Intrinsic spin-dependent thermal transport

Most studies of spin caloritronic effects to date, including spin-Seebeck effect, utilize thin films on substrates. We use patterned ferromagnetic thin film to demonstrate the profound effect of a substrate on the spin-dependent thermal transport. With different sample patterns and on varying the direction of temperature gradient, both longitudinal and transverse thermal voltages exhibit asymmetric instead of symmetric spin dependence. This unexpected behavior is due to an out-of-plane temperature gradient imposed by the thermal conduction through the substrate and the mixture of anomalous Nernst effects. Only with substrate-free samples have we determined the intrinsic spin-dependent thermal transport with characteristics and field sensitivity similar to those of the anisotropic magnetoresistance effect.     

Spin-Seebeck effects in Ni81Fe19/Pt films

The spin-Seebeck effect (SSE) converts a heat current into a spin current, a flow of spin angular momentum, and spin voltage, the driving force for nonequilibrium spin currents, in a ferromagnetic metal. In this study, the SSE in a ferromagnetic Ni₈₁Fe₁₉ film has been investigated by means of the inverse spin-Hall effect (ISHE) in a Pt film at room temperature. The experimental results measured in the Ni₈₁Fe₁₉/Pt system show that the sign of the thermally induced spin voltage is reversed between the higher- and lower-temperature ends of the Ni₈₁Fe₁₉ film. The ISHE in the Pt film allows us to detect the SSE signal with high sensitivity and to separate it from extrinsic thermoelectric effects.     

Spin Caloritronic Transport of Tree-Saw Graphene Nanoribbons

Using density functional theory combined with non-equilibrium Green's function method, we investigate the spin caloritronic transport properties of tree-saw graphene nanoribbons. These systems have stable ferromagnetic ground states with a high Curie temperature that is far above room temperature and exhibit obvious spin-Seebeck effect. Moreover, thermal colossal magnetoresistance up to 1020% can be achieved by the external magnetic field modulation. The underlying mechanism is analyzed by spin-resolved transmission spectra, current spectra and band structures.     

Amplification of spin waves by thermal spin-transfer torque

We observe amplification of spin-wave packets propagating along a film of single-crystal yttrium iron garnet subject to a transverse temperature gradient. The spin waves are excited and detected with standard techniques used in magnetostatic microwave delay lines in the 1-2 GHz frequency range. The amplification is attributed to the action of a thermal spin-transfer torque acting on the magnetization that opposes the relaxation and which is created by spin currents generated through the spin-Seebeck effect. The experimental data are interpreted with a spin-wave model that gives an amplification gain in very good agreement with the data.     

Graphene for Thermoelectric Applications: Prospects and Challenges

Thermoelectric power generators require high-efficiency thermoelectric materials to transform waste heat into usable electrical energy. An efficient thermoelectric material should have high Seebeck coefficient and excellent electrical conductivity as well as low thermal conductivity. Graphene, the first truly 2D nanomaterial, exhibits unique properties which suit it for use in thermoelectric power generators, but its application in thermoelectrics is limited by the high thermal conductivity and low Seebeck coefficient resulting from its gapless spectrum. However, with the possibility of modification of graphene's band structure to enhance Seebeck coefficient and the reduction of its thermal conductivity, it is an exciting prospect for application in thermoelectric power generation. This article examines the electronic, optical, thermal, and thermoelectric properties of graphene systems. The factors that contribute to these material properties in graphene systems like charge carriers scattering mechanisms are discussed. A salient aspect of this article is a synergistic perspective on the reduction of thermal conductivity and improvement of Seebeck coefficient of graphene for a higher thermoelectric energy conversion efficiency. In this regard, the effect of graphene nanostructuring and doping, forming of structural defects, as well as graphene integration into a polymer matrix on its thermal conductivity and Seebeck coefficient is elucidated.     

Thermal Conductivity and Thermoelectric Power of Compounds in the Cu–Ge–As–Se System

Technical Physics - The influence of temperature (in the interval of 300–400 K) and concentrations on the electrical conductivity, thermal conductivity, and thermoelectric power of copper...     

Tuning the thermal conductivity of strontium titanate through annealing treatments

Strontium titanate(SrTiO_3) is a promising n-type material for thermoelectric applications. However, its relatively high thermal conductivity limits its performance in efficiently converting heat into electrical power through thermoelectric effect.This work shows that the thermal conductivity of SrTiO_3 can be effectively reduced by annealing treatments, through an integrated study of laser flash measurement, scanning electron microscopy, Fourier transform infrared spectroscopy, x-ray absorption fine structure, and first-principles calculations. A phonon scattering model is proposed to explain the reduction of thermal conductivity after annealing. This work suggests a promising means to characterize and optimize the material for thermoelectric applications.     

Thermoelectric properties of a dilute graphite donor intercalation compound

Data on the in-plane thermal conductivity and thermoelectric power of a stage-5 potassium donor graphite intercalation compound are reported in the temperature range 3 < T < 300 K. In the lowest temperature range the electronic contribution dominates the thermal conductivity, while at higher temperature there is a dominant lattice contribution, which is much smaller than pristine graphite. The thermopower is negative in the whole temperature range.     

Mesoscopic thermal and thermoelectric measurements of individual carbon nanotubes

We discuss the mesoscopic experimental measurements of electron energy dissipation, phonon thermal transport, and thermoelectric phenomena in individual carbon nanotubes. The temperature distributions in electrically heated individual multiwalled carbon nanotubes have been measured with a scanning thermal microscope. The temperature profiles along the tube axis in nanotubes indicate the bulk dissipation of electronic energy to phonons. In addition, thermal conductivity of an individual multiwalled nanotube has been measured using a microfabricated suspended device. The observed thermal conductivity is two orders of magnitude higher than the estimation from previous experiments that used macroscopic mat samples. Finally, we present thermoelectric power (TEP) of individual single walled carbon nanotubes using a novel mesoscopic device. A strong modulation of TEP as a function of the gate electrode was observed.     

Spin Hall effect generated by a temperature gradient and heat current in a two-dimensional electron gas

We predict an intrinsic thermo-spin Hall effect, namely, that a transverse spin current is generated by the temperature gradient and the heat current in a disorder-free two-dimensional electron gas (2DEG) with finite spin–orbit coupling. There exist two classes of contributions to the thermal spin Hall effect, corresponding to a 2DEG contacting two reservoirs at different temperatures and to a 2DEG separated from the reservoirs by insulating spacers, respectively. It is shown that the thermal spin Hall current can be generated not only by the temperature gradient directly but also by the thermoelectric effect.     

Transport properties of silicon

Electrical conductivity, thermal conductivity, and thermoelectric power of single-crystalline silicon are investigated at temperatures between 2 and 300 K. From the measured data we calculate the mean free path of electrons and phonons and separate diffusion part and phonon-drag part of the thermoelectric power. Using a new method, we evaluate the mean free path of those phonons which are responsible for the phonon drag effect.Dedicated to H.-J. Queisser on the occasion of his 60th birthday     

Thermoelectric composite materials based on intermediate-valence cerium and ytterbium intermetallic compounds

An important thermoelectric characteristic-the power factor of composite thermoelectric materials—is calculated on the basis of performed measurements of the thermopower, electric conductivity, thermal conductivity, and reference data. The investigated composites consist of intermediate-valence CeNi and YbAl₃ intermetallic systems and elementary metals, nickel and chromium. On account of the specific transport properties of a heterogeneous medium, it becomes possible to increase the value of the power factor in composites at a sufficiently low concentrations of rare-earth-element-based intermetallic compounds. The dependences of the composite-thermoelectric-material power factor on the temperature and the mass fraction of valence-instable components are obtained.     

Thermoelectric power generation in a thermoacoustic refrigerator

Commercially available solid-state thermoelectric devices may be used for their electrical power generation capabilities when coupled to a thermoacoustic refrigerator or heat pump. General performance characteristics as well as bulk thermal conductivity for a selection of thermoelectric elements was first found by using a two-plate apparatus to maintain a constant temperature difference across the element. Further studies of an element’s performance when placed in series with the thermoacoustic refrigerator’s heat exchangers will be presented. Design considerations for using thermoelectric elements in a no-moving parts electrical power generation scheme will be discussed.     

Unexpected low thermal conductivity and large power factor in Dirac semimetal Cd_3As_2

Thermoelectrics has long been considered as a promising way of power generation for the next decades. So far,extensive efforts have been devoted to the search of ideal thermoelectric materials, which require both high electrical conductivity and low thermal conductivity. Recently, the emerging Dirac semimetal Cd3As2, a three-dimensional analogue of graphene, has been reported to host ultra-high mobility and good electrical conductivity as metals. Here, we report the observation of unexpected low thermal conductivity in Cd3As2, one order of magnitude lower than the conventional metals or semimetals with a similar electrical conductivity, despite the semimetal band structure and high electron mobility. The power factor also reaches a large value of 1.58 m W·m-1·K-2at room temperature and remains non-saturated up to 400 K.Corroborating with the first-principles calculations, we find that the thermoelectric performance can be well-modulated by the carrier concentration in a wide range. This work demonstrates the Dirac semimetal Cd3As2 as a potential candidate of thermoelectric materials.     

微型层式热电模块制冷特性研究

本文建立了微型层式热电模块的仿真模型,讨论了不同的层数、层厚度,层导热系数对层式热电模块制冷效果的影响。研究发现:二层热电臂结构最优,冷、热端热电臂导热系数梯度越大,制冷效果越好,且热电臂厚度在不同导热系数梯度下存在最优值。另外,在微型层式热电模块中,接触效应会显著影响其制冷性能,因此在实际设计及应用研究中不可忽略。     

Phonon considerations in the reduction of thermal conductivity in phononic crystals

Periodic porous structures offer unique material solutions to thermoelectric applications. With recent interest in phonon band gap engineering, these periodic structures can result in reduction of the phonon thermal conductivity due to coherent destruction of phonon modes characteristic in phononic crystals. In this paper, we numerically study phonon transport in periodic porous silicon phononic crystal structures. We develop a model for the thermal conductivity of phononic crystal that accounts for both coherent and incoherent phonon effects, and show that the phonon thermal conductivity is reduced to less than 4% of the bulk value for Si at room temperature. This has substantial impact on thermoelectric applications, where the efficiency of thermoelectric materials is inversely proportional to the thermal conductivity.     

Thermoelectric transport in holographic quantum matter under shear strain

We study thermoelectric transport under shear strain in two spatial dimensional quantum matter using the holographic duality.General analytic formulae for the DC thermoelectric conductivities subjected to finite shear strain are obtained in terms of black hole horizon data.Off-diagonal terms in the conductivity matrix also appear at zero magnetic field,resembling an emergent electronic nematicity,which cannot nevertheless be identified with the presence of an anomalous Hall effect.For an explici...     

Oscillations quantiques des coefficients de pouvoir thermoelectrique et de conductivite thermique: theorie et mesures dans Cd3As2

Quantum oscillations have been observed in the thermoelectric power and thermal conductivity coefficients of a semiconductor Cd₃As₂ single crystal. Theoretical expressions for these coefficients, taking into account the Landau level broadening, are derived in this paper. The good agreement found between the thermoelectric power measurements and the corresponding theoretical expressions for the temperature and magnetic field dependence of the quantum oscillation amplitudes and the absolute values of the amplitudes has allowed us to calculate effective mass values which are in agreement with Shubnikov-de Haas measurements.     

Electrical conductivity and thermoelectric power of liquid selenium doped with thallium and indium

The temperature dependence of the electrical conductivity and thermoelectric power of liquid selenium with thallium and indium additives have been studied. Large variations appear in the electrical conductivity and thermoelectric power, where Tl and In additives favourp-type conduction.     

Effect of annealing on the electric and magnetic properties of GaMnAs and Be-codoped GaMnAs

GaMnAs and Be-codoped GaMnAs films grown via molecular beam epitaxy (MBE) were heat treated and the stability of Mn in the matrix was investigated. MnAs had a stable phase at the low growth temperature, but MnGa was stable at the annealing temperature. Be-codoping did not prevent the precipitation processes, but Be itself was stable during the annealing process to maintain the GaAs matrix at the high conductivity.