The $S=1/2$ antiferromagnetic Heisenberg spin chain compound ${\mathrm{Sr}}_2{\mathrm{CuO}}_3$ doped with $1\%$ and $2\%$ of Ni impurities has been studied by means of ${}^{63}\mathrm{Cu}$ 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 $\lambda <1$ 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.

• Received 27 February 2015

DOI:https://doi.org/10.1103/PhysRevB.92.060405