Large Thermoelectric Effects Research Articles

Large transverse thermoelectric effect induced by the mixed-dimensionality of Fermi surfaces

Transverse thermoelectric effect, the conversion of longitudinal heat current into transverse electric current, or vice versa, offers a promising energy harvesting technology. Materials with axis-dependent conduction polarity, known as p × n-type conductors or goniopolar materials, are potential candidate, because the non-zero transverse elements of thermopower tensor appear under rotational operation, though the availability is highly limited. Here, we report that a ternary metal LaPt2B with unique crystal structure exhibits axis-dependent thermopower polarity, which is driven by mixed-dimensional Fermi surfaces consisting of quasi-one-dimensional hole sheet with out-of-plane velocity and quasi-two-dimensional electron sheets with in-plane velocity. The ideal mixed-dimensional conductor LaPt2B exhibits an extremely large transverse Peltier conductivity up to ∣αyx∣ = 130 A K−1 m−1, and its transverse thermoelectric performance surpasses those of topological magnets utilizing the anomalous Nernst effect. These results thus manifest the mixed-dimensionality as a key property for efficient transverse thermoelectric conversion.

Atomic‐resolution Interfacial Microstructure and Thermo‐electro‐magnetic Energy Conversion Performance of Gd/Bi0.5Sb1.5Te3 Composites

Thermo‐electro‐magnetic materials with simultaneously large magnetocaloric (MC) and thermoelectric (TE) effects are the core part for designing TE/MC all‐solid‐state cooling devices. Compositing MC phase with TE material is an effective approach. However, the elemental diffusion and chemical reaction occurring at the two‐phase interfaces could significantly impair the cooling performance. Herein, Gd/Bi0.5Sb1.5Te3 (Gd/BST) composites were prepared by a low‐temperature high‐pressure spark plasma sintering method with an aim to control the extent of interfacial reaction. The reaction of Gd with the diffusive Te and the formation of GdTe nanocrystals were identified at the Gd/BST interfaces by the atomic‐resolution microscope. The formed antisite defects and enhanced {000 l} preferential orientation in BST are responsible for the increased carrier concentration and mobility, which leads to optimized electrical properties. The heterogeneous interface phases, along with antisite defects, favor the phonon scattering enhancement and lattice thermal conductivity suppression. The optimized composite sintered at 693 K exhibited a maximum ZT of 1.27 at 300 K. Furthermore, the well‐controlled interfacial reaction has a slight impact on the magnetic properties of Gd and a high magnetic entropy change is retained in the composites. This work provides a universal approach to fabricating thermo‐electro‐magnetic materials with excellent MC and TE properties.

First-principles study of anomalous Hall effect and anomalous Nernst effect in Fe2Si

We have performed density functional calculations of the electronic structure, anomalous Hall conductivity, and anomalous Nernst conductivity for the model crystal structure of a mineral Hapkeite Fe2Si contained in a meteorite Dhofar 280, which is believed to originate from the Moon. The calculated results showed that the model structure of Hapkeite Fe2Si exhibits a larger transverse thermoelectric effect than bcc-Fe and hexagonal Fe2Si. Based on the detailed analysis of the electronic structures and transport coefficients, we found that the origin of the large transverse thermoelectric effect is due to an anomaly in the density of states at the interface between the Fe layers and the Si–Fe alternative stacked layers. This finding is promising for the design of new transverse thermoelectric materials.

Thermopower and thermophase in a d -wave superconductor

In an unconventional superconductor, the interplay of scattering off impurities and Andreev processes may lead to different scattering times for electronlike and holelike quasiparticles. Such electron-hole asymmetry appears when the impurity scattering phase shift is intermediate between the Born and unitary limits and leads to an expectation for large thermoelectric effects. Here, we examine the thermoelectric response of a $d$-wave superconductor connected to normal-metal reservoirs under a temperature bias using a fully self-consistent quasiclassical theory. The thermoelectrically induced quasiparticle current is cancelled by superflow in an open circuit setup, but at the cost of a charge imbalance induced at the contacts and extending across the structure. We investigate the resulting thermopower and thermophase and their dependencies on scattering phase shift, mean free path, and interface transparency. For crystal-axis orientations such that surface-bound zero-energy Andreev states are formed, the thermoelectric effect is reduced as a result of locally reduced electron-hole asymmetry. For a semiballistic superconductor with good contacts, we find thermopowers of order several ${\mu}V/K$, suggesting a thermovoltage measurement as a promising path to investigate thermoelectricity in unconventional superconductors.

Large anomalous Nernst effect and nodal plane in an iron-based kagome ferromagnet.

Anomalous Nernst effect (ANE), converting a heat flow to transverse electric voltage, originates from the Berry phase of electronic wave function near the Fermi energy EF. Thus, the ANE provides a sensitive probe to detect a topological state that produces large Berry curvature. In addition, a magnet that exhibits a large ANE using low-cost and safe elements will be useful to develop a novel energy harvesting technology. Here, we report our observation of a high ANE exceeding 3 microvolts per kelvin above room temperature in the kagome ferromagnet Fe3Sn with the Curie temperature of 760 kelvin. Our theoretical analysis clarifies that a “nodal plane” produces a flat hexagonal frame with strongly enhanced Berry curvature, resulting in the large ANE. Our discovery of the large ANE in Fe3Sn opens the path for the previously unexplored functionality of flat degenerate electronic states and for developing flexible film thermopile and heat current sensors.

Manipulation of the van der Waals Magnet Cr2Ge2Te6 by Spin-Orbit Torques.

We report measurements of current-induced thermoelectric and spin-orbit torque effects within devices in which multilayers of the semiconducting two-dimensional van der Waals magnet Cr2Ge2Te6 (CGT) are integrated with Pt and Ta metal overlayers. We show that the magnetic orientation of the CGT can be detected accurately either electrically (using an anomalous Hall effect) or optically (using magnetic circular dichroism) with good consistency. The samples exhibit large thermoelectric effects, but nevertheless, the spin-orbit torque can be measured quantitatively using the angle-dependent second harmonic Hall technique. For CGT/Pt, we measure the spin-orbit torque efficiency to be similar to conventional metallic-ferromagnet/Pt devices with the same Pt resistivity. The interfacial transparency for spin currents is therefore similar in both classes of devices. Our results demonstrate the promise of incorporating semiconducting 2D magnets within spin-orbitronic and magneto-thermal devices.

Quasiperiodic quantum heat engines with a mobility edge

Steady-state thermoelectric machines convert heat into work by driving a thermally-generated charge current against a voltage gradient. In this work, we propose a new class of steady-state heat engines operating in the quantum regime, where a quasi-periodic tight-binding model that features a mobility edge forms the working medium. In particular, we focus on a generalization of the paradigmatic Aubrey-Andr\'e-Harper (AAH) model, known to display a single-particle mobility edge that separates the energy spectrum into regions of completely delocalized and localized eigenstates. Remarkably, these two regions can be exploited in the context of steady-state heat engines as they correspond to ballistic and insulating transport regimes. This model also presents the advantage that the position of the mobility edge can be controlled via a single parameter in the Hamiltonian. We exploit this highly tunable energy filter, along with the peculiar spectral structure of quasiperiodic systems, to demonstrate large thermoelectric effects, exceeding existing predictions by several orders of magnitude. This opens the route to a new class of highly efficient and versatile quasi-periodic steady-state heat engines, with a possible implementation using ultracold neutral atoms in bichromatic optical lattices.

(Invited) Tuning of Catalytic Activity By Thermoelectric Effect

Thermoelectric (TE) materials convert temperature difference directly into electrical voltage via the Seebeck effect, S= -V/ΔT where V is the voltage between the two ends of the TE material and ΔT the temperature difference, S is the Seebeck coefficient. It has been used for energy harvesting from waste heat, and for silent cooling via its converse phenomenon the Peltier effect. Here we report its novel application as a catalyst promoter. Catalyst promoters can generally be divided into structural and electronic promoters. Structural promoters enhance and stabilize the dispersion of the catalyst on the support. Electronic promoters induce changes of electronic state of the catalyst near the Fermi level. These promoters are added or fixed during the catalyst preparation, therefore can subject to degradation during catalytic process. We report here the realization of in situ, controllable, reversible, and significant modification of catalytic activity, through the generation of a Seebeck voltage. A chamber reactor which combine TE effect with catalytic chemical reaction was assembled.1, 2 The chamber was placed directly on a hot-plate, and its top cap was water cooled. A disc TE BiCuSeO (BCSO) sample of 20 mm in diameter and about 2 mm in thickness was then placed into a specific glass ceramic holder and attached to the cold surface of the cap for catalytic reaction investigation under the usual TE conditions. As the back side of the disc was in contact with the water cooled stainless steel cap, its temperature was never higher than 373 K, so a large temperature gradient therefore a large Seebeck voltage across the sample thickness was created when the front surface of the disc reached a high temperature Th. Under the reduced TE (RTE) condition, the back surface of the disc was not in contact with the cap, therefore the temperature gradient across the disc thickness and also the Seebeck voltage were much smaller. Figure 1a shows the ethylene oxidation reaction rates at different front surface temperatures Th for a sample of Pt thin film of 80 nm supported on BCSO with a large TE effect (red marks, Pt(80)/BCSO TE), and under a reduced TE effect (blue marks, Pt(80)/BCSO RTE) conditions respectively. For comparison purpose, the ethylene reaction rate on a typical catalyst for this reaction Pt thin film on Y2O3-stabilized ZrO2 (YSZ, green marks, Pt(80)/YSZ) is also depicted in Fig. 1a. For Pt(80)/BCSO TE, when the front surface Th was at 666 K, the backside temperature Tc was 349 K, so a temperature difference of 317 K existed across the BCSO thickness, which generated a Seebeck voltage -71 mV. For the same sample under the reduced TE effect (Pt(80)/BCSO RTE), at 553 K, the measured Seebeck voltage was -3.1 mV, and the reaction rate was much lower than that for Pt(80)/YSZ at a similar temperature 563 K. This was probably because the surface area of the Pt(80)/BCSO was much smaller than for Pt(80)/YSZ. However, with the increasing temperature, the reaction rates for Pt(80)/BCSO under TE and RTE conditions increased much faster than for non-TE YSZ supported catalyst. Further detailed investigation demonstrated that when the total reaction rate was lower than a certain value, a linear relationship existed between the logarithm of the reaction rate and the Seebeck voltage.1, 2 We called this thermoelectric promotion of catalysis (TEPOC). To confirm the above observed TEPOC can also be observed in other reactions, the same samples were used as catalysts for carbon dioxide hydrogenation. The inlet gases were CO2 and H2, and the reaction products observed were CO and CH4, with the vast majority (>90%) being CO. Similar as was observed for ethylene oxidation, for the same sample under the same inlet gas composition, the CO2 conversion rates r under TE conditions were much higher (between 10 to 30 times) than under RTE conditions. Again, a good linear relationship between Ln(r) and -eV/kbTh was observed (Figure 1b). The effects of the Seebeck voltage on the catalytic activity under different reaction conditions, such as fuel-lean and fuel-rich conditions for the ethylene oxidation, have also been investigated. The underlying mechanisms based on metal-semiconductor contact theory and work function change, as well as the comparison with the Electrochemical Promotion of Catalysis, will be discussed.

Giant thermoelectric power factor in ultrathin FeSe superconductor

The thermoelectric effect is attracting a renewed interest as a concept for energy harvesting technologies. Nanomaterials have been considered a key to realize efficient thermoelectric conversions owing to the low dimensional charge and phonon transports. In this regard, recently emerging two-dimensional materials could be promising candidates with novel thermoelectric functionalities. Here we report that FeSe ultrathin films, a high-Tc superconductor (Tc; superconducting transition temperature), exhibit superior thermoelectric responses. With decreasing thickness d, the electrical conductivity increases accompanying the emergence of high-Tc superconductivity; unexpectedly, the Seebeck coefficient α shows a concomitant increase as a result of the appearance of two-dimensional natures. When d is reduced down to ~1 nm, the thermoelectric power factor at 50 K and room temperature reach unprecedented values as high as 13,000 and 260 μW cm−1 K−2, respectively. The large thermoelectric effect in high Tc superconductors indicates the high potential of two-dimensional layered materials towards multi-functionalization.

Colloquium : Nonequilibrium effects in superconductors with a spin-splitting field

We review the recent progress in understanding the properties of spin-split superconductors under non-equilibrium conditions. Recent experiments and theories demonstrate a rich variety of transport phenomena occurring in devices based on such materials that suggest direct applications in thermoelectricity, low-dissipative spintronics, radiation detection and sensing. We discuss different experimental situations and present a theoretical framework based on quantum kinetic equations. Within this framework we provide an accurate description of the non-equilibrium distribution of charge, spin and energy, which are the relevant non-equilibrium modes, in different hybrid structures. We also review experiments on spin-split superconductors and show how transport measurements reveal the properties of the non-equilibrium modes and their mutual coupling. We discuss in detail spin injection and diffusion and very large thermoelectric effects in spin-split superconductors.

Thermoelectric effects in superconductor-ferromagnet tunnel junctions on europium sulfide

We report on large thermoelectric effects in superconductor-ferromagnet tunnel junctions in proximity contact with the ferromagnetic insulator europium sulfide. The combination of a spin-splitting field and spin-polarized tunnel conductance in these systems breaks the electron-hole symmetry and leads to spin-dependent thermoelectric currents. We show that the exchange splitting induced by the europium sulfide boosts the thermoelectric effect in small applied fields and can therefore eliminate the need to apply large magnetic fields, which might otherwise impede applications in thermometry or cooling.

Large thermoelectric effect in ballistic Andreev interferometers

Employing quasiclassical theory of superconductivity combined with Keldysh technique we investigate large thermoelectric effect in multiterminal ballistic normal-superconducting (NS) hybrid structures. We argue that this effect is caused by electron-hole asymmetry generated by coherent Andreev reflection of quasiparticles at interfaces of two different superconductors with non-zero phase difference. Within our model we derive a general expression for thermoelectric voltages $V_{T1,2}$ induced in two different normal terminals exposed to a thermal gradient. Our results apply at any temperature difference in the subgap regime and allow to explicitly analyze both temperature and phase dependencies of $V_{T1,2}$ demonstrating that in general there exists no fundamental relation between these voltages and the equilibrium Josephson current in SNS junctions.

Nonlinear thermoelectric effects in high-field superconductor-ferromagnet tunnel junctions.

Background: Thermoelectric effects result from the coupling of charge and heat transport and can be used for thermometry, cooling and harvesting of thermal energy. The microscopic origin of thermoelectric effects is a broken electron–hole symmetry, which is usually quite small in metal structures. In addition, thermoelectric effects decrease towards low temperatures, which usually makes them vanishingly small in metal nanostructures in the sub-Kelvin regime.Results: We report on a combined experimental and theoretical investigation of thermoelectric effects in superconductor/ferromagnet hybrid structures. We investigate the dependence of thermoelectric currents on the thermal excitation, as well as on the presence of a dc bias voltage across the junction.Conclusion: Large thermoelectric effects are observed in superconductor/ferromagnet and superconductor/normal-metal hybrid structures. The spin-independent signals observed under finite voltage bias are shown to be reciprocal to the physics of superconductor/normal-metal microrefrigerators. The spin-dependent thermoelectric signals in the linear regime are due to the coupling of spin and heat transport, and can be used to design more efficient refrigerators.

Spin caloritronics with superconductors: Enhanced thermoelectric effects, generalized Onsager response-matrix, and thermal spin currents

It has recently been proposed and experimentally demonstrated that it is possible to generate large thermoelectric effects in ferromagnet/superconductor structures due to a spin-dependent particle-hole asymmetry. Here, we theoretically show that quasiparticle tunneling between two spin-split superconductors enhances the thermoelectric response manyfold compared to when only one such superconductor is used, generating Seebeck coefficients ($\mathcal{S} > 1$ mV/K) and figures of merit ($ZT \simeq 40$) far exceeding the best bulk thermoelectric materials, and also becomes more resilient toward inelastic scattering processes. We present a generalized Onsager response-matrix which takes into account spin-dependent voltage and temperature gradients. Moreover, we show that thermally induced spin currents created in such junctions, even in the absence of a polarized tunneling barrier, also become largest in the case where a spin-dependent particle-hole asymmetry exists on both sides of the barrier. We determine how these thermal spin currents can be tuned both in magnitude and sign by several parameters, including the external field, temperature, and the superconducting phase-difference.

Seeking large thermoelectric effects in MgO-based tunnel junctions

There is much controversy concerning thermoelectric effects in the MgO-based magnetic tunnel junctions (MTJs) as reported in some recent publications. To clarify the problem, we give calculations from atomic first-principles systemically. Large Seebeck coefficient (S) and up-limit of figure of merit (ZT) are predicted in double- and multi-barrier MTJs, with those in single-barrier MTJs being relatively small. By restraining the phonon thermal conductance through the introduction of one vacuum barrier or numbers of MgO barriers, the up-limit of ZT can be obtained. Room temperature and ZT are predicted in an asymmetric Fe MgO Fe Vaccum Fe MTJs. The resonant quantum-well states are suggested to be responsible for the enhanced thermoelectric effects in the MTJs with double- and multi-barrier.

Optical conductivity of layered calcium cobaltate Ca3Co4O9

We report the optical properties of layered calcium cobaltate, Ca3Co4O9, which is regarded as a promising candidate for use as a thermoelectric material. The optical conductivity shows three broad peaks related to the inter-band transition below 4 eV, which are quite similar to those in the spectra of NaxCoO2. This similarity implies that the CoO2 layer, which is an essential unit for both Ca3Co4O9 and NaxCoO2, is dominant in the energy band structure below 4 eV. In addition, we estimate the effective carrier number per Co site and find similarity between the CoO2 layers of Ca3Co4O9 and Na0.75CoO2, which is consistent with the similarity in their Seebeck coefficients. To discuss the contribution of the rocksalt-type Ca2CoO3 layer in Ca3Co4O9, we propose the concept of optical sheet conductivity in the layered materials and estimate its value in the Ca2CoO3 layer. A comparison with the spin-polarized band calculation of the LDA + Hubbard U formalism with U = 5 eV suggests that the Ca2CoO3 layer has the inter-band transition of 2.6 eV in the spin-down band structure. Evaluation of the valences of Co 3d orbitals indicates the existence of charge transfer from the Ca2CoO3 layer to the CoO2 layer and mixing of Co3+ and Co4+ in the CoO2 layer, which may be the origin of the large thermoelectric effect.

Large and tunable thermoelectric effect in single layer graphene on bilayer graphene

The conductivity and thermopower of a trilayer graphene based system have been studied within the framework of a simple model. It has been shown that kinks of the conductivity and peaks of the thermopower of the monolayer graphene formed on a tunable bilayer graphene appear near the edges of the band gap of the tunable bilayer graphene.

Electron-hole imbalance and large thermoelectric effect in superconducting hybrids with spin-active interfaces

We argue that spin-sensitive quasiparticle scattering may generate electron-hole imbalance in superconducting structures, such as, e.g., superconducting-normal hybrids with spin-active interfaces. We elucidate a transparent physical mechanism for this effect demonstrating that scattering rates for electrons and holes at such interfaces differ from each other. Explicitly evaluating the wave functions of electronlike and holelike excitations in superconducting-normal bilayers we derive a general expression for the thermoelectric current and show that---in the presence of electron-hole imbalance---this current can reach maximum values as high as the critical current of a superconductor.

Perfect spin filtering and large spin thermoelectric effects in organic transition-metal molecular junctions.

We present ab initio studies of spin-polarized transport properties and thermospin effects in cyclopentadienyl-iron molecular junctions. It is found that the spin-up transmission coefficient at the Fermi level shows an odd-even oscillating behaviour, while the spin-down transmission coefficient has an exponential decay with the molecule length. The spin polarization at the Fermi level rapidly tends toward a saturation value close to 100% with the molecule length. This is ascribed to the existence of different orbital states for different spin components at the Fermi level. In addition, we find that the spin-up Seebeck coefficient oscillates between positive and negative values, while the spin-down Seebeck coefficient always has a positive value and monotonically increases with the molecule length. Therefore in some cases, the spin Seebeck coefficient is even larger than the corresponding charge Seebeck effect. Finally, we also provide a possibility of utilizing cyclopentadienyl-iron molecular junctions to achieve the pure spin current without an accompanying charge current at about room temperature.

Systematic Conversion of Single Walled Carbon Nanotubes into n-type Thermoelectric Materials by Molecular Dopants

Thermoelectrics is a challenging issue for modern and future energy conversion and recovery technology. Carbon nanotubes are promising active thermoelectic materials owing to their narrow bandgap energy and high charge carrier mobility, and they can be integrated into flexible thermoelectrics that can recover any waste heat. We here report air-stable n-type single walled carbon nanotubes with a variety of weak electron donors in the range of HOMO level between ca. −4.4 eV and ca. −5.6 eV, in which partial uphill electron injection from the dopant to the conduction band of single walled carbon nanotubes is dominant. We display flexible films of the doped single walled carbon nanotubes possessing significantly large thermoelectric effect, which is applicable to flexible ambient thermoelectric modules.