New topological finding hinges on high-quality crystals

Combining exceptional crystal-growing skills with theoretical predictions, scientists from the University of Texas at Dallas (UT Dallas) and their collaborators have revealed new insights into unusual materials known as topological insulators (TIs). They report these insights in a paper in Physical Review X.

TIs are unusual because they behave like insulators in their interiors but are conductors on their exteriors. There are three distinctive families of TIs: strong TIs, which are common in nature; weak TIs, which are rare and difficult to produce in the lab; and another rare class called higher-order TIs.

In a cube-shaped, strong TI, for example, all six faces can conduct electrons robustly. In a weak TI, only four sides are conducting, while the top and bottom surfaces remain insulating. In a higher-order TI, electrons move only along selected hinges, where two crystal faces intersect.

In this new study, the scientists demonstrated clearly for the first time that crystals made from bismuth and iodine are not only weak TIs, but can also undergo a phase transition into a novel structure at room temperature that significantly alters the material’s electronic properties.

“We believe our study has provided the smoking gun evidence to claim this bismuth iodide material is a weak TI,” said Bing Lv, one of the paper’s corresponding authors and an assistant professor of physics in the School of Natural Sciences and Mathematics at UT Dallas. His team produced the large, high-quality bismuth iodide crystals that made the findings possible. “Other researchers have claimed to have produced a weak TI, but their evidence was not as strong.

“We also clearly demonstrate that with only a small temperature drop in the room-temperature range, this material undergoes a topological phase transition from a weak TI to a higher-order TI. This significantly changes the way it conducts electrons.”

“This work is important not only for potential electronic or computing applications at room temperature, but also because it represents the realization of two fundamentally new phases of matter where septillions of electrons organize themselves in two amazing ways,” said Fan Zhang, associate professor of physics at UT Dallas and a corresponding author of the paper.

While scientists have proposed various ways to construct a weak TI, experiments to produce and characterize such crystals have been inconclusive. Lv and his team produced the bismuth iodide crystals based on Zhang’s previous theoretical prediction about this new family of materials.

One of the major obstacles to definitively producing a weak TI is the need for crystals large enough and of sufficiently high quality that they can be cut open, or cleaved, in different ways that reveal 'clean' surfaces to examine. According to Zhang’s theory, these surfaces should exhibit specifically different physical properties.

“In order to confirm a weak TI, a testing crystal needs to be cleaved along its vertical and horizontal axes to expose clean surfaces,” Zhang explained. “But available TIs typically allow for cleavage along only one axis. Because of this, the experimental confirmation of a weak TI has been elusive.”

Over the past few years, Lv and his research team have refined their crystal-growing abilities and are now producing large, high-quality bismuth iodide crystals that can be cleaved along both axes, allowing for more detailed – and conclusive – characterization of the material’s physical properties.

Using these crystals, researchers from Rice University and the University of California, Berkeley employed a technique called angle-resolved photoemission spectroscopy (ARPES) to directly map the electronic band features of different cleavage surfaces, providing critical experimental evidence for Zhang’s theory. The layered crystals, which are shaped like clear plates or flat needles, present clearly distinguishable length-wise and width-wise edges, which allowed for optimal cleavage.

“It’s like looking at a book – it’s clear where the flat cover and back surfaces are, and the shorter edges are where you see the pages,” Zhang said.

The researchers found that, depending on external factors such as temperature, a single crystal can exist in different topological phases. At room temperature, the temperature range at which the crystals are grown, the material is a weak TI, where surface conduction is robust. At lower temperatures, it becomes a higher-order TI, where electrons conduct essentially along a one-dimensional path at the crystal’s hinges.

“As the temperature falls a few degrees, the layers of atoms in the crystal rearrange slightly, which switches the conduction pattern from its surfaces to the hinges,” Zhang said. “This change might be useful if you want to create a sensor, for example, that operates when the temperature changes. Figuring out how to exploit this superior property will be a task for our engineering colleagues.”

This research was funded in part by a grant from a US National Science Foundation program called Designing Materials to Revolutionize and Engineer our Future (DMREF). This program supports activities that significantly accelerate materials discovery and development by building the fundamental knowledge base needed to advance the understanding of materials with desirable properties or functionality.

DMREF is the primary program by which NSF participates in the Materials Genome Initiative, a federal effort to improve US global competitiveness in the advanced materials marketplace.

“Our DMREF team is trying to establish a quasi-one-dimensional material platform for its interesting physics. There are lots of amazing properties for these kinds of materials, and what we are reporting is not all there is,” Lv said. “There will be many more results with these particular materials – the physics is so rich – but we are also already studying additional related materials.”

This story is adapted from material from the University of Texas at Dallas, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.