Tag Archives: electrons

Gold’s origin in the universe due to cosmic collision

An hypothesis for gold’s origins was first mentioned here in a May 26, 2016 posting,

The link between this research and my side project on gold nanoparticles is a bit tenuous but this work on the origins for gold and other precious metals being found in the stars is so fascinating and I’m determined to find a connection.

An artist's impression of two neutron stars colliding. (Credit: Dana Berry / Skyworks Digital, Inc.) Courtesy: Kavli Foundation

An artist’s impression of two neutron stars colliding. (Credit: Dana Berry / Skyworks Digital, Inc.) Courtesy: Kavli Foundation

From a May 19, 2016 news item on phys.org,

The origin of many of the most precious elements on the periodic table, such as gold, silver and platinum, has perplexed scientists for more than six decades. Now a recent study has an answer, evocatively conveyed in the faint starlight from a distant dwarf galaxy.

In a roundtable discussion, published today [May 19, 2016?], The Kavli Foundation spoke to two of the researchers behind the discovery about why the source of these heavy elements, collectively called “r-process” elements, has been so hard to crack.

From the Spring 2016 Kavli Foundation webpage hosting the  “Galactic ‘Gold Mine’ Explains the Origin of Nature’s Heaviest Elements” Roundtable ,

Astronomers studying a galaxy called Reticulum II have just discovered that its stars contain whopping amounts of these metals—collectively known as “r-process” elements (See “What is the R-Process?”). Of the 10 dwarf galaxies that have been similarly studied so far, only Reticulum II bears such strong chemical signatures. The finding suggests some unusual event took place billions of years ago that created ample amounts of heavy elements and then strew them throughout the galaxy’s reservoir of gas and dust. This r-process-enriched material then went on to form Reticulum II’s standout stars.

Based on the new study, from a team of researchers at the Kavli Institute at the Massachusetts Institute of Technology, the unusual event in Reticulum II was likely the collision of two, ultra-dense objects called neutron stars. Scientists have hypothesized for decades that these collisions could serve as a primary source for r-process elements, yet the idea had lacked solid observational evidence. Now armed with this information, scientists can further hope to retrace the histories of galaxies based on the contents of their stars, in effect conducting “stellar archeology.”

Researchers have confirmed the hypothesis according to an Oct. 16, 2017 news item on phys.org,

Gold’s origin in the Universe has finally been confirmed, after a gravitational wave source was seen and heard for the first time ever by an international collaboration of researchers, with astronomers at the University of Warwick playing a leading role.

Members of Warwick’s Astronomy and Astrophysics Group, Professor Andrew Levan, Dr Joe Lyman, Dr Sam Oates and Dr Danny Steeghs, led observations which captured the light of two colliding neutron stars, shortly after being detected through gravitational waves – perhaps the most eagerly anticipated phenomenon in modern astronomy.

Marina Koren’s Oct. 16, 2017 article for The Atlantic presents a richly evocative view (Note: Links have been removed),

Some 130 million years ago, in another galaxy, two neutron stars spiraled closer and closer together until they smashed into each other in spectacular fashion. The violent collision produced gravitational waves, cosmic ripples powerful enough to stretch and squeeze the fabric of the universe. There was a brief flash of light a million trillion times as bright as the sun, and then a hot cloud of radioactive debris. The afterglow hung for several days, shifting from bright blue to dull red as the ejected material cooled in the emptiness of space.

Astronomers detected the aftermath of the merger on Earth on August 17. For the first time, they could see the source of universe-warping forces Albert Einstein predicted a century ago. Unlike with black-hole collisions, they had visible proof, and it looked like a bright jewel in the night sky.

But the merger of two neutron stars is more than fireworks. It’s a factory.

Using infrared telescopes, astronomers studied the spectra—the chemical composition of cosmic objects—of the collision and found that the plume ejected by the merger contained a host of newly formed heavy chemical elements, including gold, silver, platinum, and others. Scientists estimate the amount of cosmic bling totals about 10,000 Earth-masses of heavy elements.

I’m not sure exactly what this image signifies but it did accompany Koren’s article so presumably it’s a representation of colliding neutron stars,

NSF / LIGO / Sonoma State University /A. Simonnet. Downloaded from: https://www.theatlantic.com/science/archive/2017/10/the-making-of-cosmic-bling/543030/

An Oct. 16, 2017 University of Warwick press release (also on EurekAlert), which originated the news item on phys.org, provides more detail,

Huge amounts of gold, platinum, uranium and other heavy elements were created in the collision of these compact stellar remnants, and were pumped out into the universe – unlocking the mystery of how gold on wedding rings and jewellery is originally formed.

The collision produced as much gold as the mass of the Earth. [emphasis mine]

This discovery has also confirmed conclusively that short gamma-ray bursts are directly caused by the merging of two neutron stars.

The neutron stars were very dense – as heavy as our Sun yet only 10 kilometres across – and they collided with each other 130 million years ago, when dinosaurs roamed the Earth, in a relatively old galaxy that was no longer forming many stars.

They drew towards each other over millions of light years, and revolved around each other increasingly quickly as they got closer – eventually spinning around each other five hundred times per second.

Their merging sent ripples through the fabric of space and time – and these ripples are the elusive gravitational waves spotted by the astronomers.

The gravitational waves were detected by the Advanced Laser Interferometer Gravitational-Wave Observatory (Adv-LIGO) on 17 August this year [2017], with a short duration gamma-ray burst detected by the Fermi satellite just two seconds later.

This led to a flurry of observations as night fell in Chile, with a first report of a new source from the Swope 1m telescope.

Longstanding collaborators Professor Levan and Professor Nial Tanvir (from the University of Leicester) used the facilities of the European Southern Observatory to pinpoint the source in infrared light.

Professor Levan’s team was the first one to get observations of this new source with the Hubble Space Telescope. It comes from a galaxy called NGC 4993, 130 million light years away.

Andrew Levan, Professor in the Astronomy & Astrophysics group at the University of Warwick, commented: “Once we saw the data, we realised we had caught a new kind of astrophysical object. This ushers in the era of multi-messenger astronomy, it is like being able to see and hear for the first time.”

Dr Joe Lyman, who was observing at the European Southern Observatory at the time was the first to alert the community that the source was unlike any seen before.

He commented: “The exquisite observations obtained in a few days showed we were observing a kilonova, an object whose light is powered by extreme nuclear reactions. This tells us that the heavy elements, like the gold or platinum in jewellery are the cinders, forged in the billion degree remnants of a merging neutron star.”

Dr Samantha Oates added: “This discovery has answered three questions that astronomers have been puzzling for decades: what happens when neutron stars merge? What causes the short duration gamma-ray bursts? Where are the heavy elements, like gold, made? In the space of about a week all three of these mysteries were solved.”

Dr Danny Steeghs said: “This is a new chapter in astrophysics. We hope that in the next few years we will detect many more events like this. Indeed, in Warwick we have just finished building a telescope designed to do just this job, and we expect it to pinpoint these sources in this new era of multi-messenger astronomy”.

Congratulations to all of the researchers involved in this work!

Many, many research teams were  involved. Here’s a sampling of their news releases which focus on their areas of research,

University of the Witwatersrand (South Africa)

https://www.eurekalert.org/pub_releases/2017-10/uotw-wti101717.php

Weizmann Institute of Science (Israel)

https://www.eurekalert.org/pub_releases/2017-10/wios-cns101717.php

Carnegie Institution for Science (US)

https://www.eurekalert.org/pub_releases/2017-10/cifs-dns101217.php

Northwestern University (US)

https://www.eurekalert.org/pub_releases/2017-10/nu-adc101617.php

National Radio Astronomy Observatory (US)

https://www.eurekalert.org/pub_releases/2017-10/nrao-ru101317.php

Max-Planck-Gesellschaft (Germany)

https://www.eurekalert.org/pub_releases/2017-10/m-gwf101817.php

Penn State (Pennsylvania State University; US)

https://www.eurekalert.org/pub_releases/2017-10/ps-stl101617.php

University of California – Davis

https://www.eurekalert.org/pub_releases/2017-10/uoc–cns101717.php

The American Association for the Advancement of Science’s (AAAS) magazine, Science, has published seven papers on this research. Here’s an Oct. 16, 2017 AAAS news release with an overview of the papers,

https://www.eurekalert.org/pub_releases/2017-10/aaft-btf101617.php

I’m sure there are more news releases out there and that there will be many more papers published in many journals, so if this interests, I encourage you to keep looking.

Two final pieces I’d like to draw your attention to: one answers basic questions and another focuses on how artists knew what to draw when neutron stars collide.

Keith A Spencer’s Oct. 18, 2017 piece on salon.com answers a lot of basic questions for those of us who don’t have a background in astronomy. Here are a couple of examples,

What is a neutron star?

Okay, you know how atoms have protons, neutrons, and electrons in them? And you know how protons are positively charged, and electrons are negatively charged, and neutrons are neutral?

Yeah, I remember that from watching Bill Nye as a kid.

Totally. Anyway, have you ever wondered why the negatively-charged electrons and the positively-charged protons don’t just merge into each other and form a neutral neutron? I mean, they’re sitting there in the atom’s nucleus pretty close to each other. Like, if you had two magnets that close, they’d stick together immediately.

I guess now that you mention it, yeah, it is weird.

Well, it’s because there’s another force deep in the atom that’s preventing them from merging.

It’s really really strong.

The only way to overcome this force is to have a huge amount of matter in a really hot, dense space — basically shove them into each other until they give up and stick together and become a neutron. This happens in very large stars that have been around for a while — the core collapses, and in the aftermath, the electrons in the star are so close to the protons, and under so much pressure, that they suddenly merge. There’s a big explosion and the outer material of the star is sloughed off.

Okay, so you’re saying under a lot of pressure and in certain conditions, some stars collapse and become big balls of neutrons?

Pretty much, yeah.

So why do the neutrons just stick around in a huge ball? Aren’t they neutral? What’s keeping them together? 

Gravity, mostly. But also the strong nuclear force, that aforementioned weird strong force. This isn’t something you’d encounter on a macroscopic scale — the strong force only really works at the type of distances typified by particles in atomic nuclei. And it’s different, fundamentally, than the electromagnetic force, which is what makes magnets attract and repel and what makes your hair stick up when you rub a balloon on it.

So these neutrons in a big ball are bound by gravity, but also sticking together by virtue of the strong nuclear force. 

So basically, the new ball of neutrons is really small, at least, compared to how heavy it is. That’s because the neutrons are all clumped together as if this neutron star is one giant atomic nucleus — which it kinda is. It’s like a giant atom made only of neutrons. If our sun were a neutron star, it would be less than 20 miles wide. It would also not be something you would ever want to get near.

Got it. That means two giant balls of neutrons that weighed like, more than our sun and were only ten-ish miles wide, suddenly smashed into each other, and in the aftermath created a black hole, and we are just now detecting it on Earth?

Exactly. Pretty weird, no?

Spencer does a good job of gradually taking you through increasingly complex explanations.

For those with artistic interests, Neel V. Patel tries to answer a question about how artists knew what draw when neutron stars collided in his Oct. 18, 2017 piece for Slate.com,

All of these things make this discovery easy to marvel at and somewhat impossible to picture. Luckily, artists have taken up the task of imagining it for us, which you’ve likely seen if you’ve already stumbled on coverage of the discovery. Two bright, furious spheres of light and gas spiraling quickly into one another, resulting in a massive swell of lit-up matter along with light and gravitational waves rippling off speedily in all directions, towards parts unknown. These illustrations aren’t just alluring interpretations of a rare phenomenon; they are, to some extent, the translation of raw data and numbers into a tangible visual that gives scientists and nonscientists alike some way of grasping what just happened. But are these visualizations realistic? Is this what it actually looked like? No one has any idea. Which is what makes the scientific illustrators’ work all the more fascinating.

“My goal is to represent what the scientists found,” says Aurore Simmonet, a scientific illustrator based at Sonoma State University in Rohnert Park, California. Even though she said she doesn’t have a rigorous science background (she certainly didn’t know what a kilonova was before being tasked to illustrate one), she also doesn’t believe that type of experience is an absolute necessity. More critical, she says, is for the artist to have an interest in the subject matter and in learning new things, as well as a capacity to speak directly to scientists about their work.

Illustrators like Simmonet usually start off work on an illustration by asking the scientist what’s the biggest takeaway a viewer should grasp when looking at a visual. Unfortunately, this latest discovery yielded a multitude of papers emphasizing different conclusions and highlights. With so many scientific angles, there’s a stark challenge in trying to cram every important thing into a single drawing.

Clearly, however, the illustrations needed to center around the kilonova. Simmonet loves colors, so she began by discussing with the researchers what kind of color scheme would work best. The smash of two neutron stars lends itself well to deep, vibrant hues. Simmonet and Robin Dienel at the Carnegie Institution for Science elected to use a wide array of colors and drew bright cracking to show pressure forming at the merging. Others, like Luis Calcada at the European Southern Observatory, limited the color scheme in favor of emphasizing the bright moment of collision and the signal waves created by the kilonova.

Animators have even more freedom to show the event, since they have much more than a single frame to play with. The Conceptual Image Lab at NASA’s [US National Aeronautics and Space Administration] Goddard Space Flight Center created a short video about the new findings, and lead animator Brian Monroe says the video he and his colleagues designed shows off the evolution of the entire process: the rising action, climax, and resolution of the kilonova event.

The illustrators try to adhere to what the likely physics of the event entailed, soliciting feedback from the scientists to make sure they’re getting it right. The swirling of gas, the direction of ejected matter upon impact, the reflection of light, the proportions of the objects—all of these things are deliberately framed such that they make scientific sense. …

Do take a look at Patel’s piece, if for no other reason than to see all of the images he has embedded there. You may recognize Aurore Simmonet’s name from the credit line in the second image I have embedded here.

A new wave of physics: electrons flow like liquid in graphene

Unfortunately I couldn’t find a credit for the artist for the graphic (I really like it) which accompanies the news about a new physics and graphene,

Courtesy: University of Manchester

From an Aug. 22, 2017 news item on phys.org (Note: A link has been removed),

A new understanding of the physics of conductive materials has been uncovered by scientists observing the unusual movement of electrons in graphene.

Graphene is many times more conductive than copper thanks, in part, to its two-dimensional structure. In most metals, conductivity is limited by crystal imperfections which cause electrons to frequently scatter like billiard balls when they move through the material.

Now, observations in experiments at the National Graphene Institute have provided essential understanding as to the peculiar behaviour of electron flows in graphene, which need to be considered in the design of future Nano-electronic circuits.

An Aug. 22, 2017 University of Manchester press release, which originated the news item, delves further into the research (Note: Links have been removed),

Appearing today in Nature Physics, researchers at The University of Manchester, in collaboration with theoretical physicists led by Professor Marco Polini and Professor Leonid Levitov, show that Landauer’s fundamental limit can be breached in graphene. Even more fascinating is the mechanism responsible for this.

Last year, a new field in solid-state physics termed ‘electron hydrodynamics’ generated huge scientific interest. Three different experiments, including one performed by The University of Manchester, demonstrated that at certain temperatures, electrons collide with each other so frequently they start to flow collectively like a viscous fluid.

The new research demonstrates that this viscous fluid is even more conductive than ballistic electrons. The result is rather counter-intuitive, since typically scattering events act to lower the conductivity of a material, because they inhibit movement within the crystal. However, when electrons collide with each other, they start working together and ease current flow.

This happens because some electrons remain near the crystal edges, where momentum dissipation is highest, and move rather slowly. At the same time, they protect neighbouring electrons from colliding with those regions. Consequently, some electrons become super-ballistic as they are guided through the channel by their friends.

Sir Andre Geim said: “We know from school that additional disorder always creates extra electrical resistance. In our case, disorder induced by electron scattering actually reduces rather than increase resistance. This is unique and quite counterintuitive: Electrons when make up a liquid start propagating faster than if they were free, like in vacuum”.

The researchers measured the resistance of graphene constrictions, and found it decreases upon increasing temperature, in contrast to the usual metallic behaviour expected for doped graphene.

By studying how the resistance across the constrictions changes with temperature, the scientists revealed a new physical quantity which they called the viscous conductance. The measurements allowed them to determine electron viscosity to such a high precision that the extracted values showed remarkable quantitative agreement with theory.

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

Superballistic flow of viscous electron fluid through graphene constrictions by R. Krishna Kumar, D. A. Bandurin, F. M. D. Pellegrino, Y. Cao, A. Principi, H. Guo, G. H. Auton, M. Ben Shalom, L. A. Ponomarenko, G. Falkovich, K. Watanabe, T. Taniguchi, I. V. Grigorieva, L. S. Levitov, M. Polini, & A. K. Geim. Nature Physics (2017) doi:10.1038/nphys4240 Published online 21 August 2017

This paper is behind a paywall.

Why do objects feel solid when atoms are mostly empty space?

Roger Barlow (professor at University of Huddersfield, UK) has written a Feb. 16, 2017 essay for The Conversation explaining why objects feel solid (Note: A link has been removed),

Chemist John Dalton proposed the theory that all matter and objects are made up of particles called atoms, and this is still accepted by the scientific community, almost two centuries later. Each of these atoms is each made up of an incredibly small nucleus and even smaller electrons, which move around at quite a distance from the centre.

If you imagine a table that is a billion times larger, its atoms would be the size of melons. But even so, the nucleus at the centre would still be far too small to see and so would the electrons as they dance around it. So why don’t our fingers just pass through atoms, and why doesn’t light get through the gaps?

To explain why we must look at the electrons. Unfortunately, much of what we are taught at school is simplified – electrons do not orbit the centre of an atom like planets around the sun, like you may have been taught. Instead, think of electrons like a swarm of bees or birds, where the individual motions are too fast to track, but you still see the shape of the overall swarm.

In fact, electrons dance – there is no better word for it. …

Electrons are like a swarm of birds. John Holmes/Wikimedia Commons, CC BY-SA

Here’s one more excerpt from Barlow’s essay,

So why does a table also feel solid? Many websites will tell you that this is due to the repulsion – that two negatively charged things must repel each other. But this is wrong, and shows you should never trust some things on the internet. It feels solid because of the dancing electrons.

Do enjoy!

Phenomen: a future and emerging information technology project

A Sept. 19, 2016 news item on Nanowerk describes a new research project incorporating photonics, phononics, and radio frequency signal processing,

HENOMEN is a ground breaking project designed to harness the potential of combined phononics, photonics and radio-frequency (RF) electronic signals to lay the foundations of a new information technology. This new Project, funded though the highly competitive H2020 [the European Union’s Horizon 2020 science funding programme] FET [Future and Emerging Technologies]-Open call, joins the efforts of three leading research institutes, three internationally recognised universities and a high-tech SME. The Consortium members kick-offed the project with a meeting on Friday September 16, 2016, at the Catalan Institute of Nanoscience and Nanotechnology (ICN2), coordinated by ICREA Research Prof Dr Clivia M. Sotomayor-Torres, of the ICN2’ Phononic and Photonic Nanostructures (P2N) Group.

A Sept. 16, 2016 ICN2 press release, which originated the news item, provides more detail,

Most information is currently transported by electrical charge (electrons) and by light (photons). Phonons are the quanta of lattice vibrations with frequencies covering a wide range up to tens of THz and provide coupling to the surrounding environment. In PHENOMEN the core of the research will be focused on phonon-based signal processing to enable on-chip synchronisation and transfer information carried between optical channels by phonons.

This ambitious prospect could serve as a future scalable platform for, e.g., hybrid information processing with phonons. To achieve it, PHENOMEN proposes to build the first practical optically-driven phonon sources and detectors including the engineering of phonon lasers to deliver coherent phonons to the rest of the chip pumped by a continuous wave optical source. It brings together interdisciplinary scientific and technology oriented partners in an early-stage research towards the development of a radically new technology.

The experimental implementation of phonons as information carriers in a chip is completely novel and of a clear foundational character. It deals with interaction and manipulation of fundamental particles and their intrinsic dual wave-particle character. Thus, it can only be possible with the participation of an interdisciplinary consortium which will create knowledge in a synergetic fashion and add value in the form of new theoretical tools,  develop novel methods to manipulate coherent phonons with light and build all-optical phononic circuits enabled by optomechanics.

The H2020 FET-Open call “Novel ideas for radically new technologies” aims to support the early stages of joint science and technology research for radically new future technological possibilities. The call is entirely non-prescriptive with regards to the nature or purpose of the technologies that are envisaged and thus targets mainly the unexpected. PHENOMEN is one of the 13 funded Research & Innovation Actions and went through a selection process with a success rate (1.4%) ten times smaller than that for an ERC grant. The retained proposals are expected to foster international collaboration in a multitude of disciplines such as robotics, nanotechnology, neuroscience, information science, biology, artificial intelligence or chemistry.

The Consortium

The PHENOMEN Consortium is made up by:

  • 3 leading research institutes:
  • 3 universities with an internationally recognised track-record in their respective areas of expertise:
  • 1 industrial partner:

Light-captured energetics (harvesting light for optoelectronics)

Comparing graphene to a tiger is unusual but that’s what researcher Sanfeng Wu does—eventually—in a May 13, 2016 University of Washington news release (also on EurekAlert) about his work,

In the quest to harvest light for electronics, the focal point is the moment when photons — light particles — encounter electrons, those negatively-charged subatomic particles that form the basis of our modern electronic lives. If conditions are right when electrons and photons meet, an exchange of energy can occur. Maximizing that transfer of energy is the key to making efficient light-captured energetics possible.

“This is the ideal, but finding high efficiency is very difficult,” said University of Washington physics doctoral student Sanfeng Wu. “Researchers have been looking for materials that will let them do this — one way is to make each absorbed photon transfer all of its energy to many electrons, instead of just one electron in traditional devices.”

In traditional light-harvesting methods, energy from one photon only excites one electron or none depending on the absorber’s energy gap, transferring just a small portion of light energy into electricity. The remaining energy is lost as heat. But in a paper released May 13 in Science Advances, Wu, UW associate professor Xiaodong Xu and colleagues at four other institutions describe one promising approach to coax photons into stimulating multiple electrons. Their method exploits some surprising quantum-level interactions to give one photon multiple potential electron partners. Wu and Xu, who has appointments in the UW’s Department of Materials Science & Engineering and the Department of Physics, made this surprising discovery using graphene.

There has been intense research on graphene’s electrical properties but the researchers’ discovery adds a new property to be investigated (from the news release),

“Graphene is a substance with many exciting properties,” said Wu, the paper’s lead author. “For our purposes, it shows a very efficient interaction with light.”

Graphene is a two-dimensional hexagonal lattice of carbon atoms bonded to one another, and electrons are able to move easily within graphene. The researchers took a single layer of graphene — just one sheet of carbon atoms thick — and sandwiched it between two thin layers of a material called boron-nitride.

Boron-nitride is a material that has excited a great deal of interest in the last 12 to 18 months (from the news release),

“Boron-nitride has a lattice structure that is very similar to graphene, but has very different chemical properties,” said Wu. “Electrons do not flow easily within boron-nitride; it essentially acts as an insulator.”

Xu and Wu discovered that when the graphene layer’s lattice is aligned with the layers of boron-nitride, a type of “superlattice” is created with properties allowing efficient optoelectronics that researchers had sought. These properties rely on quantum mechanics, the occasionally baffling rules that govern interactions between all known particles of matter. Wu and Xu detected unique quantum regions within the superlattice known as Van Hove singularities.

Here’s an animated .gif illustrating the superlattice in action,

The Moire superlattice they created by aligning graphene and boron-nitride. Credit: Sanfeng Wu.

The Moire superlattice they created by aligning graphene and boron-nitride. Credit: Sanfeng Wu.

The news release goes on to describe the Van Hove singularities within the superlattice and to mention the ‘tiger’,

“These are regions of huge electron density of states, and they were not accessed in either the graphene or boron-nitride alone,” said Wu. “We only created these high electron density regions in an accessible way when both layers were aligned together.”

When Xu and Wu directed energetic photons toward the superlattice, they discovered that those Van Hove singularities were sites where one energized photon could transfer its energy to multiple electrons that are subsequently collected by electrodes— not just one electron or none with the remaining energy lost as heat. By a conservative estimate, Xu and Wu report that within this superlattice one photon could “kick” as many as five electrons to flow as current.

With the discovery of collecting multiple electrons upon the absorption of one photon, researchers may be able to create highly efficient devices that could harvest light with a large energy profit. Future work would need to uncover how to organize the excited electrons into electrical current for optimizing the energy-converting efficiency and remove some of the more cumbersome properties of their superlattice, such as the need for a magnetic field. But they believe this efficient process between photons and electrons represents major progress.

“Graphene is a tiger with great potential for optoelectronics, but locked in a cage,” said Wu. “The singularities in this superlattice are a key to unlocking that cage and releasing graphene’s potential for light harvesting application.”

H/t to a May 13, 2016 news item on phys.org.

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

Multiple hot-carrier collection in photo-excited graphene Moiré superlattices by Sanfeng Wu, Lei Wang, You Lai, Wen-Yu Shan, Grant Aivazian, Xian Zhang, Takashi Taniguchi, Kenji Watanabe, Di Xiao, Cory Dean, James Hone, Zhiqiang Li, and Xiaodong Xu. Science Advances 13 May 2016: Vol. 2, no. 5, e1600002 DOI: 10.1126/sciadv.1600002

This paper is open access.

Spins in artificial atoms same as spin in natural atoms

I wonder what impact this research on the spin in artificial and natural atoms will have on how we view the word ‘artificial’. (If artificial molecules/atoms are indistinguishable from natural ones, what does it mean to be artificial?)

An Aug. 7, 2015 news item on Nanowerk describes the finding about spin,

By extending the study of coupled quantum dots to five-electron systems, RIKEN [Japan] researchers have confirmed that the spin-based electron-filling rules for natural atoms apply to artificial molecules …

Systems consisting of electrons and semiconductor quantum dots—nanostructures that exhibit quantum properties—are highly intriguing artificial structures that in many ways mimic naturally occurring atoms. For example, electrons occupy the energy levels of quantum dots according to the same rules that determine how electrons fill atomic shells. Such systems are of both fundamental interest, for investigating phenomena related to nuclear spin, and applied interest, for manipulating spin in future quantum computers.

The Pauli exclusion principle, which prohibits any two electrons in an atom from having identical sets of quantum numbers, gives rise to a phenomenon known as the Pauli spin blockade in quantum-dot systems. This effect prevents electrons from following certain energetically favorable paths through a quantum-dot system since two electrons with the same spin cannot occupy the same energy level.
The Pauli spin blockade has been well studied in artificial molecules consisting of two quantum dots and two electrons. Shinichi Amaha and Seigo Tarucha from RIKEN’s Center for Emergent Matter Science, in collaboration with researchers in Japan and Canada, have extended the study of spin blockade to multilevel quantum-dot systems that have more than two electrons. This requires accessing high-spin states, which is difficult to achieve in practice.

TG Techno’s Aug. 7, 2015 posting of the identical news item fills in more details,

Using a two-quantum-dot system with three effective levels, the researchers have achieved spin blockade by exploiting Hund’s first rule, which dictates that electrons in an atom will first fill unoccupied orbitals of a subshell with greater total spin state. They used this principle to prepare the high-spin states needed for spin blockade …

The team discovered that the current of the device varied unexpectedly with the applied magnetic field. In most devices with spin effects, the current lags behind changes to the magnetic field, a phenomenon known as hysteresis. The researchers found that the hysteresis of their system follows the expected spin states based on a consideration of Hund’s rule and that in certain magnetic field regions two hysteresis effects cancelled each other out—clear evidence that competing ‘up’ and ‘down’ nuclear spin pumping processes influence the current.

These findings are expected to open the way to use arrays of such quantum dots as simulators for spin filling in real molecules. “Using an array of quantum dots as artificial atoms could assist investigations of novel spin-related phenomena in real molecules,” says Amaha.

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

Vanishing current hysteresis under competing nuclear spin pumping processes in a quadruplet spin-blockaded double quantum dot by  S. Amaha, T. Hatano, S. Tarucha, J. A. Gupta, and D. G. Austing. Appl. Phys. Lett. 106, 172401 (2015); http://dx.doi.org/10.1063/1.4919101

This paper is behind a paywall.

Putting the speed on spin, spintronics that is

This is for physics fans, if you plan on looking at the published paper. Otherwise, the July 20, 2015 news item on ScienceDaily is more accessible to the rest of us,

In a tremendous boost for spintronic technologies, EPFL scientists have shown that electrons can jump through spins much faster than previously thought.

Electrons spin around atoms, but also spin around themselves, and can cross over from one spin state to another. A property which can be exploited for next-generation hard drives. However, “spin cross-over” has been considered too slow to be efficient. Using ultrafast measurements, EPFL scientists have now shown for the first time that electrons can cross spins at least 100,000 times faster than previously thought. Aside for its enormous implications for fundamental physics, the finding can also propel the field of spintronics forward. …

A July 20, 2015 EPFL press release on EurekAlert, which originated the news item, provides context for the research,

The rules of spin

Although difficult to describe in everyday terms, electron spin can be loosely compared to the rotation of a planet or a spinning top around its axis. Electrons can spin in different manners referred to as “spin states” and designated by the numbers 0, 1/2, 1, 3/2, 2 etc. During chemical reactions, electrons can cross from one spin state to another, e.g. from 0 to 1 or 1/2 to 3/2.

Spin cross-over is already used in many technologies, e.g. optical light-emitting devices (OLED), energy conversion systems, and cancer phototherapy. Most prominently, spin cross-over is the basis of the fledgling field of spintronics. The problem is that spin cross-over has been thought to be too slow to be efficient enough in circuits.

Spin cross-over is extremely fast

The lab of Majed Chergui at EPFL has now demonstrated that spin cross-over is considerably faster than previously thought. Using the highest time-resolution technology in the world, the lab was able to “see” electrons crossing through four spin states within 50 quadrillionths of a second — or 50 femtoseconds.

“Time resolution has always been a limitation,” says Chergui. “Over the years, labs have used techniques that could only measure spin changes to a billionth to a millionth of a second. So they thought that spin cross-over happened in this timeframe.”

Chergui’s lab focused on materials that show much promise in spintronics applications. In these materials, electrons jump through four spin-states: from 0 to 1 to 2. In 2009, Chergui’s lab pushed the boundaries of time resolution to show that this 0-2 “jump” can happen within 150 femtoseconds — suggesting that it was a direct event. Despite this, the community still maintained that such spin cross-overs go through intermediate steps.

But Chergui had his doubts. Working with his postdoc Gerald Auböck, they used the lab’s world-recognized expertise in ultrafast spectroscopy to “crank up” the time resolution. Briefly, a laser shines on the material sample under investigation, causing its electrons to move. Another laser measures their spin changes over time in the ultraviolet light range.

The finding essentially demolishes the notion of intermediate steps between spin jumps, as it does not allow enough time for them: only 50 quadrillionths of a second to go from the “0” to the “2” spin state. This is the first study to ever push time resolution to this limit in the ultraviolet domain. “This probably means that it’s even faster,” says Chergui. “But, more importantly, that it is a direct process.”

From observation to explanation

With profound implications for both technology and fundamental physics and chemistry, the study is an observation without an explanation. Chergui believes that the key is electrons shuttling back-and-forth between the iron atom at the center of the material’s molecules and its surrounding elements. “When the laser light shines on the atom, it changes the electron’s spin angle, affecting the entire spin dynamics in the molecule.”

It is now up to theoreticians to develop a new model for ultrafast spin changes. On the experimental side of things, Chergui’s lab is now focusing on actually observing electrons shuttling inside the molecules. This will require even more sophisticated approaches, such as core-level spectroscopy. Nonetheless, the study challenges ideas about spin cross-over, and might offer long-awaited solutions to the limitations of spintronics.

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

Sub-50-fs photoinduced spin crossover in [Fe(bpy)3]2+ by Gerald Auböck & Majed Chergui. Nature Chemistry (2015) doi:10.1038/nchem.2305 Published online 20 July 2015

This paper is behind a paywall.

TRIUMF accelerator used by US researchers to visualize properties of nanoscale materials

The US researchers are at the University of California at Los Angeles (UCLA) and while it’s not explicitly stated I’m assuming the accelerator they mention at TRIUMF (Canada’s national laboratory for particle and nuclear physics) has something special as there are accelerators in California and other parts of the US.

A July 15, 2015 news item on Nanotechnology Now announces the latest on visualizing the properties of nanoscale materials,

Scientists trying to improve the semiconductors that power our electronic devices have focused on a technology called spintronics as one especially promising area of research. Unlike conventional devices that use electrons’ charge to create power, spintronic devices use electrons’ spin. The technology is already used in computer hard drives and many other applications — and scientists believe it could eventually be used for quantum computers, a new generation of machines that use quantum mechanics to solve complex problems with extraordinary speed.

A July 15, 2015 UCLA news release, which originated the news item, expands on the theme and briefly mentions TRIUMF’s accelerator (Note: A link has been removed),

Emerging research has shown that one key to greatly improving performance in spintronics could be a class of materials called topological insulators. Unlike ordinary materials that are either insulators or conductors, topological insulators function as both simultaneously — on the inside, they are insulators but on their exteriors, they conduct electricity.

But topological insulators have certain defects that have so far limited their use in practical applications, and because they are so tiny, scientists have so far been unable to fully understand how the defects impact their functionality.

The UCLA researchers have overcome that challenge with a new method to visualize topological insulators at the nanoscale. An article highlighting the research, which was which led by Louis Bouchard, assistant professor of chemistry and biochemistry, and Dimitrios Koumoulis, a UCLA postdoctoral scholar, was published online in the Proceedings of the National Academy of Sciences.

The new method is the first use of beta‑detected nuclear magnetic resonance to study the effects of these defects on the properties of topological insulators.

The technique involves aiming a highly focused stream of ions at the topological insulator. To generate that beam of ions, the researchers used a large particle accelerator called a cyclotron, which accelerates protons through a spiral path inside the machine and forces them to collide with a target made of the chemical element tantalum. This collision produces lithium-8 atoms, which are ionized and slowed down to a desired energy level before they are implanted in the topological insulators.

In beta‑detected nuclear magnetic resonance, ions (in this case, the ionized lithium-8 atoms) of various energies are implanted in the material of interest (the topological insulator) to generate signals from the material’s layers of interest.

Bouchard said the method is particularly well suited for probing regions near the surfaces and interfaces of different materials.

In the UCLA research, the high sensitivity of the beta‑detected nuclear magnetic resonance technique and its ability to probe materials allowed the scientists to “see” the impacts of the defects in the topological insulators by viewing the electronic and magnetic properties beneath the surface of the material.

The researchers used the large TRIUMF cyclotron in Vancouver, British Columbia.

According to the UCLA news release, there were also researchers from the University of British Columbia, the University of Texas at Austin and Northwestern University *were* involved with the work.

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

Nanoscale β-nuclear magnetic resonance depth imaging of topological insulators by Dimitrios Koumoulis, Gerald D. Morris, Liang He, Xufeng Kou, Danny King, Dong Wang, Masrur D. Hossain, Kang L. Wang, Gregory A. Fiete, Mercouri G. Kanatzidis, and Louis-S. Bouchard. PNAS July 14, 2015 vol. 112 no. 28 doi: 10.1073/pnas.1502330112

This paper is behind a paywall.

*’were’ added Jan. 20, 2016.

Fermionic atoms and the microscopes that can see them

The new fermionic microscope built at the Massachusetts Institute of Technology (MIT) allows you to image 1000 or more fermionic atoms according to a May 13, 2015 news item on ScienceDaily,

Fermions are the building blocks of matter, interacting in a multitude of permutations to give rise to the elements of the periodic table. Without fermions, the physical world would not exist.

Examples of fermions are electrons, protons, neutrons, quarks, and atoms consisting of an odd number of these elementary particles. Because of their fermionic nature, electrons and nuclear matter are difficult to understand theoretically, so researchers are trying to use ultracold gases of fermionic atoms as stand-ins for other fermions.

But atoms are extremely sensitive to light: When a single photon hits an atom, it can knock the particle out of place — an effect that has made imaging individual fermionic atoms devilishly hard.

Now a team of MIT physicists has built a microscope that is able to see up to 1,000 individual fermionic atoms. The researchers devised a laser-based technique to trap and freeze fermions in place, and image the particles simultaneously.

A May 13, 2015 MIT news release, which originated the news item, provides intriguing detail about the microscope and fascinating insight into fermions (for those who are interested but not expert and sufficiently brave to endure certain failure to understand everything in this piece),

The new imaging technique uses two laser beams trained on a cloud of fermionic atoms in an optical lattice. The two beams, each of a different wavelength, cool the cloud, causing individual fermions to drop down an energy level, eventually bringing them to their lowest energy states — cool and stable enough to stay in place. At the same time, each fermion releases light, which is captured by the microscope and used to image the fermion’s exact position in the lattice — to an accuracy better than the wavelength of light.

With the new technique, the researchers are able to cool and image over 95 percent of the fermionic atoms making up a cloud of potassium gas. Martin Zwierlein, a professor of physics at MIT, says an intriguing result from the technique appears to be that it can keep fermions cold even after imaging.

“That means I know where they are, and I can maybe move them around with a little tweezer to any location, and arrange them in any pattern I’d like,” Zwierlein says.

Zwierlein and his colleagues, including first author and graduate student Lawrence Cheuk, have published their results today in the journal Physical Review Letters.

Seeing fermions from bosons

For the past two decades, experimental physicists have studied ultracold atomic gases of the two classes of particles: fermions and bosons — particles such as photons that, unlike fermions, can occupy the same quantum state in limitless numbers. In 2009, physicist Markus Greiner at Harvard University devised a microscope that successfully imaged individual bosons in a tightly spaced optical lattice. This milestone was followed, in 2010, by a second boson microscope, developed by Immanuel Bloch’s group at the Max Planck Institute of Quantum Optics.

These microscopes revealed, in unprecedented detail, the behavior of bosons under strong interactions. However, no one had yet developed a comparable microscope for fermionic atoms.

“We wanted to do what these groups had done for bosons, but for fermions,” Zwierlein says. “And it turned out it was much harder for fermions, because the atoms we use are not so easily cooled. So we had to find a new way to cool them while looking at them.”

Techniques to cool atoms ever closer to absolute zero have been devised in recent decades. Carl Wieman, Eric Cornell, and MIT’s Wolfgang Ketterle were able to achieve Bose-Einstein condensation in 1995, a milestone for which they were awarded the 2001 Nobel Prize in physics. Other techniques include a process using lasers to cool atoms from 300 degrees Celsius to a few ten-thousandths of a degree above absolute zero.

A clever cooling technique

And yet, to see individual fermionic atoms, the particles need to be cooled further still. To do this, Zwierlein’s group created an optical lattice using laser beams, forming a structure resembling an egg carton, each well of which could potentially trap a single fermion. Through various stages of laser cooling, magnetic trapping, and further evaporative cooling of the gas, the atoms were prepared at temperatures just above absolute zero — cold enough for individual fermions to settle onto the underlying optical lattice. The team placed the lattice a mere 7 microns from an imaging lens, through which they hoped to see individual fermions.

However, seeing fermions requires shining light on them, causing a photon to essentially knock a fermionic atom out of its well, and potentially out of the system entirely.

“We needed a clever technique to keep the atoms cool while looking at them,” Zwierlein says.

His team decided to use a two-laser approach to further cool the atoms; the technique manipulates an atom’s particular energy level, or vibrational energy. Each atom occupies a certain energy state — the higher that state, the more active the particle is. The team shone two laser beams of differing frequencies at the lattice. The difference in frequencies corresponded to the energy between a fermion’s energy levels. As a result, when both beams were directed at a fermion, the particle would absorb the smaller frequency, and emit a photon from the larger-frequency beam, in turn dropping one energy level to a cooler, more inert state. The lens above the lattice collects the emitted photon, recording its precise position, and that of the fermion.

Zwierlein says such high-resolution imaging of more than 1,000 fermionic atoms simultaneously would enhance our understanding of the behavior of other fermions in nature — particularly the behavior of electrons. This knowledge may one day advance our understanding of high-temperature superconductors, which enable lossless energy transport, as well as quantum systems such as solid-state systems or nuclear matter.

“The Fermi gas microscope, together with the ability to position atoms at will, might be an important step toward the realization of a quantum computer based on fermions,” Zwierlein says. “One would thus harness the power of the very same intricate quantum rules that so far hamper our understanding of electronic systems.”

Zwierlein says it is a good time for Fermi gas microscopists: Around the same time his group first reported its results, teams from Harvard and the University of Strathclyde in Glasgow also reported imaging individual fermionic atoms in optical lattices, indicating a promising future for such microscopes.

Zoran Hadzibabic, a professor of physics at Trinity College [University of Cambridge, UK], says the group’s microscope is able to detect individual atoms “with almost perfect fidelity.”

“They detect them reliably, and do so without affecting their positions — that’s all you want,” says Hadzibabic, who did not contribute to the research. “So far they demonstrated the technique, but we know from the experience with bosons that that’s the hardest step, and I expect the scientific results to start pouring out.”

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

Quantum-Gas Microscope for Fermionic Atoms by Lawrence W. Cheuk, Matthew A. Nichols, Melih Okan, Thomas Gersdorf, Vinay V. Ramasesh, Waseem S. Bakr, Thomas Lompe, and Martin W. Zwierlein. Phys. Rev. Lett. 114, 193001 – Published 13 May 2015 (print: Vol. 114, Iss. 19 — 15 May 2015) DOI: http://dx.doi.org/10.1103/PhysRevLett.114.193001

I believe this paper is behind a paywall.

There is an earlier version available on arXiv.org,

A Quantum Gas Microscope for Fermionic Atoms by Lawrence W. Cheuk, Matthew A. Nichols, Melih Okan, Thomas Gersdorf, Vinay V. Ramasesh, Waseem S. Bakr, Thomas Lompe, Martin W. Zwierlein. (Submitted on 9 Mar 2015 (v1), last revised 10 Mar 2015 (this version, v2))

This an open access website.

Graphene, Perimeter Institute, and condensed matter physics

In short, researchers at Canada’s Perimeter Institute are working on theoretical models involving graphene. which could lead to quantum computing. A July 3, 2014 Perimeter Institute news release by Erin Bow (also on EurekAlert) provides some insight into the connections between graphene and condensed matter physics (Note: Bow has included some good basic explanations of graphene, quasiparticles, and more for beginners),

One of the hottest materials in condensed matter research today is graphene.

Graphene had an unlikely start: it began with researchers messing around with pencil marks on paper. Pencil “lead” is actually made of graphite, which is a soft crystal lattice made of nothing but carbon atoms. When pencils deposit that graphite on paper, the lattice is laid down in thin sheets. By pulling that lattice apart into thinner sheets – originally using Scotch tape – researchers discovered that they could make flakes of crystal just one atom thick.

The name for this atom-scale chicken wire is graphene. Those folks with the Scotch tape, Andre Geim and Konstantin Novoselov, won the 2010 Nobel Prize for discovering it. “As a material, it is completely new – not only the thinnest ever but also the strongest,” wrote the Nobel committee. “As a conductor of electricity, it performs as well as copper. As a conductor of heat, it outperforms all other known materials. It is almost completely transparent, yet so dense that not even helium, the smallest gas atom, can pass through it.”

Developing a theoretical model of graphene

Graphene is not just a practical wonder – it’s also a wonderland for theorists. Confined to the two-dimensional surface of the graphene, the electrons behave strangely. All kinds of new phenomena can be seen, and new ideas can be tested. Testing new ideas in graphene is exactly what Perimeter researchers Zlatko Papić and Dmitry (Dima) Abanin set out to do.

“Dima and I started working on graphene a very long time ago,” says Papić. “We first met in 2009 at a conference in Sweden. I was a grad student and Dima was in the first year of his postdoc, I think.”

The two young scientists got to talking about what new physics they might be able to observe in the strange new material when it is exposed to a strong magnetic field.

“We decided we wanted to model the material,” says Papić. They’ve been working on their theoretical model of graphene, on and off, ever since. The two are now both at Perimeter Institute, where Papić is a postdoctoral researcher and Abanin is a faculty member. They are both cross-appointed with the Institute for Quantum Computing (IQC) at the University of Waterloo.

In January 2014, they published a paper in Physical Review Letters presenting new ideas about how to induce a strange but interesting state in graphene – one where it appears as if particles inside it have a fraction of an electron’s charge.

It’s called the fractional quantum Hall effect (FQHE), and it’s head turning. Like the speed of light or Planck’s constant, the charge of the electron is a fixed point in the disorienting quantum universe.

Every system in the universe carries whole multiples of a single electron’s charge. When the FQHE was first discovered in the 1980s, condensed matter physicists quickly worked out that the fractionally charged “particles” inside their semiconductors were actually quasiparticles – that is, emergent collective behaviours of the system that imitate particles.

Graphene is an ideal material in which to study the FQHE. “Because it’s just one atom thick, you have direct access to the surface,” says Papić. “In semiconductors, where FQHE was first observed, the gas of electrons that create this effect are buried deep inside the material. They’re hard to access and manipulate. But with graphene you can imagine manipulating these states much more easily.”

In the January paper, Abanin and Papić reported novel types of FQHE states that could arise in bilayer graphene – that is, in two sheets of graphene laid one on top of another – when it is placed in a strong perpendicular magnetic field. In an earlier work from 2012, they argued that applying an electric field across the surface of bilayer graphene could offer a unique experimental knob to induce transitions between FQHE states. Combining the two effects, they argued, would be an ideal way to look at special FQHE states and the transitions between them.

Once the scientists developed their theory they went to work on some experiments,

Two experimental groups – one in Geneva, involving Abanin, and one at Columbia, involving both Abanin and Papić – have since put the electric field + magnetic field method to good use. The paper by the Columbia group appears in the July 4 issue of Science. A third group, led by Amir Yacoby of Harvard, is doing closely related work.

“We often work hand-in-hand with experimentalists,” says Papić. “One of the reasons I like condensed matter is that often even the most sophisticated, cutting-edge theory stands a good chance of being quickly checked with experiment.”

Inside both the magnetic and electric field, the electrical resistance of the graphene demonstrates the strange behaviour characteristic of the FQHE. Instead of resistance that varies in a smooth curve with voltage, resistance jumps suddenly from one level to another, and then plateaus – a kind of staircase of resistance. Each stair step is a different state of matter, defined by the complex quantum tangle of charges, spins, and other properties inside the graphene.

“The number of states is quite rich,” says Papić. “We’re very interested in bilayer graphene because of the number of states we are detecting and because we have these mechanisms – like tuning the electric field – to study how these states are interrelated, and what happens when the material changes from one state to another.”

For the moment, researchers are particularly interested in the stair steps whose “height” is described by a fraction with an even denominator. That’s because the quasiparticles in that state are expected to have an unusual property.

There are two kinds of particles in our three-dimensional world: fermions (such as electrons), where two identical particles can’t occupy one state, and bosons (such as photons), where two identical particles actually want to occupy one state. In three dimensions, fermions are fermions and bosons are bosons, and never the twain shall meet.

But a sheet of graphene doesn’t have three dimensions – it has two. It’s effectively a tiny two-dimensional universe, and in that universe, new phenomena can occur. For one thing, fermions and bosons can meet halfway – becoming anyons, which can be anywhere in between fermions and bosons. The quasiparticles in these special stair-step states are expected to be anyons.

In particular, the researchers are hoping these quasiparticles will be non-Abelian anyons, as their theory indicates they should be. That would be exciting because non-Abelian anyons can be used in the making of qubits.

Graphene qubits?

Qubits are to quantum computers what bits are to ordinary computers: both a basic unit of information and the basic piece of equipment that stores that information. Because of their quantum complexity, qubits are more powerful than ordinary bits and their power grows exponentially as more of them are added. A quantum computer of only a hundred qubits can tackle certain problems beyond the reach of even the best non-quantum supercomputers. Or, it could, if someone could find a way to build stable qubits.

The drive to make qubits is part of the reason why graphene is a hot research area in general, and why even-denominator FQHE states – with their special anyons – are sought after in particular.

“A state with some number of these anyons can be used to represent a qubit,” says Papić. “Our theory says they should be there and the experiments seem to bear that out – certainly the even-denominator FQHE states seem to be there, at least according to the Geneva experiments.”

That’s still a step away from experimental proof that those even-denominator stair-step states actually contain non-Abelian anyons. More work remains, but Papić is optimistic: “It might be easier to prove in graphene than it would be in semiconductors. Everything is happening right at the surface.”

It’s still early, but it looks as if bilayer graphene may be the magic material that allows this kind of qubit to be built. That would be a major mark on the unlikely line between pencil lead and quantum computers.

Here are links for further research,

January PRL paper mentioned above: http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.046602

Experimental paper from the Geneva graphene group, including Abanin: http://pubs.acs.org/doi/abs/10.1021/nl5003922

Experimental paper from the Columbia graphene group, including both Abanin and Papić: http://arxiv.org/abs/1403.2112. This paper is featured in the journal Science.

Related experiment on bilayer graphene by Amir Yacoby’s group at Harvard: http://www.sciencemag.org/content/early/2014/05/28/science.1250270

The Nobel Prize press release on graphene, mentioned above: http://www.nobelprize.org/nobel_prizes/physics/laureates/2010/press.html

I recently posted a piece about some research into the ‘scotch-tape technique’ for isolating graphene (June 30, 2014 posting). Amusingly, Geim argued against coining the technique as the ‘scotch-tape’ technique, something I found out only recently.