Abstract
Junctions and interfaces consisting of unconventional superconductors provide an excellent experimental playground to study exotic phenomena related to the phase of the order parameter. Not only does the complex structure of unconventional order parameters have an impact on the Josephson effects, but it also may profoundly alter the quasiparticle excitation spectrum near a junction. Here, by using spectroscopic-imaging scanning tunneling microscopy, we visualize the spatial evolution of the LDOS near twin boundaries (TBs) of the nodal superconductor FeSe. The rotation of the crystallographic orientation across the TB twists the structure of the unconventional order parameter, which may, in principle, bring about a zero-energy LDOS peak at the TB. The LDOS at the TB observed in our study, in contrast, does not exhibit any signature of a zero-energy peak, and an apparent gap amplitude remains finite all the way across the TB. The low-energy quasiparticle excitations associated with the gap nodes are affected by the TB over a distance more than an order of magnitude larger than the coherence length . The modification of the low-energy states is even more prominent in the region between two neighboring TBs separated by a distance . In this region, the spectral weight near the Fermi level () due to the nodal quasiparticle spectrum is almost completely removed. These behaviors suggest that the TB induces a fully gapped state, invoking a possible twist of the order parameter structure, which breaks time-reversal symmetry.
- Received 12 March 2015
DOI:https://doi.org/10.1103/PhysRevX.5.031022
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Published by the American Physical Society
Popular Summary
A superconducting state is characterized by a complex order parameter whose phase plays a crucial role in superconducting junctions. Besides the well-known Josephson effect, novel phase-related phenomena may occur in junctions composed of unconventional superconductors where the phase changes its sign depending on the momentum direction. Scanning tunneling microscopy is an ideal tool for searching for such phenomena, but it can only be applied to atomically sharp junction interfaces at atomically flat surfaces. The fabrication of junctions made of unconventional superconductors has furthermore been a major challenge.
Here, we utilize twin boundaries in the unconventional orthorhombic iron-based superconductor FeSe () as naturally formed superconducting junctions; twin boundaries consist of two crystallographic domains that are mirror images of one another. We conduct spectroscopic-imaging scanning tunneling microscopy of vapor-grown single crystals at 0.4 K to examine the evolution of the local density of states.
The rotation of the crystallographic orientation across the boundary twists the structure of the order parameter, causing a zero-energy bound state to potentially appear at the boundary. Even so, we find that the bound state is always at finite energies. Together with observations that the low-energy excitations are suppressed by the twin boundary over long distances, we suggest that the twin boundary in FeSe induces an additional energy gap associated with the imaginary part of the order parameter that breaks time-reversal symmetry. This conjecture is corroborated by the strongly suppressed low-energy excitations observed between the two twin boundaries. We find that our experimental results can be well described theoretically.
Our results highlight the nontrivial phase structure of an unconventional superconductor, and we anticipate that the twin boundaries in FeSe and other unconventional superconductors will yield other phase-related quantum phenomena.