Helical thermoelectrics and refrigeration

Electromagnetically tunable spin-valley-polarized current via anomalous Nernst effect in monolayer of jacutingaite

Monolayer jacutingaite (Pt2HgSe3) exhibits remarkable properties, including significant spin-orbit coupling (SOC) and a tunable band gap, attributed to its buckled honeycomb geometry and the presence of heavy atoms. In this study, we explore the spin- and valley-dependent anomalous Nernst effect (ANE) in jacutingaite under the influence of a vertical electric field, off-resonance circularly polarized light (OCPL), and an antiferromagnetic exchange field. Our findings, within the low-energy approximation, reveal the emergence of a perfectly spin-polarized ANE with the application of appropriate OCPL and a perfectly valley-polarized ANE under an antiferromagnetic exchange field. Leveraging the robust SOC inherent in monolayer jacutingaite, our study highlights the potential to attain perfectly spin-valley-polarized Nernst currents across a wide range of Fermi energy levels by combining these fields in pairs with a suitable strength. The findings can be used for the development of spin-valley-based optoelectronic devices.

Magnetoelectric tuning of spin, valley, and layer-resolved anomalous Nernst effect in transition-metal dichalcogenides bilayers

The anomalous Nernst coefficient (ANC) for transition-metal dichalcogenide (TMD) bilayers is studied with a focus on the interplay between layer pseudospin, spin, and valley degrees of freedom when electric and exchange fields are present. Breaking the inversion and time reversal symmetries via respectively electric and exchange fields results for bilayer TMDs in a spin-valley-layer polarized total ANC. Conditions are determined for controlling the spin, valley, and layer-resolved contributions via electric field tuning. Our results demonstrate the control of layer degree of freedom in bilayer TMDs magnetoelectrically which is of relevance for possible applications in spin/valley caloritronics.

Enhancement of the thermoelectric properties in bilayer graphene structures induced by Fano resonances

Fano resonances of bilayer graphene could be attractive for thermoelectric devices. The special profile presented by such resonances could significantly enhance the thermoelectric properties. In this work, we study the thermoelectric properties of bilayer graphene single and double barrier structures. The barrier structures are typically supported by a substrate and encapsulated by protecting layers, reducing considerably the phonon thermal transport. So, we will focus on the electronic contribution to the thermal transport. The charge carriers are described as massive chiral particles through an effective Dirac-like Hamiltonian. The Hybrid matrix method and the Landauer–Büttiker formalism are implemented to obtain the transmission, transport and thermoelectric properties. The temperature dependence of the Seebeck coefficient, the power factor, the figure of merit and the efficiency is analyzed for gapless single and double barriers. We find that the charge neutrality point and the system resonances shape the thermoelectric response. In the case of single barriers, the low-temperature thermoelectric response is dominated by the charge neutrality point, while the high-temperature response is determined by the Fano resonances. In the case of double barriers, Breit–Wigner resonances dominate the thermoelectric properties at low temperatures, while Fano and hybrid resonances become preponderant as the temperature rises. The values for the figure of merit are close to two for single barriers and above three for double barriers. The system resonances also allows us to optimize the output power and the efficiency at low and high temperatures. By computing the density of states, we also corroborate that the improvement of the thermoelectric properties is related to the accumulation of electron states. Our findings indicate that bilayer graphene barrier structures can be used to improve the response of thermoelectric devices.

Nonlocal Thermoelectricity in a Superconductor-Topological-Insulator-Superconductor Junction in Contact with a Normal-Metal Probe: Evidence for Helical Edge States

We consider a Josephson junction hosting a Kramers pair of helical edge states of a quantum spin Hall bar in contact with a normal-metal probe. In this hybrid system, the orbital phase, induced by a small magnetic field threading the junction known as a Doppler shift, combines with the conventional Josephson phase difference and originates an effect akin to a Zeeman field in the spectrum. As a consequence, when a temperature bias is applied to the superconducting terminals, a thermoelectric current is established in the normal probe. We argue that this purely nonlocal thermoelectric effect is a unique signature of the helical nature of the edge states coupled to superconducting leads and it can constitute a useful tool for probing the helical nature of the edge states in systems where the Hall bar configuration is difficult to achieve. We fully characterize thermoelectric response and performance of this hybrid junction in a wide range of parameters, demonstrating that the external magnetic flux inducing the Doppler shift can be used as a knob to control the thermoelectric response and the heat flow in a novel device based on topological junctions.

Optimal Thermoelectricity with Quantum Spin Hall Edge States

We study the thermoelectric properties of a Kramers pair of helical edge states of the quantum spin Hall effect coupled to a nanomagnet with a component of the magnetization perpendicular to the direction of the spin-orbit interaction of the host. We show that the transmission function of this structure has the desired qualities for optimal thermoelectric performance in the quantum coherent regime. For a single magnetic domain, there is a power generation close to the optimal bound. In a configuration with two magnetic domains with different orientations, pronounced peaks in the transmission functions and resonances lead to a high figure of merit. We provide estimates for the fabrication of this device with HgTe quantum-well topological insulators.

Thermoelectric Materials—Strategies for Improving Device Performance and Its Medical Applications

Thermoelectrics, in particular solid-state conversion of heat to electricity and vice versa, is expected to be a key energy harvesting and temperature management solution in coming years. There has been a resurgence in the search for new materials for advanced thermoelectric energy conversion applications and to enhance the properties of existing materials. In this paper, we review recent efforts on improving figure-of-merit (ZT) through alloying and nano structuring. As heatsink characteristics dictate the performance of thermoelectric modules, various types of heatsink designs has been investigated. Several reported strategies for improving ZT are critically assessed. A notable increase in figure-of-merit of thermoelectric materials (TE) has opened up new areas of applications especially in the medical field. Peltier cooling devices are widely employed for patient core temperature control, skin cooling, medical device and laboratory equipment cooling. Application of these devices in the medical field both in temperature control and power generation has been studied in detail. It is envisioned that this study will provide profound knowledge on the thermoelectric based materials and its role in medical applications.

Designing a highly efficient graphene quantum spin heat engine

We design a quantum spin heat engine using spin polarized ballistic modes generated in a strained graphene monolayer doped with a magnetic impurity. We observe remarkably large efficiency and large thermoelectric figure of merit both for the charge as well as spin variants of the quantum heat engine. This suggests the use of this device as a highly efficient quantum heat engine for charge as well as spin based transport. Further, a comparison is drawn between the device characteristics of a graphene spin heat engine against a quantum spin Hall heat engine. The reason being edge modes because of their origin should give much better performance. In this respect we observe our graphene based spin heat engine can almost match the performance characteristics of a quantum spin Hall heat engine. Finally, we show that a pure spin current can be transported in our device in absence of any charge current.