Report

Teaser, summary, work performed and final results

Periodic Reporting for period 3 - TUNNEL (Tunneling Spectroscopy in van-der-Waals Device)

Teaser

Experimental studies in physics rely on precise measurements to map the state of a given system. In the solid state, a particularly powerful kind of measurement is tunneling spectroscopy, which relies on the property of quantum-mechanical particles to propagate through...

Summary

Experimental studies in physics rely on precise measurements to map the state of a given system. In the solid state, a particularly powerful kind of measurement is tunneling spectroscopy, which relies on the property of quantum-mechanical particles to propagate through energetically forbidden regions. Electrons, for example, can travel through a very thin layer of insulating material, when such layer is in the few nm thickness range. In Solid State physics, tunneling spectroscopy is a very powerful experimental tool since such 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.

Such experiments date back to the 1960s. Then, devices consisting of two metals separated by an oxide have provided fundamentally important information for confirming one of the most important theories developed in modern condensed matter physics, namely the BCS model for superconductivity. The technique was pioneered by Giaever, who discovered that electrons tunneling through an insulating oxide barrier reveal the spectrum of superconductors in the detail of the non-linear current-voltage characteristics.

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 would allow the investigation of many types of materials and material-hybrids using tunneling, striving to precise energy resolutions. Such resulotion is temperature limited, and a major part of my effort is the commissioning of an ultra-low temperature system for measuring such devices at μeV resolution.

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.

Work performed

The work covered: (1) basic development of our device technology, mostly our ability to identify and transfer thin tunnel barriers; (2) Develop a controlled atmosphere stacking system; (3) purchase of an ultra-low temperature measurement apparatus (dilution cryostat); (4) Fabrication and measurement of a high quality tunnel device for probing the energy spectrum of a superconductor.

(1) Development of Device Technology. Our experiment relies on our ability to identify and position a thin layer of an insulator on top of another material. Such materials belong to the layered material family, often called “van-der-Waals materials” since their structure includes strongly bound atomic layers weakly bound to each other. Many of these materials peel easily, and layers down to a single atom thickness can be identified. Insulators, however, pose a special challenge, since they are mostly transparent. To this end we have spent 6 months developing methods to exfoliate materials, such as MoS2 and hexagonal Boron Nitride (hBN) down to the single-layer thickness. We have used a variety of substrates, and have ultimately been able to identify large (> 10 microns) pieces perfectly suitable for our purposes.
(2) In parallel, we have purchased and installed a nitrogen glove-box, which is a contained system allowing work in an atmosphere which does not contain oxygen and humidity, thereby suppressing the oxidation problem of many materials of interest. Within the glove-box we have installed a specially designed system allowing mechanical deposition of thin layers under a high-powered microscope. This is done using computer controlled micrometric stages.
(3) We have performed a market search for a top-of-the line cryostat designed to reach temperatures as low as 8mK. Such low temperatures are essential for the high resolution of our measurements. The cryostat selected is manufactured by BluForce cryogenics in Helsinki. It has a specialized stage for fast loading devices, and supports a 2-axis vector magnet.
(4) Using our newly installed transfer setup, we have fabricated the first round of spectroscopy devices. These consist of a nanometric-thickness flake the superconductor NbSe2, exfoliated within the glove box to protect it from degradation. On top of the superconductor we transfer a layer of insulator (hBN or MoS2) which serves as a tunnel barrier. Details on our measurements are given in the next section.

Final results

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.

At present, our devices allow us to measure the complex excitation spectrum of the superconductor, which reveals details at unprecedented clarity. We can tell, for example, that NbSe2 has two types of superconducting order, each of which forms independently. We are able to switch on a magnetic field, which creates quantum states called “vortices” in the superconductor. These vortices host a fragile and intricate spectrum of quantum states. We are then able to map the signature of such states, for the first time, within a tunneling device. These results are now being prepared for publication in a leading scientific journal.