Spin dynamics of the one-dimensional double chain spin- 12 antiferromagnet KNaCuP2O7

We combine a muon spin relaxation ($\ensuremath{\mu}\mathrm{SR}$) technique with thermodynamic measurements to explore the spin dynamics of one-dimensional (1D) $S=\frac{1}{2}$ antiferromagnetic double chain ${\mathrm{KNaCuP}}₂{\mathrm{O}}₇$. Static magnetic susceptibility and specific heat are well described by a uniform 1D spin chain model with the intrachain interaction $J/{k}_B\ensuremath{\approx}55\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and small interchain interactions. Spin excitations probed by zero-field $\ensuremath{\mu}\mathrm{SR}$ evince that high-temperature diffusive spin transport turns into ballistic behavior with decreasing temperature below 30 K. In addition, we observe that longitudinal-field $\ensuremath{\mu}\mathrm{SR}$ varies hardly with an external magnetic field. The field-independent dynamical spin susceptibility disagrees with diffusive or ballistic behaviors.