Hour-glass magnetic excitations induced by nanoscopic phase separation in cobalt oxides

Abstract

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

Electronic and Magnetic Nano Phase Separation in Cobaltates La2−x Sr x CoO4

“…2. This is unlike to the behaviour in the undoped parent material La 2 CoO 4 and also unlike to the behaviour in charge stripe ordered nickelates [13].…”

“…Using the statistical method developed in Ref. [18], we analyzed the probability density function (PDF) and observed an almost perfect power-law behaviour that reflects a scale-invariant (fractallike) domain size distribution which nicely confirms our nano phase separation scenario in cobaltates [13]. Using these new tools, it has been possible to discover a common (structural) nano scale phase separation scenario characterized by the coexistence of competing granular networks of different striped orders [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33].…”

“…However, using 100 keV hard X-rays at the synchrotron, we were able to show that in La 5/3 Sr 1/3 CoO 4 the charge correlations essentially occur with CBCO character only, see Fig. 1b [13,14]. Also, in La 1.6 Sr 0.4 CoO 4 , the charge ordering peaks were not observed at the incommensurate positions that correspond to the magnetic peak positions, but, instead at half-integer peak positions typical for CBCO [8,14].…”

“…The absence of charge stripes and Fermi surfaces in La 5/3 Sr 1/3 CoO 4 and La 1.6 Sr 0.4 CoO 4 forced us to find an alternative mechanism for the emergence of the hour-glass spectrum [8,13]. We proposed that frustration effects in disordered CBCO phases produce the incommensurabilities in La 2−x Sr x CoO 4 [8].…”

Single-crystal growth and physical properties of 50% electron-doped rhodate Sr1.5La0.5RhO4

“…Besides of a comparative study of Sr 2 RhO 4 single crystals isotypic with Sr 2 RuO 4 mainly polycrystalline Sr 2−x La x RhO 4 samples were studied over a wider doping range x = 0 ∼ 1.0 [9][10][11] . Regarding the recent observations of charge order and nano phase separation in the 3d cobaltate system La 2−x Sr x CoO 4 [12][13][14][15] also the availability of single crystals of the 4d Rhodium analogue with hole-doping levels away from the end member Sr 2 RhO 4 and, especially, around 50% electron doping (Sr 1.5 La 0.5 RhO 4 ), would be of interest for future studies of the charge correlations.…”