Fermi Surface Nesting and Nanoscale Fluctuating Charge/Orbital Ordering in Colossal Magnetoresistive Oxides

We used high resolution angle-resolved photoemission spectroscopy to reveal the Fermi surface and key transport parameters of the metallic state of the layered Colossal Magnetoresistive (CMR) oxide La1.2Sr1.8Mn2O7. With these parameters the calculated in-plane conductivity is nearly one order of magnitude larger than the measured DC conductivity. This discrepancy can be accounted for by including the pseudogap which removes at least 90% of the spectral weight at the Fermi energy. Key to the pseudogap and many other properties are the parallel straight Fermi surface sections which are highly susceptible to nesting instabilities. These nesting instabilities produce nanoscale fluctuating charge/orbital modulations which cooperate with Jahn-Teller distortions and compete with the electron itinerancy favored by double exchange

Structural distortions and model Hamiltonian parameters: from LSDA to a tight-binding description of LaMnO_3

The physics of manganites is often described within an effective two-band tight-binding (TB) model for the Mn e_g electrons, which apart from the kinetic energy includes also a local "Hund's rule" coupling to the t_{2g} core spin and a local coupling to the Jahn-Teller (JT) distortion of the oxygen octahedra. We test the validity of this model by comparing the energy dispersion calculated for the TB model with the full Kohn-Sham band-structure calculated within the local spin-density approximation (LSDA) to density functional theory. We analyze the effect of magnetic order, JT distortions, and "GdFeO_3-type" tilt-rotations of the oxygen octahedra. We show that the hopping amplitudes are independent of magnetic order and JT distortions, and that both effects can be described with a consistent set of model parameters if hopping between both nearest and next-nearest neighbors is taken into account. We determine a full set of model parameters from the density functional theory calculations, and we show that both JT distortions and Hund's rule coupling are required to obtain an insulating ground state within LSDA. Furthermore, our calculations show that the "GdFeO_3-type" rotations of the oxygen octahedra lead to a substantial reduction of the hopping amplitudes but to no significant deviation from the simple TB model.Comment: replaced with final (published) version with improved presentatio

Electronic structure of Pr0.67_{0.67}Ca0.33_{0.33}MnO3_3 near the Fermi level studied by ultraviolet photoelectron and x-ray absorption spectroscopy

We have investigated the temperature-dependent changes in the near-$E$$_F$ occupied and unoccupied states of Pr$_{0.67}$Ca$_{0.33}$MnO$_3$ which shows the presence of ferromagnetic and antiferromagnetic phases. The temperature-dependent changes in the charge and orbital degrees of freedom and associated changes in the Mn 3$d$ - O 2$p$ hybridization result in varied O 2$p$ contributions to the valence band. A quantitative estimate of the charge transfer energy ($E$$_{CT}$) shows a larger value compared to the earlier reported estimates. The charge localization causing the large $E$$_{CT}$ is discussed in terms of different models including the electronic phase separation.Comment: 19 pages, 7 figures, To be published in Phy. Rev.

Pseudogap Formation in Models for Manganites

The density-of-states (DOS) and one-particle spectral function $\rm A({\bf k}, \omega)$ of the one- and two-orbital models for manganites, the latter with Jahn-Teller phonons, are evaluated using Monte Carlo techniques. Unexpectedly robust pseudogap (PG) features were found at low- and intermediate-temperatures, particularly at or near regimes where phase-separation occurs as $\rm T$$\to$0. The PG follows the chemical potential and it is caused by the formation of ferromagnetic metallic clusters in an insulating background. It is argued that PG formation should be generic of mixed-phase regimes. The results are in good agreement with recent photoemission experiments for $\rm La_{1.2} Sr_{1.8} Mn_2 O_7$.Comment: Accepted for publication in Phys. Rev. Lett., 4 pages, Revtex, with 4 figures embedde