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orbital ordered Pr0.63Ca0.37MnO3 crystal: the charge density wave scenario
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) 388–389
Non-linear electrical response in a charge/orbital ordered Pr0.63Ca0.37MnO3 crystal: the charge density wave scenario S. Mercone*, A. Wahl, A. Pautrat, M. Pollet, Ch. Simon CRISMAT-ISMRA, 6, Bd. Du Marechal Juin, 14050 Caen, France
Abstract Non-linear conduction in a CO manganese oxide Pr0.63Ca0.37MnO3 is reported. This kind of behavior is usually interpreted as a current-induced breakdown of the CO state. We propose in this report to compare this feature of the coexistence of localized and delocalized electron states with what occurs in the Charge Density Waves systems in which the same non-linearity is observed. In such a framework the non-linear conduction arises from the motion of the CDW condensate which carries a net electrical current. r 2003 Elsevier B.V. All rights reserved. PACS: 75.10.b; 75.25.+z Keywords: Charge order; Orbital order; Charge density waves
Charge ordering (CO) is a common characteristic of transition metal oxides with perovskite structure. In colossal magnetoresistive (CMR) manganites, the formation of a Mn3þ : Mn4þ ordered phase is the most obvious manifestation of the eg electrons localization. The physics of the phase transition from a COantiferromagnet to charge-delocalized ferromagnet have been extensively studied in recent years [1,2]. Numerous experimental results have shown that the application of a moderate electric field leads to an insulator–metal (I2M) transition associated with a strong non-linearity of the voltage–current (V 2I) characteristics. To account for the non-linear conduction in CO manganese oxides, a special type of dielectric breakdown of the CO state brought about by small electric field is invoked. The system behaves in such a way that the bias current may generate metallic path giving rise to resistivity drop. One can describe this feature by considering coexistence of localized and delocalized electron states with independent path of conduction. This situation is clearly what occurs in charge density wave systems. When considering the properties of manganites with colossal magne*Corresponding author. Tel.: +33-231-45-29-16; fax: +33231-95-16-00. E-mail address: [email protected] (S. Mercone).
toresistance, one have also to take into account, besides the charge and spin degrees of freedom, the orbital structure of the transition metal ions. Therefore, the description of the collective behavior of eg carriers in connection with the specific kind of charge and orbital ordering (OO) (which depend on the doping state) might be of fundamental interest for the understanding of the non-linear electrical response in the CO/OO manganites. Recently, in Pr0.63Ca0.37MnO3, Asaka et al. [3] have revealed new forms of structural modulations. These authors have observed superlattice reflection spots with a modulation wave vector q1 ¼ ð0; 1=2; 0Þ below 230 K, 2 2 suggesting the classical formation of d3x r2 =d3y r2 orbital ordering, similar to the half-doped manganite Pr0.5Ca0.5MnO3. Below 150 K, a new modulation wave vector appears, q2 ¼ ð14; 14; 12Þ: In this temperature range, Mn3+ ions must be substituted partially on the Mn4+ sublattice in the x ¼ 12-type CO/OO in the ratio 1:3. This kind of ordering can be viewed as a quasi-onedimensional electronic structure with a reduced dimensionality compared to the 1:1 CO/OO of Pr0.5Ca0.5MnO3. Such a periodic arrangement (non-commensurate) of charge in Pr0.63Ca0.37MnO3 leads to the development of a charge density wave (CDW) condensate. The phase of this CDW could be pinned by several mechanisms; if an electric field strong enough to overcome the pinning
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.11.114
ARTICLE IN PRESS S. Mercone et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 388–389
Fig. 1. V 2I characteristics under zero applied magnetic field for various temperatures (60, 80, 120 and 140 K).
energy is applied, the CDW can be depinned and carries a current [2]. In a Pr0.63Ca0.37MnO3 crystal, nonlinearity in V 2I characteristics are observed for a wide range of temperature. Four linear contacts pads of In were soldered onto the sample in linear four-probe configuration. The electron diffraction (ED) investigation have shown the existence of twinning domains in this sample so that no alignment between current and crystallographic directions were possible. We show the V 2I characteristics in Fig. 1. On the basis of the recent experimental observations suggesting the development of charge density waves in charge and orbitally ordered manganese oxides [3], the non-linear conduction in Pr0.63Ca0.37MnO3 is described by considering a phenomenological model for CDW motion. This model is based on the consideration that the total, current density in this kind of system can be written like: j ¼ jCDW þ sE where jCDW is the current density associated with the motion of the CDW condensate and s is the ohmic conductivity at low electric field/current applied. Solving the equation of the motion, the resistance of the system is the following: " # Z I 1=2 b IC2 R ¼ Rohmic 1 PðIC Þ 1 2 dIC : 1 þ b Iohmic I Where b is an experimental parameter defined as a function of the conductivity limits, IC the mean value of the current needed to depinned the domains and PðIC Þ the statistical distribution of these critical values. This latter is necessary, according to Monceau et al. [4], in order to account a more realistic description of the sample. We consider a multiple domains system in which each domain is depinned for a given value of IC : The statistical distribution of the domains in the sample translated into a distribution of these latters. In Fig. 2 we show at 80 K the fit of the experimental data by this model. The good agreement between the experimental curves and the model is obtained by a log-normal distribution of the critical current IC. In all the CDW’s
389
Fig. 2. R versus bias current at 80 K. The solid line is the fit using the model for CDW motion.
Fig. 3. Shape of the distributions of IC for various temperatures.
systems (such as NbSe3) the distribution of the domains is exactly the same [4,5]. We show the evolution of this distribution for different temperatures in Fig. 3. Both the standard deviation and the mean value of these latters increase as the temperature increases. The crystals were grown by L. Herv!e using an image furnace technique. S. Mercone is supported by a Marie Curie fellowship of the European Community program under contract number HPMT2000-141.
References [1] [2] [3] [4] [5]
Y. Tomika, et al., Phys. Rev. B 53 (1995) R13145. M.R. Lees, et al., Phys. Rev. B 52 (1996) R14303. T. Asaka, et al., Phys. Rev. Lett. 88 (2002) 097201. P. Monceau, et al., Phys. Rev. B 25 (1982) 931. G. Gruner, et al., Rev. Mod. Phys. 60 (1988) 1129.
Journal of Magnetism and Magnetic Materials 272–276 (2004) 388–389
Non-linear electrical response in a charge/orbital ordered Pr0.63Ca0.37MnO3 crystal: the charge density wave scenario S. Mercone*, A. Wahl, A. Pautrat, M. Pollet, Ch. Simon CRISMAT-ISMRA, 6, Bd. Du Marechal Juin, 14050 Caen, France
Abstract Non-linear conduction in a CO manganese oxide Pr0.63Ca0.37MnO3 is reported. This kind of behavior is usually interpreted as a current-induced breakdown of the CO state. We propose in this report to compare this feature of the coexistence of localized and delocalized electron states with what occurs in the Charge Density Waves systems in which the same non-linearity is observed. In such a framework the non-linear conduction arises from the motion of the CDW condensate which carries a net electrical current. r 2003 Elsevier B.V. All rights reserved. PACS: 75.10.b; 75.25.+z Keywords: Charge order; Orbital order; Charge density waves
Charge ordering (CO) is a common characteristic of transition metal oxides with perovskite structure. In colossal magnetoresistive (CMR) manganites, the formation of a Mn3þ : Mn4þ ordered phase is the most obvious manifestation of the eg electrons localization. The physics of the phase transition from a COantiferromagnet to charge-delocalized ferromagnet have been extensively studied in recent years [1,2]. Numerous experimental results have shown that the application of a moderate electric field leads to an insulator–metal (I2M) transition associated with a strong non-linearity of the voltage–current (V 2I) characteristics. To account for the non-linear conduction in CO manganese oxides, a special type of dielectric breakdown of the CO state brought about by small electric field is invoked. The system behaves in such a way that the bias current may generate metallic path giving rise to resistivity drop. One can describe this feature by considering coexistence of localized and delocalized electron states with independent path of conduction. This situation is clearly what occurs in charge density wave systems. When considering the properties of manganites with colossal magne*Corresponding author. Tel.: +33-231-45-29-16; fax: +33231-95-16-00. E-mail address: [email protected] (S. Mercone).
toresistance, one have also to take into account, besides the charge and spin degrees of freedom, the orbital structure of the transition metal ions. Therefore, the description of the collective behavior of eg carriers in connection with the specific kind of charge and orbital ordering (OO) (which depend on the doping state) might be of fundamental interest for the understanding of the non-linear electrical response in the CO/OO manganites. Recently, in Pr0.63Ca0.37MnO3, Asaka et al. [3] have revealed new forms of structural modulations. These authors have observed superlattice reflection spots with a modulation wave vector q1 ¼ ð0; 1=2; 0Þ below 230 K, 2 2 suggesting the classical formation of d3x r2 =d3y r2 orbital ordering, similar to the half-doped manganite Pr0.5Ca0.5MnO3. Below 150 K, a new modulation wave vector appears, q2 ¼ ð14; 14; 12Þ: In this temperature range, Mn3+ ions must be substituted partially on the Mn4+ sublattice in the x ¼ 12-type CO/OO in the ratio 1:3. This kind of ordering can be viewed as a quasi-onedimensional electronic structure with a reduced dimensionality compared to the 1:1 CO/OO of Pr0.5Ca0.5MnO3. Such a periodic arrangement (non-commensurate) of charge in Pr0.63Ca0.37MnO3 leads to the development of a charge density wave (CDW) condensate. The phase of this CDW could be pinned by several mechanisms; if an electric field strong enough to overcome the pinning
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.11.114
ARTICLE IN PRESS S. Mercone et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 388–389
Fig. 1. V 2I characteristics under zero applied magnetic field for various temperatures (60, 80, 120 and 140 K).
energy is applied, the CDW can be depinned and carries a current [2]. In a Pr0.63Ca0.37MnO3 crystal, nonlinearity in V 2I characteristics are observed for a wide range of temperature. Four linear contacts pads of In were soldered onto the sample in linear four-probe configuration. The electron diffraction (ED) investigation have shown the existence of twinning domains in this sample so that no alignment between current and crystallographic directions were possible. We show the V 2I characteristics in Fig. 1. On the basis of the recent experimental observations suggesting the development of charge density waves in charge and orbitally ordered manganese oxides [3], the non-linear conduction in Pr0.63Ca0.37MnO3 is described by considering a phenomenological model for CDW motion. This model is based on the consideration that the total, current density in this kind of system can be written like: j ¼ jCDW þ sE where jCDW is the current density associated with the motion of the CDW condensate and s is the ohmic conductivity at low electric field/current applied. Solving the equation of the motion, the resistance of the system is the following: " # Z I 1=2 b IC2 R ¼ Rohmic 1 PðIC Þ 1 2 dIC : 1 þ b Iohmic I Where b is an experimental parameter defined as a function of the conductivity limits, IC the mean value of the current needed to depinned the domains and PðIC Þ the statistical distribution of these critical values. This latter is necessary, according to Monceau et al. [4], in order to account a more realistic description of the sample. We consider a multiple domains system in which each domain is depinned for a given value of IC : The statistical distribution of the domains in the sample translated into a distribution of these latters. In Fig. 2 we show at 80 K the fit of the experimental data by this model. The good agreement between the experimental curves and the model is obtained by a log-normal distribution of the critical current IC. In all the CDW’s
389
Fig. 2. R versus bias current at 80 K. The solid line is the fit using the model for CDW motion.
Fig. 3. Shape of the distributions of IC for various temperatures.
systems (such as NbSe3) the distribution of the domains is exactly the same [4,5]. We show the evolution of this distribution for different temperatures in Fig. 3. Both the standard deviation and the mean value of these latters increase as the temperature increases. The crystals were grown by L. Herv!e using an image furnace technique. S. Mercone is supported by a Marie Curie fellowship of the European Community program under contract number HPMT2000-141.
References [1] [2] [3] [4] [5]
Y. Tomika, et al., Phys. Rev. B 53 (1995) R13145. M.R. Lees, et al., Phys. Rev. B 52 (1996) R14303. T. Asaka, et al., Phys. Rev. Lett. 88 (2002) 097201. P. Monceau, et al., Phys. Rev. B 25 (1982) 931. G. Gruner, et al., Rev. Mod. Phys. 60 (1988) 1129.