Thermoelectric Power Coefficient Research Articles

Thermopower and Hall conductivity in the magnetic-field-driven normal state ofPr2−xCexCuO4−δsuperconductors

Measurements of thermoelectric power (TEP) and Hall coefficient ${(R}_{H})$ are carried out on ${\mathrm{Pr}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{CuO}}_{4\ensuremath{-}\ensuremath{\delta}}$ epitaxial films to probe the mechanism of charge transport in the absence of superconductivity. In the underdoped $(x=0.13)$ and optimally doped $(x=0.15)$ samples, the TEP shows a $\mathrm{ln}(1/T)$ dependence and the ${R}_{H}$ remains constant in the temperature range accessed by suppressing superconductivity. These observations and the behavior of the in-plane resistivity indicate weak localization of charge carriers on the ${\mathrm{CuO}}_{2}$ planes. The TEP and ${R}_{H}$ of the overdoped sample mimic metallic transport. This, combined with the evidence for hole and electron charge carriers, which may exist on different planes of the unit cell, suggests that the metallic transport in the overdoped samples is a consequence of a three-dimensional charge distribution.

Crystal growth, transport properties, and crystal structure of the single-crystal La2-xBaxCuO4 (x=0.11)

We have attempted the crystal growth by the traveling-solvent floating-zone (TSFZ) method of La_{2-x}Ba_xCuO_4 with x~1/8, where the superconductivity is strongly suppressed. Under flowing O_2-gas of a high pressure (4 bars), we have succeeded in growing single crystals of x=0.11 with 5 mm in diameter and 20 mm in length. It has been found that both in-plane and out-of-plane electrical resistivities of the single-crystal La_{2-x}Ba_xCuO_4 with x=0.11 exhibit a clear jump at ~53 K. The temperature corresponds to the structural phase transition temperature between the orthorhombic mid-temperature (OMT) and tetragonal low-temperature (TLT) phases, T_{d2}. It has also been found that both in-plane thermoelectric power and Hall coefficient drop rapidly at T_{d2} and decrease below T_{d2} with decreasing temperature. These results are quite similar to those observed in the single-crystal La_{1.6-x}Nd_{0.4}Sr_xCuO_4 with x~1/8, suggesting that the so-called static stripe order of holes and spins in the CuO_2 plane is formed below T_{d2} in La_{2-x}Ba_xCuO_4 with x~1/8 as well as in La_{1.6-x}Nd_{0.4}Sr_xCuO_4 with x~1/8.

Synthesis and physical properties of niobium-based oxide, Sr 2− xLa xNbO 4 (0⩽ x

Structural and physical properties of Sr 2− x La x NbO 4− δ (0⩽ x<0.2) with ordered-perovskite structure were investigated. Powder X-ray diffraction analysis showed that the samples with 0⩽ x⩽0.1 were of nearly single phase. For all the samples, electrical resistivity ( ρ) exhibited semiconducting behavior at temperatures below 310 K. On the basis of the Arrhenius plots in the temperature range between 150 and 310 K, the activation energies ( E a) were estimated at E a∼0.17 eV. The sign of thermoelectric power coefficient ( S) was confirmed to be negative for all the samples below 310 K, indicating that the dominant charge carriers were most likely electrons.

Anomalous transport properties ofRFe4P12(R= La, Ce, Pr, and Nd)

We have investigated the resistivity ($\rho$), thermoelectric power (TEP) and Hall coefficient ($R_{H}$) on high quality single crystals of REFe$_{4}$P$_{12}$. TEP in CeFe$_{4}$P$_{12}$ is extremely large ($\sim$ 0.5mV/K at 290K) with a peak of $\sim$ 0.75mV/K at around 65K. The Hall mobility also shows a peak at $\sim$ 65K, suggesting carriers with heavy masses developed at lower temperatures related with the f-hybridized band. Both Pr- and Nd- systems exhibit an apparent increase of $\rho$ with decreasing temperature far above their magnetic transition temperatures. In the same temperature ranges, TEP exhibits unusually large absolute values of -50$\mu$V/K for PrFe$_{4}$P$_{12}$ and -15$\mu$V/K for NdFe$_{4}$P$_{12}$, respectively. For PrFe$_{4}$P$_{12}$, such anomalous transport properties suggest an unusual ground state, possibly related with the Quadrupolar Kondo effect.

Magnetic and Fermi Surface Properties of the Ferromagnetic Compound UGa2

We have studied the magnetic and Fermi surface properties in a ferromagnet UGa 2 . The electrical resistivity, elastic constant, thermal expansion coefficient, thermoelectric power, magnetic susceptibility and Hall coefficient indicate highly anisotropic properties, reflecting the hexagonal structure. The 5 f 2 - and 5 f 3 -CEF schemes, based on the localized 5 f -electron nature, are discussed to understand the magnetic susceptibility and magnetization data. We have also done the magnetoresistance and de Haas-van Alphen (dHvA) experiments to investigate the Fermi surface properties. Both data are very complicated, implying that the Fermi surface is a multiply-connected one. They are explained neither by the localized 5 f 2 - and 5 f 3 -core models nor by the spin-polarized 5 f -itinerant model. The cyclotron mass is, however, rather light, ranging from 0.17 to 6.2 m 0 , which is consistent with the electronic specific heat coefficient of 11 mJ/K 2 ·mol.

Properties of Bi2Te3 single crystals doped with Sn

The effect of doping with Sn on the properties of Czochralski-grown Bi2Te3 crystals was studied. The effective segregation coefficient for Sn was determined to be 0.6. The thermoelectric power, electrical conductivity, and Hall coefficient of the doped crystals were measured at room temperature. Doping with low Sn concentrations (0.2-0.5 at. %) was found to have only a weak effect on the electrical properties ofp- type Bi2Te3. Doping with 0.7-1 at. % Sn reduces the thermoelectric power and increases the electrical conductivity and hole concentration. Lattice thermal conductivity is a nonmonotonic function of Sn concentration. The thermoelectric figure of merit of Bi2Te3 doped with less than 0.6 at. % Sn exceeds that of undoped Bi2Te3.

Conduction mechanism of BaCo 2− xNi xFe 16O 27

A series of W-type hexaferrite samples of BaCo 2− x Ni x Fe 16O 27 with ( x=0.0, 0.4, 0.8, 1.2, 1.6) were prepared by using the well known ceramic technique to investigate their electrical properties. The results of thermoelectric power coefficient and DC electrical conductivity suggested that the conduction mechanism in this system is due to hopping of electrons between Fe 2+ and Fe 3+ along with the hole transfer between Ni 2+ and Ni 3+. It was found that the conductivity increases as the replacement of Co 2+ ion by Ni 2+ ion increases. The results of real AC conductivity shows a dispersion with frequency. The dispersion decreases as the temperature increases. The dispersion of AC conductivity was explained on the basis of Koops model.

Thermoelectric power of bismuth nanowires

We report here measurements of the thermoelectric power and longitudinal magneto-Seebeck coefficient of 200 nm diameter single-crystal bismuth nanowires. Nanowires of pure Bi and of n-type-doped Bi (with Te at about $5\ifmmode\times\else\texttimes\fi{}{10}^{18}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3})$ were measured. The wires are imbedded in porous anodic alumina. The data are taken on arrays of wires connected in parallel, at temperatures from 8 to 300 K, and, between 10 and 80 K, in magnetic fields from 0 to 5 Tesla. It has been theoretically calculated that bismuth nanowires should have a strongly increased thermoelectric figure of merit over bulk Bi, when the diameter is decreased below about 10 nm. The nanowires in this study were selected because they are easier to prepare and handle. The temperature-dependent thermopower data are consistent with the partial electron and hole thermopower values calculated using the carrier Fermi energies obtained from Shubnikov--de Haas oscillations on the same samples.

Anomalous suppression of Nernst effect in La 0.7Ca 0.3MnO 3

We report on the magnetization, colossal magnetoresistance (CMR), thermoelectric power (TEP) and Nernst coefficient ( Q) measurements of a La 0.7Ca 0.3MnO 3 bulk ceramic sample ( T c∼265 K). The semiconductor—metal transition near T c is attributed to an increase in the hole density. For T≤ T c, an anomalous suppression of the Nernst coefficient was observed when the field increased from 0.38 to 1.8 T. The data suggest a field increased mobility of the charge carriers as a principal cause for the CMR.

Are high- cuprates unusual metals?

The unusual properties of high-temperature superconductors are reviewed. The structure and chemistry of the materials are described and the strong influence of the carrier concentration on their superconducting properties is illustrated. The temperature dependences of the resistivity, Hall effects and thermoelectric power coefficients are given. These properties distinguish the cuprates clearly from any ordinary metal known until recently. In particular the normal Boltzmann transport theory is no longer applicable. It was found that these anomalous properties of the normal phase are accompanied by superconducting order parameters of uncommon symmetry (d wave), as well as non-BCS temperature and magnetic field dependences. The observation of the normal-phase pseudogap in the density-of-states spectrum by means of specific heat and other measurements seems to offer an important way forward in understanding both the normal phase and the high-temperature superconductivity mechanism.

Transport properties of heavy fermion compound Ce 7Ni 3

Electrical resistivity, thermoelectric power and Hall coefficient have been measured for an antiferromagnetic heavy fermion compound Ce 7Ni 3. Above 30 K the magnetic resistivity ρ m has a logarithmic temperature dependence −ln T and the large value of ρ below the Néel temperature T N=1.8 K indicates that the magnetic state below T N is gap-type antiferromagnetic. For the thermoelectric power, the large negative peak due to magnetic contribution appears at 5 K, vanishes by slight substitution of La for Ce, and the positive peak due to Kondo contribution appears. The temperature dependence of the Hall coefficient R H above and below 30 K is explained with the skew scattering model and with the anomalous velocity model, respectively.

A fusion study of thermoelectric conversion in magnetic fields

We studied thermoelectric conversion and its application in a fusion reactor. We propose three new elements: first, a Nernst element; second, high-temperature thermoelectric materials; and third, a Peltier current lead. As these require new semiconductors, we conducted experimental and theoretical studies of semiconductor transport properties. Initially, we measured electrical resistivity, Hall coefficient, thermoelectric power, and Nernst coefficient of InSb. The Nernst coefficient is proportional to the mobility of the carrier, and InSb is a highly mobile semiconductor. The Nernst coefficient also depends on temperature, so a circular current is induced requiring Ohmic voltage considerations when estimating this coefficient. There is a microscopic effect, as electrical resistivity increases with the magnitude of the magnetic field, and a macroscopic effect, as a Hall voltage is induced, which creates a non-uniform current distribution.

Preparation and Characterization of Cu(II), Zn(II) Sulfides Obtained by Spontaneous Precipitation in Electrolyte Solutions

Transition metal sulfides, because of their electronic and spectroscopic properties, are a very interesting class of materials that may be used efficiently for the construction of photoelectric chemical solar cells. In the present work we have prepared a series of mixed Cu and Zn sulfides with the general stoichiometry CuxZn1-xS by coprecipitation in aqueous solutions. Precipitates with x = 0.10, 0.15, 0.20, 0.22, 0.25, and 0.50 were synthesized by mixing metal nitrates with ammonium sulfide at pH 2.50. The precipitates behaved as n-type semiconductors. The dependence of the conductivity of the materials on temperature and calculations of the thermoelectric power coefficient showed the presence of metallic conducting islands separated by nonconducting parts, for copper content corresponding to x = 0.15, 0.20, and 0.22. For x = 0.10, the precipitates behaved as insulators, whereas for x = 0.50, the precipitates showed metallic character. The morphology and the mean particle size of the precipitates varied with increasing Zn content, yielding for pure CuS, prismatic crystals with a mean size of ∼5 μm. Upon increasing Zn content, polycrystalline materials were obtained, forming aggregates of submicron, spherullitic particles. The measurements of the electrical properties of the materials showed that the conditions of precipitation play an important role for the design of their properties.

Thermoelectric power of delafossite-derived compounds,RCuO2+δ(R=Y, La, Pr, Nd, Sm, and Eu)

The thermoelectric power of triangle planar cuprates, $R{\mathrm{CuO}}_{2+\mathrm{\ensuremath{\delta}}}$ $(\ensuremath{\delta}g~0.5),$ of the delafossite-type with various rare-earth elements $R:$ Y, La, Pr, Nd, Sm, and Eu has been studied. The thermoelectric power coefficient $(S)$ was found to be positive for all the samples in the temperature range between 20 and 320 K. Contrary to the semiconducting behavior in the electrical conductivity, a small value of $Sl~50\ensuremath{\mu}\mathrm{V}/\mathrm{K}$ was observed at room temperature. Except for the case of $R=\mathrm{Pr},$ $S$ of each sample decreased nonlinearly down to around 50 K and then increased rapidly as the temperature decreased, suggesting a phase transition or crossover to the insulating phase. For the case of ${\mathrm{PrCuO}}_{2+\mathrm{\ensuremath{\delta}}},$ on the other hand, $S$ increased monotonically with decreasing temperature. Taking into account both the carrier-diffusion and the phonon-drag process, the nonlinear temperature dependence of $S$ between 50 and 320 K was analyzed. In spite of rather small $S$ in magnitude, which is comparable to that of metals and/or semimetals, the observed thermopower from hopping contributions may indicate that the sample is still in the nonmetallic state.

Erratum: Theory of the thermoelectric power or Seebeck coefficient: The case of phonon scattering for a degenerate free-electron gas

In order to illustrate the flexibility of the variational solution of the Boltzmann equation for transport coefficients, and in particular for the thermoelectric power of solids, we use a simple model of free electrons and consider their scattering from a spherical Fermi surface by acoustic phonons alone, without any mass enhancement effect or umklapp process. We recall the variational method, pointing out that formal constraints exist; we take two trial functions only, and we calculate the Seebeck effect in such a framework. The low-temperature behavior of this effect is known to be very complicated. Sign reversals occur which cannot be explained by a model based upon free or nearly free electrons. As shown hereby, when the inelasticity of the scattering of quasifree electrons by phonons is taken into account, nontrivial behavior in the intermediate (but still low) temperature range can be found. The Appendixes contain analytical expressions. \textcopyright{} 1995 The American Physical Society.

Evidence of the different valence states in the C14 and C15 phases CeOs2

We have measured the temperature dependence of the electrical resistivity, thermoelectric power and Hall coefficient of CeOs2 in two deferent crystallographic structures C14 and C15. We observed clear differences in all the transport properties between the two phases. The C14 phase behaves like a normal metal with a stable Ce4+, while the C15 exhibits various features characteristic of valence fluctuating compounds. This fact explains why only the C14 becomes superconductive.

Theory of the thermoelectric power or Seebeck coefficient: The case of phonon scattering for a degenerate free-electron gas.

In order to illustrate the flexibility of the variational solution of the Boltzmann equation for transport coefficients, and in particular for the thermoelectric power of solids, we use a simple model of free electrons and consider their scattering from a spherical Fermi surface by acoustic phonons alone, without any mass enhancement effect or umklapp process. We recall the variational method, pointing out that formal constraints exist; we take two trial functions only, and we calculate the Seebeck effect in such a framework. The low-temperature behavior of this effect is known to be very complicated. Sign reversals occur which cannot be explained by a model based upon free or nearly free electrons. As shown hereby, when the inelasticity of the scattering of quasifree electrons by phonons is taken into account, nontrivial behavior in the intermediate (but still low) temperature range can be found. The Appendixes contain analytical expressions. \textcopyright{} 1995 The American Physical Society.

Relation between thermoelectric power and Hall coefficient with application to the normal state of high Tc superconductors

Arguments going back to Bridgman relate longitudinal and transverse transport coefficients. In particular, Ettingshausen, Thomson and Hall coefficients are thereby interconnected. By exploiting both one- and two-band models, the Ettingshausen coefficient is microscopically connected to the Hall coefficient. This leads to a formula which relates thermoelectric power linearly to the Hall coefficient. Data for high T c superconductors in the normal state supports such a direct correlation between these properties.

Electrical transport and superconductivity of Bi 2+xSr 2−xCuO 6+d single crystals

The Bi 2+xSr 2+xCuO 6+δ single crystals with 0.11≤x≤0.4 post annealed in various atmospheres were studied using electrical resistivity, thermoelectric power and Hall coefficient measurements. The transport properties vary systematically with x and oxygen activity in annealing atmosphere. The suppression of the superconductivity for x>0.15 is explained by the carrier localization caused by the presence of bismuth cations on Sr-sites.

Inelastic-phonon-scattering effect on the behavior of the thermoelectric power of metals.

The thermoelectric power (TEP) or Seebeck coefficient is calculated along the lines of Ziman variational method taking into account explicitly the inelastic scattering of electrons or holes (occupying a single parabolic band) by the (acoustic) phonon spectrum. The metallic limit is used to show the relative signs of terms appearing in the TEP expression. It is undoubtedly and quantitatively indicated that inelastic scattering can account for the usual bump seen in TEP vs temperature data at moderate temperatures. Also, TEP does not have a uniform sign even for electronic conduction in a single band. The low-temperature expansion is also given.