A joint research team, led by Prof. Gao Hongjun of the Institute of Physics of Chinese Academy of Sciences (CAS), has published a study in Nature on June 8. It reported the observation of a large-scale, orderly and tunable Majorana zero-mode (MZM), lattice in LiFeAs' iron-based superconductor LiFeAs. This provides a pathway to future topological quantum computation.
MZMs can be described as zero-energy bound states that are confined in topological defects such as line defects or magnetic field-induced vortices. They are characterized by scanning tunneling microscopy/spectroscopy (STM/S) as zero-bias conductance peaks. They are non-Abelian in their behavior and can be used as building blocks for topological quantum computations.
MZMs have been observed in several topologically nontrivial iron-based superconductors, such as Fe (Te0.55Se0.45), (Li0.84Fe0.16)OHFeSe, and CaKFe4As4. These materials are susceptible to alloying-induced disorder, disordered vortex lattices, as well as low topological vortice yields. All of these issues hinder further research and application.
The researchers found that the naturally strained superconductor LiFeAs formed an MZM lattice. The researchers discovered that LiFeAs naturally has local strain using STM/S equipped magnetic fields. The strain produces biaxial charge density waves (CDW) along the Fe-Fe, As-As directions. It has wavevectors of L12.7 nm & L224.3nm. The CDW with wavevector L2 exhibits strong modulation of the superconductivity LiFeAs.
The vortices are forced to align along the As-As CDW strips by a magnetic field that is perpendicularly to the sample surface. This creates an ordered lattice. Reduced crystal symmetry results in a dramatic change in topological band structures at Fermi level. This transforms the vortices into topological ones that host MZMs, and forms an MZM lattice. The external magnetic field can also tinker with the MZM lattice geometry and density. Under high magnetic fields, the MZMs begin to couple.
According to researchers, the observation of an ordered, tunable MZM Lattice in LiFeAs expands on the MZM family in iron-based superconductors. This provides a promising platform for braiding and manipulating MZMs in future.
These findings could shed light on topological quantum computation using iron superconductors.