Colossal quasiparticle radiation in the Lifshitz spin liquid phase of a two-dimensional quantum antiferromagnet

Strong quantum fluctuations in magnetic systems can create disordered quantum spin liquid phases of matter which are not predicted by classical physics. The complexity of the exotic phenomena on display in spin liquids has led to a great deal of theoretical and experimental interest. However, understanding the fundamental nature of the excitations in these systems remains challenging. In this work, we consider the Lifshitz quantum critical point in a two-dimensional frustrated $XY$ antiferromagnet. At this point, quantum fluctuations destroy long-range order, leading to the formation of an algebraic Lifshitz spin liquid. We demonstrate that the bosonic magnon excitations are long-lived and well-defined in the Lifshitz spin liquid phase, though paradoxically, the dynamic structure factor has a broad non-Lorentzian frequency distribution with no single-particle weight. We resolve this apparent contradiction by showing that the Lifshitz spin liquid suffers from an infrared catastrophe: An external physical probe always excites an infinite number of arbitrarily low-energy quasiparticles, which leads to significant radiative broadening of the spectrum.

Figure 1

The leading-order contribution to the quasiparticle lifetime comes from this two-loop diagram.

Figure 2

Far from the LP, one quasiparticle can decay into three with momenta lying in a narrow cone centered around the total momentum.

Figure 3

Diagrammatic expansion of the dynamic structure factor. Dashed lines denote the source, wavy lines denote quasiparticles, and vertical dashing denotes the application of cut rules. The “magnifying” inset shows higher-order off-shell processes which generate broadening.

Figure 4

The dynamic structure factor in the LSL phase, comparing the exact solution Eqs. (30) and (37) with a naive Born approximation. Parameters: $\sqrt{b_1}{k}^2=1$, $\sqrt{b_1}{\mathrm{\Lambda }}^2=10$, ${\chi }_{\perp }=1$, $\zeta =0.2$, and ${\mathrm{\Gamma }}_{\mathbf{k}}=0.3$.