Quasiparticles, such as phonons, magnons, rotons, and heavy electrons and holes, are ubiquitous in nature. Quasiparticles usually have a long lifetime due to the weak or absence of interactions between them. However, this picture breaks down in rare conditions. In this talk, I will describe the experimental realization of magnetic field-induced spontaneous (T=0) magnon decays in an S=1/2 quantum antiferromagnet C9H18N2CuBr4 using the neutron scattering techniques [1]. The observed intriguing renormalization of one-magnon dispersion and magnon decays over a large region of the Brillouin Zone in the magnetic excitation specra can be well explained by the mechanism where the three-magnon interactions are present and the process of one-magnon decays into the two-magnon continuum is kinematically allowed [2]. Moreover, I will show that a universal scaling of the field-dependence of the Zeeman energy behaves in the same way as the temperature dependence of the spin gap in one-dimensional quantum magnets and agrees well with the calculation by the non-linear sigma model [3]. This result demonstrates the similarity between thermal and quantum fluctuations near the quantum critical point [4], where the intrinsic energy scales vanish.

 Reference:

[1] T. Hong et al., submitted.

[2] M. E. Zhitomirsky and A. L. Chernyshev, Rev. Mod. Phys. 85, 219 (2013).

[3] K. Damle and S. Sachdev, Phys. Rev. B 57, 8307 (1998).

[4] S. Sachdev, Quantum Phase Transitions, Cambridge University Press (2001).