By Carey Sargent, EPFL, NCCR MARVEL
The Dirac semimetal zirconium pentatelluride (ZrTe5) has been the subject of much research in recent years because of its curious properties. Originally attracting attention due to a resistivity anomaly—a sharp peak of resistivity at finite temperatures—more recent theoretical studies have focused on topological properties, with researchers subsequently proposing that the ZrTe5 monolayer is a large gap quantum spin Hall insulator.
In the past few years, experimental research on bulk and thin layers of the material have demonstrated topological phenomena including the magnetochiral effect, the giant planar Hall effect, pressure-induced superconductivity and optical conductivity linear in photon energy. These experimental observations contrast, however, with our current understanding of low-energy excitations in ZrTe5 and researchers have not reached a consensus on whether the material is a strong or weak topological insulator (STI or WTI).
Ab initio calculations suggest more strongly that it is an STI, but this result may not be entirely reliable in systems such as ZrTe5: theoretical studies have suggested that the bulk material is close to a topological phase transition, perhaps explaining why researchers have not been able to arrive at a consensus on the nature of the material. Experimentally, transport, ARPES and optical data have suggested both STI and WTI phases.
In the paper “Temperature dependence of the energy band gap in ZrTe5: Implications for the topological phase,” recently published in Physical Review B, researchers including NCCR MARVEL’s Ana Akrap, professor at the Department of Physics at the University of Fribourg, used THz/infrared magnetospectroscopy to investigate the temperature dependence of the band gap in ZrTe5. This information allowed them, in turn, to test the material’s topological phase. They found that the gap is nonzero at low temperatures, and increases with T monotonically.
In interpreting their results, the researchers note that, assuming that the material is close to a topological phase transition, only a relatively small increase in the unit-cell volume is theoretically needed to move ZrTe5 from the STI to the WTI regime. It should be noted too that the band gap always closes and reopens during such a topological phase transition.
Experimentally, it has been shown that the unit-cell volume in ZrTe5 increases monotonically with temperature. The temperature dependence of the band gap was subsequently proposed to be an unambiguous signature of the topological phase in ZrTe5. In the STI regime, the band gap first shrinks with increasing T and then closes completely before reopening in the WTI regime. In contrast, a monotonic increase of the band gap with temperature is expected in the WTI regime, exactly as seen in the study’s data.
I. Mohelsky, J. Wyzula, B. A. Piot, G. D. Gu, Q. Li, A. Akrap, and M. Orlita. Temperature dependence of the energy band gap in ZrTe5: Implications for the topological phase. Physical Review B 107, L041202 (2023).
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