Nernst Ettingshausen Research Articles

Large Nernst effect in a layered metallic antiferromagnet EuAl2Si2

The large Nernst effect is advantageous for developing transverse Nernst thermoelectric generators or Ettingshausen coolers within a single component, avoiding the complexity of electron- and hole-modules in longitudinal Seebeck thermoelectric devices. We report a large Nernst signal reaching 130 μV/K at 8 K and 13 T in the layered metallic antiferromagnet EuAl2Si2. Notably, this large transverse Nernst thermopower is two orders of magnitude greater than its longitudinal counterpart. The Nernst coefficient peaks around 4 and 8 K at 3 and 13 T, respectively. At similar temperatures, both the Hall coefficient and the Seebeck signal change sign. Additionally, nearly compensated electron- and hole-like carriers with high mobility (∼ 4000 cm2/V s at 4 K) are revealed from the magnetoconductivity. These findings suggest that the large Nernst effect and vanishing Seebeck thermopower in EuAl2Si2 are due to the compensated electron- and hole-like bands, along with the high mobility of the Weyl band near the Fermi level. Our results underscore the importance of band compensation and topological fermiology in achieving large Nernst thermopower and exploring potential Nernst thermoelectric applications at low temperatures.

Nernst–Ettingshausen effect in thin Pt and W films at low temperatures

As spin caloritronic measurements become increasingly common techniques for characterizing material properties, it is important to quantify potentially confounding effects. We report measurements of the Nernst–Ettingshausen response from room temperature to 5 K in thin film wires of Pt and W, metals commonly used as inverse spin Hall detectors in spin Seebeck characterization. Johnson–Nyquist noise thermometry is used to assess the temperature change in the metals with heater power at low temperatures, and the thermal path is analyzed via finite-element modeling. The Nernst–Ettingshausen response of W is found to be approximately temperature-independent, while the response of Pt increases at low temperatures. These results are discussed in the context of theoretical expectations and the possible role of magnetic impurities in Pt.

Ultrahigh transverse thermoelectric power factor in flexible Weyl semimetal WTe2

Topological semimetals are well known for their interesting physical properties, while their mechanical properties have rarely received attention. With the increasing demand for flexible electronics, we explore the great potential of the van der Waals bonded Weyl semimetal WTe2 for flexible thermoelectric applications. We find that WTe2 single crystals have an ultrahigh Nernst power factor of ~3 Wm−1K−2, which outperforms the conventional Seebeck power factors of the state-of-the-art thermoelectric semiconductors by 2–3 orders of magnitude. A unique band structure that hosts compensated electrons and holes with extremely high mobilities is the primary mechanism for this huge Nernst power factor. Moreover, a large Ettingshausen signal of ~5 × 10−5 KA−1m is observed at 23.1 K and 9 T. In this work, the combination of the exceptional Nernst–Ettingshausen performance and excellent mechanical transformative ability of WTe2 would be instructive for flexible micro-/nano-thermoelectric devices.

Spin component in the nernst–Ettingshausen effect in metals with different band structure

The spin contribution of the spin-orbit interaction to the Nernst–Ettingshausen effect in nonmagnetic metals with different band structures (Pt and W) under conditions of the coexistence of a nonzero gradient of the spin chemical potential and a temperature gradient has been studied by direct measurement. The dependence of the manifestation of the different origin spin effects (spin Nernst–Ettingshausen effect is of a thermal diffusion nature and spin Hall effect is of an electrical nature) on the band structure of the metal, which controls the profile of the spin magnetization established at the edges of the sample due to the spin accumulation is found.

Transverse Nernst–Ettingshausen Effect in the Two–Dimensional Electron Gas of a Doubly Periodic Semiconductor Superlattice

Transverse Nernst–Ettingshausen Effect in the Two–Dimensional Electron Gas of a Doubly Periodic Semiconductor Superlattice

Infinite-stage Nernst-Ettingshausen Cryocooler for Practical Applications

Recent developments in Nernst-Ettingshausen (NE) physical phenomena combined with advances in the performance of rare-earth permanent magnets make thermomagnetic (TM) cryocoolers well suited for practical applications. The device performance of a NE cryocooler depends on both the material and the geometric shape of the device. Despite continued progress in TM materials, the optimum shape is still based on a simplified infinite-stage model derived in 1963 by Harman [Adv. Energy Convers. 3(4), 667--676 (1963)]. Harman's model assumes several nonrealistic assumptions, such as temperature-independent material properties and constant current density. We relax such assumptions and derive a fully-temperature-dependent numerical model to accurately solve for the thermomagnetic features of a NE cooler with arbitrary geometry. We correct Harman's analytical function and compare its performance with the performance of devices of various shapes. The corrected shape has a higher coefficient of performance (COP) at higher temperature differentials, which indicates that when the material resistivity is a strong function of the temperature, the corrected infinite-stage device can provide better performance than Harman's geometry. Moreover, the corrected infinite-shape device can provide higher heat flow density under a similar optimum-COP condition. A case study based on a state-of-the-art TM material, $\mathrm{Bi}$-$\mathrm{Sb}$ alloy, is presented, and the critical parameters for designing an efficient thermomagnetic cooler are discussed in detail.

Comparative Study of the Nernst–Ettingshausen Coefficient in the Normal Phase in the Y1 – xPrxBa2Cu3Oy and Y0.85 – xPrxCa0.15Ba2Cu3Oy Systems

The results of the comparative study of the modification of the temperature dependences of the Nernst–Ettingshausen coefficient Q in the normal phase of two series of the samples of the Y1 – xPrxBa2Cu3Oy and Y0.85 – xPrxCa0.15Ba2Cu3Oy compositions due to praseodymium doping are presented. The specific features of dependences Q(T) and Q300 K(T) appeared as additional calcium ions exist in the YBa2Cu3Oy lattice have been revealed and analyzed. It is shown that dependences Q(T) and the temperature dependences of the thermopower coefficient obtained for the same samples previously are completely described in terms of the narrow-band model for both systems under study. The charge carrier mobilities and the degrees of asymmetry of the dispersion law are determined using a quantitative analysis of the experimental data. It is shown that the existence of additional calcium ions in the lattice does not influence a change in the asymmetry of the dispersion law with an increase in the praseodymium content, but strongly influences the character of changing the mobility. The found dependence of the mobility on the level of doping in the two systems and the observed difference in the characters of the changes in the absolute values of the thermopower coefficient and the Nernst–Ettingshausen coefficient in the Y0.85 – xPrxCa0.15Ba2Cu3Oy system are explained on the basis of the analysis of the mechanisms of influence of the energy spectrum parameters on the charge carrier mobility in the YBa2Cu3Oy system.

Features of thermomagnetic transport due to the superconducting interfaces in inclination bicrystals of Bi and 3D topological insulator Bi1−xSbx

We studied the thermomagnetic power Sii (B) and Nernst–Ettingshausen effect Sij (B) in inclination bicrystals of Bi and 3D topological insulator Bi1−xSbx (0.04 < x ≤ 0.12) with superconducting nano-width interfaces (Tc ≤ 21 K). High values of thermomagnetic effects in small disorientation angle (SDA) bicrystals far exceeding values in single-crystalline samples were found. It was established that Sii (B) linearly increases in high fields without saturation and change the sign from negative in positive in bicrystals of 3D Dirac point forming (x ∼ 0.04), specifying the signature of 3D topological semimetal. Contrarily, Sii (B) in LDA bicrystals with 0.06 ≤ x ≤ 0.12 undergoes saturation or increases smoothly, the Landau level index depends linearly on 1/Bn, and extrapolate to – 0.5 if 1/Bn → 0, what is typical for the 3D topological insulators. Two new quantum oscillation harmonics are reviled in high fields; they characterize different densities of electronic states and different levels of disorder at LDA and SDA interfaces.

On the Band Structure of Bi2Te3

For p-Bi2Te3 crystals grown by the Czochralski method, the temperature dependences of the conductivity, Hall coefficient, thermoelectric power (α), and transverse Nernst–Ettingshausen coefficient are obtained experimentally in the temperature range 77–450 K. The transmittance spectrum in the range 400–5250 cm–1 is recorded at room temperature. It is shown that, to interpret the temperature dependences of the scattering parameter r and the ratio of the thermoelectric power to temperature (α/T), it is essential to take into account the complex valence-band structure and the contribution of heavy holes to transport phenomena. Estimations of the energy band parameters in the context of the two-band model give a hole effective mass close to the free electron mass and the energy gap between nonequivalent extrema at a level of several hundredths of eV. In the absorption spectrum derived from the transmittance spectrum, a sharp increase in absorption defined by indirect interband transitions with the band gap Eg ≈ 0.14 eV is observed in the region of frequencies ν ≥ 1000 cm–1. The absorption spectrum calculated from the reflectance data using the Kramers–Kronig relations is in agreement with the experimental absorption spectrum.

О зонной структуре Bi-=SUB=-2-=/SUB=-Te-=SUB=-3-=/SUB=-

AbstractFor p -Bi_2Te_3 crystals grown by the Czochralski method, the temperature dependences of the conductivity, Hall coefficient, thermoelectric power (α), and transverse Nernst–Ettingshausen coefficient are obtained experimentally in the temperature range 77–450 K. The transmittance spectrum in the range 400–5250 cm^–1 is recorded at room temperature. It is shown that, to interpret the temperature dependences of the scattering parameter r and the ratio of the thermoelectric power to temperature (α/ T ), it is essential to take into account the complex valence-band structure and the contribution of heavy holes to transport phenomena. Estimations of the energy band parameters in the context of the two-band model give a hole effective mass close to the free electron mass and the energy gap between nonequivalent extrema at a level of several hundredths of eV. In the absorption spectrum derived from the transmittance spectrum, a sharp increase in absorption defined by indirect interband transitions with the band gap E _ g ≈ 0.14 eV is observed in the region of frequencies ν ≥ 1000 cm^–1. The absorption spectrum calculated from the reflectance data using the Kramers–Kronig relations is in agreement with the experimental absorption spectrum.

Transverse Nernst–Ettingshausen Effect in Superlattices Upon Electron-Phonon Scattering

The Nernst–Ettingshausen coefficient is calculated in superlattices with the cosine dispersion law in the case of the scattering of charge carriers at acoustic and polar optical phonons in a magnetic field in the layer plane. A significant increase in the Nernst–Ettingshausen coefficient of a degenerate quasi-three-dimensional electron gas in a weak magnetic field is shown. For polar optical-phonon scattering, the Nernst–Ettingshausen coefficient changes sign in a strong magnetic field.

Anisotropy of Nernst–Ettingshausen Effect in Superlattices During Scattering on Phonons

The paper focuses on anisotropy of the transverse Nernst–Ettingshausen effect in superlattices depending on the degree of mini-band filling and magnetic field direction during scattering on acoustic and polar optical phonons. The authors demonstrate that in the case of scattering on polar optical phonons one observes considerable anisotropy and, depending on the magnetic field direction, the Nernst–Ettingshausen coefficient may change its sign.

Поперечный эффект Нернста-Эттингсгаузена в сверхрешетках при электрон-фононном рассеянии

AbstractThe Nernst–Ettingshausen coefficient is calculated in superlattices with the cosine dispersion law in the case of the scattering of charge carriers at acoustic and polar optical phonons in a magnetic field in the layer plane. A significant increase in the Nernst–Ettingshausen coefficient of a degenerate quasi-three-dimensional electron gas in a weak magnetic field is shown. For polar optical-phonon scattering, the Nernst–Ettingshausen coefficient changes sign in a strong magnetic field.

Energy spectrum of holes in Sb2Te2.9Se0.1 solid solution according to the data on the transfer phenomena

The temperature dependence of the Hall coefficient of a single crystal of the p-Sb2Te2.9Se0.1 solid solution grown by the Czochralski technique is studied in the temperature range 77–450 K. The data on the Hall coefficient of the p-Sb2Te2.9Se0.1 are analyzed in combination with the data on the Seebeck and Nernst–Ettingshausen effects and the electrical conductivity with allowance for interband scattering. From an analysis of the temperature dependences of the four kinetic coefficients, it follows that, at T < 200 K, the experimental data are qualitatively and quantitatively described in terms of the one-band model. At higher temperatures, a complex structure of the valence band and the participation of the second-kind additional carriers (heavy holes) in the kinetic phenomena should be taken into account. It is shown that the calculations of the temperature dependences of the Seebeck and Hall coefficients performed in the two-band model agree with the experimental data with inclusion of the interband scattering when using the following parameters: effective masses of the density of states of light holes md1*≈ 0.5m0 (m0 is the free electron mass) and heavy holes md2*≈ 1.4m0, the energy gap between the main and the additional extremes of the valence band ΔEv ≈ 0.14 eV that is weakly dependent on temperature.

Nernst-Ettingshausen effect at the trivial-nontrivial band ordering in topological crystalline insulator Pb1−xSnxSe

The transverse Nernst-Ettingshausen (N-E) effect and electron mobility in Pb1−xSnxSe alloys are studied experimentally and theoretically as functions of temperature and chemical composition in the vicinity of vanishing energy gap Eg. The study is motivated by the recent discovery that, by lowering the temperature, one can change the band ordering from a trivial to nontrivial one in which the topological crystalline insulator states appear at the surface. Our work presents several new aspects. It is shown experimentally and theoretically that the bulk N-E effect has a maximum when the energy gap Eg of the mixed crystal goes through zero value. This result contradicts the claim made in the literature that the N-E effect changes sign when the gap vanishes. We successfully describe dc transport effects in the situation of extreme band’s nonparabolicity which, to the best of our knowledge, has never been tried before. A situation is reached in which both two-dimensional bands (topological surface states) and three-dimensional bands are linear in electron k vector. Various scattering modes and their contribution to transport phenomena in Pb1−xSnxSe are analyzed. As the energy gap goes through zero, some transport integrals have a singular (nonphysical) behaviour and we demonstrate how to deal with this problem by introducing damping.

Determination of parameters of a system of charge carriers in Y1–2x Ca x Pr x Ba2Cu3O y within a combined analysis of temperature dependences of thermoelectric power and Nernst–Ettingshausen coefficients

The temperature dependences of thermoelectric power S and transverse Nernst–Ettingshausen effect Q in high-temperature superconductors of the Y1–2xCaxPrxBa2Cu3Oy system with different concentrations of dopants have been investigated experimentally. It has been shown that all the specific features revealed in the behavior of the two kinetic coefficients under investigation can be described using the narrowband model with a single concept of the structure of an energy spectrum and a common set of model parameters that characterize the structure of the spectrum and properties of a system of charge carriers. Based on the quantitative analysis of the dependences S(T) and Q(T), the values of these parameters for samples of the studied composition have been determined and the character of their changes with an increase in the impurity content has been analyzed. It has been demonstrated that, for the explanation of the obtained results, it is necessary to take into account the interaction of calcium and praseodymium ions upon their simultaneous introduction into the lattice. It has been found that the mobility of charge carriers has extremely low values and almost does not depend on the level of doping, which is associated with the character of changes in the structure of the energy spectrum of Y1–2xCaxPrxBa2Cu3Oy upon doping. It has been concluded that the structure of the energy spectrum of high-temperature superconductors in the yttrium system is characterized by an asymmetry of the dispersion curve.

Anisotropic mixed hole scattering mechanism in Sb2Te3–x Se x crystals (0 ≤ x ≤ 0.1) according to the data of the Nernst–Ettingshausen and seebeck effects

The temperature dependences of three independent coefficients of the Nernst–Ettingshausen tensor Qikl are investigated along with the electrical conductivity, thermal voltage, and Hall coefficients of layered p-Sb2Te3–xSex single crystals (0 ≤ x ≤ 0.1). It is established that all Qikl < 0 for the studied alloy crystals in the temperature range 77–400 K. Data on the Nernst–Ettingshausen effect and observed anisotropy of the Seebeck coefficient are indicative of the anisotropic mixed hole scattering mechanism at acoustic phonons and impurities and defects with the dominant contribution of scattering at lattice vibrations both in the crystal cleavage plane and in the trigonal axis direction. Estimation of the scattering parameters from four kinetic coefficients showed that they decrease with an increase in temperature due to phonon-scattering enhancement. They become negative at higher temperatures (T ≳ 200 K), which is apparently associated with bandto-band hole scattering to the additional valence-band extremum.

Phase Transformation in Multicomponent Ni&lt;sub&gt;3 &lt;/sub&gt;(Mn,Ti) Alloys

The paper presents neutron diffraction analysis and X-ray investigation of ordered alloys Ni3(Mn,Ti) within the concentration range from 0 to 12.5 at.% Ti. The significant effect of the third alloying element Ti on the atomic long-range order was observed in superstructure L12 of alloy Ni3Mn. Experimental study has showed that a rapid quenching in ice water does not provide disordered Ni3(Mn,Ti) alloys. The two-phase region of internmetallic bonds was detected in compounds L12 and D024. The paper presents results estimation of electrical resistivity, normal and abnormal values of hall coefficient, Nernst–Ettingshausen effect, thermal electromotive force, and average atomic magnetic moment in quenched and annealed Ni3(Mn,Ti) alloys. Positive values of abnormal hall coefficient and Nernst–Ettingshausen effect were detected that confirms the predominant contribution of antiparallel spin 3d holes in conductivity. It is shown that in 2.2 at.% annealed Ti alloys ordering promotes 3d electron localization.

Thermo-galvanometric instabilities in magnetized plasma disks

In this work, we present a linear stability analysis of fully-ionized rotating plasma disks with a temperature gradient and a sub-thermal background magnetic field (oriented towards the axial direction). We describe how the plasma reacts when galvanometric and thermo-magnetic phenomena, such as Hall and Nernst–Ettingshausen effects, are taken into account, and meridian perturbations of the plasma are considered. It is shown how, in the ideal case, this leads to a significant overlap of the Magneto-rotational Instability and the Thermo-magnetic one. Considering dissipative effects, an overall damping of the unstable modes, although not sufficient to fully suppress the instability, appears especially in the thermo-magnetic related branch of the curve.

On the theory of Nernst–Ettingshausen oscillations in monolayer and bilayer graphene

Abstract The Landau levels in graphene in crossed magnetic and electric fields are dependent on the electric field. However, this effect is not taken into account in many theoretical studies of graphene in crossed fields. In particular, it is not considered in the Nernst–Ettingshausen effect, in which the regime of crossed fields is realized. In this Letter, we considered the Nernst–Ettingshausen effect in monolayer and bilayer graphene, taking into account the dependence of Landau levels on the electric field. We showed that the magnitude and period of the Nernst coefficient oscillations depend on the electric field. This fact is important for the fundamental theory of Nernst–Ettingshausen effect in graphene and gives the new possibility for control of this effect using an applied electric field. The latter is very interesting for practical applications.