Changing the Properties of a Topologically Insulating Film through Bending

Professor Mann-Ho Cho
Department of Physics, Yonsei University

■ Natural Sciences /

Changing the Properties of a Topologically Insulating Film through Bending

A group of researchers, including Prof. Mann-Ho Cho, examines the effects of strain on three-dimensional topological insulators


Can the properties of a topologically insulating film be tuned by bending? And are such changes reversible?

Park, Chae, Jeong, Kim, Choi, Cho, Hwang, Bae, and Kang (2015) have answered these questions in their paper, “Reversible Fermi Level Tuning of a Sb2Te3 Topological Insulator by Structural Deformation,” published in Nano Letters. Building on previous theoretical and experimental work showing the influence of structural deformation on three-dimensional (3D) topological insulators, the authors examined the effect of bending on the band gap and Fermi level of Sb2Te3 film, a chalcogenide p-type 3D topological insulator.

Topological insulators have a wide range of important potential applications, for example, in quantum computing, terahertz (THz) generators, magneto–electronic sensors, and superconductors. Therefore, various attempts at tuning the properties of these materials (in particular, their Fermi levels) have been made in the past. One of the most common approaches involves chemical doping. Unfortunately, the changes obtained through this approach are irreversible; therefore, reversible tuning techniques are in high demand.

In their study, the authors investigated the changes to the band gap and Fermi level of Sb2Te3, which has weak bonding between its layers, under the influence of bending. Sb2Te3 film specimens were placed within a 500-µm-thick polyimide sheet, which was then bent to generate a low strain of less than 2%. This strain was confirmed using transmittance X-ray scattering analysis. Further, the influence of the applied strain on the Sb2Te3 topological properties was examined using magneto-resistance and temperature-dependent resistance measurements, terahertz-time domain spectroscopy, and density functional theory calculations.

The results of these tests showed that the application of tensile strain via bending lowers the bulk carrier density and enhances the transport properties of the topological surface state of the Sb2Te3. Most strikingly, the Fermi level tuning can be reversed, as the examined sheets could be folded and unfolded repeatedly. This finding introduces the possibility of using structural deformation to achieve reversible control of a topological surface state device. Let’s take a look at some of the key results of the current study in greater detail.

In-plane strain causes Fermi level shift
Different in-plane strains were applied to the sheets using different degrees of bending; then, the sheet resistance was measured. A reversible increase in this property was recorded under increased in-plane strain, which was interpreted as being due to a shift in the Fermi level towards a lower binding energy (the midgap region). In addition, noticeable differences were observed in the sheet resistance dependence on temperature for flat and bent films, e.g., a higher maximum conductance temperature shift under bending. Increased bending also corresponded to an increased rate of change in the conductance with temperature. From these tests, the authors concluded that the bending of the Sb2Te3 film caused the Fermi level to move to a lower binding energy, which led to increased sheet resistance, enhanced topological surface state transport, and decreased bulk carrier density.

The results of a 2D transport theory analysis also showed that the electron screening factor decreased with increased bending. Further, density functional theory calculations and Hall measurements supported the conclusions that the Fermi level shifted under bending and that the carrier density was decreased.

Bending causes magneto-resistance enhancement
Another striking effect generated by the shift in the Fermi level was an enhanced change in the magneto-resistance of the Sb2Te3. Both flat and bent films were examined under a vertical magnetic field and at the same temperature. Under this magnetic field, the difference in magneto-resistance for the bent film was significantly higher than that for the flat specimen. This behavior is associated with the weak antilocalization effect and carrier density, again indicating that the greater the strain applied via bending, the lower the carrier density of the Sb2Te3 film.

Topologically insulating properties are conserved
A final important finding of the study was that the strain did not have a significant effect on the topological surface state of the Sb2Te3 film. To obtain this result, the researchers measured the THz generations of the film, finding the same spectral shape up to ~1% strain.

Conclusion
The current study has revealed that mechanical deformation, i.e., bending of a chalcogenide 3D topologically insulating material, allows the Fermi level and bulk carrier density to be tuned. The most noteworthy result is that the change in sheet resistance, which is related to the above properties, is reversible, as bending and unbending of the film lead to respective increases and decreases in this property.

The findings reported by the authors have important implications for future research on 3D topological insulators. More specifically, the topological surface state transport mechanism revealed by their investigations is interesting and worthy of further study.

Updated in Feb 2017

Reference
S.H. Park, J. Chae, K.S. Jeong, T.-H. Kim, H. Choi, M.-H. Cho, I. Hwang, M.-H. Bae, C. Kang, “Reversible Fermi Level Tuning of a Sb2Te3 Topological Insulator by Structural Deformation”, Nano Letters, doi: 10.1021/acs.nanolett.5b00553