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 the complex structure of unconventional order parameters have an impact on the Josephson effects, but also may profoundly alter the quasi-particle excitation spectrum near a junction. Here, by using spectroscopic-imaging scanning tunneling microscopy, we visualize the spatial evolution of the local density of states (LDOS) near twin boundaries (TBs) of the nodal superconductor FeSe. The $\pi/2$ 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 $\xi_{ab}$. The modification of the low-energy states is even more prominent in the region between two neighboring TBs separated by a distance $\approx7\xi_{ab}$. In this region the spectral weight near the Fermi level ($\approx\pm$0.2~meV) 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.
Highlights
When two superconductors are in close proximity, they are influenced by each other via the tunneling of Cooper pairs
We report on the visualization of the atomic scale variation of the quasiparticle states of the nodal superconductor FeSe near twin boundaries (TBs) that enforce a sign inversion of the superconducting gap
The low-energy excitation spectrum is affected by the TB over an extremely long distance, which is a few tens of times larger than ξab
Summary
When two superconductors are in close proximity, they are influenced by each other via the tunneling of Cooper pairs. The Cooper pair tunneling results in the flow of a superconducting Josephson current, which has been studied for decades and is used in various superconducting quantum devices [1]. The Josephson current is governed by the phase difference of the order parameters of the two superconductors. Josephson junctions consisting of unconventional superconductors, where the superconducting order parameter changes its sign depending on the momentum direction, serve as a unique platform where novel phase-related phenomena, e.g., spontaneous. Formation of half-flux quanta in a trijunction of cuprate superconductors [2], take place. Compared to the wellinvestigated Josephson currents, the spatial and energy dependence of the superconducting order parameter and quasiparticle states around these junctions remain to be understood
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