Suppression of the impurity-induced local magnetism by the opening of a spin pseudogap in Ni-doped Sr${}_2$CuO${}_3$

Abstract

The $S=1/2$ antiferromagnetic Heisenberg spin chain compound Sr${}_2$CuO${}_3$ doped with $1\%$ and $2\%$ of Ni impurities has been studied by means of ${}^{63}$Cu nuclear magnetic resonance. A strong decrease of the spin-lattice relaxation rate $T_1^{-1}$ at low temperatures points toward a spin gap, while a stretching exponent $\hat{I}»<1$ and a frequency dependence of $T_1^{-1}$ indicate that this spin gap varies spatially and should rather be characterized as a spin pseudogap. The magnitude of the spin pseudogap scales with doping level. Our results therefore evidence the finite-size character of this phenomenon. Moreover, an unusual narrowing of the low temperature NMR lines reveals the suppression of the impurity-induced staggered paramagnetic response with increasing doping level.

5 pages, 3 figures, 4 pages appendices