In the colossal magnetoresistance manganites polarons have been proposed as the charge carrier state which localizes across the metal-insulator transition. The character of the polarons is still under debate. We present an assessment of measurements which identify polarons in the metallic state of La 2/3 Sr 1/3 MnO3 (LSMO) and La 2/3 Ca 1/3 MnO3 (LCMO) thin films. We focus on optical spectroscopy in these films which displays a pronounced resonance in the mid-infrared. The temperature dependent resonance has been previously assigned to polaron excitations. These polaronic resonances are qualitatively distinct in LSMO and LCMO and we discuss large and small polaron scenarios which have been proposed so far. There is evidence for a large polaron excitation in LSMO and small polarons in LCMO. These scenarios are examined with respect to further experimental probes, specifically charge carrier mobility (Hall-effect measurements) and high-temperature dcresistivity.

We present detailed optical measurements of the mid-infrared (MIR) excitations in thin films of La 2/3 Sr 1/3 MnO 3 (LSMO) and La 2/3 Ca 1/3 MnO 3 (LCMO) across the magnetic transition. The shape of the excitation at about 0.2 eV in both samples is analyzed in terms of polaron models. We propose to identify the MIR resonance in LSMO as the excitation of large polarons and that in LCMO as a small polaron excitation. A scaling behavior for the low-energy side of the polaronic MIR resonance in LSMO is established. PACS numbers: 75.47.Lx, Historically, polarons were best identified in measurements with charge carriers in non-metals (e. g., from F-centers in alkali halides) or doped semiconductors. More recently, the concept of polaronic excitations has again been in the focus of solid state research with the advent of strongly correlated electronic systems, especially with the discoveries of hightemperature superconductivity and the colossal magnetoresistance in thin films of La 2/3 Ca 1/3 MnO 3 (LCMO)1 . In both classes of these correlated materials, cuprates and manganites, it is now widely accepted that, in the presence of a strong electron-phonon coupling, polaronic effects have to be considered an important ingredient to understand the complex physical properties of these compounds 2 .The fingerprints of polarons in the manganites are usually associated with the temperature dependence of hightemperature dc-resistivity 3 and the occurrence of a midinfrared (MIR) excitation in the optical conductivitytermed polaron peak in the following -which has been observed by several authors in both single crystalline 4,5,6,7,8 and thin film manganites 9,10,11 . Though evidence of the signature of polarons in the optical conductivity 12,13,14 has been reported abundantly, the question whether the charge carriers are rather delocalized (large polarons, LP) or strongly localized in a locally polarized lattice (small polarons, SP) remains still under debate in the case of manganites.Theoretically, SP and LP have been investigated in several studies 15,16,17 on doped manganites, but the observed polaron peak has been evaluated mainly in terms of the SP model or through Gaussian or Lorentzian fits for a phenomenological description 5,18 . Kim et al. reported on experimental evidence for a LP excitation in polycrystalline LCMO 6 , but unfortunately they did not identify the LP through a fit to the proposed LP model by Emin 19 .The purpose of this paper is to elucidate the nature of the polaronic charge carriers by comparison of optical spectroscopy measurements in thin films of LCMO and La 2/3 Sr 1/3 MnO 3 (LSMO). In contrast to single crystals the use of thin films has the advantage of a lower conductivity due to grain boundaries and internal strain. Consequently, screening is effectively reduced -to a level where phonons and polarons are well observable in the optical conductivity, even in the metallic phase. This allows the detailed analysis of polarons from the respective optical data.We demonstrate that the distinctive shape ...

We report on broadband frequency-dependent conductivity experiments on La 7/8 Sr 1/8 MnO 3 that cover the frequency range from 0.3 meV (Ϸ3 cm Ϫ1 ) to 5 eV (Ϸ4ϫ10 4 cm Ϫ1 ). At all frequencies investigated, the temperature dependence of Ј() reflects the structural and magnetic phase transitions. At low frequencies we find the typical characteristics of hopping conduction. The optical conductivity at frequencies above the phonon modes reveals strong changes at the structural and magnetic phase transitions and can best be described in terms of polaronic conduction. In addition, we carefully analyzed the temperature dependence of the phonon modes. Distinct anomalies in the eigenfrequencies and the appearance of side bands signal structural phase transitions.

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The complex dielectric function 1 ϩi 2 of La 1Ϫx Sr x MnO 3 has been investigated for concentrations 0.1 рxр0.175 near the metal-to-insulator transition. Quasioptical spectroscopy technique were employed for frequencies 100 GHz рр1100 GHz and temperatures 10 K рTр300 K. Both, the dielectric constant 1 and the conductivity 1 ϭ 0 2 of all samples were found to increase on cooling through the magnetic-ordering transition. From this observation and from the analysis of the frequency dependencies we conclude that the charge carriers are localized in a broader range of the phase diagram than generally accepted. We show that hopping or tunneling between localized states dominates the conductivity for Sr concentrations xр0.15 and temperatures TϽ300 K. Even for La 0.725 Sr 0.175 MnO 3 the localization effects are observed at least 10 K below the magnetic-phase transition. Finally, the dielectric constant is observed to diverge on approaching the metalinsulator transition. ͓S0163-1829͑99͒02519-9͔