In recent years, the application of thermoelectricity has become more and more widespread. Thermoelectric materials provide a simple and environmentally friendly solution for the direct conversion of heat to electricity. The development of higher performance thermoelectric materials and their performance optimization have become more important. Generally, to improve the ZT value, electrical conductivity, Seebeck coefficient and thermal conductivity must be globally optimized as a whole object. However, due to the strong coupling among ZT parameters in many cases, it is very challenging to break the bottleneck of ZT optimization currently. Beyond the traditional optimization methods (such as inducing defects, varying temperature), the Rashba effect is expected to effectively increase the S2σ and decrease the κ, thus enhancing thermoelectric performance, which provides a new strategy to develop new-generation thermoelectric materials. Although the Rashba effect has great potential in enhancing thermoelectric performance, the underlying mechanism of Rashba-type thermoelectric materials needs further research. In addition, how to introduce Rashba spin splitting into current thermoelectric materials is also of great significance to the optimization of thermoelectricity.
- Thermoelectric materials
- Rashba spin splitting
- spin-orbit coupling
- Seebeck coefficient
Thermoelectric materials can use the Seebeck and Peltier effects to directly convert heat and electricity into each other [1, 2, 3], providing a simple and environmentally friendly solution for the direct conversion of heat to electricity, and is expected to play an important role in meeting future energy challenges effect. Thermoelectric equipment can not only directly convert the heat from the sun, radioisotopes, automobiles, industrial sectors and even the human body into electrical energy, but can also implement solid-state heat pumps based on electric drive for distributed refrigeration. In recent years, the application of thermoelectricity has become more and more widespread. The equipment based on thermoelectric materials has the advantages of miniaturization, quiet operation and no emission of greenhouse gases. The development of higher performance thermoelectric materials and their performance optimization have become more important.
The dimensionless thermoelectric figure of merit (ZT value) can be used as a key indicator to quantify thermoelectric performance, which is defined as, where T is the absolute temperature, S is the Seebeck coefficient, σ is the electrical conductivity, and are the electronic and lattice thermal conductivity, respectively. Improving the ZT value has always been the main goal of thermoelectric research. Generally, to improve the ZT value, electrical conductivity, Seebeck coefficient and thermal conductivity must be globally optimized as a whole object. In order to obtain a higher ZT, a high Seebeck coefficient
As the basic science of thermoelectric matures, the research of thermoelectric materials has also begun to develop rapidly. Among the materials reported with high thermoelectric properties, the ZT value of Bi2Te3 compound and its alloy form is about 1 at room temperature, which has been regarded as the highest standard of advanced thermoelectric materials in the thermoelectric field . Until 1990, Hicks and Dresselhaus proposed that better thermoelectric performance could be designed through the “size effect”, that is, to reduce the size [5, 6]. They found that Bi2Te3 with a quantum well (two-dimensional) or nanowire (one-dimensional) structure may have the potential to further increase the ZT value, and inferred that the root cause of the improvement in its thermoelectric performance is mainly due to the increased electronic density of states due to the decrease in dimensionality (Figure 1a), resulting in an increase in Seebeck coefficient. As shown in Figure 1b, the ZT value monotonously increases with the decrease of the thickness of the quantum well or the diameter of the quantum wire. Therefore, the use of size effect provides a new strategy for increasing the ZT value, because it not only increases the density of electronic states near the Fermi level EF to increase the Seebeck coefficient, but also increases the phonon at the barrier-well interface. Boundary scattering reduces thermal conductivity.
The synergistic interaction between charge, lattice, and spin-orbit coupling is a necessary factor to further optimize thermoelectric performance (Figure 2). In the past three decades, people have discovered a variety of possible mechanisms that may affect transport performance: resonance energy levels, modulation doping, band convergence, classical and quantum size effects, anharmonicity, and spin-related effect, such as Rashba effect, spin Seebeck effect and topological state, etc., have been verified in various materials such as V2-VI3 compounds, V-VI group compounds, semi-Heusler alloys, diamond-like structure compounds and silicides. Among them, Rashba spin-orbit coupling and controllable anharmonicity may be the key to the development of next-generation thermoelectric materials.
Note that in the quantum size effect caused by the reduction of dimensionality, a key idea for optimizing the power factor
In addition, the giant Rashba spin splitting in a two-dimensional BiSb monolayer can increase the ZT value by a factor of two at room temperature compared to spin degenerate states . Furthermore, it is confirmed that the Seebeck coefficient in Rashba spin-splitting BiTeX is higher than that in traditional spin-degenerate materials [9, 10], as shown in Figure 3d. Xiao et al. also predicted from the theoretical level that the Rashba system will exhibit abnormally enhanced thermoelectric behavior . This is due to the Rashba SOC-induced scission of the density of states and extended carrier lifetime, and due to the relatively high DOS near the Fermi level. A large slope will result in a higher Seebeck coefficient, thereby increasing
Actually, the Rashba effect has attracted considerable attention in the fields of spintronics, ferroelectrics, and superconducting electronics [13, 14], Rashba spin-split generally originated from the SOC and inversion asymmetry, the SOC gives rise to a perturbing operator equal to λ
As we known, thermoelectric materials are commonly composed of heavy elements with strong SOC . In view of this, the spin-enabled mechanisms including the Rashba effect  and the spin Seebeck effect  offer new channels to manipulate and further optimize thermoelectric properties . However, the spin Seebeck effect is currently limited at cryogenic temperatures. By contrast, the Rashba effect is promising to facilitate performance enhancement in broad thermoelectric materials. It was reported that Rashba spin splitting yielded a unique constant DOS near the EF, which resulted in high
Recent classical strategies of quantum confinement effect , resonant level , band convergence , liquid-like ions , entropy engineering , anharmonicity , and modulation doping  have enhanced ZT in many thermoelectric material systems. Albeit the advances in thermoelectric theories, there’s no doubt that the ZT enhancement have already reached a bottleneck. The Rashba effect, spin-dependent band splitting, has been proved to be a new path to enhance thermoelectric performance . In detail, Hong et al. demonstrated a strong Rashba spin splitting in Sn-doped GeTe and results in the band convergence experimentally, so that
Although the Rashba effect has great potential in enhancing thermoelectric performance, the influence of Rashba spin-orbit coupling on various thermoelectric parameters, thermoelectric optimization rule and the exact mechanism are still to be explored to a large extent. In particular, the thermoelectric performance and the underlying mechanism of Rashba-type thermoelectric materials need further research, or how to introduce Rashba spin splitting into current excellent thermoelectric materials is also of great significance to the optimization of thermoelectricity.