Antiperovskite oxide superconductor Sr3−xSnO was measured by a method called muon spin rotation (μSR). It became clear that the magnetic field penetration depth is abnormally long.
Sr3−xSnO is a superconductor discovered in our laboratory in 2016. It is the first superconductor in the antiperovskite oxide, and the possibility of topological superconductivity has been theoretically proposed. We conducted a μSR experiment to investigate the properties of the superconducting state in detail. μSR is a method of irradiating a sample with muons (μ particles) with aligned spins in a magnetic field and measuring the time it takes for the spins to dissipate. Since the spin precesses under a magnetic field (the same phenomenon that a spinning top turns around before it collapses), when the direction of the spin is measured with time, the oscillation shown in the figure at the top of this page is observed. The horizontal axis of the figure is time, and when the asymmetry in the vertical axis is positive, the spin is forward, and when it is negative, the spin is backward. Since the magnetic flux is quantized in the superconducting state, the magnitude of the magnetic field varies slightly depending on the location of the sample. Therefore, it seems that the amplitude of the precession decreases as a whole because each muon precesses at different period. In the figure at the top of the page, the amplitude does not change in the red data obtained at 7 K above the superconducting transition temperature, whereas the amplitude decreases with time in the blue data obtained at 1.6 K below the transition temperature.
The figure below shows “how fast the amplitude decreases” while changing the temperature (horizontal axis is temperature, vertical axis σ is the rate of decrease). It can be seen that the decrease is faster from around 5 K, which is the superconducting transition temperature. From this result, it was found that the superconductivity of Sr3-xSnO originates from the bulk (most parts of the samples are superconducting). ..
The rate of decrease in amplitude is related to the thickness of the magnetic flux, that is, the penetration depth of the magnetic field. From this μSR experiment, the magnetic penetration depth of Sr3-−xSnO was estimated to be 320 nm or more. Considering the transition temperature of 5 K, this magnetic penetration depth is longer than other superconductors. Since the long magnetic penetration length means that the carrier density is small, Sr3-−xSnO is said to have “a high transition temperature of 5 K in spite of a small number of carriers.”
The paper can be downloaded from the links below. If you are not a researcher and have not subscribed to Physical Review B, please use the Kyoto University Research Information Repository KURENAI.
Physical Review B, 101 (17), pp. 174503, 2020.