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Tunneling Spectroscopy in van-der-Waals Device

Periodic Reporting for period 4 - TUNNEL (Tunneling Spectroscopy in van-der-Waals Device)

Reporting period: 2019-11-01 to 2020-04-30

Experimental studies in physics rely on precise measurements to map the state of a given system. In the solid state, a powerful kind of measurement is tunneling spectroscopy, which relies on the property of quantum-mechanical particles to propagate through energetically forbidden regions.
In Solid State physics, tunneling spectroscopy is a very powerful experimental tool since tunnelling electrons effectively measure the existence of quantum-mechanical states in a so-called “target system”. The subject of this project is the investigation of solid state systems using tunneling spectroscopy.

In this project, I set out to expand the range of materials and quantum systems which can be investigated using tunneling. I rely on a newly developed technique called “dry transfer”, which allows the precise deposition of atomic-thickness layers on desired locations. This allows the investigation of many types of materials and material-hybrids using tunneling, striving to precise energy resolutions.

The overall objective of this study are therefore the development and measurement of tunneling devices probing a broad range of quantum systems. These include:
(1) High resolution spectroscopy of “many-body” quantum systems. Many quantum systems at low temperature assume ordered states due electron-electron repulsive interaction. Such states can be found, for example, in graphene – an ultra-thin layer of carbon atoms, which hosts very clean electronic states. One goal of this proposal is the tunneling spectroscopy of ultra-clean graphene.
(2) Spectroscopy of superconducting states and proximity states. At very low temperatures, some conductors lose their electronic resistivity. This state is characterized by a special energy spectrum and local order. The goal of this proposal is to provide high-resolution spectra of quantum states predicted to exist in certain areas in a superconductor, but which have not been mapped before in detail.
(3) Tunneling in momentum conserving systems. Here the objective is to rely on the momentum-conservation properties of some devices to allow selective tunneling, thereby mapping the energy-momentum relation of the materials.
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.
Our tunnel devices work superbly well. For example, they exhibit a so-called “hard gap”, meaning that tunneling at low energies is strongly suppressed by an energy gap where single-electron states are forbidden. In devices, reaching such hard gap is not a trivial achievement. It requires high quality tunnel barriers which engage well to materials on both sides, and suppress non-tunneling sources of current. Hard gap tunneling is critical for a variety of future measurements where we intend to seek the spectral signature of low-energy excitations in superconductors and superconductor hybrids with materials such as graphene and topological insulators. Such states are expected to be useful for fault-tolerant quantum computation schemes.

Our studies show that superconducting properties in NbSe2 respond to magnetic field in a very interesting way. The material survives very high in-plane fields. By measuring its spectrum, we have found deviations from standard theory which point to the formation of unconventional order, induced by the in-plane field.

In parallel, we have shown that van-der-Waals stacks are an excellent platform for quantum-dot assisted spectroscopy. Making use of atomic defects within the barriers, we carry out very sensitive spectroscopic studies of graphene. The advantage of the defect dots as spectrometers is pronounced when studying interacting systems. There, new quantum many-body states form due to inter-particle interactions. Such states are fragile, however, and might disappear if interactions are screened - for example by a nearby metallic probe. The use of an atomic defect as a probe minimises this effect of screening, and reveals new types of physical effect where spectroscopy of so far impossible.