Solid state physics
A candidate excitonic insulator under pressure

Insulators, by definition, do not conduct electrical current (in theory), and have a high electrical resistivity. Still, physicists distinguish between various types of insulators, differing in how the insulating states come about. The most common insulators are materials in which electrons cannot flow freely; too much energy would be required to ‘unbind’ them. Other types include Anderson insulators, in which electrons are ‘stuck’ because of quantum interference effects, and topological insulators, which are actually conducting at their surface. But one type of insulator, the so-called excitonic insulator, is particularly special — because it has never been unambiguously observed.

In an excitonic insulator, a low number of normally mobile electrons spontaneously bind (to so-called electron holes, having a positive charge) and become immobile. They were predicted to exist in the 1960s, and have been looked for in experiments ever since. Recently, encouraging possible signatures of the elusive excitonic insulating state have been observed in a layered material containing tantalum, nickel and selenium, with the chemical formula Ta₂NiSe₅. Now, Kazuyuki Matsubayashi from the University of Electro-Communications, Tokyo, Japan, and colleagues have probed the properties of this material under pressure. Their results help to close in on settling the question whether Ta₂NiSe₅ is an excitonic insulator or not — and excitingly, they show that it probably is.

First, the researchers measured the electrical resistivity of a Ta₂NiSe₅ crystal along the three main (crystallographic) directions at ambient pressure while varying the temperature. For all three directions, the resistivity dropped with increasing temperature, with an anomalous ‘kink’ at around TC = 53 °C, the alleged temperature marking the transition to the excitonic insulating state.

The scientists then measured the temperature dependence of the resistivities with increasing applied pressure. Up to around 3 GPa, they obtained the same qualitative picture as before. But for higher pressures, resistivities first increased with increasing temperature, after which an anomalous decrease set in at around T* = -100 °C.

The results of Matsubayashi and colleagues led to two insights. First, the anomalies at T_c and T^* probably have the same origin, suggesting that also at high pressure, the insulating excitonic state can develop. Second, by looking at the temperature dependence of the resistivity ratios, it becomes clear that dimensionality aspects play an important role in the formation of excitons — within certain layers of atoms, the conductivity is significantly different from that in the perpendicular direction. Rightly so, the researchers concluded that “these results deserve further high-pressure study to complete the phase diagram on Ta₂NiSe₅”

Publication and Affiliation

• H. Arima et al, Resistive anisotropy of candidate excitonic insulator Ta2NiSe5 under pressure, J. Phys.: Conf. Ser. 1609 012001, (2020), DOI: 10.1088/1742-6596/1609/1/012001
• Affiliation: Kazuyuki Matsubayashi, Department of Engineering Science, The University of Electro Communications, Tokyo 182-8585, Japan
• Department website: http://www.m-lab.es.uec.ac.jp/Matsubayashi_Group/Home.html