The transition metal pentatellurides like ZrTe₅ and HfTe₅ have attracted considerable interest since the last 70s, because they exhibit unusual transport properties characterized by a strong resistivity peak at ~135K accompanied by a sign reversal of the Hall coefficient and thermopower across the peak temperature. The origin of such transport property anomalies has been a subject of a long-time debate but remains unclear.

Recently, it was predicted that single-layer ZrTe₅ is a two-dimensional and large bulk gap topological insulator, while bulk ZrTe₅ may host a possibility of realizing a temperature-driven topological phase transition between the weak and strong topological insulators. Many experimental results have been reported to determine the topological nature of ZrTe₅, but it remains controversial.

By using a newly developed & latest-generation angle resolved photoemission spectroscopy (ARPES) system equipped with the 6.994 eV vacuum ultraviolet laser and the time-of-flight electron energy analyser (ARToF 10K by Scienta Omicron), research team led by Prof. LIU Guodong and Prof. ZHOU X. J., in theoretical collaboration with Prof. ZHONG Fang and XI Dai’s group, systematically studied the electronic structure of ZrTe₅ and its temperature evolution.

The researchers acquired high quantity Fermi surface and band structure in ZrTe₅. They found that the overall electronic structure exhibits a marked temperature dependence that shifts down with decreasing temperature.

This results in an electronic state evolution from a p-type semimetal with a hole-like Fermi pocket at high temperature above 135K, to a semiconductor around 135K where its resistivity shows a peak, to an n-type semimetal with an electron-like Fermi pocket at low temperature.

These results constitute direct electronic evidence of a temperature-induced Lifshitz transition in ZrTe₅, which provide a natural explanation on the underlying origin of the resistivity anomaly and the sign reversal of carrier.

In addition, they observed quasi-one-dimensional electronic features that may be associated with the edge states of ZrTe₅. Such an observation, together with the persistence of bandgap opening, absence of Dirac surface states and no topological phase transition over the entire temperature range they measured, indicates that ZrTe₅ is a weak topological insulator.

This study entitled "Electronic evidence of temperature-induced Lifshitz transition and topological nature in ZrTe5" was published in Nature Communications.

The study was supported by the National Science Foundation of China, the 973 project of the Ministry of Science and Technology of China and the Strategic Priority Research Program (B) of the Chinese Academy of Sciences.

Fig. 1 Time-of- flight ARPES system (Image by Institute of Physics) 

Fig. 2 Fermi surface and band structure of ZrTe₅ measured at 195K (Image by Institute of Physics) 

Fig. 3 Temperature evolution of the band structure in ZrTe₅ (Image by Institute of Physics) 

Fig. 4 Temperature induced Lifshitz transition in ZrTe₅ (Image by Institute of Physics) 

Fig. 5 Observation of weak topological insulator feature in some ZrTe₅ samples (Image by Institute of Physics)