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

TL;DR

This study investigates how Ni doping in Sr$_2$CuO$_3$ induces a spin pseudogap that suppresses impurity-induced local magnetism, with NMR measurements revealing a doping-dependent spin gap and reduced paramagnetic response.

Contribution

It demonstrates the suppression of impurity-induced local magnetism by a doping-dependent spin pseudogap in Ni-doped Sr$_2$CuO$_3$, highlighting finite-size effects.

Findings

Spin pseudogap scales with doping level.

Suppression of impurity-induced paramagnetic response.

Evidence of finite-size spin gap phenomena.

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 $\lambda < 1$ 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.