Colossal zero-field Nernst effect in topological pentagonal PB2

With the increase of integration scale, heat dissipation becomes one of the major problems in high density electronic devices and circuits. Controlling and reusing the heat energy in such miniaturized structures are essential topics for current and future technologies. With the development of microfabrication technology and low-temperature measurement technology in the last two decades, the thermoelectric measurement in low-dimensional sample has been feasible, and the thermal transport has received more and more attention. For the multi-terminal device, there is a novel thermoelectric phenomenon, called the spin Nernst effect, in which spin currents (or spin voltages) are generated perpendicularly to the temperature gradient. The spin Nernst effect has been confirmed experimentally, and has been theoretically studied in a variety of materials. In this paper, the spin and charge Nernst effect in a pair of vertically aligned quantum dots attached to four leads are studied in the Coulomb blockade regime based on the nonequilibrium Green's function technique. We focus on the influences of magnetic configuration and intra-dot (inter-dot) Coulomb interaction on the spin and charge Nernst effect. It is found that the signs and the magnitudes of spin and charge Nernst effect can be modulated by adjusting the magnetization directions of ferromagnetic electrodes. When the magnetic moments in the 1 and 3 electrodes are turned to antiparallel alignment, the pure spin Nernst (without charge Nernst) effect can occur by applying a transverse temperature gradient. Conversely, the spin and charge Nernst effect disappear if the magnetic moments of lead 1 and lead 3 are in the case of parallel configuration. Except for left and right thermal leads, we investigate the effect of the middle lead (lead 4) on the property of the Nernst effect. We find that when the normal metal lead 4 is transferred to ferromagnetic metal, the spin and charge Nernst effect both can be obtained simultaneously. In the end of the paper, we study the influences of intra-dot and inter-dot Coulomb interaction on the spin dependent Nernst coefficient. Through numerical calculations, we demonstrate that the magnitude of the Nernst effect is less dependent on the polarization strength of ferromagnetic electrodes, but can be remarkably enhanced by the Coulomb blockade. The spin Nernst coefficient is predicted to be more than two orders of magnitude larger than that of the case of zero Coulomb interaction. All the results indicate that the proposed four-terminal double quantum dot nano system is a promising candidate for spin caloritronic device.

On one Sb2Te3 single crystal, the temperature dependences of all three independent components of the Nernst-Ettingshausen tensor (Qikl) are measured in the temperature range of 85–450 K, all three components being negative. Alongside with the Nernst-Ettingshausen effect, the anisotropy of the Hall (Rikl) and Seebeck (Sij) coefficients and the conductivity (σii) is also investigated. The carried-out analysis of the experimental data on the Nernst-Ettingshausen and Seebeck effects indicates that there is the mixed scattering mechanism with the participation of acoustic phonons and impurity ions, the relative contributions of these mechanisms varying with temperature. In the relaxation-time-tensor approximation, the values of the effective scattering parameter (r) are determined. The obtained values point to the dominant scattering at acoustic phonons in the cleavage plane and to the substantial contribution of charged ions to the scattering along the trigonal axis c3. It is shown that it is possible to explain the major features of experimental data on the Nernst-Ettingshausen effect within the two-valence-band model with the participation of several groups of holes in the transport phenomena.

Vortex movements parallel and perpendicular to twin boundaries seen through the Nernst and Seebeck effects of YBa 2Cu 3O 7−δ

The Nernst and Ettingshausen effects in germanium single crystals of different conductivity type and with various impurity densities have been measured between 300 and 750\ifmmode^\circ\else\textdegree\fi{}K in magnetic fields of 9000 gauss; the Nernst effect also at 2100 gauss. The experimental results are compared with theoretical expressions for the Nernst and Ettingshausen coefficients and with previously reported values for the thermal conductivity, which links the two effects through the Bridgman relationship. The qualitative agreement is very good, quantitative discrepancies are explained in terms of deviations of the sample mobilities from the theoretically assumed pure low-field lattice mobilities and by uncertainties in the numerical values used for the theoretical calculations. A derivation of the Nernst constant in the range of negligible phonon-drag effect is given, based on the analogy between thermo- and photomagnetoelectric effects.

The electronic structure of compensated antiferromagnets (CAF) creates large functional responses, reminiscent of ferromagnets and suitable for data storage and readout, despite (nearly) net-zero spontaneous magnetization. Many experimental signatures of CAF - such as giant thermoelectric Nernst effects - should be enhanced when two or more electronic bands are nearly degenerate in vicinity of the Fermi energy. Here, we report a zero-field, thermoelectric Nernst effect >1 μV/K in the CAF CoNb3S6 despite its tiny net magnetization ~2 milli − μB. As drivers of the functional Nernst and Hall effects, we identify near-degeneracies of electron bands at the upper and lower boundaries of the first Brillouin zone, which are vestiges of nodal planes enforced by a screw axis symmetry in the paramagnetic state. Hot spots of emergent, or fictitious, magnetic fields are formed at the slightly gapped nodal planes. Taking into account more than six hundred Wannier orbitals, our theoretical model reproduces the observed spontaneous Nernst effect, emphasizes the role of proximate spin-space group symmetries and nodal planes for the electronic structure of CAF, and demonstrates the promise of ab-initio search for functional responses in a wide class of materials with reconstructed unit cells (supercells) due to spin or charge order.

The thermoelectric power, including the Nernst and Seebeck effects, in graphene nanoribbon is studied. By using the non-equilibrium Green function combining with the tight-binding Hamiltonian, the Nernst and Seebeck coefficients are obtained. Due to the electron-hole symmetry, the Nernst coefficient is an even function of the Fermi energy while the Seebeck coefficient is an odd function regardless of the magnetic field. In the presence of a strong magnetic field, the Nernst and Seebeck coefficients are almost independent of the chirality and width of the nanoribbon, and they show peaks when the Fermi energy crosses the Landau levels. The height of $n$-th (excluding $n=0$) peak is $[\ln2/|n|]$ for the Nernst effect and is $\ln2/n$ for the Seebeck effect. For the zeroth peak, it is abnormal with height $[2\ln2]$ for the Nernst effect and the peak disappears for the Seebeck effect. When the magnetic field is turned off, however, the Nernst effect is absent and only Seebeck effect exists. In this case, the Seebeck coefficient strongly depends on the chirality of the nanoribbon. The peaks are equidistant for the nanoribbons with zigzag edge but are irregularly distributed for the armchair edge. In particular, for the insulating armchair ribbon, the Seebeck coefficient can be very large near the Dirac point. When the magnetic field varies from zero to large values, the differences among the Seebeck coefficients for different chiral ribbons gradually vanish and the nonzero value of Nernst coefficient appears first near the Dirac point then gradually extents to the whole energy region.

A series of thinfilms Fe4–xGexN (x = 0 –1) were fabricated onto MgO substrates by magnetron sputtering withthe aim of studying the possible enhancement of the anomalous Nernsteffect (ANE), envisaged based on density functional theory (DFT) calculations.The Nernst and Hall effects of the series were systematically analyzed,complemented with resistivity, magnetic, electron microscopy, andMössbauer experiments, and DFT calculations including elasticproperties. The Fe4N phase crystallizes in the cubic symmetrywith Pm3̅m space group, whereasa small tetragonal distortion is realized in Fe4–xGexN films for x > 0.35. From the comparison of the experimental isomershift with DFT calculations, we conclude that Ge occupies the 4b site in the tetragonal I4/mcm structure. The ferromagnetic Curie temperature decreases rapidlyfrom ∼750 K for x = 0 to ∼100 K for x = 1. The tetragonal samples with x =0.8 and 1 display two-component behavior in the Hall and Nernst effectshysteresis loops, which can be analyzed as a sum of positive and negativeloops with different saturation fields. This unusual behavior is aproduct of a combination of several factors: (1) coexistence of twodifferent crystallographic orientations in the tetragonal thin film,namely with the majority of c-axis and minority of a-axis normal to the film surface; (2) opposite sign ofthe anomalous Hall and Nernst effects for the direction of magnetizationalong the a- and c-axes revealedby DFT calculation; and (3) the magnetocrystalline anisotropy characterizedby an easy ab-plane, which is responsible for thedifferent saturation fields for a- and c-axes. The maximum ANE was determined to be 0.9 μV/K for x = 0 at room temperature and −0.85 μV/K for x = 1 at T = 50 K. The rapid increaseof ANE of Fe3GeN from low temperatures indicates that,were it not for its low Curie temperature, it could surpass the ANEof Fe4N. This observation is consistent with our theoreticalassumptions and motivates further research of doped Fe4N for which ANE enhancement is predicted by DFT calculations.

Anisotropy of transport entropy and of Hall angle are reported for the first time through various electrical and thermal transport properties, such as the electric resistivity and the Seebeck and the Nernst effects (in the presence of a magnetic field ranging between 0 and 4 T), measured for a Pb-free textured Bi-2223 system. The field and temperature influence in the mixed state region is reported only for the H//a and H//c cases and the temperature gradient applied along the b direction. A much stronger influence of magnetic field along the c than the a axis is demonstrated. The relative evolution of the so-called "excess transport effects" is discussed. We show the laws relating the respective losses to the applied magnetic field. This leads us to the understanding that anisotropic vortex kinetics exist in these systems. We consequently propose two original routes of theoretical investigations to be considered for the calculation of Nernst and TEP effects in a field, in particular to consider a vortex-carrier drag phenomenon.

The antidynamo theorems of Namikawa and Matsushita (1970) preclude maintenance of a steady magnetic field, or non-steady poloidal magnetic field, by a spherically symmetric radial velocity. A simple reinterpretation of these theorems also precludes such field maintenance by a spherically symmetric Nernst-Ettingshausen thermomagnetic effect. For any non-steady magnetic field supported by the above mechanisms, we show (a) that the poloidal component has a decay time less than five free-decay times, and (b) that any harmonic of the toroidal component with angular length scale less than a critical value, will decay. The models of Namikawa and Matsushita for toroidal fields in radially contracting stars do not decay simply by (b) above, but nevertheless are shown to decay with lives shorter than a free-decay time. These models are seen to be special cases of a larger class of decaying models. A brief discussion of the maintenance of axisymmetric fields by a compressible flow or thermomagnetic effect i...

Spin caloritronics, origin and outlook

The Hall effect can be extended by inducing a temperature gradient in lieu of electric field that is known as the Nernst (-Ettingshausen) effect. The recently discovered spin Nernst effect in heavy metals continues to enrich the picture of Nernst effect-related phenomena. However, the collection would not be complete without mentioning the valley degree of freedom benchmarked by the valley Hall effect. Here we show the experimental evidence of its missing counterpart, the valley Nernst effect. Using millimeter-sized WSe{}_{2} mono-multi-layers and the ferromagnetic resonance-spin pumping technique, we are able to apply a temperature gradient by off-centering the sample in the radio frequency cavity and address a single valley through spin-valley coupling. The combination of a temperature gradient and the valley polarization leads to the valley Nernst effect in WSe{}_{2} that we detect electrically at room temperature. The valley Nernst coefficient is in good agreement with the predicted value.

The Nernst effect of ${\mathrm{Pr}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{CuO}}_{4}$ $(x=0.13,$ 0.15, and 0.17) has been measured on thin film samples between 5--120 K and 0--14 T. In comparison to recent measurements on hole-doped cuprates that showed an anomalously large Nernst effect above the resistive ${T}_{c}$ and ${H}_{c2}$ [Z. A. Xu et al., Nature (London) 406, 486 (2000); Yayu Wang et al., Phys. Rev. B 64, 224519 (2001); Yayu Wang et al., Phys. Rev. Lett. 88, 257003 (2002); C. Caplan et al., ibid 88, 056601 (2002)], we find a normal Nernst effect above ${T}_{c}$ and ${H}_{c2}$ for all dopings. The lack of an anomalous Nernst effect in the electron-doped compounds supports the models that explain this effect in terms of amplitude and phase fluctuations in the hole-doped cuprates. In addition, the ${H}_{c2}(T)$ determined from the Nernst effect shows a conventional behavior for all dopings. The energy gap determined from ${H}_{c2}(0)$ decreases as the system goes from underdoping to overdoping in agreement with the recent tunneling experiments.

The transverse thermoelectric (Nernst) effect on pyrochlore molybdates is investigated experimentally. In Nd(2)Mo(2)O(7) and Sm(2)Mo(2)O(7) with the spin chirality, the Nernst signal, which mostly arises from the transverse heat current (or equivalently the transverse Peltier coefficient alpha(xy)), shows a low-temperature (20-30 K) positive extremum, whereas it is absent in (Gd(0.95)Ca(0.05))(2)Mo(2)O(7) with no single-spin anisotropy of the rare-earth ion and hence with no spin chirality. The correlation between the Hall conductivity sigma(xy) and alpha(xy) in Nd(2)Mo(2)O(7) also indicates the spin chirality plays a significant role in the spontaneous (anomalous) Nernst effect.

The Nernst and Hall effects in single crystals of very pure gallium were measured at 4.2 and 1.71K in magnetic fields up to about 40 kG. The Nernst effect shows oscillations when the magnetic field lies along, or very near, the c axis and the temperature gradient Del T is parallel to the b axis, while the Hall effect does not exhibit any oscillation in the same orientations.

Transport coefficients and flux motion in Bi 2Sr 2CaCu 2O x single crystals