Defects in low dimensional quantum magnets*

Quantum systems are known to be extremely susceptible to defects when confined in one dimension. Spin chains and ladders are no exception to this rule. In this talk I will demonstrate the dramatic effect that a depletion with nonmagnetic impurities has on several such compounds. Due to fundamentally different low-energy degrees of freedom, impurities in spin chains and spin ladders play very different roles. In spin chains, scattering by defects leads to a confinement of low energy quasiparticles already present in the unperturbed system. As a result, a magnetic excitation spectrum acquires a concentration-dependent ``pseudogap'' which bears a universal description~[1]. In contrast, the unperturbed spin ladder has no low-energy excitations of its own. Instead, new local degrees of freedom are released upon the introduction of defects. Strong antiferromagnetic correlations shape them as spatially extended ``spin islands''. Although these spin islands are localized, they strongly interact and thereby give rise to unusual thermodynamic properties and novel collective modes~[2]. In both cases, inelastic neutron scattering allows to see the corresponding defect-induced transformations of the spin excitation spectra \emph{directly}. Good understanding of the clean ground states makes it possible to describe these metamorphoses of gapped and gapless behavior in quantitative ways. ~\\[4pt] [1] G. Simutis, S. Gvasaliya \emph{et al.}; Phys. Rev. Lett. \textbf{111}, 067204 (2013)\\ ~[2] D. Schmidiger, K. Yu. Povarov \emph{et al.}; Phys. Rev. Lett. \textbf{116}, 257203 (2016)