Ning Mao

Humboldt Fellow as of August 2024

Overview of my Research Project:

Traditional thermoelectric materials suffer from a long-standing problem of low efficiency. This efficiency can be captured by a thermoelectric figure-of-merit ZT = S2𝜎T/k, where S, 𝜎, T, and k correspond to thermopower, electrical conductivity, temperature, and thermal conductivity. A fundamental roadblock in conventional materials exists between thermopower S and electrical conductivity 𝜎 due to their conflicting requirements. Specifically, metals showcase substantial electrical conductivity yet have a small thermopower. Similarly, insulators, due to their extremely low electrical conductivity, are deemed unsuitable as thermoelectric materials. On the contrary, topological materials present a pair of unique advantages. Firstly, Fu Liang et al. proposed a novel quantized thermoelectric Hall effect using topological Weyl semimetals under extreme quantum limit, which has been experimentally confirmed in ZrTe5 and TaP systems. Despite the realization of a large and non-saturating Seebeck coefficient, these materials demand a significant magnetic field to reach the quantum limit. Additionally, the observed regime occurs at approximately 40 K, which is far away from conditions of daily utilization. While the value in traditional metals and insulators may be negligible, the emergence of topological materials has successfully overcome this limitation. Therefore, our goal is to bring the substantial thermoelectric Hall effects or Nernst effects to room temperature and eliminate the need for an external magnetic field.

My research, funded by the Alexander von Humboldt foundation, is focus on the investigation of thermopower effects and transport properties of topological heterostructures. The first step is to construct ferromagnetic heterostructures with matched lattice constants, where the magnetism originates from the magnetic proximity effect of the ferromagnetic substrate. Additionally, the relationship between magnetic direction and thermopower will be studied. For the second step, we plan to construct misfit/twist heterostructures to investigate the potential application of orbital polarization. Previous studies have demonstrated that in a graphene system, orbital polarization can induce a magnetic field ranging from 10 to 300 T. However, most research aiming at enhancing thermopower has focused on spin-polarized magnetic fields, neglecting the potential impact of orbital polarization on thermoelectric transport.

Those investigations will be performed in Prof. Claudia Felser group at the Max Planck Institute for Chemical Physics of Solids in Dresden. Furthermore, her group boasts a lot of specialists skilled in thermoelectric experimentation, offering fresh perspectives on the topological heterostructures that I am investigating. Collaborating with her team provides an invaluable opportunity to compare my theoretical computations with experimental results, thereby enriching my comprehension of the underlying concepts and principles.



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