ferromagnetic liquid blue metal
 ferromagnetic liquid blue metal

Superconductivity of ultrathin materials revealed

Quantum computing is the future generation of data processing devices, solving problems faster than classical computers do. One of the approaches is to use the quantum properties of superconductors, often in small dimensions and large magnetic fields. However, there are open questions about how the superconducting properties change at the nanometer scale. Physicists at the Institute for Molecules and Materials (IMM) of Radboud University discovered that for ultrathin aluminum, grown layer by layer on a silicon surface, the temperature at which these layers become superconducting increase as the material gets thinner. This study improves the understanding of two-dimensional superconductivity and may help the development of high-performance quantum computing devices. The results of the study have recently been published in Science Advances.

Superconductors are an essential part of our future technology, in particular to create devices used for quantum computing. Superconductors are materials that can conduct electricity without any electronic resistance, which means that electrons can move through the material without any loss of energy. The behavior of bulk superconducting materials is described by the BCS theory, stating that superconductivity occurs when two electrons are held together by lattice vibrations. However, when superconductors are very small, their behavior can change and is hard to predict.

Superconductivity of ultrathin layers

The main goal of the study was to understand how superconductivity changes in reduced dimensions. “For the case of aluminum, the superconducting state in thin films can survive up to higher temperatures compared to the bulk material, but we do not exactly know why this happens. A very thin superconductor becomes sensitive to its environment and therefore, it is hard to determine the intrinsic properties of the material”, researcher Werner van Weerdenburg explains.

The research team used state-of-the-art growth techniques to grow films of aluminum under very clean conditions. Using a scanning tunneling microscope, the morphology of these films as well as the superconducting properties could be measured. Interestingly, they found that the very thin and clean layers of aluminum show a larger temperature at which the material becomes superconducting. This finding suggests that the enhanced superconductivity state is intrinsic to aluminum. Moreover, the researchers showed that the superconducting state of these films is extremely robust against the application of in-plane magnetic fields. “The combination of high fields and superconductivity is a unique regime where unconventional types of pairing can exist. By studying the shape of the vortex in large magnetic field, we could infer the presence of such unconventional pairs.”


Structural and spectroscopic properties of ultrathin epitaxial Al films

New functionality of devices

The results of the study show how small dimensions can affect superconductors and allow for new ways to study them in large magnetic fields. This could be interesting for future devices since the superconducting properties of very thin materials can become tunable, e.g. in this study the critical temperature can be ‘tuned’ by making the material thinner or thicker. Gaining a better understanding of how superconductivity behaves in these regimes may ultimately help the development of such devices, or lead to new functionality. “So far, we managed to grow superconducting films with a minimum thickness of four atomic layers, but in principle it should be possible to grow a film with a thickness of just one atomic layer. It would be very interesting to see if the trend continues and the critical temperature increases further. We are also excited to explore the high-field regime of superconductivity further, in particular at the atomic scale”, van Weerdenburg concludes.

Scanning Probe Microscopy

Werner van Weerdenburg is researcher in the Scanning Probe Microscopy group at Radboud University. The group, led by Professor Alex Khajetoorians, is part of the IMM. The research focus is on advancing the state-of-the-art in scanning probe techniques in order to understand both fundamental and technological problems in condensed matter physics as well as in surface chemistry. “Our aim is to understand fundamental problems in material science, to ultimately find innovative approaches for technological applications.”

Article information

Science Advances publication 

Contact information

IMM Communications: imm-communication [at] ru.nl (imm-communication[at]ru[dot]nl)