In 2012, theorists predicted a new class of quantum material: the topological crystalline insulator (TCI), in which the protection of conducting edge states comes not from time-reversal symmetry — as in conventional topological insulators — but from the symmetry of the crystal lattice itself. The prediction opened a tantalising possibility: atomically thin sheets of certain materials could conduct electricity along their edges without dissipation, even at room temperature.
It has taken more than a decade to make the prediction real. Now, researchers at the University of Jyväskylä and Aalto University in Finland have fabricated the first two-dimensional topological crystalline insulator — a bilayer of tin telluride (SnTe) grown on a niobium diselenide (NbSe₂) crystal. The work was published July 11 in Nature Communications.
“This material was predicted more than ten years ago, and many groups have tried to make it,” said corresponding author Peter Liljeroth, a professor at Aalto University’s Department of Applied Physics. “The challenge was finding the right substrate to provide the necessary strain.”
Strain as a control knob
The key to the breakthrough is strain. SnTe in its bulk form is a three-dimensional TCI, but when thinned to just a few atomic layers, the topological protection is lost — unless the film is placed under compression. The Finnish team grew bilayer SnTe (four atomic layers in total, roughly 0.8 nanometres thick) on a substrate of 2H-NbSe₂ using molecular beam epitaxy, a technique that deposits atoms one layer at a time in ultrahigh vacuum. The lattice mismatch between SnTe and NbSe₂ — the two crystal structures do not quite line up — creates an intrinsic compressive strain in the SnTe film that is the key to unlocking its topological properties.
Using scanning tunnelling microscopy and spectroscopy at 4.7 kelvin, the team observed two distinct pairs of conducting edge states along the boundaries of the SnTe islands — one at low energy (around 0.5 electron-volts) and one at higher energy (around 1.55 eV). These edge states are the hallmark of a topological crystalline insulator: they are one-dimensional channels along which electrons can flow freely, even though the interior of the material is insulating.
The band gap — the energy range in which no bulk electronic states exist — measured between 0.2 and 0.3 eV, more than eight times the thermal energy at room temperature (roughly 25 meV). This is a critical threshold: it means the topological protection should persist at ordinary temperatures, without the extreme cooling required by most topological materials.
“Most topological insulators require liquid helium temperatures,” said Liwei Jing, the study’s first author and a doctoral researcher at the University of Jyväskylä. “A gap of 0.2 eV means this material could work at room temperature, which is essential for practical applications.”
Confirming the topology
The team confirmed the topological nature of the edge states through several lines of evidence. Density functional theory calculations showed that the strained bilayer SnTe undergoes a Lifshitz transition — a change in the topology of its electronic band structure — into a phase with a time-reversal-mirror Chern number of ±2, a precise topological invariant that characterises the TCI phase. Atomic-scale defects at the island edges were observed to locally break the mirror symmetry that protects the edge states, opening a small gap in their spectrum — a direct demonstration that the protection mechanism is indeed the crystal symmetry and not time-reversal symmetry.
Adjacent edge states closer than about 5 nanometres to each other were seen to hybridise, with their energies shifting as they coupled. This coupling length sets a lower bound on how closely topological channels could be packed in a device.
The group also made their computational tools available: the strain extraction code and model Hamiltonian software are published on GitHub, and the DFT data have been deposited in the NOMAD repository.
Next steps
The immediate next step is to demonstrate electrical transport through the edge channels — measuring actual conductance rather than inferring it from spectroscopy. The researchers also propose doping the SnTe with ferromagnetic atoms to create a Chern insulator, a platform for topological quantum computing, or using the superconducting nature of the NbSe₂ substrate to induce topological superconductivity and host Majorana fermions.
Several caveats apply. The definitive observation of edge states was made at cryogenic temperature (4.7 K); room-temperature operation is predicted from the band gap energy but not yet demonstrated experimentally. No transport measurements have been performed. The system relies on a specific substrate (NbSe₂) to provide the necessary strain, and integration with other platforms may require different approaches.
Nevertheless, the work closes a decade-long gap between prediction and realisation. The first 2D topological crystalline insulator now exists in a laboratory — and its edge states are stable enough to build on.
Source: Jing, L., Amini, M., Fumega, A.O. et al. “Bilayer SnTe on NbSe₂: a two-dimensional topological crystalline insulator.” Nature Communications 17, 817 (2026). DOI: 10.1038/s41467-025-67520-y