Computers Math

A team of researchers from the Faculty of Pure and Applied Sciences at the University of Tsukuba filmed the ultrafast motion of electrons with sub-nanoscale spatial resolution. This work provides a powerful tool for studying the operation of semiconductor devices, which can lead to more efficient electronic devices.
The ability to construct ever smaller and faster smartphones and computer chips depends on the ability of semiconductor manufacturers to understand how the electrons that carry information are affected by defects. However, these motions occur on the scale of trillionths of a second, and they can only be seen with a microscope that can image individual atoms. It may seem like an impossible task, but this is exactly what a team of scientists at the University of Tsukuba was able to accomplish.
The experimental system consisted of Buckminsterfullerene carbon molecules — which bear an uncanny resemblance to stitched soccer balls — arranged in a multilayer structure on a gold substrate. First, a scanning tunneling microscope was set up to capture the movies. To observe the motion of electrons, an infrared electromagnetic pump pulse was applied to inject electrons into the sample. Then, after a set time delay, a single ultrafast terahertz pulse was used to probe the location of the elections. Increasing the time delay allowed the next “frame” of the movie to be captured. This novel combination of scanning tunneling microscopy and ultrafast pulses allowed the team to achieve sub-nanoscale spatial resolution and near picosecond time resolution for the first time. “Using our method, we were able to clearly see the effects of imperfections, such as a molecular vacancy or orientational disorder,” explains first author Professor Shoji Yoshida. Capturing each frame took only about two minutes, which allows the results to be reproducible. This also makes the approach more practical as a tool for the semiconductor industry.
“We expect that this technology will help lead the way towards the next generation of organic electronics” senior author Professor Hidemi Shigekawa says. By understanding the effects of imperfections, some vacancies, impurities, or structural defects can be purposely introduced into devices to control their function.

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Materials provided by University of Tsukuba. Note: Content may be edited for style and length. More

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Quantum nanowires — which have length but no width or height-provide a unique environment for the formation and detection of a quasiparticle known as a Majorana zero mode.
A new UNSW-led study overcomes previous difficulty detecting the Majorana zero mode, and produces a significant improvement in device reproducibility.
Potential applications for Majorana zero modes include fault-resistant topological quantum computers, and topological superconductivity.
MAJORANA FERMIONS IN 1D WIRES
A Majorana fermion is a composite particle that is its own antiparticle.

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Antimatter explainer: Every fundamental particle has a corresponding antimatter particle, with the same mass but opposite electrical charge. For example, the antiparticle of an electron (charge -1) is a positron (charge +1)
Such unusual particle’s interest academically and commercially comes from their potential use in a topological quantum computer, predicted to be immune to the decoherence that randomises the precious quantum information.
Majorana zero modes can be created in quantum wires made from special materials in which there is a strong coupling between their electrical and magnetic properties.
In particular, Majorana zero modes can be created in one-dimensional semiconductors (such as semiconductor nanowires) when coupled with a superconductor.
In a one-dimensional nanowire, whose dimensions perpendicular to length are small enough not to allow any movement of subatomic particles, quantum effects predominate.

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NEW METHOD FOR DETECTING NECESSARY SPIN-ORBIT GAP
Majorana fermions, which are their own antiparticle, have been theorised since 1937, but have only been experimentally observed in the last decade. The Majorana fermion’s ‘immunity’ to decoherence provides potential use for fault-tolerant quantum computing.
One-dimensional semiconductor systems with strong spin-orbit interaction are attracting great attention due to potential applications in topological quantum computing.
The magnetic ‘spin’ of an electron is like a little bar magnet, whose orientation can be set with an applied magnetic field.
In materials with a ‘spin-orbit interaction’ the spin of an electron is determined by the direction of motion, even at zero magnetic field. This allows for all electrical manipulation of magnetic quantum properties.
Applying a magnetic field to such a system can open an energy gap such that forward -moving electrons all have the same spin polarisation, and backward-moving electrons have the opposite polarisation. This ‘spin-gap’ is a pre-requisite for the formation of Majorana zero modes.
Despite intense experimental work, it has proven extremely difficult to unambiguously detect this spin-gap in semiconductor nanowires, since the spin-gap’s characteristic signature (a dip in its conductance plateau when a magnetic field is applied) is very hard to distinguish from unavoidable the background disorder in nanowires.
The new study finds a new, unambiguous signature for the spin-orbit gap that is impervious to the disorder effects plaguing previous studies.
“This signature will become the de-facto standard for detecting spin-gaps in the future,” says lead author Dr Karina Hudson.
REPRODUCIBILITY
The use of Majorana zero modes in a scalable quantum computer faces an additional challenge due to the random disorder and imperfections in the self-assembled nanowires that host the MZM.
It has previously been almost impossible to fabricate reproducible devices, with only about 10% of devices functioning within desired parameters.
The latest UNSW results show a significant improvement, with reproducible results across six devices based on three different starting wafers.
“This work opens a new route to making completely reproducible devices,” says corresponding author Prof Alex Hamilton UNSW). More

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