Understanding Hydrogen Bonding and Low-Energy Magnetic Excitations in VOHPO$_4$$\cdot$$\frac{1}{2}$H$_2$O

We report the variable temperature vibrational properties of the S=1/2, quasi-one-dimensional quantum Heisenberg antiferromagnet VOHPO$_4$$\cdot$$\frac{1}{2}$H$_2$O. Vibrational splitting points toward a weak local symmetry breaking near 180 K, and the low-temperature redshift of V-O and H-O related modes demonstrates enhanced low-temperature hydrogen bonding. Due to spin-orbit interaction, the singlet to triplet gap also appears in the infrared response. We compare this value to those obtained via magnetic susceptibility, electron-spin resonance, and neutron scattering, and we point out the existence of a spectral feature that supports weak interaction between traditional``isolated V-V dimers.'' Both magnon dispersion calculations and the experimental data suggest $\alpha$=J$^\prime$/J is $\sim$ 7\%.