Spectroscopic Studies of Inhomogeneous Electronic Phases in Colossal Magnetoresistance and Charge-Ordering Compounds
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Mat. Res. Soc. Symp. Proc. Vol. 602 © 2000 Materials Research Society
matter physics, as there is growing evidence that the physics associated with this phenomenon is at the root of some of the most dramatic properties discovered in this field over the past few decades, including (a) "electronic phase separation" behavior, i.e., the coexistence of meso-scale magnetic phase and "colossal polaron formation [1,2], (b) magnetic regions magnetoresistance" (CMR) [3,4]; and (c) "charge-ordering" (CO) behavior, i.e., the spontaneous ordering of charge in periodic patterns on the lattice [5,6,7]. An understanding of these important phenomena, and the relationship among the diverse ground states exhibited by these systems, demands an understanding of the processes by which electronic phase separation, polaron formation, and charge ordering occurs in various doped insulators. In this paper, we juxtapose three different forms of electronic inhomogeneity in various strongly-coupled systems: (a) pure spin polaron
development in EuB6, a ferromagnetic metal exhibiting CMR-type behavior with weak electron-lattice coupling; (b) small magnetoelastic polarons, and their persistence into the ferromagnetic metal phase, in the strongly electronlattice coupled CMR system Lal_ (Sr,Ca)xMnO 3 (x < 0.5); and (c) electronic phase separation into coexisting ferromagnetic and antiferromagnetic regions in the CO system Bil.xCa.MnO 3 (x > 0.5). OPTICAL PROBES OF THE METAL-INSULATOR TRANSITION Both infrared and Raman spectroscopy were used in these studies to
investigate metal-insulator (MI) transitions, and inhomogeneous electronic phases, in colossal magnetoresistance and charge-ordering systems. Infrared spectroscopy is the typical spectroscopic means by which metal-insulator transitions have been studied in the past: infrared spectroscopy provides a measure of the optical spectral weight, - cop 2 (- n/m*), which is the natural order parameter for the Mott transition, as it approaches a zero value for either of the two paths by which a Mott transition is achieved: a diminution of the carrier density n, or a divergence of the carrier effective mass m *. However, there are several strong limitations to probing metal-insulator
transitions using conventional techniques such as infrared spectroscopy and transport, some of which reveal themselves particularly clearly in strongly spin-lattice-charge coupled systems. In particular, infrared spectroscopy is primarily sensitive to the charge response, and hence provides little information about changes in the spin degrees of freedom through metalinsulator transitions. This is a severe limitation in systems with complex magnetic, structural, as well as electronic phase diagrams. On the other hand, Raman scattering has evolved as a powerful probe of metal-insulator transitions in strongly correlated systems [4,8,9]. Raman scattering distinguishes itself from more conventional probes of phase transitions such as infrared reflectivity and neutron scattering in that it (a) affords a means of simultan
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