Static spin freezing at $T\sim 11\mathrm{K}$ was detected by muon spin relaxation ( $\mu \mathrm{SR}$) measurements of ${\mathrm{NaV}}_2{\mathrm{O}}_5$, which is widely considered to have a spin-singlet state below 35 K. This spin freezing is suppressed in charge-doped Na deficient samples ${\mathrm{Na}}_x{\mathrm{V}}_2{\mathrm{O}}_5$ with $x=0.99\mathrm{}$ and 0.90, in which spin gap formation is also suppressed. We discuss a few different scenarios to interpret this spin freezing.

• Received 27 January 1999

DOI:https://doi.org/10.1103/PhysRevLett.83.3301