Thermoelectric properties of rare-earth doped Fe2VAl Heusler alloys.

The low-temperature electrical transport properties of the rare-earth (RE) Ce, Dy, Sm element doped Fe2VAl Heusler alloys have been investigated. A significant enhancement in the Seebeck coefficient S (peak values of about -125 to -160 µV K-1) is observed as compared to the pure Fe2VAl (peak value of about 40 µV K-1). It is observed that the thermal conductivity reduced by 50% in RE-doped samples. The single parabolic band model has been used to analyze the experimental data and to understand the role of fundamental parameters like the Lorenz number. The lattice contribution to the total thermal conductivity was analyzed through the Callaway model, which in turn provided the insight into the phonon scattering in these alloys. Finally, we demonstrate a significant improvement in power factor and figure of merit at all temperatures for the RE-doped Fe2VAl alloys.

Role of electron-magnon interaction in non-Fermi liquid behavior of SrRuO3.

SrRuO3 is a popular material extensively used as a bottom electrode in various applications, however, a few problems which will certainly change the interface band structure and greatly alter the device's property are still not fully understood, such as the change of carrier types at a certain temperature and the quasiparticle scattering for non-Fermi liquid behavior below ferromagnetic transition temperature. In this study, magnetic, transport (electrical and thermal) properties and x-ray photoemission spectra have been used to understand the role of quasiparticle interactions in the SrRuO3 bulk system. At the Fermi level, the hybridization of Ru4dt 2g ↓ and O2p  bands form a typical two band system. In order to explain the problems as mentioned, our present work reveals that there must be an impurity band that couples with the bands around Fermi level and serves as a charge reservoir. In the present case, the impurity is attributed to the Ru vacancies. As a result, the conduction electrons scatter strongly with the Ru vacancies and couple with the Ru magnons to give rise to a dominant electron-magnon coupling that overwhelms the electron-phonon coupling in the temperature range of 90-150 K.

Thermoelectric properties of Ag-doped CuS nanocomposites synthesized by a facile polyol method.

We report the first thermoelectric properties of Cu1-xAgxS, x = 0-0.75 nanocomposites, synthesized by using a facile polyol method. Systematic characterizations using powder XRD, Rietveld refinement of XRD, EDAX, XPS and Raman spectroscopy confirmed their single phase, hexagonal crystal structure with the space group P63/mmc, nominal elemental composition, valence states of the constituent elements and stoichiometric nature. The TEM images showing the CuS formation of nearly perfect hexagonal disk-like particles of average thickness 26.7 nm and breadth ranging in a few hundreds of nanometers with nanorods stacked from these hexagonal nanodisks (NDs) elongated along the c axis corroborate the FESEM images. Attributed to structural phase transition, an anomaly at 55 K is clearly observed in both the thermopower and Hall resistivity data. By increasing x, a systematic reduction in thermal conductivity was observed near 300 K. Consequently, a 50% enhancement in figure of merit was observed for Cu0.9Ag0.1S as compared to pure CuS at 300 K. These results therefore are expected to provide a new direction in improving ZT near 300 K.

The effect of stoichiometry on the structural, thermal and electronic properties of thermally decomposed nickel oxide.

A thermal decomposition route with different sintering temperatures was employed to prepare non-stoichiometric nickel oxide (Ni1-δ O) from Ni(NO3)2·6H2O as a precursor. The non-stoichiometry of samples was then studied chemically by iodometric titration, wherein the concentration of Ni3+ determined by chemical analysis, which is increasing with increasing excess of oxygen or reducing the sintering temperature from the stoichiometric NiO; it decreases as sintering temperature increases. These results were corroborated by the excess oxygen obtained from the thermo-gravimetric analysis (TGA). X-ray diffraction (XRD) and Fourier transformed infrared (FTIR) techniques indicate the crystalline nature, Ni-O bond vibrations and cubic structural phase of Ni1-δ O. The change in oxidation state of nickel from Ni3+ to Ni2+ were seen in the X-ray photoelectron spectroscopy (XPS) analysis and found to be completely saturated in Ni2+ as the sintering temperature reaches 700 °C. This analysis accounts for the implication of non-stoichiometric on the magnetization data, which indicate a shift in antiferromagnetic ordering temperature (T N) due to associated increased magnetic disorder. A sharp transition in the specific heat capacity at T N and a shift towards lower temperature are also evidenced with respect to the non-stoichiometry of the system.

Electrical and thermal transport properties of intermetallic RCoGe2 (R = Ce and La) compounds.

To investigate the electronic structure of the intermetallic compound CeCoGe2, we performed electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) measurements in a temperature range of 10-300 K. For comparison, the non-magnetic counterpart LaCoGe2 is also studied. It is found that CeCoGe2 exhibits a broad maximum in the S(T) near 75 K, at which the sudden drop in the ρ(T) is observed. Temperature-dependent electrical resistivity and the Seebeck coefficient of CeCoGe2 can be described well by a two-band model, which reveals the signature of Kondo scattering in CeCoGe2. On the other hand, a typical metallic-like behavior is seen in the non-magnetic LaCoGe2 from the ρ(T) and S(T) studies. Analysis of the thermal conductivity indicates that the electronic contribution dominates thermal transport above 100 K in both CeCoGe2 and LaCoGe2. In addition, it is found that the variation in low-temperature lattice thermal conductivity of CeCoGe2 as compared to that of LaCoGe2 is most likely due to the phonon-point-defect scattering.

The effect of Al/Si ratio on the transport properties of the layered intermetallic compound CaAl(2)Si(2).

We report the results of the electrical resistivity and Seebeck coefficient as well as thermal conductivity measurements on the stoichiometric CaAl(2)Si(2) and non-stoichiometric CaAl(1.75)Si(2.25), CaAl(1.9)Si(2.1), CaAl(2.1)Si(1.9), and CaAl(2.25)Si(1.75) compounds in the temperature range 10-300 K. It has been found that the magnitude of electrical resistivity decreases for the non-stoichiometric samples, attributed to the shift of Fermi energy from the dip of the density of states as a consequence of the changed Si/Al content. In addition, a systematic change in the magnitude of Seebeck coefficient as a function of Al/Si concentration has been observed. The results have been associated with the effect of hole/electron doping on the Fermi level density of states. A detailed analysis of the electrical resistivity and Seebeck coefficient suggests the presence of two types of charge carrier and the temperature dependent changes in their mobility. From the thermal conductivity results, we correlated the extent of disorder and Al/Si ratio with various thermal scattering mechanisms in the investigated temperature range.

Weak charge-density-wave transition in LaAgSb(2) investigated by transport, thermal, and NMR studies.

We report a study of the charge-density-wave (CDW) behavior in LaAgSb(2) by means of electrical resistivity, Seebeck coefficient, thermal conductivity, specific heat, and nuclear magnetic resonance (NMR) measurements. Except for the Seebeck coefficient, apparent indications of CDW ordering at around 207 K were noticed in the physical quantities investigated. On the other hand, all measured physical properties are insensitive to the second CDW formation (∼184 K), as the transition character is considerably weaker than the high-temperature one. Further, analyses of the thermal conductivity and NMR Knight shift data revealed that the observed variations are essentially of electronic origin. The present findings are in good agreement with the previous results, indicating that the high-temperature CDW ordering is associated with a small gapping of the Fermi surface with a minor periodic displacement of the crystal lattice in LaAgSb(2).

Phase properties of a zigzag chain lattice gas with Coulomb interactions.

It has been reported that the phase transitions found in the quasi-one-dimensional sulfide KCu(7-x)S4 are most likely due to vacancy ordering involving Cu+-ion diffusion along the Cu(2)-Cu(2) zigzag chains. Our previous studies with both a self-consistent method and Monte Carlo simulations confirmed that phase transitions indeed exist in a one-dimensional (1D) lattice gas system in which vacancy ordering is involved. In this paper, we calculate the more nearly real case of KCu6.88S4 and further investigate the angular dependence of the phase properties in a partially occupied 1D zigzag chain with various particle occupancies. The calculated results suggest that the phase transitions that occur in the quasi-one-dimensional material KCu(7-x)S4 are presumably due to both intrachain and interchain interactions between the partially occupied Cu+ zigzag chains. Most interestingly, we found that the average particle distribution of the lowest free energy state is a linear superposition of two other solutions with different particle distributions for occupancy n(av)=1/2.

Numerical study of passive Q switching of a tunable alexandrite laser with a Cr:Y2SiO5 solid-state saturable absorber.

Passive Q switching of a tunable alexandrite solid-state laser with a Cr:Y2SiO5 solid-state saturable absorber is numerically studied over a major portion of the alexandrite tuning range. The theory of passive Q switching with a slowly relaxing saturable absorber is studied and utilized for evaluating the performance of the Cr:YSO Q-switched alexandrite laser system. With a typical laser configuration, a Q-switched laser pulse of 262 mJ in 23 ns is obtained.

Ho:YV0(4) solid-state saturable-absorber Q switch for a 2-µm Tm, Cr:Y(3)Al(5)O(12) laser.

The holmium-doped yttrium orthovanadate (Ho:YVO(4)) crystal was shown to be an effective solid-state saturable-absorber Q switch for a flash-lamp-pumped Tm,Cr:Y(3)Al(5)O(12) laser operating at 2.017 µm. A single Q-switched laser pulse of 3.5 mJ in energy and 45 ns in duration was observed with a 1-mm-thick 4% Ho:YVO(4) solid-state saturable absorber, which was grown with the high-temperature-solution-growth method.

Ho:CaF(2) solid-state saturable-absorber Q switch for the 2-µm Tm,Cr:Y(3)Al(5)O(12) laser.

The holmium-doped calcium fluoride (Ho:CaF(2)) crystal was shown to be an effective solid-state saturable-absorber Q-switch for a flash-lamp-pumped Tm,Cr:Y(3)Al(5)O(12) laser at 2.017 µm. With a 1-cm-thick Ho(0.5%),Er(5%):CaP(2) saturable absorber and a 6.3% output coupler, a single Q-switched laser pulse of 51 mJ in energy and 60 ns in duration was obtained at a flash-lamp input energy of 85 J. With a 14.6% output coupler, a typical Q-switched laser pulse of 84 mJ and 82 ns was observed.

Characteristics of ruby passive Q switching with a Dy(2+):CaF(2) solid-state saturable absorber.

Characteristics of ruby passive Q switching with a Dy(2+):CaF(2) solid-state saturable absorber are investigated with output couplers of various reflectivities and saturable absorbers of different thicknesses. Numerical simulation is used to investigate the behavior of ruby passive Q switching with a Dy(2+):CaF(2) saturable absorber and to interpret the experimental results.

Dy(2+):CaF(2) saturable-absorber Q switch for the ruby laser.

Passive Q switching of a flash-lamp-pumped ruby laser has been demonstrated with a Dy(2+):CaF(2) saturable absorber at room temperature. Typically a single Q-switched pulse of 4.5 mJ and 94 ns was observed. At 694 nm, the ground-state absorption cross section and the excited-state absorption cross section of the Dy(2+):CaF(2) absorber were found to be 1.2 × 10(-18) cm(2) and 0.9 × 10(-18) cm(2) respectively, with a four-level slow-relaxing saturable-absorber model.