Team from The University of Tokyo discovers nonlinear transverse response, superconducting rectification effect in noncentrosymmetric trigonal crystal

A research group led by graduate student Yuki Itahashi, School of Engineering, Assistant Professor Toshiya Ideue, Department of Applied Physics, the University of Tokyo, and Professor Yoshihiro Iwasa, in collaboration with groups at Saitama University and the Tokyo Institute of Technology, has discovered that the layered superconductor PbTaSe₂ (a single crystal composed of lead: Pb, tantalum: Ta, selenium: Se) with three-fold rotation symmetry and broken spatial inversion symmetry has been found to exhibit giant rectification properties even in the absence of an external magnetic field or magnetic ordering.

The current rectification effect that occurs in a single substance is a new phenomenon that has the potential to be the principle for superconducting diodes as it can be realized in special states, such as superconductivity, unlike the rectification properties that occur in conventional semiconductor junction structures. Although rectification effects have been reported in superconductors, they all require the application of a magnetic field.

In this research, the group has successfully observed a new superconducting rectification effect that does not require an external magnetic field by applying the findings of previous studies to precise measurements of electrical conduction in a PbTaSe₂ superconductor, which has noncentrosymmetric trigonal symmetry. Based on this, they measured material-specific rectification properties in PbTaSe₂ that do not require an external magnetic field.

The group subsequently observed the characteristic rectification effect reflecting crystal symmetry in both the normal (non-superconducting) and superconducting states and found that the magnitude of the rectification effect is enhanced to a greater extent in the superconducting state than in the normal state. To interpret the observed superconducting rectification effect, the group formulated a vortex-antivortex motion in a superconductor with threefold rotational symmetry and proposed a new principle for superconducting diodes in which the asymmetry of the motion generates superconducting rectification properties.

This suggests that the rectification effect is a transport phenomenon that reflects the detailed dynamics of the superconducting vortex and can be an important tool for understanding the excited states and vortex dynamics of superconductors with broken spatial inversion symmetry.

"Moving forward, we hope to realize similar effects in various superconductors, improve the efficiency of rectification, and search for other unique superconducting functions, which will lead to the realization of electronic circuits with extremely low energy loss," says Dr. Ideue.

■ Rotational symmetry: The property of a material to have a state that does not change when it is rotated around an axis by a specific angle. Threefold rotational symmetry is the kind of symmetry that equilateral triangles have, which does not change when rotated 120 degrees around an axis.

■ Vortex-anti-vortex: Generally, magnetic flux is excluded inside a superconductor (Meissner effect), but in a Type II superconductor, increasing the external magnetic field allows quantized magnetic flux to enter the interior. This is called a vortex. On the other hand, it is possible to generate a pair of vortexes and an antivortex, a quantized magnetic flux in the opposite direction, in a superconductor to which no magnetic field is applied. Although it is mainly noticeable in two-dimensional superconductors, such excitations are also expected to exist in layered materials near the superconducting transition temperature or under conditions of large current flow.