New progress in the study of the anomalous thermoelectric effect of ZrTe5 under strong magnetic field

[ Instrument network instrument research and development ] Zhang Jianlei, researcher of the Strong Magnetic Field Science Center of the Chinese Academy of Sciences, researcher Tian Tianliang, Professor Lu Haizhou of Southern University of Science and Technology, Professor Wang Chunming of Shanghai Normal University, researched the topological material ZrTe5 with strong steady magnetic field device. The anomalous thermoelectric effect under magnetic field, the related research results are published in the Journal of the American Physical Society "Physical Review Letters" under the topic of Anomalous thermoelectric effects of ZrTe5 in and beyond the quantum limit.

ZrTe5 is a topological material that combines the characteristics of one-dimensional chain and two-dimensional layer structure. Previous studies have shown that ZrTe5 can be switched between different topologies by applying pressure, stress or changing temperature. In addition to the rich topological state, ZrTe5 has a relatively low quantum limit and requires only a small magnetic field to reach its quantum criticality. In recent years, the novel physical phenomena exhibited by ZrTe5 under strong magnetic fields have also received extensive attention in the academic community.

In this work, the researchers conducted an in-depth study of the thermoelectric effects (Seebeck effect and Nernst effect) of ZrTe5 single crystal samples under strong magnetic fields. The anomalous Nernst effect caused by Dirac fermion at low temperatures was observed experimentally. As the magnetic field increases, both the Seebeck coefficient and the Nernst effect of ZrTe5 show significant quantum oscillations. When the magnetic field reaches 5.2 Tesla, all electrons of ZrTe5 occupy the lowest Rank level, that is, the system is at the quantum limit. The previous theoretical predictions show that after entering the quantum limit, the thermoelectric potential of the Dirac/German semimetal will increase non-saturated with the increase of the magnetic field, but the Seebeck coefficient of ZrTe5 has an abnormal "bullet" above 5 Tesla. .

In order to reveal the mechanism of ZrTe5's anomalous thermoelectric effect above the quantum limit, Zhang Zhenlei overcomes the difficulties of vibration and noise of water-cooled magnets by continuously improving the test scheme, and successfully realized the single crystal sample under the highest magnetic field of the water-cooled magnet with a maximum of 33 Tesla. Thermoelectric effect measurement. It is found that the Seebeck coefficient of ZrTe5 reaches a minimum at a critical magnetic field B* of 14 Tesla, converges to zero, and then begins to rise as the magnetic field increases (Fig. (a)). At the same time, the ZrTe5 Nernst signal undergoes a sign change near the critical magnetic field (Fig. (b)). Theoretical calculations show that the ZrTe5 body conduction band and the valence band have the characteristics of band inversion. After entering the quantum limit, the zero-throw energy levels of ZrTe5 electrons and holes will meet at the critical magnetic field (as shown in Figure (c). Show). Therefore, in the vicinity of the critical magnetic field, the ZrTe5 Seebeck coefficient exhibits a zero valley value, and the Nernst signal undergoes a sign transition. Under normal circumstances, as a feature of the topological non-mean state, the band inversion feature is difficult to detect directly by electrical transport experiments. This work shows that the measurement of thermoelectric effect under strong magnetic field can be used as a direct experimental means to detect the energy band reversal of topological materials.

The research has received strong support from the National Natural Science Foundation of China, the Chinese Academy of Sciences Youth Promotion Association, the Chinese Academy of Sciences Research Equipment Development Project, and the Hefei Material Science and Technology Center Innovation Project Cultivation Fund.

Figure: ZrTe5(a) Seebeck coefficient and (b) Nernst effect as a function of magnetic field. (c) Theoretically calculate the relationship between the ZrTe5 Landau level and the magnetic field. Upper right: Water-cooled magnet thermoelectric effect test device.