When the scanning tunnelling microscope made its debut in the 1980s the result was an explosion in nanotechnology and quantum-device research. Since then, other types of scanning probe microscopes have been developed and together they have helped researchers flesh out theories of electron transport. But these techniques probe electrons at a single point, thereby observing them as particles and only seeing their wave nature indirectly. Now researchers at the Weizmann Institute of Science in Israel have built a new scanning probe – the quantum twisting microscope – that detects the quantum wave characteristics of electrons directly.
“It’s effectively a scanning probe tip with an interferometer at its apex,” says Shahal Ilani, the team leader. The researchers overlay a scanning probe tip with ultrathin graphite, hexagonal boron nitride and a van der Waals crystal such as graphene, which conveniently flop over the tip like a tent with a flat top about 200 nm across. The flat end is key to the device’s interferometer function. Instead of an electron tunnelling between one point in the sample and the tip, the electron wave function can tunnel across at multiple points simultaneously.
“Quite surprisingly we found that the flat end naturally pivots so that it is always parallel with the sample,” says John Birkbeck, the corresponding author of a paper describing this work. This is fortunate because any tilt would alter the tunnelling distance and hence strength from one side of the plateau to the other. “It is the interference of these tunnelling paths, as identified in the measured current, that gives the device its unique quantum wave probing function,” says Birkbeck.
Double slit experiment
This interference is analogous to the effects of firing electrons at a screen with two slits in it, like the famous Young’s double slit experiment, as Erez Berg explains. Berg, together with Ady Stern, Binghai Yan and Yuval Oreg led the theoretical understanding of the new instrument.
If you measure which slit the particle passes through – like what happens with the measurements of other scanning probe techniques – the wave behaviour is lost and all you see is the particle. However, if you leave the particle to pass with its crossing position undetected, the two available paths produce a pattern of constructive and destructive interference like the waves that ripple out from two pebbles dropped in a pond side by side.
“Since the electron can only tunnel where its momentum matches between the probe and sample, the device directly measures this parameter, which is key for theories explaining collective electron behaviour,” says Berg.
In fact the idea of measuring the momentum of an electron using the interference of its available tunnelling routes dates back to the work of Jim Eisenstein at Caltech in the 1990s. However, the Weizmann researchers move things up several gears with some key innovations thanks to two explosive developments since. These are the the isolation of graphene prompting research into similar atomically thin van der Waals crystals; and the subsequent experimentally observed effects of a twist in the orientation of layered van der Waals materials.
When layered with a twist, materials like graphene form a moiré lattice, so named after textiles where the mesh of the fabric is slightly out of register and has funny effects on your eyes. The electrons in these moiré 2D materials are subjected to the potential of this additional artificial moiré lattice, which has a period determined by the twist angle. Hence twisting through the relative angles between two layers of van der Waals crystal using a piezoelectric rotator on the quantum twisting microscope, makes it possible to measure a much wider range in momentum than was possible with the magnetic fields used previously, as well as exploring many other electronic phenomena too. The natty device also makes it easy to study a range of different van der Waals crystals and other quantum materials.
From problem to solution
Following the discovery of twist effects, people were keen to experiment with materials at different twist angles. However they had to go through the painstaking process of producing each device afresh for each twist angle. Although it had been possible to twist through angles is a single device, the twist tends to get locked at certain angles where, it’s basically game over for the experiment. In the quantum twisting microscope the atomically thin material on the tip has strong adhesion along the tip sides as well as the end, so that the net forces easily outweigh the attraction between the two van der Waal crystal layers of probe and sample, even for these most attractive twist angles. It was fabrication challenges like these that the Weizmann researchers had originally set out to tackle.
Twisted graphene pioneer Cory Dean, who was not involved with this research, describes how some of the most detailed understanding of twisted layer systems is coming from scanning probes over them. This way each region with its unique albeit uncontrolled twist can be identified and treated as its own device. “In the Weizmann approach, they have taken this step to a really creative new direction where the twist angle control and spectroscopic analysis are integrated into the same platform,” says Dean, who is at Columbia University. “This idea, that the device is also the instrument, is a rare and exciting combination in condensed matter systems.” He also highlights that the device is not limited to twisted layer systems.
Ilani says of his team’s invention, “To be honest every week we discover a new type of measurement that you can do with the quantum twisting microscope – it’s a very versatile tool”. For example, the researchers can also press the tip down to explore the effects of pressure, which decreases the distance between van der Waals layers. “There are experiments on 2D materials done with pressure, also in the context of magic angle graphene,” says Birkbeck, as he refers to experiments with pistons in oil chambers plunged to low temperatures that need to be reset from scratch for each pressure value. “We’ve reached comparable pressures with the quantum twisting microscope but now with the ability to quickly and continuously tune it in situ.”
The results are reported in Nature.
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