  We report on the crystal structure and magnetic behavior of the spin-1/2
compound AgVOAsO4. Magnetic susceptibility, high-field magnetization, and
electron spin resonance measurements identify AgVOAsO4 as a gapped quantum
magnet with a spin gap Delta ~ 13 K and a saturation field H_s ~ 48.5 T.
Extensive band structure calculations establish the microscopic magnetic model
of spin chains with alternating exchange couplings J ~ 40 K and J' ~ 26 K.
However, the precise evaluation of the spin gap emphasizes the role of
interchain couplings which are frustrated due to the peculiar crystal structure
of the compound. The unusual spin model and the low energy scale of the
exchange couplings make AgVOAsO4 a promising candidate for an experimental
investigation of Bose-Einstein condensation and other exotic ground states in
high magnetic fields.
