Physica B 329–333 (2003) 846–847 Optical reflectivity study on magnetoresistive manganese perovskites: impurity effect on the ferromagnetic–metallic and charge-ordered states Shunsuke Okuyama*, Koshi Takenaka 1 , Ryozo Shiozaki, Yuko Sawaki, Shunji Sugai Department of Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan Abstract Impurityeffectonmanganeseperovskitesareinvestigatedbytheopticalreflectivitymeasuredoncleavedsurfacesof the single crystals. The Al impurity affects optical conductivity sðoÞ of ferromagnetic–metallic La 1x Sr x MnO 3 overa wide energy range up to 1 eV or higher, contrary to the simple-Drude picture. The energy gap of the charge-ordered (CO)stateinNd 0:5 Sr 0:5 MnO 3 isabout0:5eV; much larger than that suggested by the previous studies using polished surfaces. This indicates that the CO state can be easily damaged by external disorder. r 2003 Elsevier Science B.V. All rights reserved. Keywords: Manganese perovskite; CMR; Optical conductivity; Impurity effect Physical properties of manganese perovskites are sensitive to impurities at the Mn site. A small amount ofCrmeltsthecharge-ordered(CO)state [1] andasmall amount of Al destroys the ferromagnetic–metallic (FM) state [2] and induces the unconventional localization state [3]. Therefore, Mn-site substitution is a promising method to design the properties of these materials, whichhavebeeninvestigatedindetailduetotheobvious potentialforindustrialapplicationsutilizingthecolossal magnetoresistance (CMR). In order to get more insight into how the electronic states vary with doping of impurities and/or randomness, we have investigated the optical spectra. All of the single crystals were grown by a floating- zone method. Electrical resistivity rðT Þ was measured using a DC four-probe method. Near-normal incident reflectivity RðoÞ was measured on the cleaved surfaces using a Fourier-type interferometer (0.005–1:6 eV), a grating spectrometer (0.8–6:6 eV), and a Seya–Namio- ka-type spectrometer for vacuum-ultraviolet synchro- tron radiation (4.0–40eV) at Okazaki National Research Institutes. Optical conductivity sðoÞ was deduced from RðoÞ via a Kramers–Kronig transforma- tion. The details were described elsewhere [2–5]. Fig.1 shows sðoÞ ofAl-dopedLa 0:825 Sr 0:175 MnO 3 and the inset shows the rðT Þ data. The sðoÞ spectra of the Al-free crystal is characterized by the coherent-to- incoherent crossover [5]. sðoÞ exhibitsaslowlydecaying quasi-Drude peak only below a certain temperature T n C200K; which is lower than Curie temperature T C ¼ 283K: Above T n sðoÞ ischaracterizedbyafinite- energy peak regardless of metallic rðT Þ: sðoÞ of the Al 1%-dopedcrystalat10Kalmostcoincideswiththatof the Al-free crystal at 100 K over the entire o range relevant to the charge dynamics (up to 1 eV or higher). Thesecharacteristicsaremoreclearlydescribedbythe extended-Drude analysis. The quasi-Drude response below T n can be numerically expressed by a monotonic o-linear dependence of scattering rate g n over a wide energy range up to 0:4eV(Fig.2). The Al doping does not alter this characteristic behavior; g n of the Al 1%- doped crystal at 10 K coincides with g n of the Al-free crystalat100K: TheDCconductivity s DC ¼ r 1 ofthe Al 1%-doped crystal at 10 K is almost identical to that *Corresponding author. E-mail addresses: f010215m@mbox.media.nagoya-u.ac.jp (S. Okuyama), k46291a@nucc.cc.nagoya-u.ac.jp (K. Takenaka). 1 Also for correspondence. 0921-4526/03/$-see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-4526(02)02550-4