Hour-glass magnetic excitations
A. C. Komarek, Z. W. Li, Y. Drees, H. Guo, L. H. Tjeng
Charge stripes are widely believed to play an important role for the physics of high-temperature superconducting (HTSC) cuprates. However, these vertical/horizontal charge stripes were known for a long time only for certain copper oxides with a corrugated low-temperature tetragonal (LTT) structure in a small range of hole-concentration around 1/8-doping, i.e. in La2−xBaxCuO4 and La2−x−yNdySrxCuO4 with x ∼ 1/8 . Initially, in the prototypical cuprate material La2−xSrxCuO4 (LSCO) charge stripes have not been observed; even not around 1/8-doping. However, we were able to detect these long-sought charge stripe phases for the ﬁrst time in LSCO within a combined resonant and hard X-ray diﬀraction study . Aftewards, charge stripes or charge density waves (CDW) were found in many other copper oxide materials without LTT structure (for compositions with hole-concentrations around 1/8-doping). This shows that cuprates are susceptible to charge stripe / CDW instabilities in this certain range of hole-doping but does not yet give any conclusive evidence for the relevance of ﬂuctuating charge stripes for the superconducting pairing mechanism. (Note, that static charge stripes tend to suppress superconductivity.) Surprisingly, an hour-glass magnetic spectrum has been recently observed in the cobalt oxide material La5/3Sr1/3CoO4  which is isostructural to these HTSC cuprates. Since the magnetic peak positions were found at the same positions in reciprocal space like the ones in the isostructural nickelates with well-known robust diagonal charge stripe order, also in these cobalt oxides the presence of charge stripes was assumed. Hence, it was conjectured that this ”system has stripe correlations and is an insulator, which means that its magnetic dynamics can conclusively be ascribed to stripes. The results provide compelling evidence that the hour-glass spectrum in the copper oxide superconductors arises from ﬂuctuating stripes.” . In a very recent study on La1.6Sr0.4CoO4 we found no indications for charge stripes neither in diﬀraction measurements nor in the Co-O bond-stretching phonon dispersion . However, we were also able to observe hour-glass shaped magnetic excitations in this cobaltate. Hence, we could show that besides Fermi surface eﬀects also charge stripes are not needed for the emergence of hour-glass spectra . In a subsequent study we re-analyzed La5/3Sr1/3CoO4. Also in this material we were able to exclude any signiﬁcant role of charge stripes and unraveled a new nano-phase separation scenario for the emergence of hour-glass spectra in these cobaltates [5,6]. One conclusion from our new model is, that the hour-glass spectrum consists of excitations with distinct origin on the nanometer scale, see Fig. 1.
Whereas the low-energy regime is dominated by excitations within nanometer-sized hole-doped regions, at higher energies only the nanoscopic undoped regions can be excited since J is much higher in these regions. Our results show that nano phase separation plays an important role for the emergence of hour-glass spectra which can no longer be adequately described as a magnetic excitation spectrum arising from one single phase in the cobaltates. Hence, our findings in the isostructural cobaltates shed a new light also on the hour-glass spectra in HTSC cuprates, which were found to be the unifying property of these intriguing materials.