Hour-glass magnetic excitations induced by nanoscopic phase separation in cobalt oxides La$_{2-x}$Sr$_x$CoO$_4$

摘要

The magnetic excitations in the cuprate superconductors might be essentialfor an understanding of high-temperature superconductivity. In these cupratesuperconductors the magnetic excitation spectrum resembles an hour-glass andcertain resonant magnetic excitations within are believed to be connected tothe pairing mechanism which is corroborated by the observation of a universallinear scaling of superconducting gap and magnetic resonance energy. So far,charge stripes are widely believed to be involved in the physics of hour-glassspectra. Here we study an isostructural cobaltate that also exhibits anhour-glass magnetic spectrum. Instead of the expected charge stripe order weobserve nano phase separation and unravel a microscopically split origin ofhour-glass spectra on the nano scale pointing to a connection between themagnetic resonance peak and the spin gap originating in islands of theantiferromagnetic parent insulator. Our findings open new ways to theories ofmagnetic excitations and superconductivity in cuprate superconductors.