Observation of Orbital Currents in CuO Scagnoli, V.; Staub, U.; Bodenthin, Y. ...
Science (American Association for the Advancement of Science),
05/2011, Letnik:
332, Številka:
6030
Journal Article
Recenzirano
Orbital currents are proposed to be the order parameter of the pseudo-gap phase of cuprate high-temperature superconductors. We used resonant x-ray diffraction to observe orbital currents in a ...copper-oxygen plaquette, the basic building block of cuprate superconductors. The confirmation of the existence of orbital currents is an important step toward the understanding of the cuprates as well as materials lacking inversion symmetry, such as magnetically induced multiferroics. Although observed in the antiferromagnetic state of cupric oxide, we show that orbital currents can occur even in the absence of long-range magnetic moment ordering.
An experimental approach to the analysis of charge, magnetic and orbital ordering in 3d transition‐metal oxides is presented. The technique combines two important components: azimuthal rotations ...around the Bragg wavevector and polarization analysis of the Bragg intensities in the range 500–900 eV. The polarization analysis is performed using graded multilayers, which are translated and rotated in the vacuum chamber. It is shown why these two components are important to determine the origin of the Bragg scattered signals and how they allow us to separate the different contributions. Examples are given for the oxygen K and the Mn, Co, Ni and Cu L2,3‐edges, and the advantages and drawbacks of this experimental technique are discussed.
We report the doping dependence of the ground state of A-site ordered manganites below and above half doping. Energy and polarization dependence of the orbital reflection, taken by resonant ...soft-x-ray powder diffraction, at both Mn L(2,3) and O K edges, provides direct evidence for orbital order at Mn(3+) and oxygen sites and absence of Zener polaron formation. For x > or = 0.2 anomalous melting of the orbital order is observed, which is coupled neither to magnetic ordering nor to a structural transition, indicating a two-dimensional character of the interactions.
We report the observation of superstructures associated with the oxygen 2p states in two prototypical manganites using x-ray diffraction at the oxygen K edge. In the stripe order system ...Bi0.31Ca0.69MnO3, hole-doped O states are orbitally ordered, at the same propagation vector as the Mn orbital ordering, but no oxygen charge stripes are found at this periodicity. In La7/8Sr1/8MnO3, we observe a 2p charge ordering described by alternating hole-poor and hole-rich MnO planes that is consistent with some of the recent predictions.
Based on measurements of soft x-ray magnetic diffraction under in situ applied electric field, we report on significant manipulation and exciting of commensurate magnetic order in multiferroic ...ErMn2O5. The induced magnetic scattering intensity arises at the commensurate magnetic Bragg position whereas the initial magnetic signal almost persists. We demonstrate the possibility to imprint a magnetic response function in ErMn2O5 by applying an electric field.
A detailed structural analysis of a Langmuir−Blodgett (LB) multilayer composed of a polyelectrolyte−amphiphile complex (PAC) is presented. The PAC is self-assembled from metal ions, ditopic ...bis-terpyridines, and amphiphiles. The vertical structure of the LB multilayer is investigated by X-ray reflectometry. The multilayer has a periodicity of 57 Å, which corresponds to an architecture of flat lying metallo-supramolecular coordination polyelectrolyte (MEPE) rods and upright-standing amphiphiles (dihexadecyl phosphate, DHP). In-plane diffraction reveals hexagonal packing of the DHP molecules. Using extended X-ray absorption fine structure (EXAFS) experiments, we prove that the central metal ion is coordinated to the terpyridine moieties in a pseudo-octahedral coordination environment. The Fe−N bond distances are 1.82 and 2.0 Å, respectively. Temperature resolved measurements indicate a reversible phase transition in a temperature range up to 55 °C. EXAFS measurements indicate a lengthening of the average Fe−N bond distance from 1.91 to 1.95 Å. The widening of the coordination cage upon heating is expected to lower the ligand field stabilization, thus giving rise to spin transitions in these composite materials.
A phase transition in an amphiphilic mesophase is explored to deliberately induce mechanical strain in an assembly of tightly coupled metal ion coordination centers. Melting of the alkyl chains in ...the amphiphilic mesophase causes distortion of the coordination geometry around the central transition metal ion. As a result, the crystal field splitting of the d-orbital subsets decreases resulting in a spin transition from a low-spin to a high-spin state. The diamagnetic−paramagnetic transition is reversible. This concept is demonstrated in a metallo-supramolecular coordination polyelectrolyte−amphiphile complex self-assembled from ditopic bis-terpyridines, Fe(II) as central transition metal, and dialkyl phosphates as amphiphiles. The magnetic properties are studied in a Langmuir−Blodgett multilayer. The modularity of this concept provides extensive control of structure and function from molecular to macroscopic length scales and gives access to a wide range of new molecular magnetic architectures such as nanostructures, thin films, and liquid crystals.
The combination of liquid crystals and spin-crossover (SCO) in one compound offers exciting perspectives for magnetic materials. The liquid crystalline organization of SCO compounds adds another ...dimension to magnetic materials. The SCO compounds can be for instance reoriented through external fields, which alters the optical properties. In addition, the LC phase transition can induce a SCO in the embedded compounds by inducing mechanical strain, which can perturb the coordination geometry of the central metal ions thus giving rise to a change in the electronic and magnetic properties. So far, this concept has only been implemented in a few material combinations. Here, we review the magnetic properties in metallo-supramolecular coordination polyelectrolyte-amphiphile complexes (PACs).
Multilayers of an Fe(II)‐polyelectrolyte‐amphiphile complex (Fe‐PAC) were investigated simultaneously by energy‐dispersive X‐ray reflectivity and in‐plane diffraction at the bending‐magnet beamline ...at BESSY II. By recording spectra between room temperature and about 348 K with a time resolution of about 60 s, two phase transitions were identified and the respective activation energies were determined. Owing to a fixed geometrical set‐up, an energy‐dispersive experiment is suitable for measuring X‐ray reflectivity and in‐plane X‐ray diffraction of thin organic films simultaneously. Installed at a bending magnet at BESSY II and using an energy range of 4 < E < 25 keV, both reflectivity and diffraction spectra can be collected within about 60 s. The performance is demonstrated by temperature‐ and time‐resolved measurements of the phase transition behaviour of multilayers of an Fe‐PAC deposited on a silicon support by means of the Langmuir–Blodgett (LB) technique. By measuring the X‐ray reflectivity while increasing the temperature, it is shown that the original LB phase is not stable and transforms irreversibly into a liquid‐crystalline (LC)‐like phase at about 318 K. At the same temperature the in‐plane diffraction signal vanishes reversibly, reflecting rotational disorder of the hexagonal arrangement of amphiphilic chains. Its activation energy is determined to be about 1.3 eV. At about 338 K a second irreversible phase transition occurs to another LC phase with smaller vertical layer spacing. This transition is reversible between 329 K and 338 K.
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