Conducting channels visualised with STM


Rebecca Pool

Thursday, February 7, 2019 - 19:00
Scanning tunnelling microscopy image of boundary between regions with different electron orbit orientations. [Ali Yazdani, Princeton University]
Using scanning tunnelling microscopy, US-based researchers have detected electron flow in 'quantum wires'.
Professor Ali Yazdani, Director of the Princeton Center for Complex Materials, and colleagues, observed channels of conducting electrons between two quantum states on the surface of a bismuth crystal subjected to a high magnetic field.
These two quantum states of electrons were moving in elliptical orbits with different orientations.
And to the researchers' surprise, they found that the current flow in these channels could be turned on and off, making these channels a new type of controllable quantum wire.
"These channels are remarkable because they spontaneously form at the boundaries between different quantum states in which electrons collectively align their elliptical orbits," explains Yazdani. "It is exciting to see how the interaction between electrons in the channels strongly dictates whether or not they can conduct."
To visualise electron behaviours on the surface of the bismuth crystal, the researchers used an ultra-high vacuum scanning tunnelling microscope operating at dilution refrigerator temperatures and high magnetic fields.
Measurements were performed at 250 mK using a tungsten tip, and in this way, the researchers could image the electrons' motions in the presence of magnetic field, thousands of times larger that of a refrigerator magnet.
Conducting channels visualized with scanning tunnelling microscopy. [Ali Yazdani, Princeton University]
What's more, applying the huge magnetic field forced the electrons to move in elliptical orbits, instead of the more typical flow of electrons parallel to the direction of an electric field.
Yazdani and colleagues discovered that the conducting channels formed at the boundary, which they call a valley-polarized domain wall, between two regions on the crystal where the electron orbits switch orientations abruptly.
As Yazdani writes in Nature: "Our experimental approach was to leverage the accessibility of quantum Hall states formed on the surface of bismuth and use STM measurements to directly visualize domain walls between different valley-polarized phases."
"We used the tunability of valley occupation in this system in combination with high-resolution STM spectroscopy to probe the properties of valley-polarized topological boundary modes," he says.
"We found that there are two-lane and four-lane channels in which the electrons can flow, depending on the precise value of the magnetic field," adds Mallika Randeria from Yazdani's lab. 
The researchers noted that when electrons are tuned to move in a four-lane channel, they get stuck, but they can flow unimpeded when they are confined to only a two-lane channel.
In trying to understand this behaviour, the researchers uncovered new rules by which the laws of quantum mechanics dictate repulsion between electrons in these multi-channel quantum wires.
While the larger number of lanes would seem to suggest better conductivity, the repulsion between electrons counter-intuitively causes them to switch lanes, change direction, and get stuck, resulting in insulating behaviour.
With fewer channels, electrons have no option to change lanes and must transmit electrical current even if they have to move 'through' each other - a quantum phenomenon only possible in such one-dimensional channels.
The theoretical explanation for the new finding builds on earlier work carried out by two members of the team, Siddharth Parameswaran, who was then a graduate student at Princeton and is now an associate professor of physics at Oxford University, and Princeton's Shivaji Sondhi, professor of physics, and collaborators.
"Although some of the theoretical ideas we used have been around for a while, it's still a challenge to see how they fit together to explain an actual experiment, and a real thrill when that happens," says Parameswaran. "This is a perfect example of how experiment and theory work in tandem: Without the new experimental data we would never have revisited our theory, and without the new theory it would have been difficult to understand the experiments."
Research is published in Nature.
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