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...

The magnetic and electronic properties of multiferroic TbMnO 3 in the paramagnetic, antiferromagnetic, sinusoidal, and spiral-spin phases were studied by spectral generalized magneto-optical ellipsometry. The measurements show a strong anisotropy of the dielectric tensor. A redistribution of spectral weight was observed in the diagonal components of the dielectric tensor for the temperature range from 110 to T N =46 K. In the off-diagonal elements, spectral generalized magneto-optical ellipsometry shows sensitivity to the antiferromagnetic and ferroelectric phase transitions at T N = 46 K and T F = 29 K, respectively, and a persistent signal up to 6T N . DOI: 10.1103/PhysRevB.77.193105 PACS number͑s͒: 75.47.Lx, 75.25.ϩz, 75.50.Ee, 77.22.Ej Multiferroic materials exhibit phases with simultaneous magnetic and ferroelectric properties. The close coupling of the magnetization and the electric polarization in multiferroics makes these materials excellent candidates for demonstrating giant magnetoelectrical behavior, 1,2 in which large magnetic and electric responses can be induced with weak electric and magnetic fields, respectively. 3,4 Spectral generalized magneto-optical ellipsometry ͑SGME͒ is an ideal technique for studying the interplay between electric and magnetic properties in multiferroic systems, as it probes both the diagonal xx and off-diagonal xy components of the dielectric tensor. 5 Multiferroicity in TbMnO 3 is induced by spin-charge coupling. Below the Néel temperature T N = 46 K, the wavelength of the incommensurate spin density decreases with decreasing temperature down to the ferroelectric transition at T F = 29 K. In the absence of a magnetic field, this spin-density modulation gets locked in at T F . Below T F , the inversion symmetry of the lattice is broken by the bc-cycloidal arrangement of the spins, inducing a ferroelectric polarization along the c axis while the antiferromagnetic ordering along the b axis persists. 3,4,6 Here, we study the coupling between spin and charge degrees of freedom in multiferroic TbMnO 3 by examining the temperature-dependent dielectric responses xx and xy , which are the diagonal and off-diagonal components of the dielectric tensor. The magnetization of a material breaks the symmetry between left and right circularly polarized ͑LCP and RCP͒ light. While 2xx is proportional to the sum of absorptions of LCP and RCP light, 1xy is proportional to the difference. 7 Consequently, a complete magneto-optical characterization of 2xx and 1xy in a magnetic material provides detailed information regarding the coupling between the material's electronic and magnetic properties.The temperature-dependent ellipsometry was done with a custom-made setup by using an extended Sentech SE850 spectral ellipsometer inside a He flow cryostat. The cryostat has stress-free mounted windows and maintains a base pressure of about 3 ϫ 10 −8 mbar at room temperature. 8 Standard ellipsometry was performed for temperatures from 10 to 470 K in the spectral range from 0.5 to 5.5 eV. In SGME, on...

We study the dynamics of the superconducting order parameter in the high-Tc cuprate Bi2Sr2CaCu2O8+delta by employing a novel time-resolved pump-probe Raman experiment. We find two different coupling mechanisms that contribute equally to the pair-breaking peak. One coupling sets in very fast at 2 ps and relaxes slowly, while the other one is delayed and sets in roughly at 5 ps and relaxes fast. A model that couples holes through phonons is able to reproduce one part of the condensate dynamics; thus, we argue that hole-spin interactions are of importance as well.