The research team composed of Zhang Jinglei, Researcher Tian Mingliang, Research Center of Strong Magnetic Field, Hefei Institute of Material Science, Chinese Academy of Sciences, Professor Lu Haizhou, Southern University of Science and Technology, and Wang Chunming, Professor of Shanghai Normal University, studied the topological material ZrTe5 in Regarding the anomalous thermoelectric effect under the magnetic field, related research results were published in the Physical Review Letters of the American Physical Society Journal under the title of Anomalous thermoelectric effects of ZrTe5 in and beyond the quantum limit.
ZrTe5 is a topological material with both one-dimensional chain and two-dimensional layered structure. The previous research results show that ZrTe5 can switch between different topological states by applying pressure, stress or changing the temperature. In addition to the rich topological states, the quantum limit of ZrTe5 is relatively low, and only a small magnetic field is needed to reach quantum criticality. In recent years, the novel physical phenomena exhibited by ZrTe5 under strong magnetic fields have also attracted widespread attention in academia.
In this work, the researchers conducted in-depth studies on the thermoelectric effects (Seebeck effect and Nernst effect) of ZrTe5 single crystal samples under a strong magnetic field. The experiment observed the abnormal Nernst effect caused by Dirac fermions at low temperature. With the enhancement of the magnetic field, the Seebeck coefficient and the Nernst effect of ZrTe5 show obvious quantum oscillations. When the magnetic field reaches 5.2 Tesla, all electrons of ZrTe5 occupy the lowest Landau energy 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 / Weir semimetal will increase unsaturatedly with the increase of the magnetic field, but the Seebeck coefficient of ZrTe5 is abnormally "swollen" above 5 Tesla. .
In order to reveal the mechanism of the abnormal thermoelectric effect of ZrTe5 above the quantum limit, Zhang Jinglei continuously improved the test scheme to overcome the difficulties of vibration and noise of the water-cooled magnet, and successfully achieved the single crystal sample under the strong magnetic field of up to 33 Tesla Thermoelectric effect measurement. The experiment found that the Seebeck coefficient of ZrTe5 reached a minimum at the critical magnetic field B * of 14 Tesla, converged to zero, and then began to rise as the magnetic field increased (Figure (a)). At the same time, the sign of the ZrTe5 Nernst signal changes around this critical magnetic field (Figure (b)). Theoretical calculations show that the ZrTe5 body conduction band and valence band have the characteristics of band reversal. After entering the quantum limit, the zeroth Landau energy level of ZrTe5 electrons and holes will meet at the critical magnetic field (as shown in (c)). Show). Therefore, in the vicinity of the critical magnetic field, the ZrTe5 Seebeck coefficient has a valley of zero, and the sign of the Nernst signal changes. Under normal circumstances, as a characteristic of topological non-trivial posture, the characteristic of band reversal is difficult to directly detect through electrical transport experiments. This work shows that the measurement of the thermoelectric effect under a strong magnetic field can be used as a direct experimental method to detect the band reversal of topological materials.
The research was strongly supported by the National Natural Science Foundation of China, the Youth Promotion Association of the Chinese Academy of Sciences, the research and development equipment of the Chinese Academy of Sciences, and the innovation project cultivation fund of the Hefei Material Science and Technology Center.
Figure: ZrTe5 (a) Seebeck coefficient and (b) Nernst effect as a function of magnetic field. (c) Theoretical calculation of the relationship between ZrTe5 Landau energy level and magnetic field. Upper right: Thermoelectric effect test device for water-cooled magnets.
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