Direct Observation of Small-Polaron Ordering in Manganites

Abstract

Transmission-electron-microscopy measurements characterizing the structural features of chargeordering (CO) phases in La͑Ca͒MnO 3 and Pr͑Ca͒MnO 3 are presented. A rich variety of structural phenomena resulting from CO transition have been observed at low temperatures. The most interesting of all is the presence of a new structural modulation on the b ‫ء‬ -c ‫ء‬ plane. This modulation shows some structural properties in sharp contrast with those of the known d z 2 ͑Mn 31 ͒-orbital-ordering state. We herein interpret this new structural modulation in terms of the small-polaron ordering.[S0031-9007(99)08685-8] PACS numbers: 75.30.Kz, 64.70.Rh, 71.38. + i, 75.50.Cc The nature of the quasiparticles that originate from the doped holes in the high-temperature superconductors and colossal magnetoresistance manganites is believed to be of central importance for the understanding of the unusual physical properties unveiled in these systems [1][2][3]. Charge localization and the stripe correlations of concentrated holes and spins have attracted much attention in the recent studies, particularly because of their possible role on high-T c superconductivity [4][5][6]. In manganese systems, charge ordering (CO), as well as its magnetic-field-melting mechanism, has also been extensively investigated in connection with the colossal magnetoresistance [7-10]. An ordered structure observed in the CO states of manganites has been successfully explained in terms of the ionic ordering of Mn 31 and Mn 41 associated with the d z 2 ͑Mn 31 ͒ orbital ordering of e g electrons [10-12]. Furthermore, a number of physical evidences have strongly suggested the presence of small polarons in the hole-doped manganites [13,14]. However, direct observations of this kind of small polarons and their structural effects have not yet been achieved. In this Letter, we report a new structural modulation observed in the ͑La, Ca͒MnO 3 and ͑Pr, Ca͒MnO 3 materials and explain it in terms of the small-polaron ordering.Both ceramic and single-crystalline samples of ͑La, Ca͒MnO 3 and ͑Pr, Ca͒MnO 3 were used for the present study. The ceramics samples were prepared by a conventional solid-state reaction method and the singlecrystalline samples were melt grown by the floatingzone method. At low temperatures, the samples of La 12x Ca x MnO 3 with 0.5 # x # 1.0 and Pr 12x Ca x MnO 3 with 0.3 # x # 1.0 are known as CO insulators [15,16]. Experimental details of transmission-electron-microscopy (TEM) observations will be described elsewhere [17].In order to reveal the structural distortion in correlation with the CO transition in manganites, we have performed extensive low-temperature TEM studies on a series of ͑La, Ca͒MnO 3 and ͑Pr, Ca͒MnO 3 samples. A rich variety of microstructural phenomena have been observed. The most important of all is the presence of incommensurate structural modulations in the low-temperature CO phases. Based on the systematic analysis of the experimental data, two distinctive structural modulations were determined. The first is the well-kno...

Stripe charge ordering in SrO-terminated SrTiO3(001) surfaces

“…Furthermore, a small-polaron ordering was observed along with the charge ordering, which indicates a very strong electronphonon coupling in this system [26]. Polaronic quasiparticle excitations through inelastic tunneling processes were reported in manganite systems, with energy gaps evident in the tunneling spectra even in the metallic phase [27,28].…”

Electronic ferroelectricity, charge ordering and structural phase transitions in LuFe2O4(LuFeO3)n (n = 0 and 1)

“…The results suggest that structural features of q 2 appear chiefly in the FeO 1.5 layers, while, on the other hand, the q 1 modulation occurs chiefly in the charge-frustrated W layers. The correlation between the q 1 and q 2 modulations is likely to be governed by the commensurate effects, which are known to play an essential role in the charge ordering in the manganites and high-T c superconductors [19]. A common relationship can be approximately written as (…”

Crystal and magnetic structure of theLa1−xCaxMnOPissas1, Kallias2, Hofmann3 et al. 2002Phys. Rev. B561221View full textAdd to dashboardCite

“…Moreover, transmission electron microscopy studies in La 1−x Ca x MnO 3 with x = 0.50, 0.625, 0.67, 0.75 and 0.80 have shown superlattice reflections and their analysis pointed to a linear dependence between the magnitude of the modulation wave vector q s along the a axis and x, that is, q s = 1−x. 28 However, the superlattice reflections of the charge-ordering for x = 0.8 19,28 were noticeably broader than lower compositions, indicating a very short coherence length.…”

“…19,28 In the present paper we discuss the crystal and magnetic structure for doping levels near the end members of the La 1−x Ca x MnO 3 homologous series using elastic neutron diffraction data from polycrystalline La 1−x Ca x MnO 3 samples with x = 0.8 and 0.85. An important question that is been posed is whether charge ordering is actually present or not.…”