Blending Superconductivity and Magnetism to
Create Composite Quantum Excitations

Christos Panagopoulos and Alexander Petrović

The team of Professor Christos Panagopoulos recently created a novel "hybrid" material in which superconductivity and magnetism interact via their respective topological solitons:vortices and skyrmions. This interaction is achieved by experimentally coupling chiral magnetism and superconductivity in [IrFeCoPt]/Nb heterostructures. They also detected a thermally tunable Rashba-Edelstein exchange coupling in the isolated skyrmion phase. This realization of a strongly interacting skyrmion-(anti)vortex system opens a new path toward controllable topological hybrid materials (Fig. 5).

Fig. 5: A novel material combining superconductivity with magnetism was developed and studied in Professor C. Panagopoulos' laboratory. The cryostat pictured here maintains the material at milliKelvin temperatures while ultra-low noise electrical transport and spectroscopy measurements are performed.

Chiral magnets and superconductors host topological excitations known as skyrmions and vortices, whose duality was recognized in the 1980s [5]. Topological solitons are particle-like phenomena which are unusually stable against external perturbations. Just as a knot on a string cannot be undone by pulling on the ends of the string, a topological soliton cannot be easily destroyed by disturbing the material. While this may sound like an esoteric concept, various types of topological solitons are known to exist in different materials, from magnets to protein chains.

Two types of topological solitons which are of special interest to physicists are superconducting vortices and magnetic skyrmions. Vortices are topological solitons which appear in many superconductors—materials that conduct electricity with zero resistance—upon exposure to a magnetic field. They consist of tubes of magnetic flux, surrounded by nanometer-sized, tornado-like swirls of electric current. Skyrmions, on the other hand, are topological solitons which appear in certain magnetic materials. Ordinary magnets are composed of nanoscopic magnetic "spins" which all point in the same direction. In magnets displaying a chiral instability however, the spins prefer to twist relative to their neighbors rather than aligning, thus creating intricate patterns called skyrmions.

In a paper published in Physical Review Letters [6, 7], the team of Professor Panagopoulos announced the successful combination of two different topological solitons—vortices and skyrmions—in a material. This breakthrough is exciting because theoretical physicists have predicted that combining magnetism and superconductivity in this manner can yield Majorana fermions—exotic particles that act as their own antiparticles. If a superconductor is brought into proximity with a chiral magnet containing skyrmions, the field from the magnet is expected to change the symmertry of the electron pairing responsible for superconductivity, resulting in the formation of a "topological superconductor" whose vortices each contain a single Majorana fermion.

Majorana fermions have potentially revolutionary implications for quantum computing. In a Majorana-based quantum computer, information can be encoded using pairs of Majorana fermions (i.e., two vortices). Such pairs would be much more resilient to environmental disturbances than the quantum bits used in existing quantum computers.

A major challenge was that magnetic vortices and skyrmions do not simultaneously appear, let alone interact, in ordinary materials. To get around this, the team developed a hybrid material combining niobium, a known superconductor, with an atomically-precise magnetic multilayer made of iridium, iron, cobalt and platinum.

This is a completely new material architecture exploiting a stack of magnets and a superconductor. The carefully-designed properties of this structure allow coupling between topological solitons from two distinct quantum orders—chiral magnetism and superconductivity—for the very first time.

The properties of the new material were investigated using many different scientific methods in a joint effort by researchers in Nanyang Technological University, Singapore, the University of Geneva in Switzerland, the Technion in Israel, and the University of Antwerp in Belgium. Each of the different analytical techniques was chosen to expose a different aspect of the hybrid behavior. Electronic snapshots of the superconducting vortices were taken using scanning tunneling spectroscopy at temperatures a fraction of a degree above absolute zero, while magnetic force microscopy tracked the formation and morphological evolution of skyrmions within the magnetic layer. An extremely sensitive detector known as a Superconducting Quantum Interference Device (SQUID) captured the nucleation of vortices by skyrmions, while the influence of skyrmions on the flux dynamics was deduced from the tiny voltages induced by vortices moving in an applied electrical current (Fig. 6).

Fig. 6: The first author of the article, Dr. Alexander Petrović at work on the experimental apparatus.

Computer simulations of the material matched the key experimental results, hence verifying the ability of individual skyrmions to generate vortices in the neighboring superconductor. These simulations also exposed intriguing flow characteristics for the vortices at high currents, which could enlighten studies of particle dynamics in other complex systems.

The beauty of these [IrFeCoPt]/Nb heterostructures is that they show stable vortices above elongated chiral spin textures, as well as isolated skyrmions. This is an ideal geometry in which to attempt manipulation of individual vortices, a prerequisite for computational operations. In the future, the team aims to explore the electronic properties of their samples for signatures of the anticipated Majorana fermions.