51V-NMR study on the S=1/2 square lattice antiferromagnet K2V3O8

Abstract

Static and dynamic properties of the quasi-two-dimensional antiferromagnet K${}_2$V${}_3$O${}_8$ have been investigated by ${}^{51}$V-NMR experiments on nonmagnetic V${}^{5+}$ sites. Above the structural transition temperature $T_{\mathrm{S}}$ = 115 K, NMR spectra are fully compatible with the $P4bm$ space group symmetry. The formation of superstructure below $T_{\mathrm{S}}$ causes splitting of the NMR lines, which get broadened at lower temperatures so that individual peaks are not well resolved. Evolution of NMR spectra with magnetic field along $c$-axis below the magnetic transition temperature $T_{\mathrm{N}}\sim 4$ K is qualitatively consistent with a simple Néel order and a spin flop transition. However, broad feature of the spectra does not rule out possible incommensurate spin structure. The spin-lattice relaxation rate $1/T_1$ below $T_{\mathrm{N}}$ shows huge enhancement for a certain range of magnetic field, which is independent of temperature and attributed to cross relaxation due to anomalously large nuclear spin-spin coupling between V${}^{5+}$ and magnetic V${}^{4+}$ sites. The results indicate strong gapless spin fluctuations, which could arise from incommesurate orders or complex spin textures.

13 pages, 14 figures