A recent study by scientists at the Paul Scherrer Institute PSI demonstrates how spatially resolved X-ray diffraction can image electrical switching in ultrathin cryogenic memory devices. By tracking how the atomic layers of a van der Waals crystal reorganize during device operation, the research provides insights into a new class of ultraefficient memory elements.
Large-scale facilities like the Swiss Light Source SLS synchrotron at PSI are essential for characterizing quantum materials and next-generation electronics. A recent example comes from the groups of Simon Gerber at the PSI Center for Photon Science and Dragan Mihailovic at the Jožef Stefan Institute in Slovenia where the team peeked inside a switchable electronic device using the microXAS beamline of the SLS.
Ultrathin layered crystal flakes were prepared by exfoliation in the PICO clean room of PSI. "Their electronic properties can be flipped with high efficiency and speed between insulating off and conducting on states," says Corinna Burri, first author of the study. At low temperature, the ground state is insulating, but light or current pulses can make the crystal flakes electrically conducting. By tracking how atom layers rearrange during this process, the team has now learned how to control the switching more effectively. This behaviour is promising for future devices, such as the cryogenic memory needed for the classical control electronics of solid-state quantum computers," adds Corinna Burri.
As traditional silicon electronics approach their physical limits, researchers are increasingly exploring alternative materials with tunable electronic properties that require minimal energy to change. "van der Waals materials - composed of weakly bound atomic layers - are especially promising," explains Simon Gerber. Some of these materials host a variety of correlated electronic phases that can be switched with short electrical or optical pulses, making them candidates for highly energy-efficient memory devices.
The compound 1T-TaS₂ is such a layered van der Waals material. At low temperatures, it forms an electrically insulating state that is governed by a collective ordering of electrons known as a charge-density wave. Remarkably, during the last decade Dragan Mihailovic's team in Slovenia has established that this insulating ground state can be transformed into a long-lived metallic “hidden” state by applying an ultrashort light or current pulse. "This switchable behavior has been demonstrated in transport measurements, but a central question has remained unanswered: Where and how does the switching actually occur inside the material?" notes Corinna Burri.
In many conventional memory devices, switching is driven by the formation of narrow, filament-like conduction paths that concentrate current in small regions. Whether a similar mechanism is at work in 1T-TaS₂ cryomemory devices, or whether switching instead involves a collective reorganization of the crystal, has important implications for device reliability, scalability, and efficiency. But addressing this question requires a way to look inside a functioning device without destroying it.
To tackle this challenge, Corinna Burri fabricated ultrathin 1T-TaS₂ flakes into microscopic devices in the PICO clean room of PSI. The devices were then cooled to cryogenic temperatures, and a combination of electrical transport measurements with spatially resolved X-ray diffraction and fluorescence was performed at the microXAS beamline of the SLS. While short current pulses were applied to switch the device, a tightly focused X-ray beam scanned across the samples, enabling the team to map both the electronic state and the atomic structure in three dimensions.
The study reveals that electrical switching of 1T-TaS₂ differs fundamentally from the filamentary process of conventional memories. Rather than forming localized conductive paths, the metallic hidden state appears as an extended, well-ordered region that spans a significant volume of the device. This region penetrates deeply into the material and even extends beneath the metal electrodes, showing that switching is a bulk, collective process involving many atomic layers.
© Paul Scherrer Institute PSI/Corinna Burri
By reconstructing the stacking of the van der Waals layers before and after switching, the team showed that electrical pulses change how the layers are arranged on top of each other. This structural rearrangement stabilizes the metallic state and explains its nonvolatile character at low temperatures.
Simon Gerber adds that "beyond the insights into the nonthermal switching process of the material 1T-TaS2, this work highlights the power of non-destructive X-ray imaging for the development of next-generation electronic devices." Being able to visualize where and how switching occurs inside an operating device provides a new level of feedback for device design and optimization.
Text: Corinna Burri and Simon Gerber
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Original publication
Imaging of electrically controlled van der Waals layer stacking in 1T-TaS2
C. Burri, N. Hua, D. Ferreira Sanchez, W. Hu, H. G. Bell, R. Venturini, S.-W. Huang, A. G. McConnell, F. Dizdarevic, A. Mraz, D. Svetin, B. Lipovsek, M. Topic, D. Kazazis, G. Aeppli, D. Grolimund, Y. Ekinci, D. Mihailovic, and S. Gerber
Nature Communications 16, 10296 (2025)
DOI: 10.1038/s41467-025-65212-1