Unusual state in the superconductor-insulator transition

Field-induced SMIT detected by resistance measurements.

Scientists at Tokyo Institute of Technology (Tokyo Tech) approach the two decade-old mystery of why an anomalous metallic state appears in the superconductor-insulator transition in 2-D superconductors. Through experimental measurements of a thermoelectric effect, they found that the quantum liquid state of quantum vortices causes the anomalous metallic state. The results clarify the nature of the transition and could help in the design of superconducting devices for quantum computers.

The superconductor-insulator transition (SIT) in two-dimensional (2-D) materials is an amazing phenomena. If one cools down thin films of certain materials to near absolute-zero temperature and applies an external magnetic field, the effects of thermal fluctuations are suppressed enough so 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 intervening between both phases.

The scientists employed an amorphous molybdenum-germanium (MoGe) thin film cooled down to an extremely low temperature of 0.1 K and applied an external magnetic field. They measured a traverse thermoelectric effect through the film called the “Nernst effect,” which can sensitively and selectively probe superconducting fluctuations caused by mobile magnetic flux. The results revealed something important about the nature of the anomalous metallic state: the “quantum liquid state” of quantum vortices causes the anomalous metallic state. The quantum liquid state is the peculiar state where the particles are not frozen even at zero temperature because of the quantum fluctuations.

Most importantly, the experiments uncovered 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. 

Detecting superconducting fluctuations with precision in a purely quantum regime opens a new way to next-generation superconducting devices, including qubits for quantum computers. (Tokyo Tech and Phys.org)

The study has been published in Physical Review Letters.

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