Chang Hwan Park

Dominant role of a pseudogap in the physics of the colossal magnetoresistive oxides

We have performed direct electronic structure measurements of the colosssal magnetoresistive oxides using high energy resolution photoemission spectroscopy. A key result of these measurements is the discovery of a pseudogap which drastically supresses or completely removes the spectral weight (density of states) at the Fermi energy EF. It is shown that the pseudogap plays a dominant role in the colossal conductivity changes which occur across the magnetic ordering temperature. We briefly discuss possible origins for the pseudogap.

Experimental Determination of the key Energy Scales in the Colossal Magnetoresistive Manganites

The authors have performed X-ray absorption (XAS) and angle-resolved photoemission (ARPES) on single crystals of both the layered and cubic colossal magnetoresistive manganites to determine the electronic structure and the relevant energy scales in the problem: the intra-atomic exchange energy J ({approximately} 2.7 eV), the Jahn-Teller energy gain E{sub J-T} ( 3 eV for layered compounds) and the lattice relaxation or polaron binding energy E{sub B} (.65 eV ferromagnetic phase and .8 eV paramagnetic phase). Lattice polarons are deemed important especially in the paramagnetic but also to a degree in the ferromagnetic phase. Due to the energy scale mismatch, the Jahn-Teller effect is unlikely to be the cause for these lattice polarons, at least for the layered samples.

COMPARISON OF THE MEASURED ELECTRONIC STRUCTURE OF THE COLOSSAL MAGNETORESISTIVE MANGANITES AND HIGH Tc SUPERCONDUCTORS: BAND STRUCTURE, PHOTOEMISSION LINESHAPES, AND A PSEUDOGAP

We have used high energy-resolution angle-resolved photoemission spectroscopy to study the k-dependent electronic structure of single-crystalline samples of the manganese-based perovskites as a function of doping level, temperature, and layer number. An especially important aspect of the data is a temperature and sample-type dependent pseudogap which drastically depletes the spectral weight at EF. In contrast to the pseudogap observed in the high Tc superconductors, the pseudogap in the manganites is essentially isotropic in k-space, implying that a local effect such as a lattice distortion should be responsible.

Electronic Structure Measurements of Colossal Magnetoresistive Manganese-Oxides: Polaronic Effects on the Band Structure

We have used high energy-resolution angle-resolved photoemission spectroscopy(ARPES) to map the k-dependent electronic structure of single-crystallinesamples of the manganese-based perovskites as a function of doping level, temperature,and layer number. The measured near-Fermi energy states display E vs. kand symmetry relationships which agree relatively well with the LSDA band theoryprediction through much of the Brillouin zone, and the locus of lowest energy excitations matches the predicted large Fermi surface quite well. However, thespectralfeatures are too broad to be well described as Fermi-Liquid likequasiparticles, and they are strongly suppressed from the Fermi energy, e.g., there is apseudogap in the excitation spectrum. We discuss the spectral properties interms of strong coupling to a local effect such as a lattice distortion.

Low Energy K -Dependent Electronic Structure of the Layered Magnetoresistive Oxide La 1.2 Sr 1.8 Mn 2 O 7

We have studied the fc-dependent electronic structure of the layered colossal magnetoresistive manganite La 1.2 Sr 1.8 Mn 2 O 7 using high-resolution angle-resolved photoemission spectroscopy. We found dispersive energy bands as a function of the crystal momentum k near the Fermi level (E F ) . We have also performed local spin density approximation (LSDA)+ U band-structure calculations on the current system. The overall experimental dispersion relation is basically in agreement with the band-structure calculations yet close to E F there is a significant deviation from the predicted dispersions. Instead of clear Fermi-surface (FS) crossings, we observe a depression of the features as the FS is approached as if there is a “pseudo” gap in the excitation spectrum. The pseudogap continuously opens with temperature and does not show further significant opening above T c , corresponding to the metal-insulator transition. Those unusual aspects of the spectra has been discussed from the viewpoint of the strong electron-lattice coupling model.