Anomalous thermoelectric power behaviour in PrSn3 and NdSn3

Results of the thermoelectric power (TEP) measurements done on monocrystalline samples of RESn₃ compounds (RE=La, Pr, Nd, and Gd) are presented for the temperature range of 5.5-300 K. It was found that the TEP is positive and weakly temperature dependent at temperatures T>100 K. For T<100 K pronounced anomalies have been observed for the PrSn₃ and the NdSn₃ compounds in the vicinity of 10 K.We argue that the Kondo and crystal field effects cause these anomalies. A shape of the TEP anomaly found for PrSn₃ resembles very much that observed in the electrical resistivity.     

TEP studies on Fe80B20 under pressure

The amorphous to crystalline transformation in the ferromagnetic metallic glass Fe₈₀B₂₀ has been studied up to 30 kbar pressure and 1000K. A previous study at ambient pressure revealed no change in thermoelectric power (TEP) at the crystallization temperature (T_x) while the resistivity showed a sudden decrease at the same temperature. The present experimental results show a distinct anomaly in TEP at T_x even at ambient pressure. This anomaly gets enhanced under pressure.     

Phase transition of [(C6H5)3PCH3]+(TCNQ)-2 and electrical transport properties

The electrical conductivity at 10GHz, the dielectric constant, and the thermoelectric power (TEP) of [(C₆H₅)₃PCH₃]⁺(TCNQ)^-₂, from 230 up to 400 K, have been measured. This organic quasi-one-dimensional solid undergoes a first order phase transition at 314 K. At the transition the conductivity increases by a factor of 2.2 and the activation energy drops to 0.26 from 0.31 eV. At 314 K TEP decreases abruptly from -75 to -60μVK^-1 and remains almost constant for T > 314 K. The dielectric permeability ?₀ is constant and equal to 5 in the low temperature phase, increases abruptly by 7% at the transition, and then depends strongly on temperature in the high temperature phase. Results of the high temperature phase are interpreted in terms of a strongly correlated salt.     

Conduction behaviour and thermoelectric power of Agx (As0.4Se0.6) 100−x chalcogenide system

Chalcogenide bulk alloys of Ag_x (As_0.4Se_0.6) _100−x (x=5, 7.5, 10, 12.5, 15 and 17.5) system were prepared by the conventional melt-quench technique. The d.c. electrical conductivity (σ) and thermoelectric power (TEP) measurements were carried out in the temperature range from 83 to 373 K and from 253 to 373 K, respectively. Variations of both σ and TEP with ambient temperature proved the p-type semiconducting behaviour of these materials. The current density-electric field characteristics were found to be linear. The activation energies, calculated from both the electrical conductivity E_σ and thermoelectric power E_s, were found to be dependent on composition.     

The thermoelectric power of charge-density-wave conductors Rb0.3MoO3 and Rb0.15K0.15MoO3

The thermoelectric power (TEP) of the quasi-one-dimensional charge-density-wave (CDW) conductors rubidium blue bronze Rb_0.3MoO₃ and its alloy Rb_0.15K_0.15MoO₃ were measured in the temperature range 80-280 K. The result showed a sign change from a small positive value to a great negative value where the Peierls transition temperatures (T_p) are 183 and 180 K for Rb_0.3MoO₃ and Rb_0.15K_0.15MoO₃, respectively. Above T_p, the TEP for both samples can be described with the empirical relation S=AT+B; while below T_p, the TEP fits well the relation S=AT+B/T based on the experimental data. The Fermi energies ε_F for Rb_0.3MoO₃ and Rb_0.15K_0.15MoO₃ are estimated to be 1.55 and 0.53 eV, respectively.     

The thermoelectric power of MEM(TCNQ)2

MEM(TCNQ)₂ undergoes a first order semiconductor to metal transition at 340.8 K. We have measured the thermoelectric power (TEP) of MEM(TCNQ)₂ in the temperature range above 335 K. Above the transition the TEP is ?65 μV/°K, in the low temperature phase it is strongly temperature dependent and approaches zero near the transition. The indicated loss of spin entropy at the transition is discussed.     

Low concentration of the superconducting phase and thermoelectric power in 90 K superconductors

The thermoelectrical power (TEP) and the electrical resistivity behaviour of three 90 K superconductors viz, Y₁Ba₂Cu₃O_7−x , Sm₁Ba₂Cu₃O_7−x and Gd₁Ba₂Cu₃O_7−x , after the specimens were quenched from the sintering temperature (920°C) to 77 K, are reported. Interestingly the Y₁₂₃ specimen, which has the presence of trace amount of the orthorhombic phase in an otherwise tetragonal phase and does not show a superconducting transition down to 77 K shows zero TEP around 82 K, theT _c for the well oxygenated specimen. The Sm and Gd specimens on the other hand show completely tetragonal structure, semiconducting behaviour in resistivity and no zero TEP up to 77 K. It is argued that the critical concentration of the superconducting phase necessary to make the TEP zero is much smaller than that required for zero resistivity.     

Thermoelectric power of Na0.33V2O5

Thermoelectric power (TEP) and conductivity measurements are reported on the highly anisotropic conductor Na_0.33V₂O₅. The high temperature TEP is interpreted in terms of spin entropy, characteristic of systems with strong on-site Coulomb correlations, with nearest neighbor correlations important at low temperatures (T < 130 K). The conductivity is suggested to reflect the importance of disorder and/or impurity effects.     

Transport and magnetic properties of cobalt doped CeNiAl4

Electrical resistivity and thermoelectric power (TEP) have been measured in polycrystalline sample of CeNi_0.75Co_0.25Al₄. The magnetization measurements have been performed in oriented powder with a-axis of the crystallites parallel to the external magnetic field. All the data have been compared with its parent compound CeNiAl₄. In the range 120-300 K, the Seebeck coefficient is significantly higher for alloy than for the parent material and exhibits a peak at ∼150 K. R(T)/R(300) follows a logarithmic temperature dependence for all samples above 100 K, and rapidly decreases as the temperature is lowered. The decrement is much faster in the cobalt doped samples than in the parent compound, suggesting stronger electronic correlations in the former. Inverse magnetic susceptibility in oriented powder follows a Curie-Weiss law above 100 K and shows p_eff=2.7 slightly higher than that of the free Ce³⁺ ion value of 2.5.     

Electrical and optical properties of single crystal In2Te3 and Ga2Te3

Electrical conductivity and thermopower measurements are reported for the defect semiconductors p-In₂Te₃ and n-Ga₂Te₃. The hole mobility μ_p in the former varied as Tⁿwheren=+5.98 forT<350 K and n=-4.13 forT>350 K showing a maximum of 210 cm²V^-1 sec^-1 at 350 K. Electron mobility in n-Ga₂Te₃ also went through a maximum at 320 K. The optical band-gaps for both were found to be direct, with values of 1.01 and 1.08 eV for In₂Te₃ and Ga₂Te₃ respectively at 300 K. Pronounced effects of annealing on TEP and optical absorption gave evidence of defect ordering at low temperatures followed by defect creation at T>350 K.     

THERMOELECTRIC POWER OF A LOCAL PAIR MODEL AND ITS APPLICABILITY TO YBa2Cu3O7

A formula of thermoelectric power (TEP) for the local pair model has been derived. It has been found that from T_c to about 125K the TEP data of YBa₂Cu₃O₇ (Y-123) can be described by this formula. We have also found that the formula of the diffusion thermoelec-tricity in the free electron approximation can be used to express the TEP data of Y-123 in the region of 220 to about 300 K. It seems that a general transition occurs in the intermediate temperature region, that is from the regime in which the local pair subsystem is dominating to the regime in which the itinerant carrier subsystem is dominating.     

The impact of Y substitution on the 110 K high Tc phase in a Bi (Pb):2223 superconductor

The structural and superconducting properties of Bi_1.7Pb_0.3Sr₂Ca_2−xY_xCu₃O_y superconducting samples are investigated by X-ray diffraction (XRD), resistivity and thermoelectric power (TEP) measurements. XRD results reveal that the volume percentage of the 2223 high T_c phase decreases with an increase in Y content. The replacement of the Ca²⁺ ion by the Y³⁺ ion does not influence the tetragonal structure of the pure Bi (Pb): 2223 system and the lattice parameters vary with Y content. The results of resistivity indicate that the critical temperatures T_c of the samples decrease monotonically with an increase in Y content. Further, the critical concentration of Y to completely suppress superconductivity in the Y-doped Bi (Pb):2223 system is higher (0.60) than that reported (0.20) for the other rare-earth elements. On the other hand, the values of TEP at room temperature are found to be negative for Y=0.00 and 0.10 samples, and it changed to positive with further increase in Y content. The hole-carrier concentration per Cu ion (P) is deduced by using two different ways: the first in terms of T_c values in the superconducting state and the other in terms of TEP values in the normal state. Interestingly, it is found that the values of P deduced from the formal way are not consistent with the reported parabolic behavior for superconducting systems in the under-doped region, and consequently disagree with the general roles of substitution. However, the vice versa is recorded for the values of P deduced from the latter way. The results are discussed in terms of the possible reasons for the suppression of superconductivity in the considered system.     

Electrical and thermal properties of Pr2/3(Ba1−xCsx)1/3MnO3 manganites

Electrical and thermal properties of Pr_2/3(Ba_1?x Cs _x )_1/3MnO₃ (0 ≤ x ≤ 0.25) manganite perovskites are reported here. Two insulator-metal (I-M) transitions (T _P1 &T _P2) are observed in the electrical resistivity (ρ) of the pristine Pr_2/3Ba_1/3MnO₃ (PBMO) sample, and they are systematically shifted to lower temperatures with increasing Cs substitution. An upturn in ρ is noticed below 50 K in these perovskites, presumably due to the combined effect of weak localization, electron-electron and electron-phonon scattering. It is found that the absolute value of room-temperature thermoelectric power (TEP) gradually decreases with increasing Cs content, implying the annihilation of the charge carriers with doping. An analysis of the electrical resistivity and thermoelectric power data indicates that the paramagnetic insulating state above T _P1 is governed by the small polaron hopping due to a non-adiabatic process. It is argued that the electron-magnon scattering processes are responsible for low temperature metallic behavior of TEP. A distinct specific heat peak below T _P1 is observed, attributed to the magnetic ordering, and its broadening with Cs-doping corresponds to the increase of magnetic inhomogeneity. Further, the temperature variation of thermal conductivity and the low temperature plateau in κ has been associated with the delocalization of Jahn-Teller polarons and transition from Umklapp scattering to a defect-limited scattering, respectively.     

Magnetic field dependence of the thermoelectric power of n - type gallium arsenide in the extreme quantum limit

The effect of the quantization of the electron energy levels in a strong transverse magnetic field H on the low-temperature thermoelectric power (TEP) S of a high-purity isotropic semiconductor ($\text{n}$ - type gallium arsenide GaAs) is investigated theoretically. The “electron-diffusion” (S_e) and “phonon-drag” (S_p) components of S( = S_e + S_p) are calculated in the extreme quantum limit, when all the electrons in the conduction band are concentrated in the lowest Landau level. The transition to nondegeneracy, which takes place when the bottom of the lowest Landau level is driven through the Fermi level, has a large effect on the variations of S_e and S_p with magnetic field. The results are illustrated with numerical calculations for $\text{n}$ - type GaAs at 4.2 K with 1.2 × 10¹⁶ cm^-3 electrons.     

Anomalously large contributions of vacancies and dislocations to Hall coefficient and thermoelectric power of copper films

Effect of vacancies and dislocations on Hall coefficient (R_H) and thermoelectric power (TEP) of copper films has been deduced from annealing behaviour of various transport properties. The large contributions of these defects may be attributed to variations in the density of hole-like states and/or the energy dependence of conductivity (at the Fermi surface), caused by changes in the extent and/or degree of curvature of the necks at Fermi surface.     

Structural, electrical and optical properties of TiO2 doped WO3 thin films

TiO₂ doped WO₃ thin films were deposited onto glass substrates and fluorine doped tin oxide (FTO) coated conducting glass substrates, maintained at 500 °C by pyrolytic decomposition of adequate precursor solution. Equimolar ammonium tungstate ((NH₄)₂WO₄) and titanyl acetyl acetonate (TiAcAc) solutions were mixed together at pH 9 in volume proportions and used as a precursor solution for the deposition of TiO₂ doped WO₃ thin films. Doping concentrations were varied between 4 and 38%. The effect of TiO₂ doping concentration on structural, electrical and optical properties of TiO₂ doped WO₃ thin films were studied. Values of room temperature electrical resistivity, thermoelectric power and band gap energy (E_g) were estimated. The films with 38% TiO₂ doping in WO₃ exhibited lowest resistivity, n-type electrical conductivity and improved electrochromic performance among all the samples. The values of thermoelectric power (TEP) were in the range of 23-56 μV/K and the direct band gap energy varied between 2.72 and 2.86 eV.     

Studies of thermodynamic fluctuations of thermoelectric power of Tl2Ba2Ca0.8Y0.2Cu2O8+y superconductor

Excess thermoelectric power (TEP) of Tl₂Ba₂Ca_0.8Y_0.2Cu₂O_8+y ceramic superconductor has been studied. Variation of excess TEP with temperature has been found to follow an equation similar to that of Aslamazov and Larkin derived for the variation of excess conductivity with temperature. The superconducting behaviour of the sample, as determined from excess TEP, has been found to be 2-dimensional in nature.     

Analysis of thermoelectric power of some insulating high-T c superconductors using extended hubbard model

The thermoelectric power (TEP) for a one-dimensional lattice has been studied using the extended Hubbard model in the limitU ≠ ∞, whereU is the on-site Coulomb interaction. A new expression for TEP has been derived in this study. Our study shows that if theV-dependent term in the extended Hubbard model Hamiltonian is considered as attractive, the new expression for TEP could successfully reproduce the TEP results of high-T _c hole-doped insulating systems of Tl₂Ba₂Ca_1−x Y _x Cu₂O_8+y (Tl-2212) and Bi₂Sr₂Ca_−x Y _x Cu₂O_8+y (Bi-2212).     

Intensity and half-width measurements in the 1·525 μm band of acetylene

Line intensities at 150°K and 295°K, self-broadened half-widths at 171°K, 200°K, 250°K and 295°K, and hydrogen-broadened half-widths at 171°K, 200°K and 295°K have been measured in the ν₁+v₃ band of C₂H₂ at 1·525 μm. The absolute intensity of the band has been determined independently by employing the Wilson-Wells-Penner-Weber technique. Our best estimate for the absolute intensity of the band is S_v=7·82 ± 0·07 cm^?2 atm^?1 at 295°K. Line intensities calculated using this value of S_v are in good agreement with the measured intensities at the two extreme temperatures of 150°K and 295°K considered in the present study, thereby not suggesting any significant intensity anomalies. Line positions have been measured for the first time for this band for R(29)?P(25).     

Critical behavior of heat capacity in the vicinity of phase transitions in [NH2(CH3)2]5Cd3Cl11

The heat capacity of a [NH₂(CH₃)₂]₅Cd₃Cl₁₁ crystal was studied calorimetrically in the temperature interval 100–300 K. The C _p (T) dependence indicates that, as the temperature is lowered, phase transitions occur at temperatures T ₁ = 176.5 K and T ₂ = 123.5 K. The thermodynamic characteristics of this crystal were determined. It is shown that the transition at T ₂ = 123.5 K is an incommensurate-commensurate phase transformation and that the transition at T ₁ = 176.5 K is a normal-incommensurate phase transition.