Ankam Bhaskar

High-temperature transport properties of Ca0.98RE0.02MnO3−δ (RE=Sm, Gd, and Dy)

We report measurements of electrical resistivity and thermopower on CaMnO3−δ and Ca0.98RE0.02MnO3−δ (RE=Sm, Gd, and Dy) prepared by solid state reaction. CaMnO3−δ exhibits nonmetal-like temperature dependence of resistivity while metal-like temperature dependence of thermopower. This inconsistency can be explained by the extrinsic carriers arising from oxygen defects using two-band model. Ca0.98RE0.02MnO3−δ exhibits metal-like temperature dependence in both resistivity and thermopower. The transition to metal-like behavior resembles the case in degenerate semiconductors and can be attributed to an impurity band formation with characteristic of hybridized Mn 3d eg and O 2p states due to electron doping via partial substitution of lanthanides for Ca2+ and oxygen defects.

Transport properties of Bi-doped FeSe superconductor up to 700 K

Polycrystalline samples of Fe1-xBixSe with x = 0.00, 0.02, 0.05, and 0.08 are prepared by conventional solid state reaction. The resistivity and thermopower are measured up to 700 K. The onset superconducting transition temperature decreases slightly with increasing Bi content. Transport behavior of electrical resistivity in the normal state is quite complex. A linear temperature dependence is found between 20 and 100 K. Above 150 K, the electrical resistivity behavior resembles misfit-layered cobalt oxides Ca3Co4O9+δ; a Fermi-liquid behavior of T2 dependence is observed between 150 and 210 K with the Fermi-liquid transport coefficient A having the size between 4.3 × 10−6 and 10 × 10−6 mΩ-cm/K2, followed by an incoherent metal and a high-temperature nonmetal-like behavior. Thermopower behavior is nonconventional. Sign crossover of thermopower occurs both below and above room temperature. According to the dynamical mean field calculations, the negative dip feature of thermopower between 110 and 130 K seems to be associated with excitations of a pseudogap ground state.

Effects of Mn doping on the normal-state transport of tetragonal FeSe superconductor up to 700 K

A series of Fe1?xMnxSe polycrystalline samples with , 0.02, 0.05 and 0.08 is prepared by conventional solid-state reaction. Normal-state transport of electrical resistivity and thermopower are investigated up to 700 K. The electrical resistivity monotonically increases with increasing Mn content. The temperature dependence of resistivity is analyzed using a power-law equation . At 150\ \text{K}

Effects of Fe doping on the thermal hysteresis of the La0.5Ca0.5MnO3 system

SRC=http://ej.iop.org/images/0295-5075/108/1/17011/epl16585ieqn3.gif/>, the electrical resistivity behavior resembles misfit-layered cobalt oxides Ca3Co4O9+?. The electrical resistivity follows a T2-dependence between 150 and 220 K with the Fermi-liquid transport coefficient having the size between and , followed by a transition to an incoherent metal in the vicinity of Mott transition. Metal-nonmetal transition is observed at 315 K, 370 K, 304 K and 275 K for , 0.02, 0.05 and , respectively. The resistivity follows a power law for , which might comply with the prediction of the spin fluctuation theories for the behavior in the vicinity of an antiferromagnetic transition. Thermopower exhibits unconventional behavior. Above room temperature, the thermopower shows a sign crossover to positive at 352 K, 537 K, and 374 K for , 0.05, and 0.08, respectively. Unlike the undoped and Bi-doped FeSe, the thermopower of Mn-doped samples increases with increasing temperature in the high-temperature regime up to 700 K, which might arise from the thermally activated conduction of hole carriers. Being consistent with earlier reports, the sharp dip anomaly of thermopower occurs between 100 and 120 K and seems to be associated with the excitations of a pseudogap ground state.

Thermoelectric Properties of Ca3−xDyxCo3.95Ga0.05O9+δ

A series of polycrystalline La0.5Ca0.5Mn1−xFexO3 (x = 0.010, 0.025, 0.050, 0.075, 0.100, 0.125, 0.150, 0.175 and 0.200) was synthesized using a solid state reaction. We investigated the electrical resistivity, thermopower, and magnetization as a function of temperature. La0.5Ca0.5MnO3 exhibits a large thermal hysteresis in its electrical resistivity, thermopower, and magnetization, which can be attributed to the charge density waves pinned by impurities. The thermal hysteresis decreases with increasing Fe content up to x = 0.050 and disappears at even higher x. La0.5Ca0.5MnO3 shows nonmetal-like behavior in terms of its electrical resistivity within the entire investigated temperature range of 80–300 K, while the x = 0.010 and 0.025 samples show metal–nonmetal transitions in their electrical resistivity at about 137–149 K. The metal–nonmetal transition can be attributed to the reduction of charge ordering at small Fe content values. However, there is no metal–nonmetal transition observed for x ≥ 0.050, which arises from the suppression of the double exchange mechanism at high Fe content values. The activation energy derived from electrical resistivity differs from that derived from thermopower, indicating that the conduction mechanism is polaronic transport in La0.5Ca0.5Mn1−xFexO3. The magnetic transition temperature is observed at ∼168 K and ∼135 K for x = 0.010 and 0.025, respectively. There is no magnetic transition observed for x = 0.100 and 0.200.

Thermoelectric and magnetic properties of Ca{sub 3}Co{sub 4–x}Cu{sub x}O{sub 9+δ} with x = 0.00, 0.05, 0.07, 0.10 and 0.15

Ca3Co4O9+δ and Ca3−xDyxCo3.95Ga0.05O9+δ (x = 0.05, 0.10) samples were prepared by conventional solid-state synthesis, and their thermoelectric properties measured at 25 K to 300 K. The x-ray diffraction patterns revealed that all the samples are single phase. The thermopower of all the samples was positive, indicating that the predominant carriers are holes over the entire temperature range. The highest power factor among all the samples (2.45 μW cm−1 K−2 at 132 K) was obtained for Ca2.9Dy0.1Co3.95Ga0.05O9+δ, being about 80% higher than that of the undoped sample. Ca2.9Dy0.1Co3.95 Ga0.05O9+δ had the highest dimensionless figure of merit of 0.033 at 300 K, representing an improvement of about 74% compared with undoped Ca3Co4O9+δ.

Thermoelectric Properties of Ca3−x Dy x Co3.95Ga0.05O9+δ

Graphical abstract: - Highlights: • Resistivity of all the samples exhibits nonmetallic to metallic behavior in the low temperature region. • Ca{sub 3}Co{sub 3.85}Cu{sub 0.15}O{sub 9+δ} shows the highest dimensionless figure of merit. • The observed effective magnetic moments decrease with increasing Cu content. - Abstract: Ca{sub 3}Co{sub 4–x}Cu{sub x}O{sub 9+δ} (x = 0.00, 0.05, 0.07, 0.10 and 0.15) samples were prepared by conventional solid-state synthesis and their thermoelectric properties were systematically investigated. The thermopower of all the samples was positive, indicating that the predominant carriers are holes over the entire temperature range. Ca{sub 3}Co{sub 3.85}Cu{sub 0.15}O{sub 9+δ} had the highest power factor of 2.17 μW cm{sup −1} K{sup −2} at 141 K, representing an improvement of about 64.4% compared to undoped Ca{sub 3}Co{sub 4}O{sub 9+δ}. Magnetization measurements indicated that all the samples exhibit a low-spin state of cobalt ions. The observed effective magnetic moments decreased with increasing copper content.

Thermoelectric Properties of Ca 3− x Dy x Co 3.95 Ga 0.05 O 9+ δ

Ca3Co4O9+δ and Ca3−xDyxCo3.95Ga0.05O9+δ (x = 0.05, 0.10) samples were prepared by conventional solid-state synthesis, and their thermoelectric properties measured at 25 K to 300 K. The x-ray diffraction patterns revealed that all the samples are single phase. The thermopower of all the samples was positive, indicating that the predominant carriers are holes over the entire temperature range. The highest power factor among all the samples (2.45 μW cm−1 K−2 at 132 K) was obtained for Ca2.9Dy0.1Co3.95Ga0.05O9+δ, being about 80% higher than that of the undoped sample. Ca2.9Dy0.1Co3.95 Ga0.05O9+δ had the highest dimensionless figure of merit of 0.033 at 300 K, representing an improvement of about 74% compared with undoped Ca3Co4O9+δ.

Thermoelectric properties of Ca2.95Bi0.05Co4−xFexO9+δ (0 ⩽ x ⩽ 0.15)

Ca3Co4O9+δ and Ca3−xDyxCo3.95Ga0.05O9+δ (x = 0.05, 0.10) samples were prepared by conventional solid-state synthesis, and their thermoelectric properties measured at 25 K to 300 K. The x-ray diffraction patterns revealed that all the samples are single phase. The thermopower of all the samples was positive, indicating that the predominant carriers are holes over the entire temperature range. The highest power factor among all the samples (2.45 μW cm−1 K−2 at 132 K) was obtained for Ca2.9Dy0.1Co3.95Ga0.05O9+δ, being about 80% higher than that of the undoped sample. Ca2.9Dy0.1Co3.95 Ga0.05O9+δ had the highest dimensionless figure of merit of 0.033 at 300 K, representing an improvement of about 74% compared with undoped Ca3Co4O9+δ.