Interacting chains of orbital polarons in the colossal magnetoresistance material La1-xSrxMnO3 revealed by spin and lattice dynamics

Article

关键词: MAGNETIC-PROPERTIES;    TRANSITION;    LIQUID;    PHASE;    CHARGE;   

DOI  :  10.1103/PhysRevB.99.214416

来源: SCIE

【 摘 要 】

The origin of the effect of colossal magnetoresistance (CMR) remains still unexplained. In this work we revisit the spin dynamics of the pseudocubic La1-xSrxMnO3 along the Mn-O-Mn bond direction at four x doping values (x <= 0.5) and various temperatures and report a lattice dynamics study at x(0) = 0.2, representative of 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 E-n(mag)(q) with n levels, characteristic of the internal excitations of orbital polarons defined by Mn3+ neighbors surrounding a central Mn4+ hole. Because of its hopping, the hole mixes dynamically all the possible orbital configurations of its surrounding Mn3+ with degenerate energies. The E-n(mag) values indicate a lifting of orbital degeneracy by phonon excitations. The n value and the q range are used to characterize these orbital polarons in direct space. At x = 0.125 and x = 0.3 the spectrum reveals two-dimensional polarons coupled by exchange and three-dimensional free polarons, respectively, with sizes l = 1.67a <= 2a in all bond directions. At x(0) = 0.2, the spin and the lattice dynamics provide evidence for chains of orbital polarons of size l = 2a with a periodic distribution over approximate to 3a and an interaction energy approximate to 3 meV. At T <= T-c 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 nonmetallic matrix may be crucial for CMR.

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