Spin Thermoelectric Effects Research Articles

Graph theoretic analysis of three-terminal quantum dot thermocouples: Onsager relations and spin-thermoelectric effects

Graph theoretic analysis of three-terminal quantum dot thermocouples: Onsager relations and spin-thermoelectric effects

Energy-resolved spin filtering effect and thermoelectric effect in topological-insulator junctions with anisotropic chiral edge states

Energy-resolved spin filtering effect and thermoelectric effect in topological-insulator junctions with anisotropic chiral edge states

Spin-thermoelectric effects in a quantum dot hybrid system with magnetic insulator

We investigate spin thermoelectric properties of a hybrid system consisting of a single-level quantum dot attached to magnetic insulator and metal electrodes. Magnetic insulator is assumed to be of ferromagnetic type and is a source of magnons, whereas metallic lead is reservoir of electrons. The temperature gradient set between the magnetic insulator and metallic electrodes induces the spin current flowing through the system. The generated spin current of magnonic (electric) type is converted to electric (magnonic) spin current by means of quantum dot. Expanding spin and heat currents flowing through the system, up to linear order, we introduce basic spin thermoelectric coefficients including spin conductance, spin Seebeck and spin Peltier coefficients and heat conductance. We analyse the spin thermoelectric properties of the system in two cases: in the large ondot Coulomb repulsion limit and when these interactions are finite.

Perfect spin filtering effect, tunnel magnetoresistance and thermoelectric effect in metals-adsorbed blue phosphorene nanoribbons

The thermoelectric transport properties of Fe- and Ti-adsorbed blue phosphorene nanoribbons are studied using the first principles approach. The research indicates that Fe- and Ti-adsorbed blue phosphorene nanoribbons exhibit spin-dependent Seebeck effect. The devices based on the Fe-adsorbed blue phosphorene nanoribbons show perfect spin filtering effect caused by 100% spin polarization and 106% tunnel magnetoresistance. The spin-up current and the spin-down current are reversed in Ti-adsorbed blue phosphorene nanoribbons, indicating that a large spin current and a small charge current can be generated. These systems also exhibit negative differential resistance induced by the current-voltage curve. The band structures and transport spectrums are utilized to explain the physical mechanism of spin current generation. Metals-adsorbed blue phosphorene nanoribbons can be considered novel thermoelectric materials and used to make multifunctional thermal spintronic devices.

Spin thermoelectric effects on aluminum or phosphorus doped zigzag silicene nanoribbons

Based on the density-functional theory (DFT) combined with nonequilibrium Green’s function (NGF), this paper investigates the effects of either single aluminum (Al) or single phosphorus (P) atom substitutions at different edge positions of zigzag-edged silicene nanoribbons (ZGNRs) in the ferromagnetic state on the spin-dependent transport properties and spin thermoelectric effects. It has been found that the spin polarization at the Fermi level can reach 100% or –100% in the doped ZSiNRs. Meanwhile, the spin-up Seebeck effect (for -100% case) and spin-down Seebeck effect (for 100% case) are also enhanced. Moreover, the spin Seebeck coefficient is much larger than the corresponding charge Seebeck coefficient at a special doping position and electron energy. Therefore, the study shows that the Al or P doped ZSiNRs can be used to prepare the ideal thermospin devices.

Spin thermoelectric effects of new-style one-dimensional carbon-based nanomaterials

Based on first-principles methods, the authors of this paper investigate spin thermoelectric effects of one-dimensional spin-based devices consisting of zigzag-edged graphene nanoribbons (ZGNRs), carbon chains and graphene nanoflake. It is found that the spin-down transmission function is suppressed to zero, while the spin-up transmission function is about 0.25. Therefore, an ideal half-metallic property is achieved. In addition, the phonon thermal conductance is obviously smaller than the electronic thermal conductance. Meantime, the spin Seebeck effects are obviously enhanced at the low-temperature regime (about 80K), resulting in the fact that spin thermoelectric figure of merit can reach about 40. Moreover, the spin thermoelectric figure of merit is always larger than the corresponding charge thermoelectric figure of merit. Therefore, the study shows that they can be used to prepare the ideal thermospin devices.

Spin-thermoelectric transport in nonuniform strained zigzag graphene nanoribbons

We study a nonuniform strain in zigzag graphene nanoribbons for producing the spin-thermoelectric effects, using the mean-field Hubbard approximation and a Green's function approach. Our theoretical results show that a sinusoidal-shaped inhomogeneous strain with electron-electron interaction could induce a different effect on each edge of zigzag nanoribbons and finally generate a spin semiconductor with a tunable spin-dependent band gap. The strength of strain also controls the magnitude of magnetization in each edge. Interestingly, pure spin current and a giant spin Seebeck coefficient can be produced even at low values of strain by applying a thermal gradient and without magnetic elements. These results pave a practical way toward improved design for spin-thermoelectric applications through strain engineering.

Circularly-polarized light controlled thermal spin transport in stanene nanoribbon

The major challenge of spintronics lies in how to generate, manipulate, and detect spin current. Multiple methods, such as using magnetic materials, magnetic field, and polarized light field to manipulate the spin of electrons, have been proposed. Owing to the possible applications in spintronic devices, there is currently great interest in the field of spin caloritronics, which focuses on the interplay of spin and heat currents. Stanene is a type of two-dimensional topological insulator consisting of a single layer of Sn atoms arranged in a hexagonal lattice. In this paper, the effects of light and electric fields on the spin-dependent thermoelectric effect of the stanene nanoribbon are studied theoretically based on the non-equilibrium Green’s function method. The results show that the properties and intensity of the thermoelectric current can be effectively controlled by the intensity and the polarization direction of the circularly polarized light field. Under the joint action of a strong circularly-polarized light field and an electric field, the stanene can transform from a quantum spin-Hall insulator into a spin-polarized quantum Hall insulator. When the left-circularly-polarized light field is applied, the spin-down edge states of stanene undergo a phase transition to form a bandgap, and a 100% spin-polarized spin-down current driven by temperature gradient can be obtained. When the right-circularly-polarized light is applied, the edge states of spin-up electrons are destroyed, and a completely polarized spin-up thermal current can be generated. In the weak external field, the properties of the edge state do not change, and the system does not output a thermoelectric current. In addition, the study shows that the intensity of the thermal spin current is related to the width of the bandgap, and a moderate increase in temperature can significantly increase the peak value of the current, but the higher equilibrium temperature and temperature gradient will restrain the spin thermoelectric effect.

Amplification of Spin Thermoelectric Signals in Multilayer Spin Thermopiles

Spin thermoelectric effect, an electric voltage generation by a thermal spin current in magnetic systems, has been investigated as a new candidate for thermal energy harvesting, but its thermoelect...

Spin thermoelectric transport in a four-quantum-dot ring: Influence of quantum dots magnetization

Non-equilibrium Green’s function method is applied to study the spin thermoelectric effects in a four-quantum-dot ring with two of the quantum dots magnetized. The influence of magnetic configuration on the spin thermoelectric transport through the system is investigated. In some magnetic configurations, a significant spin Seebeck coefficient [Formula: see text] can be generated with the variation of the quantum dot (QD) energy level. Appropriately tuning the QD energy level can let the present device work as a pure spin-up (spin-down) or a pure-spin-current thermal generator. The combined effect of the magnetization and magnetic flux (or Rashba spin-orbit coupling) is also investigated. Finally, the charge figure of merit [Formula: see text] and spin figure of merit [Formula: see text] are evaluated, it is found that the magnitude of the spin figure of merit can be comparable to that of the charge counterpart.

Spin Thermoelectric Effects in a Three-Terminal Double-Dot Interferometer**Supported by the National Natural Science Foundation of China under Grant No. 11604113 and the Natural Science Foundation of the Jiangsu Higher Education Institutions of China under Grant No. 17KJB140004

We theoretically investigate the thermoelectric properties of a three-terminal double-dot interferometer with Rashba spin-orbit interaction. It is found that with some temperature distributions a thermal spin current can even be produced without the help of magnetic flux and by tuning the spin interference effect in the system, a pure spin or fully spin-polarized current can be driven by temperature differences. For the cases that two of the terminals are held at the same temperature, the charge (spin) thermopower and the charge (spin) figure of merit are defined and calculated in the linear response regime. With some choices of the system parameters the calculated spin and charge thermopowers are of the same order of magnitude and the charge figure of merit can exceed 1.

Spin thermoelectric effects in organic single-molecule devices

The spin thermoelectric performance of a polyacetylene chain bridging two zigzag graphene nanoribbons (ZGNRs) is investigated based on first principles method. Two different edge spin arrangements in ZGNRs are considered. For ferromagnetic (FM) ordering, transmission eigenstates with different spin indices distributed below and above Fermi level are observed, leading directly to a strong spin thermoelectric effect in a wide temperature range. With the edge spins arranged in the antiferromagnetic (AFM) ordering, an obvious transport gap appears in the system, which greatly enhances the thermoelectric effects. The presence of a small spin splitting also induces a spin thermoelectric effect greater than the charge thermoelectric effect in certain temperature range. In general, the single-molecule junction exhibits the potential to be used for the design of perfect thermospin devices.

Spin-dependent thermoelectric effects in a strongly correlated double quantum dot

We investigate spin-dependent thermoelectric transport through a system of two coupled quantum dots attached to reservoirs of spin-polarized electrons. Generally, we focus on the strongly correlated regime of transport. To this end, a slave-boson method for finite $U$ is employed. Our main goal is to show that, apart from complex low-temperature physics, such a basic multilevel system provides a possibility to examine various quantum interference effects, with particular emphasis put on the influence of such phenomena on thermoelectric transport. Apart from the influence of interference effects on spin-degenerate charge transport, we show how spin-dependent transport, induced by ferromagnetic leads, can be modified as well. Finally, we also consider the case where the spin-relaxation time in the ferromagnetic leads is relatively long, which leads to the so-called spin thermoelectric effects.

Carbon-based molecular devices: Fano effects controlled by the molecule length and the gate voltage.

Fano effect is an important quantum phenomenon in mesoscopic systems, which arises from an interference between the localized state and the extended state. Here we observe an obvious Fano effect near the Fermi level in an all-carbon molecular device consisting of an acene molecule sandwiched between two zigzag graphene nanoribbon (ZGNR) electrodes. By increasing the length of the molecule, an extended state gradually evolves into a localized state. With the aid of the nearby extended state, a Fano effect is achieved. Using a gate voltage, we can easily tune the Fano effect induced by the single-transmission channel. When the spin degree of freedom is involved, the all-carbon device can show a half-metallic property with positive or negative 100% spin polarization at the Fermi level under the gate voltage; meanwhile the spin thermoelectric effect can also be enhanced.

Enhancement of thermospin effect in germanene based normal/ferromagnetic stub/normal junction

Spin thermoelectric effects in ferromagnetic (FM) germanene are theoretically investigated by using the nonequilibrium Green's function method. It is found that the spin Seebeck effect can be generated by temperature bias ΔT when a FM germanene is considered in the central region. However, the obtained spin resolved Seebeck coefficients is quite low with maximum value of Sσ≃700μV/K. The spin Seebeck effect is shown to increase enormously in different energy states with the assistance of electric field or stub structure. By modulating the geometric parameters of stub, the spin thermopower Ss has distinct peak values in the bulk states. Moreover, varying the Fermi energy within the bulk gap by the gate, Ss can be significantly enhanced by increasing the strength of electric field. The spin thermopower obtained by each method is predicted to be 2500μV/K, which is more than 300% larger relative to the case without electric field or stub. In addition, the magnitude and sign of spin thermopower can be manipulated by adjusting the Fermi energy. The results show that such FM germanene stub device exhibits much better thermoelectric performance and may be used as a wide energy range tunable spin thermoelectric generator.

Tunable half-metallic properties and spin Seebeck effects in zigzag-edged graphene nanoribbons adsorbed with V atom or V-benzene compound

We investigate the spin transport and thermospin properties of zigzag-edged graphene nanoribbons (ZGNRs) adsorbed with a V atom or V-benzene compound using the spin-polarized density functional theory. It is found that the spin polarization of the transmittance at the Fermi level can be obviously enhanced as the V atom moves from the edge to middle hollow sites. In some cases, ZGNRs with V adsorption can possess half-metallicity with −100% spin polarization, which is mainly attributed to the appearance of a spin-up localized state from the d orbital of the V atom. When a benzene is covered on the V atom, the sign of spin polarization is altered, and even it approaches 100%. In addition, compared with the ideal ZGNRs, the spin thermoelectric effects of these adsorbed ZGNRs are obviously enhanced in some cases. Meantime, the magnitude of the spin Seebeck coefficient at the Fermi level is nearly equal to that of the charge Seebeck coefficient. Moreover, we also find that the spin thermoelectric properties are rather sensitive to the width of the nanoribbon, and the sign and magnitude of the spin Seebeck coefficient can be modified by the width. Therefore, these adsorbed ZGNRs can be used to design perfect spin-filtering systems and high-performance thermopsin devices.

Spin power and efficiency in an Aharnov-Bohm ring with an embedded magnetic impurity quantum dot

Spin thermoelectric effects in an Aharnov-Bohm ring with a magnetic impurity quantum dot (QD) are theoretically investigated by using the nonequilibrium Green's function method. It is found that due to the exchange coupling between the impurity and the electrons in QD, spin output power, and efficiency can be significant and be further modulated by the gate voltage. The spin thermoelectric effect can be modulated effectively by adjusting the Rashba spin-orbit interaction (RSOI) and the magnetic flux. The spin power and efficiency show zigzag oscillations, and thus spin thermoelectric effect can be switched by adjusting the magnetic flux phase factor and RSOI ones. In addition, the spin efficiency can be significantly enhanced by the coexistence of the RSOI and the magnetic flux, and the maximal value of normalized spin efficiency ηmax/ηC = 0.35 is obtained. Our results show that such a QD ring device may be used as a manipulative spin thermoelectric generator.

Spin Thermoelectric Effects in a Strongly Correlated Double Quantum Dot System

Spin Thermoelectric Effects in a Strongly Correlated Double Quantum Dot System

Enhanced spin thermoelectric effects in BN-embedded zigzag graphene nanoribbons

Spin thermoelectric effects in BN-embedded zigzag graphene nanoribbons (ZGNRs) are studied by a first-principles method. The multiple spin-up (spin-down) quasi-bound states below (above) the Fermi level can be introduced by the BN bonded pairs, leading to the dip structures in the transmission spectra. We further find that the spin thermoelectric effect at the Fermi level is obviously stronger than the corresponding charge thermoelectric effect. As the more BN bonded pairs are gradually doped from the edge to middle regions, the spin thermoelectric figure of merit (FOM) can be improved up to ten times than the charge thermoelectric FOM. A pure spin current can also be achieved in the BN-embedded ZGNRs under an external thermal bias.

Spin thermoelectric effects in a double quantum dot embedded in an Aharonov–Bohm ring coupled to nonmagnetic leads

Spin thermoelectric effects in a Rashba double quantum dot (QD) embedded in one arm of an Aharonov–Bohm (AB) ring are investigated theoretically in the linear response regime with the use of Green's function method. We found that by applying an external thermal bias to QD-1, the spin Seebeck effect can be induced due to the presence of Heisenberg exchange coupling between QD-1 and QD-2. A high spin thermopower with a maximum as large as 1.5kB/e and spin figure of merit with a maximum as large as 1.4 can be obtained with the coaction of exchange coupling and interference effect, but without the need for any extra magnetic field or ferromagnetic electrodes. In addition, the spin thermoelectric effect can be manipulated effectively by the spin–orbit interaction and the magnetic flux. The results indicate that such a double dot ring system may lead to potential applications in spin thermoelectric devices.