A New Route to the Realization of Topological Superconductivity

Superconductivity is a phenomenon observed in certain metals and alloys where electrical resistance vanishes at low temperatures. This property is utilized in superconducting magnets, which are currently used in applications such as MRI machines and maglev railway systems. Superconductivity is considered to originate due to the formation of Cooper pairs of electrons. Cooper pairs can have various shapes (symmetries) depending on the materials. Although the superconductors that are currently used in practical applications have isotropic s-wave symmetry, anisotropic superconductivity has attracted considerable attention in recent studies. For example, high-T_c cuprate superconductors are known to be anisotropic d-wave superconductors hosting line nodes.

Topological superconductivity, which is a kind of anisotropic superconductivity, has been energetically studied for over a decade. Topological superconductors have special surface states called Majorana states, which are theoretically considered to be applicable to quantum computing. However, few superconductors have been experimentally confirmed to possess the topological property so far. Therefore, further research is needed to explore setups that can realize the topological superconductivity.

In this paper, we theoretically demonstrate the usefulness of a supercurrent for controlling the quantum states in superconductors. We introduce a three-dimensional tight-binding model of a tetragonal superconductor in a D+p-wave pairing state with a finite center-of-mass momentum. The results show that topological Weyl superconductivity can be realized by applying an infinitesimal supercurrent to the noncentrosymmetric spin–orbit-coupled superconductors hosting line nodes.

The Weyl superconductivity has point nodes (Weyl points) that are protected by a topological monopole charge. Ordinarily, the structure of the superconducting nodes does not vary greatly since this property is inherent in each superconductor. Nevertheless, the above result indicates that the line-nodal to point-nodal transition occurs due to the cooperation between the applied supercurrent and the spin–orbit coupling arising from the inversion symmetry breaking. This transition drastically changes the topology of the wavefunction in the superconductor. Indeed, the point nodes are Weyl nodes characterized by the monopole charge, and surface arc states connecting the Weyl nodes appear. We also propose CeRhSi₃ and CeIrSi₃ as candidate superconductors for the supercurrent-induced Weyl superconductivity.

This study proposes the use of supercurrent as a possible method to realize topological superconductivity, which could potentially lead to further developments, such as quantum computing and some electronics/spintronics devices, using the surface states obtained by the controlling method.

(written by S. Sumita on behalf of all authors.)