Magnetotransport properties of alkali metal doped La-Ca-Mn-O system under pulsed magnetic field: Decrease of small polaron coupling constant and melting of polarons in the high temperature phase

S Bhattacharya, S Pal, A Banerjee, HD Yang, BK Chaudhuri

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Abstract

Pulsed magnetic field (0–4.4 T) was used to study the magnetic field dependent resistivity (12–350 K) and thermoelectric power (0–1.5 T) of Na and K-doped La0.7Ca0.7−yAyMnO3 (0.0⩽y⩽0.3, A=Na, K) system showing semiconducting to metallic transitions around temperature Tp. Na/K-doping increases both conductivity and Tp. In La1−xCaxMnO3, an increase of Tp and conductivity with an increase of Ca (for x⩽0.33) are small and the small polaron coupling constant (γ) and hence the electron-lattice (phonon) interaction is strong. But in the Na/K doped system, γ is small and for y⩾0.05, Motts’ condition of strong el–ph interaction breaks down in the high temperature (T>Tp) phase. Increase of conductivity in the Na/K doped system is caused by the decrease of γ, binding energy (Wp), hopping energy (WH), and effective mass (mp) of the polarons leading to the melting (we call it) of polarons in the T>Tp phase. This melting results in an increase of exchange coupling constant between spins. Field dependent thermoelectric power (TEP) of the samples (measured between 80–300 K) also supports the small polaron hopping conduction. The resistivity data are well fitted with the variable range hopping model for a limited range of temperature (Tp<T<θD/2, θD being the Debye temperature) while thermally activated small polaron hopping model is found valid for T>θD/2. With the application of a magnetic field, the density of states at the Fermi level increases. The TEP data indicate the importance of electron-magnon contribution in the low temperature (T<Tp) ferromagnetic metallic phase. Estimated polaron bandwidth (J) satisfies Holstein’s condition of the adiabatic “small polaron” hopping conduction mechanism for the region T>Tp.
Original languageEnglish
Article number3972
JournalJournal of Chemical Physics
Volume119
Issue number7
DOIs
Publication statusPublished - 2003
MoE publication typeA1 Journal article-refereed

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Alkali Metals
Galvanomagnetic effects
Gene Conversion
Polarons
polarons
alkali metals
Thermoelectric power
Melting
melting
Magnetic fields
conductivity
magnetic fields
electrical resistivity
Temperature
Exchange coupling
Electrons
electrons
breakdown
binding energy
Fermi level

Cite this

@article{2ad0b7319e274132a29adf61e8571349,
title = "Magnetotransport properties of alkali metal doped La-Ca-Mn-O system under pulsed magnetic field: Decrease of small polaron coupling constant and melting of polarons in the high temperature phase",
abstract = "Pulsed magnetic field (0–4.4 T) was used to study the magnetic field dependent resistivity (12–350 K) and thermoelectric power (0–1.5 T) of Na and K-doped La0.7Ca0.7−yAyMnO3 (0.0⩽y⩽0.3, A=Na, K) system showing semiconducting to metallic transitions around temperature Tp. Na/K-doping increases both conductivity and Tp. In La1−xCaxMnO3, an increase of Tp and conductivity with an increase of Ca (for x⩽0.33) are small and the small polaron coupling constant (γ) and hence the electron-lattice (phonon) interaction is strong. But in the Na/K doped system, γ is small and for y⩾0.05, Motts’ condition of strong el–ph interaction breaks down in the high temperature (T>Tp) phase. Increase of conductivity in the Na/K doped system is caused by the decrease of γ, binding energy (Wp), hopping energy (WH), and effective mass (mp) of the polarons leading to the melting (we call it) of polarons in the T>Tp phase. This melting results in an increase of exchange coupling constant between spins. Field dependent thermoelectric power (TEP) of the samples (measured between 80–300 K) also supports the small polaron hopping conduction. The resistivity data are well fitted with the variable range hopping model for a limited range of temperature (Tp<T<θD/2, θD being the Debye temperature) while thermally activated small polaron hopping model is found valid for T>θD/2. With the application of a magnetic field, the density of states at the Fermi level increases. The TEP data indicate the importance of electron-magnon contribution in the low temperature (T<Tp) ferromagnetic metallic phase. Estimated polaron bandwidth (J) satisfies Holstein’s condition of the adiabatic “small polaron” hopping conduction mechanism for the region T>Tp.",
author = "S Bhattacharya and S Pal and A Banerjee and HD Yang and BK Chaudhuri",
year = "2003",
doi = "10.1063/1.1582433",
language = "English",
volume = "119",
journal = "Journal of Chemical Physics",
issn = "0021-9606",
publisher = "American Institute of Physics AIP",
number = "7",

}

Magnetotransport properties of alkali metal doped La-Ca-Mn-O system under pulsed magnetic field: Decrease of small polaron coupling constant and melting of polarons in the high temperature phase. / Bhattacharya, S; Pal, S; Banerjee, A; Yang, HD; Chaudhuri, BK.

In: Journal of Chemical Physics, Vol. 119, No. 7, 3972, 2003.

Research output: Contribution to journalArticleScientificpeer-review

TY - JOUR

T1 - Magnetotransport properties of alkali metal doped La-Ca-Mn-O system under pulsed magnetic field: Decrease of small polaron coupling constant and melting of polarons in the high temperature phase

AU - Bhattacharya, S

AU - Pal, S

AU - Banerjee, A

AU - Yang, HD

AU - Chaudhuri, BK

PY - 2003

Y1 - 2003

N2 - Pulsed magnetic field (0–4.4 T) was used to study the magnetic field dependent resistivity (12–350 K) and thermoelectric power (0–1.5 T) of Na and K-doped La0.7Ca0.7−yAyMnO3 (0.0⩽y⩽0.3, A=Na, K) system showing semiconducting to metallic transitions around temperature Tp. Na/K-doping increases both conductivity and Tp. In La1−xCaxMnO3, an increase of Tp and conductivity with an increase of Ca (for x⩽0.33) are small and the small polaron coupling constant (γ) and hence the electron-lattice (phonon) interaction is strong. But in the Na/K doped system, γ is small and for y⩾0.05, Motts’ condition of strong el–ph interaction breaks down in the high temperature (T>Tp) phase. Increase of conductivity in the Na/K doped system is caused by the decrease of γ, binding energy (Wp), hopping energy (WH), and effective mass (mp) of the polarons leading to the melting (we call it) of polarons in the T>Tp phase. This melting results in an increase of exchange coupling constant between spins. Field dependent thermoelectric power (TEP) of the samples (measured between 80–300 K) also supports the small polaron hopping conduction. The resistivity data are well fitted with the variable range hopping model for a limited range of temperature (Tp<T<θD/2, θD being the Debye temperature) while thermally activated small polaron hopping model is found valid for T>θD/2. With the application of a magnetic field, the density of states at the Fermi level increases. The TEP data indicate the importance of electron-magnon contribution in the low temperature (T<Tp) ferromagnetic metallic phase. Estimated polaron bandwidth (J) satisfies Holstein’s condition of the adiabatic “small polaron” hopping conduction mechanism for the region T>Tp.

AB - Pulsed magnetic field (0–4.4 T) was used to study the magnetic field dependent resistivity (12–350 K) and thermoelectric power (0–1.5 T) of Na and K-doped La0.7Ca0.7−yAyMnO3 (0.0⩽y⩽0.3, A=Na, K) system showing semiconducting to metallic transitions around temperature Tp. Na/K-doping increases both conductivity and Tp. In La1−xCaxMnO3, an increase of Tp and conductivity with an increase of Ca (for x⩽0.33) are small and the small polaron coupling constant (γ) and hence the electron-lattice (phonon) interaction is strong. But in the Na/K doped system, γ is small and for y⩾0.05, Motts’ condition of strong el–ph interaction breaks down in the high temperature (T>Tp) phase. Increase of conductivity in the Na/K doped system is caused by the decrease of γ, binding energy (Wp), hopping energy (WH), and effective mass (mp) of the polarons leading to the melting (we call it) of polarons in the T>Tp phase. This melting results in an increase of exchange coupling constant between spins. Field dependent thermoelectric power (TEP) of the samples (measured between 80–300 K) also supports the small polaron hopping conduction. The resistivity data are well fitted with the variable range hopping model for a limited range of temperature (Tp<T<θD/2, θD being the Debye temperature) while thermally activated small polaron hopping model is found valid for T>θD/2. With the application of a magnetic field, the density of states at the Fermi level increases. The TEP data indicate the importance of electron-magnon contribution in the low temperature (T<Tp) ferromagnetic metallic phase. Estimated polaron bandwidth (J) satisfies Holstein’s condition of the adiabatic “small polaron” hopping conduction mechanism for the region T>Tp.

U2 - 10.1063/1.1582433

DO - 10.1063/1.1582433

M3 - Article

VL - 119

JO - Journal of Chemical Physics

JF - Journal of Chemical Physics

SN - 0021-9606

IS - 7

M1 - 3972

ER -