The single-layer perovskite cobaltates have attracted enormous attention due to the recent observation of hour-glass shaped magnetic excitation spectra which resemble the ones of the famous high-temperature superconducting cuprates. Here, we present an overview of our most recent studies of the spin and charge correlations in floating-zone grown cobaltate single crystals. We find that frustration and a novel kind of electronic and magnetic nano phase separation are intimately connected to the appearance of the hour-glass shaped spin excitation spectra. We also point out the difference between nano phase separation and conventional phase separation.

We observe quasi-static incommensurate magnetic peaks in neutron scattering experiments on layered cobalt oxides La 2− x Sr x CoO 4 with high Co oxidation states that have been reported to be paramagnetic. This enables us to measure the magnetic excitations in this highly hole-doped incommensurate regime and compare our results with those found in the low-doped incommensurate regime that exhibit hourglass magnetic spectra. The hourglass shape of magnetic excitations completely disappears given a high Sr doping. Moreover, broad low-energy excitations are found, which are not centered at the incommensurate magnetic peak positions but around the quarter-integer values that are typically exhibited by excitations in the checkerboard charge ordered phase. Our findings suggest that the strong inter-site exchange interactions in the undoped islands are critical for the emergence of hourglass spectra in the incommensurate magnetic phases of La 2− x Sr x CoO 4 .

We observe quasi-static incommensurate magnetic peaks in neutron scattering experiments on layered cobalt oxides La2-xSrxCoO4 with high Co oxidation states that have been reported to be paramagnetic. This enables us to measure the magnetic excitations in this highly hole-doped incommensurate regime and compare our results with those found in the low-doped incommensurate regime that exhibit hourglass magnetic spectra. The hourglass shape of magnetic excitations completely disappears given a high Sr doping. Moreover, broad low-energy excitations are found, which are not centered at the incommensurate magnetic peak positions but around the quarter-integer values that are typically exhibited by excitations in the checkerboard charge ordered phase. Our findings suggest that the strong inter-site exchange interactions in the undoped islands are critical for the emergence of hourglass spectra in the incommensurate magnetic phases of La2-xSrxCoO4.

We observe quasi-static incommensurate magnetic peaks in neutron scattering experiments on layered cobalt oxides La2-xSrxCoO4 with high Co oxidation states that have been reported to be paramagnetic. This enables us to measure the magnetic excitations in this highly hole-doped incommensurate regime and compare our results with those found in the low-doped incommensurate regime that exhibit hourglass magnetic spectra. The hourglass shape of magnetic excitations completely disappears given a high Sr doping. Moreover, broad low-energy excitations are found, which are not centered at the incommensurate magnetic peak positions but around the quarter-integer values that are typically exhibited by excitations in the checkerboard charge ordered phase. Our findings suggest that the strong inter-site exchange interactions in the undoped islands are critical for the emergence of hourglass spectra in the incommensurate magnetic phases of La2-xSrxCoO4.

SmFeO3 has attracted considerable attention very recently due to its reported multiferroic properties above room temperature. We have performed powder and single crystal neutron diffraction as well as complementary polarization dependent soft X-ray absorption spectroscopy measurements on floating-zone grown SmFeO3 single crystals in order to determine its magnetic structure. We found a k=0 G-type collinear antiferromagnetic structure that is not compatible with inverse Dzyaloshinskii-Moriya interaction driven ferroelectricity. While the structural data reveal a clear sign for magneto-elastic coupling at the Néel-temperature of ∼675  K, the dielectric measurements remain silent as far as ferroelectricity is concerned.

C.-Y. Kuo, Y. Drees, M. T. Fernández-Díaz, L. Zhao, L. Vasylechko, D. Sheptyakov, A. M. T. Bell, T. W. Pi, H.-J. Lin, M.-K. Wu, E. Pellegrin, S. M. Valvidares, Z.W. Li, P. Adler, A. Todorova, R. Küchler, A. Steppke, L. H. Tjeng, Z. Hu, and A. C. Komarek Max-Planck-Institute for Chemical Physics of Solids, Nöthnitzer Strasse 40, 01187 Dresden, Germany Institut Laue-Langevin, 38042 Grenoble, France Institute of Physics, Academia Sinica, Taipei 11529, Taiwan Lviv Polytechnic National University, 12 Bandera Street, 79013 Lviv, Ukraine Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland HASYLAB at DESY, Notkestrasse 85, 22607 Hamburg, Germany National Synchrotron Radiation Research Center (NSRRC), 101 Hsin-Ann Road, Hsinchu 30077, Taiwan CELLS-ALBA Synchrotron Radiation Facility, Carretera BP 1413, km 3.3, E-08290 Cerdanyola del Vallès, Barcelona, Spain (Received 18 June 2014; published 20 November 2014)

C.-Y. Kuo, Y. Drees, M. T. Fernández-Dı́az, L. Zhao, L. Vasylechko, 4 D. Sheptyakov, A. M. T. Bell, T. W. Pi, H.-J. Lin, M.-K. Wu, E. Pellegrin, S. M. Valvidares, Z. W. Li, P. Adler, A. Todorova, R. Küchler, A. Steppke, L. H. Tjeng, Z. Hu, and A. C. Komarek ∗ Max-Planck-Institute for Chemical Physics of Solids, Nöthnitzer Str. 40, 01187 Dresden, Germany Institut Laue-Langevin, 38042 Grenoble, France Institute of Physics. Academia Sinica, Taipei 11529, Taiwan Lviv Polytechnic National University, 12 Bandera St., 79013 Lviv, Ukraine Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland HASYLAB at DESY, Notkestrasse 85, 22607 Hamburg, Germany National Synchrotron Radiation Research Center (NSRRC), 101 Hsin-Ann Road, Hsinchu 30077, Taiwan CELLS-ALBA Synchrotron Radiation Facility, Carretera BP 1413, km 3.3, E-08290 Cerdanyola del Vall‘es, Barcelona, Spain (Dated: December 10, 2014)

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.

An hour-glass-shaped magnetic excitation spectrum appears to be a universal characteristic of the high-temperature superconducting cuprates. Fluctuating charge stripes or alternative band structure approaches are able to explain the origin of these spectra. Recently, an hour-glass spectrum has been observed in an insulating cobaltate, thus favouring the charge stripe scenario. Here we show that neither charge stripes nor band structure effects are responsible for the hour-glass dispersion in a cobaltate within the checkerboard charge-ordered regime of La(2-x)Sr(x)CoO(4). The search for charge stripe ordering reflections yields no evidence for charge stripes in La(1.6)Sr(0.4)CoO(4), which is supported by our phonon studies. With the observation of an hour-glass-shaped excitation spectrum in this stripeless insulating cobaltate, we provide experimental evidence that the hour-glass spectrum is neither necessarily connected to charge stripes nor to band structure effects, but instead, probably intimately coupled to frustration and arising chiral or non-collinear magnetic correlations.

Charge stripes have been predicted as a combined charge and spin-density wave phenomenon [1-3] before being experimentally observed in La2-xSrxNiO4 [4] and in the Nd codoped La2xSrxCuO4 [5]. However, the role of this charge stripe instability for the superconducting pairing mechanism still remains a matter of debate [6,7]. Remarkable attention has been reattracted to the physics of stripe phases due to a recent discovery of a “hour-glass”-shaped magnetic spectrum in La2-xSrxCoO4 [8] similar to the observations in the superconducting cuprates [8,9]. Apart from the cobaltates [10] also the nickelates could be a useful reference system since a rather stable diagonal charge stripe order has been observed in these systems at higher hole-doping [4]. However, so far only the higher Sr-/hole-doped La2-xSrxNiO4 compounds have been studied in detail and no “hourglass”dispersion has been found in these systems. The exact onset of stripe ordering is not known properly and has not been studied systematically (to the best of our knowledge). Therefore, we are interested in studying charge stripe phases in the nickelates La2-xSrxNiO4 systematically. In particular we were interested in the interplay of stripe ordering and structural distortions. Hence, we measured a whole series of La2-xSrxNiO4 samples with low Sr-doping at beamline P02 HRPD in order to study the symmetry and lattice parameters as a function of Sr-/hole-doping.

Magnetic correlations in superconducting LiFeAs were studied by elastic and by inelastic neutron-scattering experiments. There is no indication for static magnetic ordering, but inelastic correlations appear at the incommensurate wave vector (0.5±δ,0.5-/+δ,0) with δ~0.07 slightly shifted from the commensurate ordering observed in other FeAs-based compounds. The incommensurate magnetic excitations respond to the opening of the superconducting gap by a transfer of spectral weight.

The crystal structure and magnetic structure of the chromate series $A$CrO${}_{3}$ ($A$ $=$ Ca, Sr, Pb) where Cr exhibits the unusual Cr${}^{4+}$ oxidation state have been studied by comprehensive diffraction experiments. In addition we present a detailed analysis of the infrared optical properties of polycrystalline CaCrO${}_{3}$ which is found to be metallic along all crystallographic directions. CaCrO${}_{3}$ forms the rare case of a three-dimensional antiferromagnetic and metallic transition-metal oxide in which the occurrence of magnetic ordering is closely coupled to a flattening of the lattice. The same $C$-type magnetic order is also found in the Sr material and again is coupled to a structural distortion associated with lattice flattening. However, in SrCrO${}_{3}$ only a part of the sample transforms into the antiferromagnetic distorted phase. In contrast to CaCrO${}_{3}$ and SrCrO${}_{3}$, PbCrO${}_{3}$ clearly is insulating with a significantly larger unit-cell volume and exhibits $G$-type magnetic order.

High-resolution and high-flux neutron as well as x-ray powder-diffraction experiments were performed on the oxypnictide series LaO1-xFxFeAs with 0 <= x <= 0.15 in order to study the crystal and magnetic structure. The magnetic symmetry of the undoped compound corresponds to those reported for REOFeAs (with RE a rare earth) and for AFe(2)As(2)(A=Ba, Sr) materials. We find an ordered magnetic moment of 0.63(1) mu B at 2 K in LaOFeAs, which is significantly larger than the values previously reported for this compound. A sizable ordered magnetic moment is observed up to a F doping of 4.5% whereas there is no magnetic order for a sample with a F concentration of x=0.06. In the undoped sample, several interatomic distances and FeAs4 tetrahedra angles exhibit pronounced anomalies connected with the broad structural transition and with the onset of magnetism supporting the idea of strong magnetoelastic coupling in this material.