The hole-doped manganite La 0.7 Ca 0.3 MnO 3 and the electron-doped manganite La 0.7 Ce 0.3 MnO 3 undergo an insulator to metal transition at around 250 K, above which both behave as a polaronic semiconductor. We have successfully fabricated an epitaxial trilayer (La 0.7 Ca 0.3 MnO 3 /SrTiO 3 /La 0.7 Ce 0.3 MnO 3 ), where SrTiO 3 is an insulator. At room temperature, i.e. in the semiconducting regime, it exhibits asymmetric current-voltage (I-V) characteristics akin to a p-n diode. The observed asymmetry in the I-V characteristics disappears at low temperatures where both the manganite layers are metallic. To the best of our knowledge, this is the first report of such a p-n diode, using the polaronic semiconducting regime of doped manganites.

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...

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We investigate the low-temperature electron, lattice, and spin dynamics of LaMnO3 (LMO) and La0.7Ca0.3MnO3 (LCMO) by resonant pump-probe reflectance spectroscopy. Probing the high-spin d-d transition as a function of time delay and probe energy, we compare the responses of the Mott insulator and the double-exchange metal to the photoexcitation. Attempts have previously been made to describe the sub-picosecond dynamics of CMR manganites in terms of a phenomenological three temperature model describing the energy transfer between the electron, lattice and spin subsystems followed by a comparatively slow exponential decay back to the ground state. However, conflicting results have been reported. Here we first show clear evidence of an additional component in the long term relaxation due to film-to-substrate heat diffusion and then develop a modified three temperature model that gives a consistent account for this feature. We confirm our interpretation by using it to deduce the bandgap in LMO. In addition we also model the non-thermal sub-picosecond dynamics, giving a full account of all observed transient features both in the insulating LMO and the metallic LCMO.

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