Tag Archives: Ettore Majorana

Graphene-based nanoelectronics platform, a replacement for silicon?

A December 31, 2022 news item on phys.org describes research into replacing silicon in the field of electronics, Note: Links have been removed,

A pressing quest in the field of nanoelectronics is the search for a material that could replace silicon. Graphene has seemed promising for decades. But its potential has faltered along the way, due to damaging processing methods and the lack of a new electronics paradigm to embrace it. With silicon nearly maxed out in its ability to accommodate faster computing, the next big nanoelectronics platform is needed now more than ever.

Walter de Heer, Regents’ Professor in the School of Physics at the Georgia Institute of Technology [Georgia Tech], has taken a critical step forward in making the case for a successor to silicon. De Heer and his collaborators have developed a new nanoelectronics platform based on graphene—a single sheet of carbon atoms. The technology is compatible with conventional microelectronics manufacturing, a necessity for any viable alternative to silicon.

In the course of their research, published in Nature Communications, the team may have also discovered a new quasiparticle. Their discovery could lead to manufacturing smaller, faster, more efficient and more sustainable computer chips, and has potential implications for quantum and high-performance computing.

A January 3, 2023 Georgia Institute of Technology news release (also on EurekAlert but published December 21, 2022] by Catherine Barzler, which originated the news item, delves further into the work

“Graphene’s power lies in its flat, two-dimensional structure that is held together by the strongest chemical bonds known,” de Heer said. “It was clear from the beginning that graphene can be miniaturized to a far greater extent than silicon — enabling much smaller devices, while operating at higher speeds and producing much less heat. This means that, in principle, more devices can be packed on a single chip of graphene than with silicon.”

In 2001, de Heer proposed an alternative form of electronics based on epitaxial graphene, or epigraphene — a layer of graphene that was found to spontaneously form on top of silicon carbide crystal, a semiconductor used in high power electronics. At the time, researchers found that electric currents flow without resistance along epigraphene’s edges, and that graphene devices could be seamlessly interconnected without metal wires. This combination allows for a form of electronics that relies on the unique light-like properties of graphene electrons.

“Quantum interference has been observed in carbon nanotubes at low temperatures, and we expect to see similar effects in epigraphene ribbons and networks,” de Heer said. “This important feature of graphene is not possible with silicon.”

Building the Platform

To create the new nanoelectronics platform, the researchers created a modified form of epigraphene on a silicon carbide crystal substrate. In collaboration with researchers at the Tianjin International Center for Nanoparticles and Nanosystems at the University of Tianjin, China, they produced unique silicon carbide chips from electronics-grade silicon carbide crystals. The graphene itself was grown at de Heer’s laboratory at Georgia Tech using patented furnaces.

The researchers used electron beam lithography, a method commonly used in microelectronics, to carve the graphene nanostructures and weld their edges to the silicon carbide chips. This process mechanically stabilizes and seals the graphene’s edges, which would otherwise react with oxygen and other gases that might interfere with the motion of the charges along the edge.

Finally, to measure the electronic properties of their graphene platform, the team used a cryogenic apparatus that allows them to record its properties from a near-zero temperature to room temperature.

Observing the Edge State

The electric charges the team observed in the graphene edge state were similar to photons in an optical fiber that can travel over large distances without scattering. They found that the charges traveled for tens of thousands of nanometers along the edge before scattering. Graphene electrons in previous technologies could only travel about 10 nanometers before bumping into small imperfections and scattering in different directions.

“What’s special about the electric charges in the edges is that they stay on the edge and keep on going at the same speed, even if the edges are not perfectly straight,” said Claire Berger, physics professor at Georgia Tech and director of research at the French National Center for Scientific Research in Grenoble, France.

In metals, electric currents are carried by negatively charged electrons. But contrary to the researchers’ expectations, their measurements suggested that the edge currents were not carried by electrons or by holes (a term for positive quasiparticles indicating the absence of an electron). Rather, the currents were carried by a highly unusual quasiparticle that has no charge and no energy, and yet moves without resistance. The components of the hybrid quasiparticle were observed to travel on opposite sides of the graphene’s edges, despite being a single object.

The unique properties indicate that the quasiparticle might be one that physicists have been hoping to exploit for decades — the elusive Majorana fermion predicted by Italian theoretical physicist Ettore Majorana in 1937.

“Developing electronics using this new quasiparticle in seamlessly interconnected graphene networks is game changing,” de Heer said.

It will likely be another five to 10 years before we have the first graphene-based electronics, according to de Heer. But thanks to the team’s new epitaxial graphene platform, technology is closer than ever to crowning graphene as a successor to silicon.

Here’s a link to and a citation for the paper,

An epitaxial graphene platform for zero-energy edge state nanoelectronics by Vladimir S. Prudkovskiy, Yiran Hu, Kaimin Zhang, Yue Hu, Peixuan Ji, Grant Nunn, Jian Zhao, Chenqian Shi, Antonio Tejeda, David Wander, Alessandro De Cecco, Clemens B. Winkelmann, Yuxuan Jiang, Tianhao Zhao, Katsunori Wakabayashi, Zhigang Jiang, Lei Ma, Claire Berger & Walt A. de Heer. Nature Communications volume 13, Article number: 7814 (2022) DOI: https://doi.org/10.1038/s41467-022-34369-4 Published 19 December 2022

This paper is open access.

Proposed nanodevice made possible by particle that is its own antiparticle (Majorana particle)

I’m not sure how much the mystery of Ettore Majorana’s disappearance in 1938 has to do with the latest research from Brazil on Majorana particles but it’s definitely fascinating,. From an April 6, 2018 news item on ScienceDaily,

In March 1938, the young Italian physicist Ettore Majorana disappeared mysteriously, leaving his country’s scientific community shaken. The episode remains unexplained, despite Leonardo Scascia’s attempt to unravel the enigma in his book The Disappearance of Majorana (1975).

Majorana, whom Enrico Fermi called a genius of Isaac Newton’s stature, vanished a year after making his main contribution to science. In 1937, when he was only 30, Majorana hypothesized a particle that is its own anti-particle and suggested that it might be the neutrino, whose existence had recently been predicted by Fermi and Wolfgang Pauli.

Eight decades later, Majorana fermions, or simply majoranas, are among the objects most studied by physicists. In addition to neutrinos — whose nature, whether or not they are majoranas, is one of the investigative goals of the mega-experiment Dune — another class not of fundamental particles but of quasi-particles or apparent particles has been investigated in the field of condensed matter. These Majorana quasi-particles can emerge as excitations in topological superconductors.

An April 6, 2018 Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) press release on EurekAlert, which originated the news item,  reveals more about the Brazilian research (Note: Links have been removed),

A new study by PhD student Luciano Henrique Siliano Ricco with a scholarship from the São Paulo Research Foundation – FAPESP, in collaboration with his supervisor Antonio Carlos Ferreira Seridonio and others, was conducted on the Ilha Solteira campus of São Paulo State University (UNESP) in Brazil and described in an article in Scientific Reports.

“We propose a theoretical device that acts as a thermoelectric tuner – a tuner of heat and charge – assisted by Majorana fermions,” Seridonio said.

The device consists of a quantum dot (QD), represented in the Figure A by the symbol ε1. QDs are often called “artificial atoms.” In this case, the QD is located between two metallic leads at different temperatures.

The temperature difference is fundamental to allowing thermal energy to flow across the QD. A quasi-one-dimensional superconducting wire – called a Kitaev wire after its proponent, Russian physicist Alexei Kitaev, currently a professor at the California Institute of Technology (Caltech) in the US – is connected to the QD.

In this study, the Kitaev wire was ring- or U-shaped and had two majoranas (η1 and η2) at its edges. The majoranas emerge as excitations characterized by zero-energy modes.

“When the QD is coupled to only one side of the wire, the system behaves resonantly with regard to electrical and thermal conductance. In other words, it behaves like a thermoelectric filter,” said the principal investigator for the FAPESP fellowship.

“I should stress that this behavior as a filter for thermal and electrical energy occurs when the two majoranas ‘see’ each other via the wire, but only one of them ‘sees’ the QD in the connection.”

Another possibility investigated by the researchers involved making the QD “see” the two majoranas at the same time by connecting it to both ends of the Kitaev wire.

“By making the QD ‘see’ more of η1 or η2, i.e., by varying the system’s asymmetry, we can use the artificial atom as a tuner, where the thermal or electrical energy that flows through it is redshifted or blueshifted,” Seridonio said (see Figure B for illustrative explanation).

This theoretical paper, he added, is expected to contribute to the development of thermoelectric devices based on Majorana fermions.

Here’s a link to and a citation for the paper,

Tuning of heat and charge transport by Majorana fermions by L. S. Ricco, F. A. Dessotti, I. A. Shelykh, M. S. Figueira & A. C. Seridonio. Scientific Reportsvolume 8, Article number: 2790 (2018) doi:10.1038/s41598-018-21180-9 Published online: 12 February 2018

This paper is open access.

As I prepared to publish this piece I stumbled across a sad Sept. 3, 2018 article about Brazil and its overnight loss of heritage in a fire by Henry Grabar for slate.com (Note: Links have been removed),

On Sunday night, a fire ripped through Brazil’s National Museum in Rio de Janeiro, destroying the country’s most valuable storehouse of natural and anthropological history within hours.

Most of the 20 million items housed inside—including the skull of Luzia, the oldest human remains ever found in the Americas; one of the world’s largest archives of South America’s indigenous cultures; more than 26,000 fossils, 55,000 stuffed birds, and 5 million insect specimens; and a library of more than 500,000 books—are thought to have been destroyed.

The loss is a symptom of a larger problem as Grabar notes in his article.

‘Nano-hashtags’ for Majorana particles?

The ‘nano-hashtags’ are in fact (assuming a minor leap of imagination) nanowires that resemble hashtags.

Scanning electron microscope image of the device wherein clearly a ‘hashtag’ is formed. Credit: Eindhoven University of Technology

An August 23, 2017 news item on ScienceDaily makes the announcement,

In Nature, an international team of researchers from Eindhoven University of Technology [Netherlands], Delft University of Technology [Netherlands] and the University of California — Santa Barbara presents an advanced quantum chip that will be able to provide definitive proof of the mysterious Majorana particles. These particles, first demonstrated in 2012, are their own antiparticle at one and the same time. The chip, which comprises ultrathin networks of nanowires in the shape of ‘hashtags’, has all the qualities to allow Majorana particles to exchange places. This feature is regarded as the smoking gun for proving their existence and is a crucial step towards their use as a building block for future quantum computers.

An August 23, 2017 Eindhoven University press release (also on EurekAlert), which originated the news item, provides some context and information about the work,

In 2012 it was big news: researchers from Delft University of Technology and Eindhoven University of Technology presented the first experimental signatures for the existence of the Majorana fermion. This particle had been predicted in 1937 by the Italian physicist Ettore Majorana and has the distinctive property of also being its own anti-particle. The Majorana particles emerge at the ends of a semiconductor wire, when in contact with a superconductor material.

Smoking gun

While the discovered particles may have properties typical to Majoranas, the most exciting proof could be obtained by allowing two Majorana particles to exchange places, or ‘braid’ as it is scientifically known. “That’s the smoking gun,” suggests Erik Bakkers, one of the researchers from Eindhoven University of Technology. “The behavior we then see could be the most conclusive evidence yet of Majoranas.”

Crossroads

In the Nature paper that is published today [August 23, 2017], Bakkers and his colleagues present a new device that should be able to show this exchanging of Majoranas. In the original experiment in 2012 two Majorana particles were found in a single wire but they were not able to pass each other without immediately destroying the other. Thus the researchers quite literally had to create space. In the presented experiment they formed intersections using the same kinds of nanowire so that four of these intersections form a ‘hashtag’, #, and thus create a closed circuit along which Majoranas are able to move.

Etch and grow

The researchers built their hashtag device starting from scratch. The nanowires are grown from a specially etched substrate such that they form exactly the desired network which they then expose to a stream of aluminium particles, creating layers of aluminium, a superconductor, on specific spots on the wires – the contacts where the Majorana particles emerge. Places that lie ‘in the shadow’ of other wires stay uncovered.

Leap in quality

The entire process happens in a vacuum and at ultra-cold temperature (around -273 degree Celsius). “This ensures very clean, pure contacts,” says Bakkers, “and enables us to make a considerable leap in the quality of this kind of quantum device.” The measurements demonstrate for a number of electronic and magnetic properties that all the ingredients are present for the Majoranas to braid.

Quantum computers

If the researchers succeed in enabling the Majorana particles to braid, they will at once have killed two birds with one stone. Given their robustness, Majoranas are regarded as the ideal building block for future quantum computers that will be able to perform many calculations simultaneously and thus many times faster than current computers. The braiding of two Majorana particles could form the basis for a qubit, the calculation unit of these computers.

Travel around the world

An interesting detail is that the samples have traveled around the world during the fabrication, combining unique and synergetic activities of each research institution. It started in Delft with patterning and etching the substrate, then to Eindhoven for nanowire growth and to Santa Barbara for aluminium contact formation. Finally back to Delft via Eindhoven for the measurements.

Here’s a link to and a citation for the paper,

Epitaxy of advanced nanowire quantum devices by Sasa Gazibegovic, Diana Car, Hao Zhang, Stijn C. Balk, John A. Logan, Michiel W. A. de Moor, Maja C. Cassidy, Rudi Schmits, Di Xu, Guanzhong Wang, Peter Krogstrup, Roy L. M. Op het Veld, Kun Zuo, Yoram Vos, Jie Shen, Daniël Bouman, Borzoyeh Shojaei, Daniel Pennachio, Joon Sue Lee, Petrus J. van Veldhoven, Sebastian Koelling, Marcel A. Verheijen, Leo P. Kouwenhoven, Chris J. Palmstrøm, & Erik P. A. M. Bakkers. Nature 548, 434–438 (24 August 2017) doi:10.1038/nature23468 Published online 23 August 2017

This paper is behind a paywall.

Dexter Johnson has some additional insight (interview with one of the researchers) in an Aug. 29, 2017 posting on his Nanoclast blog (on the IEEE [institute of Electrical and Electronics Engineers] website).

Majorana, matter, anti-matter, and nanowires

This is one of my favourite types of science story and I’m going to start with the quantum physics part of this (from the April 13, 2012 news item on Nanowerk),

Scientists at TU Delft’s Kavli Institute and the Foundation for Fundamental Research on Matter (FOM Foundation) have succeeded for the first time in detecting a Majorana particle. In the 1930s, the brilliant Italian physicist Ettore Majorana deduced from quantum theory the possibility of the existence of a very special particle, a particle that is its own anti-particle: the Majorana fermion. That ‘Majorana’ would be right on the border between matter and anti-matter.

The researchers have made a video about the Majorana fermion and nanowires (from the April 12, news release on the TU Delft website),

Here’s a little more about the Majorana fermion and why the researchers as so excited (from the TU Delft news release),

Majorana fermions are very interesting – not only because their discovery opens up a new and uncharted chapter of fundamental physics; they may also play a role in cosmology. A proposed theory assumes that the mysterious ‘dark matter, which forms the greatest part of the universe, is composed of Majorana fermions. Furthermore, scientists view the particles as fundamental building blocks for the quantum computer. Such a computer is far more powerful than the best supercomputer, but only exists in theory so far. Contrary to an ‘ordinary’ quantum computer, a quantum computer based on Majorana fermions is exceptionally stable and barely sensitive to external influences.

This breakthrough was achieved not with the Large Hadron Collider at CERN (European Particle Physics Laboratory) but with nanowires (from the TU Delft news release),

For the first time, scientists in Leo Kouwenhoven’s research group managed to create a nanoscale electronic device in which a pair of Majorana fermions ‘appear’ at either end of a nanowire. They did this by combining an extremely small nanowire, made by colleagues from Eindhoven University of Technology, with a superconducting material and a strong magnetic field. ‘The measurements of the particle at the ends of the nanowire cannot otherwise be explained than through the presence of a pair of Majorana fermions’, says Leo Kouwenhoven.

The device is made of an Indium Antemonide nanowire, covered with a Gold contact and partially covered with a Superconducting Niobium contact. The Majorana fermions are created at the end of the Nanowire. (from the TU Delft website)

At the end of the TU Delft news release, they mention more about Ettore Majorana and this is where the story gets quite intriguing,

The Italian physicist Ettore Majorana was a brilliant theorist who showed great insight into physics at a young age. He discovered a hitherto unknown solution to the equations from which quantum scientists deduce elementary particles: the Majorana fermion. Practically all theoretic particles that are predicted by quantum theory have been found in the last decades, with just a few exceptions, including the enigmatic Majorana particle and the well-known Higgs boson. But Ettore Majorana the person is every bit as mysterious as the particle. In 1938 he withdrew all his money and disappeared during a boat trip from Palermo to Naples. Whether he killed himself, was murdered or lived on under a different identity is still not known. No trace of Majorana was ever found.

Here’s the citation for the article describing the discovery of the Majorana fermion (from the TU Delft news release),

The article is published in Science Express on 12 April: Signatures of Majorana fermions in hybrid superconductor-semiconductor nanowire devices, V. Mourik, K. Zuo, S.M. Frolov, S.R. Plissard, E.P.A.M. Bakkers, L.P. Kouwenhoven

There’s more information and there are more images with the April 12, 2012 TU Deflt news release.