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Coulomb correlations and two-channel conduction in CuIr2S4 and CuIr2Se4 compounds
Journal of Magnetism and Magnetic Materials 226}230 (2001) 244}245
Coulomb correlations and two-channel conduction in CuIr S and CuIr Se compounds
K. Yagasaki *, T. Nakama , M. Hedo , A.T. Burkov , N. Matsumoto, S. Nagata Department of Physics, Faculty of Science, University of the Ryukyus, Okinawa 903-01, Japan Department of Materials Science and Engineering, Muroran Institute of Technology, Muroran, Hokkaido 050-8585, Japan A.F. Iowe Physical-Technical Institute, Russian Academy of Sciences, Sankt-Petersburg 194021, Russia
Abstract The resistivity of CuIr S and CuIr Se have been measured at temperatures from 2 to 300 K under hydrostatic pressures up to 2 GPa. The conductivity at low temperature in the insulating phase of CuIr S is expressed as follows: "exp[!(T*/T ) ]. The pressure dependence of the activation energy and metal}insulator transition temperature give evidence that Coulomb correlations are important. Appearance of the insulating phase in CuIr Se is at a lower pressure than it had been assumed. The total resistivity under pressure can be represented as the resistivity in two parallel channels: the metallic resistivity which is exponentially dependent on temperature and the semiconducting one. The e!ective cross-section of the semiconducting channel rapidly increases with pressure above 1 GPa. 2001 Elsevier Science B.V. All rights reserved. Keywords: Phase transitions*metal}insulator; Phase transitions*electronic; Pressure e!ect*spinel
Spinel-type compound CuIr S displays metal}insula tor transition (MIT) at T +230 K [1]. The MIT in +'2 CuIr S is associated with a structural transition from the high-temperature cubic symmetry to the low-temperature tetragonal phase [2]. The isostructural (cubic) compound CuIr Se remains metallic at ambient pressure at temperatures down to 0.5 K. However, MIT can be induced in CuIr Se by application of pressure of about 4 GPa [3]. Despite rather extensive studies, the precise driving force of the transition in CuIr S remains un known. The instability of the metallic phase due to the closeness of both these compounds to MIT may result in a non-trivial transport property behavior [4]. Indeed, it was shown recently [5] that the conductivity of CuIr S in its low temperature (T(50 K) in the insulating phase was found to follow closely to Efros}Shklovskii hopping conductivity mechanism 1 " J exp[!(¹ H/¹) ], G
(1)
* Corresponding author. Tel.: #81-98-895-8523; fax: #8198-895-8523. E-mail address: [email protected] (K. Yagasaki).
indicating that Coulomb correlation may play an important role in the stabilization of the insulating phase. It should be mentioned that the thermopower in the same range of the temperature is positive and linear as for nomal metals. Its resistivity at higher temperatures follows ! J exp[!(¹H/¹) ]. (2) G The temperature dependence of logarithmic derivative of the resistivity is shown in Fig. 1. The conductivity at the low-temperature phase under hydrostatic pressure reveals the same temperature variation as that found at the ambient pressure. The activation temperature ¹ H inG creases with the pressure P approximately as 1/¹ HJP, G as shown in Fig. 2. This means that the activation temperature is related to the Coulomb potential ¹ H&e/r"e/(r !cP); here c is compressibility. At G the same time the MIT transition temperature increases with pressure at the rate of 24(K/GPa). All these facts suggest that the conductivity follows the Efros}Shklovskii hopping type. On the other hand, the resistivity of metallic CuIr Se has very unusual exponential temperature dependence: ! J exp[!(¹ H /¹)L].
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 6 4 4 - 2
(3)
K. Yagasaki et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 244}245
Fig. 1. Temperature dependence of the conductivity (¹)¹ ) +'2 or resistivity (¹*¹ ) of CuIr S in plots of temperature +'2 dependence of logarithmic derivatives. The linear potion indicates conductivity follows in Eq. (1).
Fig. 2. Pressure dependence of the inverse activation energy of CuIr S .
Here n"0.5 for T)T "200 K, and n"1 for T*T , which express the new type of the phase transition at almost the same temperature as MIT temperature of CuIr S . The thermopower of CuIr Se exhibits also unusual temperature dependence: it follows to 1.5 powers of temperature in the low temperature phase and also in the high-temperature phase. The striking similarity and singularity of the resistivity temperature dependence of CuIr Se and of CuIr S indicates an unknown conduc tion mechanism. A model such as Efros}Shklovskii hopping mechanism combined with Mott's s}d scattering for the metallic phase of CuIr S and for all temperature of
245
Fig. 3. Temperature dependence of the resistivity of CuIr Se under several constant pressures open circles: experimental data, solid curves: "tting results of the two-channel model.
CuIr Se is suggested. The temperature dependence of the resistivity of CuIr Se under pressures indicated in Fig. 3. The evidence of insulator phase is coming from higher temperature side and its appearance is at a lower pressure than it had been assumed. These curves can be "tted by two-channel model: 1/"1/ #1/ , where is expressed as Eq. (2) and for : J exp[$(¹H /¹)]. (4)
This model comes from Efros}Shklovskii hopping mechanism combined with Mott's s}d scattering. The activation energy in is almost constant irrespective of the pressure. The minus sign in is for P)1.6 GPa and the positive sign, P*1.6 GPa. The e!ective cross-section of the channel sharply increases around 1.0 GPa. The mechanism of the conduction of these compounds should be discussed further with more information in additional experiments.
References [1] S. Nagata, T. Hagino, Y. Seki, T. Bitoh, Physica B 194 (1994) 1077. [2] T. Furubayashi, N. Matsumoto, S. Nagata, J. Phys. Soc. Jpn. 63 (1994) 3333. [3] T. Furubayashi, T. Kosaka, J. Tang, T. Matsumoto, Y. Kato, S. Nagata, J. Phys. Soc. Jpn. 66 (1997) 1563. [4] M. Imada, A. Fujimori, Y. Tokura, Rev. Mod. Phys. 70 (1998) 1039. [5] A.T. Burkov, T. Nakama, M. Hedo, K. Shintani, K. Yagasaki, N. Matsumoto, S. Nagata, Phys. Rev. B 61 (2000) 10049.
Coulomb correlations and two-channel conduction in CuIr S and CuIr Se compounds
K. Yagasaki *, T. Nakama , M. Hedo , A.T. Burkov , N. Matsumoto, S. Nagata Department of Physics, Faculty of Science, University of the Ryukyus, Okinawa 903-01, Japan Department of Materials Science and Engineering, Muroran Institute of Technology, Muroran, Hokkaido 050-8585, Japan A.F. Iowe Physical-Technical Institute, Russian Academy of Sciences, Sankt-Petersburg 194021, Russia
Abstract The resistivity of CuIr S and CuIr Se have been measured at temperatures from 2 to 300 K under hydrostatic pressures up to 2 GPa. The conductivity at low temperature in the insulating phase of CuIr S is expressed as follows: "exp[!(T*/T ) ]. The pressure dependence of the activation energy and metal}insulator transition temperature give evidence that Coulomb correlations are important. Appearance of the insulating phase in CuIr Se is at a lower pressure than it had been assumed. The total resistivity under pressure can be represented as the resistivity in two parallel channels: the metallic resistivity which is exponentially dependent on temperature and the semiconducting one. The e!ective cross-section of the semiconducting channel rapidly increases with pressure above 1 GPa. 2001 Elsevier Science B.V. All rights reserved. Keywords: Phase transitions*metal}insulator; Phase transitions*electronic; Pressure e!ect*spinel
Spinel-type compound CuIr S displays metal}insula tor transition (MIT) at T +230 K [1]. The MIT in +'2 CuIr S is associated with a structural transition from the high-temperature cubic symmetry to the low-temperature tetragonal phase [2]. The isostructural (cubic) compound CuIr Se remains metallic at ambient pressure at temperatures down to 0.5 K. However, MIT can be induced in CuIr Se by application of pressure of about 4 GPa [3]. Despite rather extensive studies, the precise driving force of the transition in CuIr S remains un known. The instability of the metallic phase due to the closeness of both these compounds to MIT may result in a non-trivial transport property behavior [4]. Indeed, it was shown recently [5] that the conductivity of CuIr S in its low temperature (T(50 K) in the insulating phase was found to follow closely to Efros}Shklovskii hopping conductivity mechanism 1 " J exp[!(¹ H/¹) ], G
(1)
* Corresponding author. Tel.: #81-98-895-8523; fax: #8198-895-8523. E-mail address: [email protected] (K. Yagasaki).
indicating that Coulomb correlation may play an important role in the stabilization of the insulating phase. It should be mentioned that the thermopower in the same range of the temperature is positive and linear as for nomal metals. Its resistivity at higher temperatures follows ! J exp[!(¹H/¹) ]. (2) G The temperature dependence of logarithmic derivative of the resistivity is shown in Fig. 1. The conductivity at the low-temperature phase under hydrostatic pressure reveals the same temperature variation as that found at the ambient pressure. The activation temperature ¹ H inG creases with the pressure P approximately as 1/¹ HJP, G as shown in Fig. 2. This means that the activation temperature is related to the Coulomb potential ¹ H&e/r"e/(r !cP); here c is compressibility. At G the same time the MIT transition temperature increases with pressure at the rate of 24(K/GPa). All these facts suggest that the conductivity follows the Efros}Shklovskii hopping type. On the other hand, the resistivity of metallic CuIr Se has very unusual exponential temperature dependence: ! J exp[!(¹ H /¹)L].
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 6 4 4 - 2
(3)
K. Yagasaki et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 244}245
Fig. 1. Temperature dependence of the conductivity (¹)¹ ) +'2 or resistivity (¹*¹ ) of CuIr S in plots of temperature +'2 dependence of logarithmic derivatives. The linear potion indicates conductivity follows in Eq. (1).
Fig. 2. Pressure dependence of the inverse activation energy of CuIr S .
Here n"0.5 for T)T "200 K, and n"1 for T*T , which express the new type of the phase transition at almost the same temperature as MIT temperature of CuIr S . The thermopower of CuIr Se exhibits also unusual temperature dependence: it follows to 1.5 powers of temperature in the low temperature phase and also in the high-temperature phase. The striking similarity and singularity of the resistivity temperature dependence of CuIr Se and of CuIr S indicates an unknown conduc tion mechanism. A model such as Efros}Shklovskii hopping mechanism combined with Mott's s}d scattering for the metallic phase of CuIr S and for all temperature of
245
Fig. 3. Temperature dependence of the resistivity of CuIr Se under several constant pressures open circles: experimental data, solid curves: "tting results of the two-channel model.
CuIr Se is suggested. The temperature dependence of the resistivity of CuIr Se under pressures indicated in Fig. 3. The evidence of insulator phase is coming from higher temperature side and its appearance is at a lower pressure than it had been assumed. These curves can be "tted by two-channel model: 1/"1/ #1/ , where is expressed as Eq. (2) and for : J exp[$(¹H /¹)]. (4)
This model comes from Efros}Shklovskii hopping mechanism combined with Mott's s}d scattering. The activation energy in is almost constant irrespective of the pressure. The minus sign in is for P)1.6 GPa and the positive sign, P*1.6 GPa. The e!ective cross-section of the channel sharply increases around 1.0 GPa. The mechanism of the conduction of these compounds should be discussed further with more information in additional experiments.
References [1] S. Nagata, T. Hagino, Y. Seki, T. Bitoh, Physica B 194 (1994) 1077. [2] T. Furubayashi, N. Matsumoto, S. Nagata, J. Phys. Soc. Jpn. 63 (1994) 3333. [3] T. Furubayashi, T. Kosaka, J. Tang, T. Matsumoto, Y. Kato, S. Nagata, J. Phys. Soc. Jpn. 66 (1997) 1563. [4] M. Imada, A. Fujimori, Y. Tokura, Rev. Mod. Phys. 70 (1998) 1039. [5] A.T. Burkov, T. Nakama, M. Hedo, K. Shintani, K. Yagasaki, N. Matsumoto, S. Nagata, Phys. Rev. B 61 (2000) 10049.