Impurities in the weakly coupled quantum spin chains Sr2CuO3 and SrCuO2

We study the effect of nonmagnetic ${\mathrm{Zn}}²⁺$ (spin-0) and magnetic ${\mathrm{Ni}}²⁺$ (spin-1) impurities on the ground state and low-lying excitations of the quasi-one-dimensional spin-$1/2$ Heisenberg antiferromagnet ${\mathrm{Sr}}₂{\mathrm{CuO}}₃$ using inelastic neutron scattering, specific heat, and bulk magnetization measurements. We show that 1% ${\mathrm{Ni}}²⁺$ doping in ${\mathrm{Sr}}₂{\mathrm{CuO}}₃$ results in a sizable spin gap in the spinon excitations, analogous to the case of Ni doped ${\mathrm{SrCuO}}₂$ previously reported [Simutis et al., Phys. Rev. Lett. 111, 067204 (2013)]. However, a similar level of ${\mathrm{Zn}}²⁺$ doping in ${\mathrm{SrCuO}}₂$, investigated here for comparison, did not reveal any signs of a spin gap. Magnetic ordering temperature was found to be suppressed in the presence of both ${\mathrm{Zn}}²⁺$ and ${\mathrm{Ni}}²⁺$ impurities; however, the rate of suppression due to ${\mathrm{Ni}}²⁺$ was found to be much more pronounced than for ${\mathrm{Zn}}²⁺$. Effect of magnetic field on the ordering temperature is investigated. We found that with increasing magnetic field, not only the magnetic ordering temperature gradually increases but the size of specific heat anomaly associated with the magnetic ordering also progressively enhances, which can be qualitatively understood as due to the field induced suppression of quantum fluctuations.