The electronic response of doped manganites at the transition from the paramagnetic insulating to the ferromagnetic metallic state in La 1−x Ca x MnO 3 for x = 0.3 and 0.2 was investigated by dc conductivity, ellipsometry, and vacuum ultraviolet reflectance for energies between 0 and 22 eV. A stabilized Kramers-Kronig transformation yields the optical conductivity and reveals changes in the optical spectral weight up to 22 eV at the metal-to-insulator transition. In the observed energy range, the spectral weight is conserved within 0.3%. The redistribution of spectral weight in this surprisingly broad energy range has important ramifications for the effective low-energy physics. We discuss the importance of the charge-transfer, Coulomb on-site, Jahn-Teller, and long-range Coulomb screening effects to the electronic structure. Among strongly correlated materials, the manganites exhibit a wealth of novel properties. For example, some hexagonal insulating materials exhibit multiferroic behavior and the cubic doped manganites show charge ordering and the colossal magnetoresistance ͑CMR͒ effect.1,2 It is clear that the two key ingredients responsible for these diverse phenomena are, first, the high geometrical and spin frustration and, second, the large number of competing interactions, the most important of which are the electron-electron and electron-phonon interactions. 1,[3][4][5][6][7][8][9] There is a deep disagreement as to which of these interactions is the primary driving force behind either the insulating phase of the manganites or the metal-to-insulator transition in the doped manganites. Models describing these phenomena involve double exchange, Jahn-Teller ͑JT͒, superexchange, and Coulomb on-site ͑Hubbard U͒ interactions that yield effective low-energy Hamiltonians, which predict different types of quasiparticle excitations, such as spin excitations, lattice polarons, spin polarons, or orbitons. 3,4,[6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] However, the effective Hamiltonians used to describe the manganites typically ignore the oxygen p bands and consider only an effective manganese d band. It is also generally assumed that the high-energy degrees of freedom can be neglected by a "down folding" of the large number of bands into a single effective band. This implies that there is no redistribution of electronic states between low-energy and high-energy degrees of freedom. On the other hand, if one considers the importance of local interactions and hybridization in correlated materials, one would expect quite pronounced effects at higher energies that are connected to charge-transfer or Mott-Hubbard physics. 15,[22][23][24] Thus, the important test for the effective low-energy picture is to study whether one finds strong exchanges of spectral weight between low and high energies.Therefore, it is crucial to test the complex nature of the band structure explicitly. The most direct experiment is to measure the dielectric response of a material as a function of temperature and doping. Unfort...

Multiferroicity in LiCu 2 O 2 single crystals is studied using resonant soft x-ray magnetic scattering, hard x-ray diffraction, heat capacity, magnetic susceptibility, and electrical polarization. Two magnetic transitions are found at 24.6 K ͑T 1 ͒ and 23.2 K ͑T 2 ͒. Our data are consistent with a sinusoidal spin structure at T 2 Ͻ T Ͻ T 1 and with a helicoidal spin structure at T Ͻ T 2 , giving rise to ferroelectricity. Surprisingly, above T 2 , the correlation lengths of the spin structures increase as the temperature increases with dramatic changes of ϳ42% occurring along the c axis. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2787973͔ Low-dimensional spin ͑S͒ =1/ 2 copper-oxides have posed some of the most challenging problems in solid state physics. The interplay between frustration and quantum spin fluctuations results in a rich phase diagram and unusual magnetic properties. In a model with quantum S =1/ 2 chains and competing nearest-neighbor interactions J 1 and next-nearestneighbor interactions J 2 , one expects, depending on J 1 / J 2 , a gapless collinear phase, a gapped disordered dimer liquid phase, or a quasi long-range ordered helicoidal spin structure. has shown the presence of a spin-singlet state with a spin gap of about 6 meV at 23 K. Specific heat 4 and nuclear magnetic resonance 7 show phase transitions at 24.2, 22.5, and 9 K. Recent neutron diffraction 5 found one transition to IC magnetic superstructure below 22 K with a propagation vector ͓͑2n +1͒ / 2,k + ␦ , l͔, where n, k, and l are integers and ␦ ϳ 0.174. It is concluded that the magnetic superstructure is a helical in the ab plane. This is inconsistent with the observation of ferroelectricity with an electric polarization along the c direction below ϳ23 K. 6 In an attempt to understand the coupling between lattice, charge, and spin degrees we have studied LiCu 2 O 2 single crystal using polarization dependent resonant soft x-ray magnetic scattering ͑RSXMS͒, hard x-ray diffraction ͑HXD͒, heat capacity, magnetic susceptibility, and electrical polarization, from the same sample.The LiCu 2 O 2 single crystal was grown by the self-flux method. 6 The HXD was done at the beamline BW5 of HA-SYLAB with photon energy of 100.5 keV. The lattice parameters a = 5.6963, b = 2.8497, and c = 12.417 Å of the orthorhombic structure at low temperature were verified. This confirms the high crystalline perfection and the absence of impurity phases. The crystal is found to be microscopia͒ Electronic

A new excitation is observed at 201 meV in the doped-hole ladder cuprate Sr14Cu24O41, using ultraviolet resonance Raman scattering with incident light at 3.7 eV polarized along the direction of the rungs. The excitation is found to be of charge nature, with a temperature independent excitation energy, and can be understood via an intra-ladder pair-breaking process. The intensity tracks closely the order parameter of the charge density wave in the ladder (CDWL), but persists above the CDWL transition temperature (TCDW L ), indicating a strong local pairing above TCDW L . The 201 meV excitation vanishes in La6Ca8Cu24O 41+δ , and La5Ca9Cu24O41 which are samples with no holes in the ladders. Our results suggest that the doped holes in the ladder are composite bosons consisting of paired holons that order below TCDW .