Spin gap in the weakly interacting quantum spin chain antiferromagnet KCuPO4·H2O

The $S=1/2$ Heisenberg linear chain antiferromagnet is the simplest spin model; nevertheless it serves as a platform for various quantum many-body phenomena. Here, we report the magnetic behavior of a quasi-one-dimensional antiferromagnet ${\mathrm{KCuPO}}_{4}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathrm{H}}_{2}\mathrm{O}$. A long-range commensurate antiferromagnetic order with ordered moment 0.31(1) ${\ensuremath{\mu}}_{\mathrm{B}}$ per spin occurs at ${T}_{\mathrm{N}}=11.7(1)$ K. Above ${T}_{\mathrm{N}}$, the inelastic neutron excitation is characterized by a two spinon continuum. The intrachain interaction $J$ and interchain interaction $|{J}^{\ensuremath{'}}|$ are estimated to be 172 and 4.25(4) K, respectively; thus the ratio of $|{J}^{\ensuremath{'}}|/J=0.0247(3)$. At lower energies, below ${T}_{\mathrm{N}}$, a spin gap is observed in the dispersive excitations. These results are consistent with characteristics observed in a weakly interacting $S=1/2$ Heisenberg chain system.