This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Superconductors, materials that can carry electric current with an electrical resistance of zero, have proved to be promising for the development of various technologies, including medical devices, particle accelerators and quantum computers. Studying these materials could help researchers uncover new physical states that could be useful for specific applications.
Magnetic fields can enter some superconductors, known as type-II superconductors, via regions called vortices. Each of these vortices carries a fixed amount of magnetic flux (i.e., a measure of the magnetic field passing through an area), with electrical currents circulating around its center. Researchers at Tsinghua University, Southern University of Science and Technology, Boston College and other institutions investigated electronic states trapped near magnetic vortex centers in an iron-based superconductor.
Their paper, published in Physical Review Letters, reports the observation of charge stripes (periodic modulations of electronic charge along one direction) that become stronger around vortex centers. These stripes were closely linked to two different types of vortex states, one of which was identified as hosting a so-called Majorana zero mode. A Majorana zero mode is a localized, zero-energy collective electronic excitation that behaves as a particle that is its own antiparticle.
Such states could be advantageous in quantum computing, as information stored across separated Majorana modes could be protected against some local disturbances. "Our work grew out of our long-term effort to understand high-temperature superconductivity with atomic-scale precision," Can-Li Song, senior author of the paper, told Phys.org. "Magnetic vortices are particularly interesting because their cores provide a natural platform for exploring emergent electronic states in these unconventional superconductors." Song and his colleagues originally set out to search for electronic states trapped in and around the center of a magnetic vortex in an iron-based superconductor.
They specifically decided to examine thin films of a type-II superconductor called cobalt-doped barium iron arsenide, with the formula Ba(Fe₀.₉₄Co₀.₀₆)₂As₂. "Our initial goal was to search for vortex bound states and possible Majorana zero modes in 122-type iron pnictides," said Song. "Unexpectedly, we discovered charge stripes closely intertwined with different vortex states, which, with insights from our theoretical collaborators, became the central focus of this work." The superconducting films studied by the researchers contain barium, iron, cobalt and arsenic, with cobalt replacing 6% of the iron sites in their crystal lattice.
The team studied the films using spectroscopic-imaging scanning tunneling microscopy, which moves an extremely fine tip just above a material's surface to map its electronic properties at different energies. "We grew optimally doped Ba(Fe0.94Co0.06)2As2 superconducting films by molecular beam epitaxy and studied them using low-temperature scanning tunneling microscopy and spectroscopy," explained Yu Liu, the first author of the paper. "By mapping the electronic states around many vortices with atomic-scale precision, we identified charge stripes and two distinct types of vortex states, including those hosting Majorana zero modes, and revealed their intimate spatial correlation." The researchers uncovered repeating electronic charge stripes in their films, which became stronger around vortex centers.
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