Electron Spin Resonance as a Direct Probe of Spinon Interactions in a Quantum Spin Chain

The strong, yet well-hidden backscattering interaction between fractionalized spinon excitations has been known as a part of the S=1/2 chain physics for a long time. However, its dramatic consequences for the dynamic properties, such as the appearance of the collective spin 1 mode at small momenta (analogue of the Fermi liquid's Silin mode),  were realized only recently [1]. These fine details, occuring only in the magnetized state and only at nonzero momenta, are challenging for experimental observation. We have succeeded in experimental detection of these features using Electron Spin Resonance as the probe [2]. This was only possible due to the specific "momentum boost” present in the spectrum of a unique spin chain material K₂CuSO₄Br₂ featuring the uniform Dzyaloshinskii-Moriya interaction pattern [3,4]. Description of the observed spectrum requires accounting for the backscattering interaction already on the qualitative level. Quantitative analysis allows us to estimate the backscattering constant to be as large as 2.38J, in units of the exchange interaction J. Yet the corresponding dimensionless interaction constant, that determines the Renormalization Group flow [5], is quite small, and allows for a successful direct comparison of our data with the RG theory predictions.