Phonon Drag Effect Research Articles

Phonon-Drag Effect on Seebeck Coefficient in Co-Doped Si Wire with Submicrometer-Scaled Cross Section

Phonon-Drag Effect on Seebeck Coefficient in Co-Doped Si Wire with Submicrometer-Scaled Cross Section

Magnetophonon oscillations of thermoelectric power and combined resonance in two-subband electron systems

By measuring the thermoelectric effect in high-mobility quantum wells with two occupied subbands in perpendicular magnetic field, we detect magnetophonon oscillations due to interaction of electrons with acoustic phonons. These oscillations contain specific features identified as combined resonances caused by intersubband phonon-assisted transitions of electrons in the presence of Landau quantization. The quantum theory of phonon-drag magnetothermoelectric effect, generalized to the case of multi-subband occupation, describes our experimental findings.

Colossal Seebeck effect enhanced by quasi-ballistic phonons dragging massive electrons in FeSb2.

Phonon transport is an essential property of thermoelectric materials. Although the phonon carries heat, which reduces the thermoelectric efficiency, it contributes positively to the Seebeck coefficient S through the phonon-drag effect, as typified by the high-purity semiconductors, which show fairly large S at cryogenic temperatures. Although such a large S is attractive in terms of Peltier cooling, a clear guiding principle for designing thermoelectric materials enriched by the phonon-drag effect remains to be established. Here we demonstrate that a correlated semiconductor, FeSb2, is a promising thermoelectric material featuring quasi-ballistic phonons dragging d electrons with large effective mass. By changing the sample size within the sub-millimetre order for high-purity single crystals, we succeed in substantially increasing S to as much as −27 mV K−1 at low temperatures. Our results exemplify a strategy for exploring phonon-drag-based thermoelectric materials, the performance of which can be maximized by combining heavy electrons with ballistic phonons.

Thermoelectric coefficients ofn-doped silicon from first principles via the solution of the Boltzmann transport equation

We present a first-principles computational approach to calculate thermoelectric transport coefficients via the exact solution of the linearized Boltzmann transport equation, also including the effect of nonequilibrium phonon populations induced by a temperature gradient. We use density functional theory and density functional perturbation theory for an accurate description of the electronic and vibrational properties of a system, including electron-phonon interactions; carriers' scattering rates are computed using standard perturbation theory. We exploit Wannier interpolation (both for electronic bands and electron-phonon matrix elements) for an efficient sampling of the Brillouin zone, and the solution of the Boltzmann equation is achieved via a fast and stable conjugate gradient scheme. We discuss the application of this approach to $n$-doped silicon. In particular, we discuss a number of thermoelectric properties such as the thermal and electrical conductivities of electrons, the Lorenz number and the Seebeck coefficient, including the phonon drag effect, in a range of temperatures and carrier concentrations. This approach gives results in good agreement with experimental data and provides a detailed characterization of the nature and the relative importance of the individual scattering mechanisms. Moreover, the access to the exact solution of the Boltzmann equation for a realistic system provides a direct way to assess the accuracy of different flavors of relaxation time approximation, as well as of models that are popular in the thermoelectric community to estimate transport coefficients.

BEC-polaron gas in a boson-fermion mixture: A many-body extension of Lee-Low-Pines theory

We investigate the ground state properties of the gaseous mixture of a single species of bosons and fermions at zero temperature, where bosons are major in population over fermions, and form the Bose-Einstein condensate (BEC). The boson-boson and boson-fermion interactions are assumed to be weakly repulsive and attractive respectively, while the fermion-fermion interaction is absent due to the Pauli exclusion for the low energy $s$-wave scattering. We treat fermions as a gas of polarons dressed with Bogoliubov phonons, which is an elementary excitation of the BEC, and evaluate the ground state properties with the method developed by Lemmens, Devreese, and Brosens (LDB) originally for the electron polaron gas, and also with a general extension of the Lee-Low-Pines theory for many-body systems (eLLP), which incorporates the phonon drag effects as in the original LLP theory. The formulation of eLLP is developed and discussed in the present paper. The binding (interaction) energy of the polaron gas is calculated in these methods, and shown to be finite (negative) for the dilute gas of heavy fermions with attractive boson-fermion interactions, though the suppression by the many-body effects exists.

High Seebeck Coefficient of Porous Silicon: Study of the Porosity Dependence.

In-plane Seebeck coefficient of porous Si free-standing membranes of different porosities was accurately measured at room temperature. Quasi-steady-state differential Seebeck coefficient method was used for the measurements. A detailed description of our home-built setup is presented. The Seebeck coefficient was proved to increase with increasing porosity up to a maximum of ~1 mV/K for the ~50 % porosity membrane, which is more than a threefold increase compared to the starting highly doped bulk c-Si substrate. By further increasing porosity and after a maximum is reached, the Seebeck coefficient sharply decreases and stabilizes at ~600 μV/K. The possible mechanisms that determine this behaviour are discussed, supported by structural characterization and photoluminescence measurements. The decrease in nanostructure size and increase in carrier depletion with increasing porosity, together with the complex structure and morphology of porous Si, are at the origin of complex energy filtering and phonon drag effects. All the above contribute to the observed anomalous behaviour of thermopower as a function of porosity and will be discussed.

Effect of Phonon Drag on the Thermopower in a Parabolic Quantum Well

The theory of phonon-drag thermopower resulting from a temperature gradient in the plane of a two-dimensional electron gas layer in a parabolic quantum well is developed. The interaction mechanisms between electrons and acoustic phonons are considered, taking into account potential screening of the interaction. It is found that the effect of electron drag by phonons makes a significant contribution to the thermopower of the two-dimensional electron gas. It is shown that the consideration of screening has a significant effect on the drag thermopower. For the temperature dependence of the thermopower in a parabolic GaAs/AlGaAs quantum well in the temperature range of 1–10 K, good agreement between the obtained theoretical results and experiments is shown.

Thermal Generation of Spin Current in an Antiferromagnet.

The longitudinal spin Seebeck effect has been investigated for a uniaxial antiferromagnetic insulator Cr(2)O(3), characterized by a spin-flop transition under magnetic field along the c axis. We have found that a temperature gradient applied normal to the Cr(2)O(3)/Pt interface induces inverse spin Hall voltage of spin-current origin in Pt, whose magnitude turns out to be always proportional to magnetization in Cr(2)O(3). The possible contribution of the anomalous Nernst effect is confirmed to be negligibly small. The above results establish that an antiferromagnetic spin wave can be an effective carrier of spin current.

Coarse-grained elastodynamics of fast moving dislocations

The fundamental mechanism of dynamic plasticity in metallic materials subjected to shock loading remains unclear because it is difficult to obtain the precise information of individual fast moving dislocations in metals from the state-of-the-art experiments. In this work, the dynamics of sonic dislocations in anisotropic crystalline materials is explored through a concurrent atomistic-continuum modeling method. We make a first attempt to characterize the complexity of nonuniformly moving dislocations in anisotropic crystals from atomistic to microscale, including the energy intensities as well as the wavelengths of acoustic phonons emitted from sonic dislocations, and the velocity-dependent stress fluctuations around the core of nonuniformly moving dislocations. Instantaneous dislocation velocities and phonon drag effects on the dislocation motions are quantified and analyzed. Mach cones in a V-shaped pattern of the phonon wave-fronts are observed in the wake of the sonic dislocations. Analysis of simulation results based on a wavelet transform show that the faster a dislocation is moving, the longer the emitted phonon wavelength. The dislocation velocity drops dramatically with the occurrence of the interactions between dislocations and phonon waves reflected from the boundaries of specimens. The concurrent atomistic-continuum modeling framework is demonstrated to be the first multiscale method that explicitly treats the strong coupling between the long-range elastic fields away from the dislocation core, the highly nonlinear time-dependent stress field within the core, and the evolutions of the atomic-scale dislocation core structures. As such, it is shown that this method is capable in predicting elastodynamics of dislocations in the presence of inertia effects associated with sonic dislocations in micron-sized anisotropic crystalline materials from the atomic level, which is not directly accessible to the recent elastodynamic discrete dislocation model.

Ab initio optimization of phonon drag effect for lower-temperature thermoelectric energy conversion

Although the thermoelectric figure of merit zT above 300 K has seen significant improvement recently, the progress at lower temperatures has been slow, mainly limited by the relatively low Seebeck coefficient and high thermal conductivity. Here we report, for the first time to our knowledge, success in first-principles computation of the phonon drag effect--a coupling phenomenon between electrons and nonequilibrium phonons--in heavily doped region and its optimization to enhance the Seebeck coefficient while reducing the phonon thermal conductivity by nanostructuring. Our simulation quantitatively identifies the major phonons contributing to the phonon drag, which are spectrally distinct from those carrying heat, and further reveals that although the phonon drag is reduced in heavily doped samples, a significant contribution to Seebeck coefficient still exists. An ideal phonon filter is proposed to enhance zT of silicon at room temperature by a factor of 20 to ∼ 0.25, and the enhancement can reach 70 times at 100 K. This work opens up a new venue toward better thermoelectrics by harnessing nonequilibrium phonons.

Theoretical study of magnetotransport properties of colossal magnetoresistive manganites (Re1−xAxMnO3): A variational treatment

Abstract The magnetotransport properties like electrical resistivity and Thermoelectric power of rare earth manganites doped with alkaline earths namely Re 1− x A x MnO 3 (where Re=La, Pr, Nd etc., and A=Ca, Sr, Ba etc.) which exhibit colossal magnetoresistance (CMR), metal–insulator transition and many other poorly understood phenomena, have been studied in this work by means of a Variational method. We have used two band (l-b) model for manganites in the strong electron–lattice Jahn–Teller (JT) coupling regime, an approach similar to the two-fluid models along with (l-b) hybridization. We have already used this Variational method to study the zero field electrical resistivity ρ ( T ) of doped CMR manganites. In the present study we find that the resistivity decreases with increasing magnetic field and the metal–insulator transition temperature ( T ρ ) shifts towards higher temperature region. This may happen because of the delocalized charge carriers, induced by applied magnetic field, decreasing the resistivity which in turn leads to the local ordering of the electron spins. Due to this ordering, the ferromagnetic metallic state might have suppressed the paramagnetic insulating regime resulting in the observed increase in T ρ under applied magnetic field. We have also shown the temperature dependent magneto-resistance (MR) ratio at different magnetic fields. We obtained large MR value at low temperatures. An interesting feature of the Magnetic field dependent thermoelectric power Q ( T ) is that it rises gradually with decreasing temperatures and a broad peak appears at low temperatures and as the temperature is lowered further, the value of Q ( T ) drops sharply (i.e. Q →0 as T →0 ), which is suggested to originate from the weak carrier localization effect. In the external magnetic field, the phonon-drag effect is expected to become weaker so that value of Q ( T ) reduces.

Unified Picture for the Colossal Thermopower Compound FeSb2.

We identify the driving mechanism of the gigantic Seebeck coefficient in FeSb2 as the phonon-drag effect associated with an in-gap density of states that we demonstrate to derive from excess iron. We accurately model electronic and thermoelectric transport coefficients and explain the so far ill-understood correlation of maxima and inflection points in different response functions. Our scenario has far-reaching consequences for attempts to harvest the spectacular power factor of FeSb2.

Optical and low-temperature thermoelectric properties of phase-pure p-type InSe thin films

Polycrystalline phase-pure p-type InSe thin films were deposited on glass substrates by reactive evaporation at an optimized substrate temperature of 473 ± 5 K and pressure of 10−5 mbar. The as-prepared InSe thin films were analyzed by X-ray diffractometry, energy-dispersive X-ray spectroscopy, atomic force microscopy, UV–Vis–NIR spectroscopy, electrical conductivity and Hall measurements. The lattice parameters, particle size, dislocation density, number of crystallites per unit area and the lattice strain of the prepared InSe thin films were calculated and found as a = 4.00 ± 0.002 A and c = 16.68 ± 0.002 A, 48 ± 2 nm, 4.34 × 1010 lines cm−2, 15.37 × 1010 cm−2 and 1.8 × 10−3, respectively. The as-deposited InSe thin films showed a direct allowed transition with an optical band gap of 1.35 ± 0.02 eV and high absorption coefficient of about 105 cm−1. The oscillator energy (E o) and dispersion energy (E d) were calculated using the single-oscillator Wemple and DiDomenico model. The p-type conductivity and photosensitivity of the as-prepared InSe thin films confirmed their potential application in photovoltaic devices. The mean free path, relaxation time, density of states, Fermi energy and effective mass of holes in the film were determined by correlating the results of thermopower and Hall measurements. The sudden and sharp increase in thermopower from 80 to 37 K was explained as due to the effect of phonon drag on charge carriers.

Thermoelectric properties of 50-nm-wide n- and p- type silicon nanowires

For the evaluation of thermoelectric properties in silicon nanowires (SiNWs), thermoelectric test structures are manufactured, including 50-nm-wide n- and p-type SiNWs, micro-heater and temperature sensors using a conventional lithography method on 8 in. silicon wafer. For the optimization of thermoelectric properties in SiNWs, we have evaluated Seebeck coefficients and power factors of n- and p-type SiNWs by varying the nanowire length 10, 40 μm and temperature (from 310 to 450 K). The results show that the maximum Seebeck coefficients and power factors are $$146.37 \,{\upmu} \hbox {V/K}, 1.15\,\times \,10^{3}\, \hbox {W}\,\hbox {m}^{-1}\,\hbox {K}^{-2}, 113.83\; {\upmu} \hbox {V/K}, 0.67\,\times \,10^{3}\, \hbox {W}\,\hbox {m}^{-1}\,\hbox {K}^{-2}$$ and $$-113.25\; {\upmu} \hbox {V/K}, 0.59\,\times \,10^{3}\,\hbox {W}\,\hbox {m}^{-1}\,\hbox {K}^{-2}$$ for $$10, 40\; {\upmu} \hbox {m}$$ long p-type and $$40\; {\upmu}\hbox {m}$$ long n-type SiNWs, respectively. The contribution of phonon-drag effect to thermoelectric power is discussed in the highly doped SiNWs.

Superconductivity in Lu3Os4Ge13

We report the normal and superconducting state properties of high quality single crystal of Lu3Os4Ge13 grown by Czochralski method. Lu3Os4Ge13 crystallises in the cubic crystal structure (space group Pm3n) . Lu3Os4Ge13 is a type-II superconductor (Tc = 3.1 K) as determined by the electrical transport, magnetisation and heat capacity measurements. The thermopower measurement shows a large phonon drag effect at low temperatures. The electronic band structure calculations show a multi-valley type band structure and a complex Fermi surface in Lu3Os4Ge13, making it a possible candidate to observe a magnetic field induced triplet superconductivity at ultra-low temperatures.

Synthesis and physical properties of Cr–P-based intermetallic compounds: Cr3P, Cr3PC, and Cr3PN

We report the synthesis, crystal structures and electrical/thermal transport properties of polycrystalline Cr3PX (X=C, N, and vacancy) compounds. As a result, Cr3PC and Cr3PN have an orthorhombic crystal structure with space group Cmcm, while Cr3P shows a tetragonal structure with space group I−4. All samples show good metallic behavior between 2 and 350K in the measurement of electrical transport. At the same time, the Fermi-liquid behavior is observed at low temperatures for Cr3PC (N) (2–50K) and Cr3P (2–22K). Based on the fitting results of low-temperature resistivity and specific heat, large Kadowaki–Woods ratio of Cr3PC (N) and Cr3P were obtained, suggesting a considerably strong electron–electron correlation for all our samples. By analyzing the thermal conductivity data, it is found that the electron thermal conductivity is predominantly stronger than the phonon one at low temperatures for Cr3P. As to Cr3PC and Cr3PN, however, both contributions are comparable in the whole temperature range investigated. Besides, phonon-drag effect was observed in the Seebeck coefficient α(T) curves for all the samples, which may explain the crossover of sign at 70K in α(T) of Cr3PN.

Phonon-drag effect inFeGa3

The thermoelectric properties of single- and polycrystalline FeGa3 are systematically investigated over a wide temperature range. At low temperatures, below 20 K, previously not known pronounced peaks in the thermal conductivity (400-800 W K^-1m-1) with corresponding maxima in the thermopower (in the order of -16000 microV/K) were found in single crystalline samples. Measurements in single crystals along [100] and [001] directions indicate only a slight anisotropy in both the electrical and thermal transport. From susceptibility and heat capacity measurements, a structural or magnetic phase transition was excluded. Using density functional theory-based calculations, we have revisited the electronic structure of FeGa3 and compared the magnetic (including correlations) and non-magnetic electronic density of states. Thermopower at fixed carrier concentrations are calculated using semi-classical Boltzmann transport theory, and the calculated results match fairly with our experimental data and exclude the possibility of strong electronic correlations as an explanation for the low temperature enhancement. Eventually, after a careful review, we assign the peaks in the thermopower as a manifestation of the phonon-drag effect, which is supported by thermopower measurements in a magnetic field.

Superconducting properties of indium-doped topological crystalline insulator SnTe

We report on the superconducting properties of indium-doped SnTe. SnTe has recently been explored as a topological crystalline insulator. Single crystals of Sn0.5In0.5Te have been synthesized by a modified Bridgman method. Resistivity measurement performed in the range 1.6–300 K shows a metallic normal state with onset of superconducting transition at . Bulk superconductivity has also been confirmed by DC magnetization, AC susceptibility and rf penetration depth measurements. Zero-temperature upper critical field, lower critical field, coherence length, and penetration depth are estimated to be 1.6 T, 1 mT, 143.5 Å and 853 nm, respectively. The temperature dependence of the low-temperature penetration depth indicates S-wave fully gapped characteristics with BCS (Bardeen-Cooper-Schrieffer) gap . Hall and Seebeck coefficient measurements confirm the dominance of hole conduction with possible phonon-drag effects around . Resistive transition studied under applied magnetic field shows thermally activated flux flow behaviour.

Phonon drag effect on Seebeck coefficient of ultrathin P-doped Si-on-insulator layers

The contribution of the phonon drag effect to the Seebeck coefficient of P-doped ultrathin Si-on-insulator (SOI) layers with a thickness of 9–100 nm is investigated for near-room-temperature applications. The contribution is found to be significant in the lightly doped region and to depend on the carrier concentration with increasing carrier concentration above ∼5 × 1018 cm−3. Moreover, the contribution is not influenced by SOI thickness above 9 nm. On the basis of phonon mean-free-path calculations considering phonon scattering processes, the phonon drag part of the SOI Seebeck coefficient in the lightly doped region is mainly governed by phonon-phonon scattering. Furthermore, in higher concentration regions, the dependence of phonon drag can be qualitatively explained by the interaction between phonons and doped impurities.

Comprehensive Study of the Low-Temperature Transport and Thermodynamic Properties of the Cluster Compounds AgxMo9Se11 (3.41 ≤ x ≤ 3.78)

We present a detailed study of the evolution of the electrical, galvanomagnetic, and thermodynamic properties of polycrystalline AgxMo9Se11 compounds for 3.4 ≤ x ≤ 3.8 at low temperatures (2–350 K). In agreement with density functional theory calculations, the collected data show an overall gradual variation in the transport properties from metallic to semiconducting behavior on going from x = 3.4 to 3.8. The results evidence subtle variations in the electronic properties with the Ag content, typified by both positive and negative phonon-drag effects together with thermopower and Hall coefficient of opposite signs. Analysis of the data suggests that these features may be due to peculiarities of the dispersion of the valence bands in the vicinity of the chemical potential. A drastic influence of the Ag content on the thermal transport was evidenced by a pronounced change in the temperature dependence of the specific heat below 10 K. Nonlinearities in the Cp(T3) data are correlated to the concentration of Ag atoms, with an increase in x resulting in a more pronounced departure from a Debye law. The observed behavior mirrors that of ionic conductors, suggesting that AgxMo9Se11 for x ≥ 3.6 might belong to this class of compounds.