"There are theories that try to explain the origin of dissipative resistance at zero temperature in 2D superconductors, but no definitive experimental demonstrations using resistance measurements have been made to unambiguously clarify why the SIT differs from the expected quantum phase transition models."Koichiro Ienaga, Tokyo Institute of Technology
A team of scientists at Tokyo Institute of Technology (Tokyo Tech) in Japan has gone some way to solving the two decade-old mystery of why an anomalous metallic state appears at the superconductor-insulator transition in 2D superconductors. Through experimental measurements of a thermoelectric effect, they found that this anomalous metallic state is caused by the 'quantum liquid state' of quantum vortices. Their findings, reported in a paper in Physical Review Letters, clarify the nature of the transition and could help in the design of superconducting devices for quantum computers.
The superconducting state, in which current flows with zero electrical resistance, has fascinated physicists since its discovery in 1911. It has been extensively studied, not only because of its potential applications but also to gain a better understanding of quantum phenomena. Though scientists know much more about this peculiar state now than they did in the 20th century, there seems to be no end to the mysteries of superconductors.
A famous, technologically relevant example is the superconductor-insulator transition (SIT) in two-dimensional (2D) materials. If thin films of certain materials are cooled to near absolute-zero and an external magnetic field is applied, thermal fluctuations are suppressed enough that purely quantum phenomena (such as superconductivity) dominate macroscopically. Although quantum mechanics predicts that the SIT is a direct transition from one state to the other, multiple experiments have shown the existence of an anomalous metallic state between these two states in 2D superconductors.
The origin of this mysterious intermediate state has eluded scientists for over two decades, which is why a team of scientists from the Department of Physics at Tokyo Tech set out to find an answer.
"There are theories that try to explain the origin of dissipative resistance at zero temperature in 2D superconductors, but no definitive experimental demonstrations using resistance measurements have been made to unambiguously clarify why the SIT differs from the expected quantum phase transition models," said Koichiro Ienaga, an assistant professor at Tokyo Tech, who led the team.
For this study, the scientists employed an amorphous molybdenum-germanium (MoGe) thin film. They cooled this film down to an extremely low temperature of 0.1K and applied an external magnetic field. Next, they measured a traverse thermoelectric effect known as the 'Nernst effect' through the film, which allowed them to sensitively and selectively probe superconducting fluctuations caused by mobile magnetic flux.
Their results revealed something important about the nature of the anomalous metallic state: it's caused by the 'quantum liquid state' of quantum vortices. The quantum liquid state describes the peculiar situation where subatomic particles are not frozen, even at zero temperature, because of quantum fluctuations.
Most importantly, the experiments revealed that the anomalous metallic state emerges from quantum criticality; the peculiar broadened quantum critical region at zero temperature corresponds to the anomalous metallic state. This is in a sharp contrast to the quantum critical 'point' at zero temperature in the ordinary SIT.
Phase transitions mediated by purely quantum fluctuations (quantum critical points) have been a long-standing puzzle in physics, and this study brings scientists one step closer to understanding the SIT for 2D superconductors. "Detecting superconducting fluctuations with precision in a purely quantum regime, as we have done in this study, opens a new way to next-generation superconducting devices, including q-bits for quantum computers," said Ienaga.
Now that this study has shed light on the two-decade old SIT mystery, further research will be required to obtain a more precise understanding of the contributions of quantum vortices to the anomalous metallic state.
This story is adapted from material from Tokyo Institute of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.