[Paper] Unquantized superconducting vortices

日本語/English

Our new study about superconducting vortices carrying a part of magnetic flux quantum in a multiband superconductor has been published in Science on June 1, 2023. The press releases are issued by my collaborators at KTH Royal Institute of Technology and TD Lee Institute. 

Y. Iguchi et al., Science 380, 1244-1247 (2023). (Open access link)

The total magnetic flux in an isolated vortex in a single crystal of the type-II superconductors has discrete values, $n\Phi_0=nh/2e$ where n is an integer, h is the Planck constant, and e is the electron charge, because of the $2n\pi$ phase winding of Cooper pairs around the vortex. The half integer ($n=1/2$) vortex has been observed at the intrinsic Josephson Junction formed on the grain boundaries of d-wave superconductors because of the $\pi$ phase winding.

Let's consider the effect of multiband in a single crystal of the type-II multiband superconductors. We theoretically expect that the quantum phase winding is only in one of the bands at the level of Ginzburg-Landau theory, inducing fractional vortices, whose total flux may be a part of $\Phi_0$ and temperature dependent. However, this vortex type had never been observed experimentally since its first theoretical discovery in 2002.

We first report similar isolated vortices in a multiband superconductor K$_x$Ba$_{1-x}$Fe$_2$As$_2$, which carries only a part of $\Phi_0$. By manipulating vortices in K$_x$Ba$_{1-x}$Fe$_2$As$_2$ with the scanning SQUID microscope, we demonstrate that some vortices carry temperature-dependent unquantized fractions of a flux quantum, which is universal at the same temperature over millimeter scale range. These are clearly different phenomena from the half integer vortex at the intrinsic Josephson junctions.