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Register a new userUltrafast Polaron Dynamics in Layered and Perovskite Manganites: 2D and 3D Polarons
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We have studied the sub-picosecond quasiparticle dynamics in the perovskite manganite La0.7Ca0.3MnO3 and the layered manganite La1.4Sr1.6Mn2O7 using ultrafast optical spectroscopy. We found that for T > TC, initial relaxation proceeds on the time scale of several hundred femtoseconds and corresponds to the redressing of a photoexcited electron to its polaronic ground state. The temperature and dimensionality dependence of this polaron redressing time provides insight into the relationship between polaronic motion and spin dynamics on a sub-picosecond time scale. We also observe a crossover to a more conventional electron-phonon relaxation in the ferromagnetic metallic phase below Tc.
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We report x-ray scattering studies of broad peaks located at a (0.5 0 0)/(0 0.5 0)-type wavevector in the paramagnetic insulating phases of La_{0.7}Ca_{0.3}MnO_{3} and Pr_{0.7}Ca_{0.3}MnO_{3}. We interpret the scattering in terms of correlated polarons and measure isotropic correlation lengths of 1-2 lattice constants in both samples. Based on the wavevector and correlation lengths, the correlated polarons are found to be consistent with CE-type bipolarons. Differences in behavior between the samples arise as they are cooled through their respective transition temperatures and become ferromagnetic metallic (La_{0.7}Ca_{0.3}MnO_{3}) or charge and orbitally ordered insulating (Pr_{0.7}Ca_{0.3}MnO_{3}). Since the primary difference between the two samples is the trivalent cation size, these results illustrate the robust nature of the correlated polarons to variations in the relative strength of the electron-phonon coupling, and the sensitivity of the low-temperature ground state to such variations.
Using the Lanczos method in linear chains we study the double exchange model in the low concentration limit, including an antiferromagnetic super-exchange K. In the strong coupling limit we find that the ground state contains ferromagnetic polarons whose length is very sensitive to the value of K/t. We investigate the dispersion relation, the trapping by impurities, and the interaction between these polarons. As the overlap between polarons increases, by decreasing K/t, the effective interaction between them changes from antiferromagnetic to ferromagnetic. The scaling to the thermodynamic limit suggests an attractive interaction in the strong coupling regime (J_h > t) and no binding in the weak limit (J_h simeq t).
We argue that in lightly hole doped perovskite-type Mn oxides the holes (Mn$^{4+}$ sites) are surrounded by nearest neighbor Mn$^{3+}$ sites in which the occupied $3d$ orbitals have their lobes directed towards the central hole (Mn$^{4+}$) site and with spins coupled ferromagnetically to the central spin. This composite object, which can be viewed as a combined orbital-spin-lattice polaron, is accompanied by the breathing type (Mn$^{4+}$) and Jahn-Teller type (Mn$^{3+}$) local lattice distortions. We present calculations which indicate that for certain doping levels these orbital polarons may crystallize into a charge and orbitally ordered ferromagnetic insulating state.
Angle-resolved photoemission spectroscopy data for the bilayer manganite La1.2Sr1.8Mn2O7 show that, upon lowering the temperature below the Curie point, a coherent polaronic metallic groundstate emerges very rapidly with well defined quasiparticles which track remarkably well the electrical conductivity, consistent with macroscopic transport properties. Our data suggest that the mechanism leading to the insulator-to-metal transition in La1.2Sr1.8Mn2O7 can be regarded as a polaron coherence condensation process acting in concert with the Double Exchange interaction.
The nature of the polarons in the optimally doped colossal magnetoresistive (CMR) materials La0.7Ba0.3MnO3 (LBMO) and La0.7Sr0.3MnO3 (LSMO) is studied by elastic and inelastic neutron scattering. In both materials, dynamic nanoscale polaron correlations develop abruptly in the ferromagnetic state. However, the polarons are not able to lock-in to the lattice and order, in contrast to the behavior of La0.7Ca0.3MnO3. Therefore ferromagnetic order in LBMO and LSMO survives their formation, explaining the conventional second order nature of the ferromagnetic--paramagnetic transition. Nevertheless, the results demonstrate that the fundamental mechanism of polaron formation is a universal feature of these ferromagnetic perovskite manganites.
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