lifetime.