Spin pseudogap in the $S=\frac{1}{2}$ chain material Sr${}_2$CuO${}_3$ with impurities

Abstract

The low energy magnetic excitation spectrum of the Heisenberg antiferromagnetic $S=1/2$ chain system Sr${}_2$CuO${}_3$ with Ni- and Ca-impurities is studied by neutron spectroscopy. In all cases, a defect-induced spectral pseudogap is observed and shown to scale proportionately to the number of scattering centers in the spin chains.