Unusual magnetic excitations in the weakly ordered spin-12chain antiferromagnetSr2CuO3: Possible evidence for Goldstone magnon coupled with the amplitude mode

We report on an electron spin resonance (ESR) study of a nearly one-dimensional (1D) spin-$\frac{1}{2}$ chain antiferromagnet, ${\mathrm{Sr}}₂{\mathrm{CuO}}₃$, with extremely weak magnetic ordering. The ESR spectra at $T>{T}_N$, in the disordered Luttinger-spin-liquid phase, reveal nearly ideal Heisenberg-chain behavior with only a very small, field-independent linewidth, $\ensuremath{\sim}1/T$. In the ordered state, below ${T}_N$, we identify field-dependent antiferromagnetic resonance modes, which are well described by pseudo-Goldstone magnons in the model of a collinear biaxial antiferromagnet. Additionally, we observe a major resonant mode with unusual and strongly anisotropic properties, which is not anticipated by the conventional theory of Goldstone spin waves. We propose that this unexpected magnetic excitation can be attributed to a field-independent magnon mode renormalized due to its interaction with the high-energy amplitude (Higgs) mode in the regime of weak spontaneous symmetry breaking.