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Exploring an unusual state in the superconductor-insulator transition

The superconducting state, in which current flows with zero electrical resistance, has entranced physicists since its revelation in 1911. It has been broadly considered as a result of its expected applications and to get a better understanding of quantum phenomena. Although scientists know considerably more about this particular state now than in the twentieth century, there seems to be no limit to the mysteries that superconductors hold.

A relevant example is a superconductor-insulator transition (SIT) in two-dimensional (2-D) materials. Suppose one cools down thin films of certain materials to near absolute zero temperature and applies an external magnetic field. In that case, the effects of thermal fluctuations are suppressed enough so that purely quantum phenomena (such as superconductivity) dominate macroscopically.

According to quantum mechanics, 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.

Until now, the source of this baffling intermediate state has evaded scientists for more than twenty years.

Thus, scientists from the Department of Physics at Tokyo Tech, Japan, recently set out to find an answer to the question of why a strange metallic state appears in the superconductor-insulator transition in 2-D superconductors.

Assistant Professor Koichiro Ienaga, who led the study, explains their motivation said, “There are theories that try to explain the origin of dissipative resistance at zero temperature in 2-D 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.”

Scientists used a thin film of an amorphous molybdenum-germanium (MoGe). The film was cooled down to a shallow temperature of 0.1 K. Scientists then applied an external magnetic field on the film and 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 outcomes uncovered that something important about the anomalous metallic state’s nature: 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.

Notable, 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 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 long-standing puzzles in physics, and this study puts us one step closer to understanding the SIT for 2-D superconductors.

Ienaga, who led the study, said, “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.”

Despite that the study sheds light on the two-decade old SIT mystery, further research will be required to get a more precise understanding of the quantum vortices’ contributions in the strange metallic state.

Journal Reference:
  1. K. Ienaga et al., Quantum Criticality inside the Anomalous Metallic State of a Disordered Superconducting Thin Film, Physical Review Letters (2020). DOI: 10.1103/PhysRevLett.125.257001

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