Scaling Behavior of Spectral Weight Changes in Perovskite ManganitesLa0.7−yPry

Abstract

Optical conductivity spectra of La0.7−y Pry Ca0.3MnO3 were systematically investigated. For metallic samples, the spectral weight below 0.5 eV, whose magnitude can be represented by the effective carrier number N eff (0.5 eV), increases as temperature becomes lower. Regardless of the Pr doping, all the measured values of N eff (0.5 eV)/TC fall into one scaling curve. This scaling behavior could be explained by the theoretical model by Röder et al. (Phys. Rev. Lett. 76, 1356), which includes spin double exchange and Jahn-Teller lattice coupling to holes. With the Pr doping, far-infrared conductivities were found to be suppressed, probably due to the Anderson localization. PACS number; 72.15. Gd, 75.50.Cc, 75.30.Kz, 78.20.Ci Physics of doped manganites, R 1−x A x MnO 3 (R=rare earth and A=alkaline earth ions) with 0.2≤x≤0.5, has been investigated extensively since the recent discovery of colossal magnetoresistance (CMR) phenomena in these compounds. Even if the double exchange (DE) interaction can qualitatively explain most of their physical properties [1], a lot of recent works have revealed that some additional degrees of freedom should be included to describe them more accurately [2][3][4][5][6][7][8][9][10][11]. Optical techniques have been useful to address such additional degrees of freedom [2,3,[8][9][10].However, there still remain some controversies on how to explain temperature(T)-dependent spectral weight (SW) changes in optical spectra of the doped manganites. Especially, there have been numerous interpretations on the mid-infrared (IR) absorption peak below 0.5 eV, which appears below the Curie temperature T C and becomes stronger at lower temperatures. Okimoto et al.claimed that the SW changes should be understood in the spin-split band picture, and they assigned the mid-IR feature to an intraband excitation within an e g band which was merged below T C from two spin-split bands [2,3]. Other workers suggested that an orbital degree of freedom should be included to explain the SW changes [4,5], and de Brito and Shiba attributed the mid-IR absorption to an interband transition between the e g orbital states [5]. Millis et al. showed that the dynamic Jahn-Teller (JT) interaction could play an important role in the SW changes [6,7], and Kaplan et al. assigned the mid-IR feature to a JT type small polaron absorption [8]. Recently, through detailed studies on optical properties of La 0.7 Ca 0.3 MnO 3 (LCMO), we proposed that the evolution of the low frequency feature below 0.5 eV come from a crossover from small to large polaron states [10]. In other words, the mid-IR feature should be attributed to an incoherent absorption of a large polaron state, whose existence was predicted by Röder et al. [11].To get further insights on this interesting issue, we investigated optical properties of La 0.7−y Pr y Ca 0.3 MnO 3 (LPCMO). Hwang et al. showed that dc transport properties of LPCMO were affected by a carrier hopping parameter which could be varied systematically by the Pr doping [12]. In this paper, we rep...

Lattice effects in manganites

“…Instead, at low temperatures, in the ferromagnetic phase the system shows two types of optical response: a Drude term and a broad absorption band due respectively to the coherent and incoherent motion of large polarons. The results obtained are consistent with experimental data [12,13,30].…”

“…In the intermediate electron-phonon coupling regime, the free energy (13) gives rise to a region of coexisting phases [22] characterized by different electron densities q 1 and q 2 but the same value of chemical potential m. The quantities q 1 and q 2 depend on the temperature and correspond to homogeneous phases of the system constituted by large and small polarons, respectively. Hence, near the metal-insulator transition (see Fig.…”

Electronic structure and carrier dynamics of the ferromagnetic semiconductorGa1−xMnxAs<

“…While the data for individual samples is rather sparse, the overall trend of several Mn dopings indicates a linear scaling between changes in the spectral weight and the square of the magnetization. This form of scaling relation is also observed in the manganites 54,55 and can be accounted for within the double exchange model. 59 The data in Fig.…”

Charge dynamics in the half-metallic ferromagnetCrO2

“…9 In other systems with double-exchange coupling a strong correlation exists between the magnetization and intraband spectral weight. 27 In the inset of Fig. 4 we also plot the temperature-dependent magnetization of CrO 2 normalized to its saturation value.…”