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Register a new userPressure-induced melting of the orbital polaron lattice in La1-xSrxMnO3
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We report on the pressure effects on the orbital polaron lattice in the lightly doped manganites $mathrm{La_{1-x}Sr_xMnO_{3}}$, with $xsim 1/8$. The dependence of the orbital polaron lattice on $negative$ chemical pressure is studied by substituting Pr for La in $mathrm{(La_{1-y}Pr_y)_{7/8}Sr_{1/8}MnO_{3}}$. In addition, we have studied its hydrostatic pressure dependence in $mathrm{(La_{0.9}Pr_{0.1})_{7/8}Sr_{1/8}MnO_{3}}$. Our results strongly indicate that the hopping $t$ significantly contributes to the stabilization of the orbital polaron lattice and that the orbital polarons are ferromagnetic objects which get stabilized by local double exchange processes. The analysis of short range orbital correlations and the verification of the Grueneisen scaling by hard x-ray, specific heat and thermal expansion data reinforces our conclusions.
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By resonant x-ray scattering at the Mn K-edge on La7/8Sr1/8MnO3, we show that an orbital polaron lattice (OPL) develops at the metal-insulator transition of this compound. This orbital reordering explains consistently the unexpected coexistence of ferromagnetic and insulating properties at low temperatures, the quadrupling of the lattice structure parallel to the MnO2-planes, and the observed polarization and azimuthal dependencies. The OPL is a clear manifestation of strong orbital-hole interactions, which play a crucial role for the colossal magnetoresistance effect and the doped manganites in general.
We have explored spin, charge and orbitally ordered states in La1-xSrxMnO3 (0 < x < 1/2) using model Hartree-Fock calculations on d-p-type lattice models. At x=1/8, several charge and orbitally modulated states are found to be stable and almost degenerate in energy with a homogeneous ferromagnetic state. The present calculation indicates that a ferromagnetic state with a charge modulation along the c-axis which is consistent with the experiment by Yamada et al. might be responsible for the anomalous behavior around x = 1/8.
The origin of the effect of colossal magneto-resistance (CMR) remains still unexplained. In this work we revisit the spin dynamics of the pseudo-cubic La1-xSrxMnO3 along the Mn-O-Mn bond direction at four x doping values (x < 0.5) and various temperatures and report a new lattice dynamics study at x0=0.2, representative of the optimal doping for CMR. We propose an interpretation of the spin dynamics in terms of orbital polarons. This picture is supported by the observation of a discrete magnetic energy spectrum Enmag (q) characteristic of the internal excitations of orbital polarons defined by Mn3+ neighbors surrounding a Mn4+ center with a hole. Because of its hopping, the hole mixes up dynamically all the possible orbital configurations of its surrounding Mn3+ sites with degenerate energies. The Enmag values indicate a lift of orbital degeneracy by phonon excitations. The number n varies with the spatial dimension D of the polaron and the q-range determines its size. At x=0.125 and x=0.3 the spectrum reveals 2D polarons coupled by exchange and 3D free polarons respectively, with sizes l=1.67a < 2a in all bond directions. At x0=0.2, the spin and the lattice dynamics provide evidence for chains of orbital polarons of size l=2a with a periodic distribution over ~ 3a and an interaction energy ~ 3 meV. At T < Tc the charges propagate together with the longitudinal acoustic phonons along the chains enhancing their ferromagnetic character. The phase separation between metallic and ferromagnetic chains in a non-metallic matrix may be crucial for CMR.
We report on the discovery of a novel triangular phase regime in the system La1-xSrxMnO3 by means of electron spin resonance and magnetic susceptibility measurements. This phase is characterized by the coexistence of ferromagnetic entities within the globally paramagnetic phase far above the magnetic ordering temperature. The nature of this phase can be understood in terms of Griffiths singularities arising due to the presence of correlated quenched disorder in the orthorhombic phase.
According to standard theory the magnetoresistance magnitude in ferromagnetic manganites crucially depends on the electron-phonon coupling strength. We showed that in La0.7Sr0.3MnO3 the phonon renormalization is strong, despite its relatively small magnetoresistance. Here, we report results of a similar inelastic neutron scattering investigation of a closely related compound, La0.8Sr0.2MnO3, where the magnetoresistance is enhanced. We find similar phonon renormalization and dynamic CE-type polaron correlations as in La0.7Sr0.3MnO3. However, quantitative comparison of the results for the two samples shows that only polaron lifetime is well correlated with the strength of the CMR.
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