Understanding low-energy magnetic excitations and hydrogen bonding in VOHP O₄ * 1 / 2 H₂O

We report the variable temperature vibrational properties of single crystals of the S=1/2 Heisenberg antiferromagnet VOHP O₄ *1/2 H₂O. A pair of peaks in the far infrared spectral response may be due to magnetic excitations. We invoke a dynamic Dzyaloshinskii-Moriya mechanism to explain the activation and polarization dependence of the singlet-to-triplet gap in the far infrared, and we identify the low-energy phonons that likely facilitate this coupling. The spin-gap values are compared to those obtained via magnetic susceptibility, electron spin resonance, and neutron scattering. Vibrational mode splitting in VOHP O4 * 1 / 2 H2 O indicates 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. The low lattice symmetry is important for the proposed magnetoelastic interactions.