In our work under the TUNNEL project, we have developed new methods to study the spectrum of solid state systems, relying on the stacking of atomically thin layers, forming stacks of of 2,3,4 or more distinct materials.
In our first study, we addressed two dimensional superconductors, focusing on NbSe2, a superconductor which can be exfoliated down to the two-dimensional limit. Using these devices, we have discovered that the electrons which carry the supercurrent belong to two distinct populations, and experience different dynamics. This, in turn, affects their stability as superconducting carriers. We have measured electronic levels which reside at the center of superconducting vortices, and have found that these vortices have a unique structure, a consequence of the two-carrier nature of the material.
Using the same method, we have studied another material, the superconductor FeTe(x)Se(1-x), suspected to be a topological superconductor. We have found that the material retains an energy gap up to high magnetic fields, a property which confirms the sparsity of states within superconducting vortices it harbors.
During the project, we have discovered that within the ultrathin tunnel barriers we often find evidence for atomic defects. These defects function, electronically, like stepping stones allowing electrons traverse the gap. Since such states are sharply defined in energy, they open a number of interesting research opportunities. We have found, in one study, that electrons residing in defects may inherit properties of the nearby superconductor. This type of proximity effect gives rise to low-lying spectral features called Andreev bound states (ABS). ABS have been studied extensively in nanowires coupled to conventional superconductors. We have shown, in our work, that defects in barriers form ABS coupled to unconventional superconductors – and thus open an entirely new research avenue for the study of localized proximity of superconducting states.
Another exciting application of defect states is their use as spectrometers. Placing a defect-carrying barrier near graphene, we have found that the defect “quantum dot” provides a very clean and stable spectrum of the graphene states at high magnetic fields. Defect dots allow electrons to tunnel at very narrow energy windows. They reside at very close proximity to the graphene layer, and are sensitive to a nanometer sized region. We have found that at high magnetic fields, the graphene spectra break into 4 components, and effect known as “degeneracy lifting”. The presence of all 4 components, we believe, is due to the fact that the defect dot is “minimally invasive” – providing minimal screening to interactions, and thereby avoiding the disruption of fragile, interaction-driven states.
Finally, we have developed a new type of superconducting device, which we call a "two dimensional Josephson junction" (2DJJ). 2DJJs are constructed using similar methods to the ones used for the tunnel devices. Using cracked NbSe2 as source and drain superconductors, placed on top of graphene, we arrive at a superconducting devices which consists only of layered materials, and is entirely flat. As a result, the device sustains supercurrent up to parallel fields of 8.5 Tesla - an unprecedented technical achievement. We have shown that the devices allow the study of the interplay between the spin degree of freedom, and the superconducting state in graphene.
