Controllable strain-driven topological phase transition and dominant surface-state transport in HfTe5

Abstract

The fine-tuning of topologically protected states in quantum materials holds great promise for novel electronic devices. However, there are limited methods that allow for the controlled and efficient modulation of the crystal lattice while simultaneously monitoring the changes in the electronic structure within a single sample. Here, we apply significant and controllable strain to high-quality HfTe5 samples and perform electrical transport measurements to reveal the topological phase transition from a weak topological insulator phase to a strong topological insulator phase. After applying high strain to HfTe5 and converting it into a strong topological insulator, we found that the resistivity of the sample increased by 190,500% and that the electronic transport was dominated by the topological surface states at cryogenic temperatures. Our results demonstrate the suitability of HfTe5 as a material for engineering topological properties, with the potential to generalize this approach to study topological phase transitions in van der Waals materials and heterostructures.

Probing the Spatial Homogeneity of Exfoliated HfTe5 Films

“…Due to the sensitivity of the transition metal pentatellurides’ electronic structure to lattice strain, our findings have important consequences for electronic transport measurements on such contacted and exfoliated films. Several studies provided evidence, including photoemission spectroscopy and electron transport, that the fundamental gaps of HfTe 5 , and similarly ZrTe 5 , can be strain-tuned across a weak to a strong topological insulator phase. ,,, Moreover, the location of additional, trivial carrier pockets depends on strain as well. , Therefore, our results may explain partly the considerable variation of results seen in the literature. While some studies reported a quasi-quantized Hall resistance of gapped Dirac Fermions with well-developed Shubnikov-de-Haas oscillations, , other studies reported signatures of anomalous Hall effects with largely suppressed Shubnikov-de-Haas oscillations, ,, sometimes even in nominally identical materials, and further studies described the transport features taking into account additional carrier pockets. − Based on the substantial inhomogeneity of exfoliated films observed here, it may be advantageous to carefully and systematically differentiate between measurements on bulk crystal (mm-scale) and exfoliated films (μm-scale).…”

“…On the one hand, our results imply that inhomogeneities within the films have important implications for the interpretation of sometimes seemingly disparate results on microscale contacted films. On the other hand, we argue that Raman microscopy can be a valuable tool to pre- or postselect films with optimized homogeneity for further advanced experiments, e.g., on the topological properties of the materials and their circuits. , …”

“… 9 , 16 − 18 The topological character of the gap has been shown to be strain-tunable with evidence from band structure calculations as well as transport and spectroscopic experiments. 19 − 21 While these studies demonstrate the general possibility to dynamically and reversibly control transitions between topological phases via strain in HfTe 5 and ZrTe 5 , the important aspect of sample homogeneity in micro- to nanoscale devices has not been elucidated so far.…”

“…Particularly for weakly gapped or semimetallic systems, applied strain induces topological band inversions already by comparatively small shifts of band energies . In this context, the transition metal pentatelluride HfTe 5 , and similarly ZrTe 5 , is an interesting material with a low-energy bulk dispersion described by weakly gapped 3D Dirac Fermions at the Γ-point with gap sizes on the order of 5–10 meV, − with low carrier densities down to 8 × 10 15 cm –3 , and with low-temperature mobilities reaching 1 × 10 6 cm 2 V –1 s –1 . ,, Notably, their electronic structure resides at the transition between a strong and weak topological insulator. ,− The topological character of the gap has been shown to be strain-tunable with evidence from band structure calculations as well as transport and spectroscopic experiments. − While these studies demonstrate the general possibility to dynamically and reversibly control transitions between topological phases via strain in HfTe 5 and ZrTe 5 , the important aspect of sample homogeneity in micro- to nanoscale devices has not been elucidated so far.…”

“…On the other hand, we argue that Raman microscopy can be a valuable tool to pre- or postselect films with optimized homogeneity for further advanced experiments, e.g., on the topological properties of the materials and their circuits. 20 , 21 …”