Tag Archives: superconductivity

Graphene and its magnetism

I have two news bits about graphene and magnetism. If I understood what I was reading, one is more focused on applications and the other is focused on further establishing the field of valleytronics.

University of Cambridge and superconductivity

A February 8, 2021 news item on Nanowerk announces ‘magnetic work’ from the University of Cambridge (Note: A link has been removed),

The researchers, led by the University of Cambridge, were able to control the conductivity and magnetism of iron thiophosphate (FePS3), a two-dimensional material which undergoes a transition from an insulator to a metal when compressed. This class of magnetic materials offers new routes to understanding the physics of new magnetic states and superconductivity.

Using new high-pressure techniques, the researchers have shown what happens to magnetic graphene during the transition from insulator to conductor and into its unconventional metallic state, realised only under ultra-high pressure conditions. When the material becomes metallic, it remains magnetic, which is contrary to previous results and provides clues as to how the electrical conduction in the metallic phase works. The newly discovered high-pressure magnetic phase likely forms a precursor to superconductivity so understanding its mechanisms is vital.

Their results, published in the journal Physical Review X, also suggest a way that new materials could be engineered to have combined conduction and magnetic properties, which could be useful in the development of new technologies such as spintronics, which could transform the way in which computers process information.

A February 8, 2021 University of Cambridge press release (also on EurekAlert), which originated the news item, delves into the topic,

Properties of matter can alter dramatically with changing dimensionality. For example, graphene, carbon nanotubes, graphite and diamond are all made of carbon atoms, but have very different properties due to their different structure and dimensionality.

“But imagine if you were also able to change all of these properties by adding magnetism,” said first author Dr Matthew Coak, who is jointly based at Cambridge’s Cavendish Laboratory and the University of Warwick. “A material which could be mechanically flexible and form a new kind of circuit to store information and perform computation. This is why these materials are so interesting, and because they drastically change their properties when put under pressure so we can control their behaviour.”

In a previous study by Sebastian Haines of Cambridge’s Cavendish Laboratory and the Department of Earth Sciences, researchers established that the material becomes a metal at high pressure, and outlined how the crystal structure and arrangement of atoms in the layers of this 2D material change through the transition.

“The missing piece has remained however, the magnetism,” said Coak. “With no experimental techniques able to probe the signatures of magnetism in this material at pressures this high, our international team had to develop and test our own new techniques to make it possible.”

The researchers used new techniques to measure the magnetic structure up to record-breaking high pressures, using specially designed diamond anvils and neutrons to act as the probe of magnetism. They were then able to follow the evolution of the magnetism into the metallic state.

“To our surprise, we found that the magnetism survives and is in some ways strengthened,” co-author Dr Siddharth Saxena, group leader at the Cavendish Laboratory. “This is unexpected, as the newly-freely-roaming electrons in a newly conducting material can no longer be locked to their parent iron atoms, generating magnetic moments there – unless the conduction is coming from an unexpected source.”

In their previous paper, the researchers showed these electrons were ‘frozen’ in a sense. But when they made them flow or move, they started interacting more and more. The magnetism survives, but gets modified into new forms, giving rise to new quantum properties in a new type of magnetic metal.

How a material behaves, whether conductor or insulator, is mostly based on how the electrons, or charge, move around. However, the ‘spin’ of the electrons has been shown to be the source of magnetism. Spin makes electrons behave a bit like tiny bar magnets and point a certain way. Magnetism from the arrangement of electron spins is used in most memory devices: harnessing and controlling it is important for developing new technologies such as spintronics, which could transform the way in which computers process information.

“The combination of the two, the charge and the spin, is key to how this material behaves,” said co-author Dr David Jarvis from the Institut Laue-Langevin, France, who carried out this work as the basis of his PhD studies at the Cavendish Laboratory. “Finding this sort of quantum multi-functionality is another leap forward in the study of these materials.”

“We don’t know exactly what’s happening at the quantum level, but at the same time, we can manipulate it,” said Saxena. “It’s like those famous ‘unknown unknowns’: we’ve opened up a new door to properties of quantum information, but we don’t yet know what those properties might be.”

There are more potential chemical compounds to synthesise than could ever be fully explored and characterised. But by carefully selecting and tuning materials with special properties, it is possible to show the way towards the creation of compounds and systems, but without having to apply huge amounts of pressure.

Additionally, gaining fundamental understanding of phenomena such as low-dimensional magnetism and superconductivity allows researchers to make the next leaps in materials science and engineering, with particular potential in energy efficiency, generation and storage.

As for the case of magnetic graphene, the researchers next plan to continue the search for superconductivity within this unique material. “Now that we have some idea what happens to this material at high pressure, we can make some predictions about what might happen if we try to tune its properties through adding free electrons by compressing it further,” said Coak.

“The thing we’re chasing is superconductivity,” said Saxena. “If we can find a type of superconductivity that’s related to magnetism in a two-dimensional material, it could give us a shot at solving a problem that’s gone back decades.”

The citation and link to the paper are at the end of this blog posting.

Aalto University’s valleytronics

Further north in Finland, researchers at Aalto University make some advances applicable to the field of valleytronics, from a February 5, 2021 Aalto University press release (also on EurekAltert but published February 8, 2021),

Electrons in materials have a property known as ‘spin’, which is responsible for a variety of properties, the most well-known of which is magnetism. Permanent magnets, like the ones used for refrigerator doors, have all the spins in their electrons aligned in the same direction. Scientists refer to this behaviour as ferromagnetism, and the research field of trying to manipulate spin as spintronics.

Down in the quantum world, spins can arrange in more exotic ways, giving rise to frustrated states and entangled magnets. Interestingly, a property similar to spin, known as “the valley,” appears in graphene materials. This unique feature has given rise to the field of valleytronics, which aims to exploit the valley property for emergent physics and information processing, very much like spintronics relies on pure spin physics.

‘Valleytronics would potentially allow encoding information in the quantum valley degree of freedom, similar to how electronics do it with charge and spintronics with the spin.’ Explains Professor Jose Lado, from Aalto’s Department of applied physics, and one of the authors of the work. ‘What’s more, valleytronic devices would offer a dramatic increase in the processing speeds in comparison with electronics, and with much higher stability towards magnetic field noise in comparison with spintronic devices.’

Structures made of rotated, ultra-thin materials provide a rich solid-state platform for designing novel devices. In particular, slightly twisted graphene layers have recently been shown to have exciting unconventional properties, that can ultimately lead to a new family of materials for quantum technologies. These unconventional states which are already being explored depend on electrical charge or spin. The open question is if the valley can also lead to its own family of exciting states.

Making materials for valleytronics

For this goal, it turns out that conventional ferromagnets play a vital role, pushing graphene to the realms of valley physics. In a recent work, Ph.D. student Tobias Wolf, together with Profs. Oded Zilberberg and Gianni Blatter at ETH Zurich, and Prof. Jose Lado at Aalto University, showed a new direction for correlated physics in magnetic van der Waals materials.

The team showed that sandwiching two slightly rotated layers of graphene between a ferromagnetic insulator provides a unique setting for new electronic states. The combination of ferromagnets, graphene’s twist engineering, and relativistic effects force the “valley” property to dominate the electrons behaviour in the material. In particular, the researchers showed how these valley-only states can be tuned electrically, providing a materials platform in which valley-only states can be generated. Building on top of the recent breakthrough in spintronics and van der Waals materials, valley physics in magnetic twisted van der Waals multilayers opens the door to the new realm of correlated twisted valleytronics.

‘Demonstrating these states represents the starting point towards new exotic entangled valley states.’ Said Professor Lado, ‘Ultimately, engineering these valley states can allow realizing quantum entangled valley liquids and fractional quantum valley Hall states. These two exotic states of matter have not been found in nature yet, and would open exciting possibilities towards a potentially new graphene-based platform for topological quantum computing.’

Citations and links

Here’s a link to and a citation for the University of Cambridge research,

Emergent Magnetic Phases in Pressure-Tuned van der Waals Antiferromagnet FePS3 by Matthew J. Coak, David M. Jarvis, Hayrullo Hamidov, Andrew R. Wildes, Joseph A. M. Paddison, Cheng Liu, Charles R. S. Haines, Ngoc T. Dang, Sergey E. Kichanov, Boris N. Savenko, Sungmin Lee, Marie Kratochvílová, Stefan Klotz, Thomas C. Hansen, Denis P. Kozlenko, Je-Geun Park, and Siddharth S. Saxena. Phys. Rev. X 11, 011024 DOI: https://doi.org/10.1103/PhysRevX.11.011024 Published 5 February 2021

This article appears to be open access.

Here’s a link to and a citation for the Aalto University research,

Spontaneous Valley Spirals in Magnetically Encapsulated Twisted Bilayer Graphene by Tobias M. R. Wolf, Oded Zilberberg, Gianni Blatter, and Jose L. Lado. Phys. Rev. Lett. 126, 056803 DOI: https://doi.org/10.1103/PhysRevLett.126.056803 Published 4 February 2021

This paper is behind a paywall.

The latest ‘golden’ age for electronics

I don’t know the dates for the last ‘golden’ age of electronics but I can certainly understand why these Japanese researchers are excited about their work. In any event, I think the ‘golden age’ is more of a play on words. From a June 25, 2019 news item on Nanowerk (Note: A link has been removed),

One way that heat damages electronic equipment is it makes components expand at different rates, resulting in forces that cause micro-cracking and distortion. Plastic components and circuit boards are particularly prone to damage due to changes in volume during heating and cooling cycles. But if a material could be incorporated into the components that compensates for the expansion, the stresses would be reduced and their lifetime increased.

Everybody knows one material that behaves like this: liquid water expands when it freezes and ice contracts when it melts. But liquid water and electronics don’t mix well – instead, what’s needed is a solid with “negative thermal expansion” (NTE).

Although such materials have been known since the 1960s, a number of challenges had to be overcome before the concept would be broadly useful and commercially viable. In terms of both materials and function, these efforts have only had limited success.

The experimental materials had been produced under specialized laboratory conditions using expensive equipment; and even then, the temperature and pressure ranges in which they would exhibit NTE were well outside normal everyday conditions.

Moreover, the amount they expanded and contracted depended on the direction, which induced internal stresses that changed their structure, meaning that the NTE property would not last longer than a few heating and cooling cycles.

A research team led by Koshi Takenaka of Nagoya University has succeeded in overcoming these materials-engineering challenges (APL Materials, “Valence fluctuations and giant isotropic negative thermal expansion in Sm1–xRxS (R = Y, La, Ce, Pr, Nd)”).

A June 22, 2019 Nagoya University press release (also on EurekAlert but published on June 25, 2019), which originated the news item, provides more technical detail,

Inspired by the series of work by Noriaki Sato, also of Nagoya University – whose discovery last year of superconductivity in quasicrystals was considered one of the top ten physics discoveries of the year by Physics World magazine – Professor Takenaka took the rare earth element samarium and its sulfide, samarium monosulfide (SmS), which is known to change phase from the “black phase” to the smaller-volume “golden phase”. The problem was to tune the range of temperatures at which the phase transition occurs. The team’s solution was to replace a small proportion of samarium atoms with another rare earth element, giving Sm1-xRxS, where “R” is any one of the rare earth elements cerium (Ce), neodymium (Nd), praseodymium (Pr) or yttrium (Y). The fraction x the team used was typically 0.2, except for yttrium. These materials showed “giant negative thermal expansion” of up to 8% at ordinary room pressure and a useful range of temperatures (around 150 degrees) including at room temperature and above … . Cerium is the star candidate here because it is relatively cheap.

The nature of the phase transition is such that the materials can be powdered into very small crystal sizes around a micron on a side without losing their negative expansion property. This broadens the industrial applications, particularly within electronics.

While the Nagoya University group’s engineering achievement is impressive, how the negative expansion works is fascinating from a fundamental physics viewpoint. During the black-golden transition, the crystal structure stays the same but the atoms get closer together: the unit cell size becomes smaller because (as is very likely but perhaps not yet 100% certain) the electron structure of the samarium atoms changes and makes them smaller – a process of intra-atomic charge transfer called a “valence transition” or “valence fluctuation” within the samarium atoms … . “My impression,” says Professor Takenaka, “is that the correlation between the lattice volume and the electron structure of samarium is experimentally verified for this class of sulfides.”

More specifically, in the black (lower temperature) phase, the electron configuration of the samarium atoms is (4f)6, meaning that in their outermost shell they have 6 electrons in the f orbitals (with s, p and d orbitals filled); while in the golden phase the electronic configuration is (4f)5(5d)1 -an electron has moved out of a 4f orbital into a 5d orbital. Although a “higher” shell is starting to be occupied, it turns out – through a quirk of the Pauli Exclusion Principle – that the second case gives a smaller atom size, leading to a smaller crystal size and negative expansion.

But this is only part of the fundamental picture. In the black phase, samarium sulfide and its doped offshoots are insulators – they do not conduct electricity; while in the golden phase they turn into conductors (i.e. metals). This is suggesting that during the black-golden phase transition the band structure of the whole crystal is influencing the valance transition within the samarium atoms. Although nobody has done the theoretical calculations for the doped samarium sulfides made by Professor Takenaka’s group, a previous theoretical study has indicated that when electrons leave the samarium atoms’ f orbital, they leave behind a positively charged “hole” which itself interacts repulsively with holes in the crystal’s conduction band, affecting their exchange interaction. This becomes a cooperative effect that then drives the valence transition in the samarium atoms. The exact mechanism, though, is not well understood.

Nevertheless, the Nagoya University-led group’s achievement is one of engineering, not pure physics. “What is important for many engineers is the ability to use the material to reduce device failure due to thermal expansion,” explains Professor Takenaka. “In short, in a certain temperature range – the temperature range in which the intended device operates, typically an interval of dozens of degrees or more – the volume needs to gradually decrease with a rise in temperature and increase as the temperature falls. Of course, I also know that volume expansion on cooling during a phase transition [like water freezing] is a common case for many materials. However, if the volume changes in a very narrow temperature range, there is no engineering value. The present achievement is the result of material engineering, not pure physics.”

Perhaps it even heralds a new “golden” age for electronics.

I worked in a company for a data communications company that produced hardware and network management software. From a hardware perspective, heat was an enemy which distorted your circuit boards and cost you significant money not only for replacements but also when you included fans to keep the equipment cool (or as cool as possible).

Enough with the reminiscences, here’s a link to and a citation for the paper,

Valence fluctuations and giant isotropic negative thermal expansion in Sm1–xRxS (R = Y, La, Ce, Pr, Nd) by D. Asai, Y. Mizuno, H. Hasegawa, Y. Yokoyama, Y. Okamoto, N. Katayama, H. S. Suzuki, Y. Imanaka, and K. Takenaka. Applied Physics Letters > Volume 114, Issue 14 > 10.1063/1.5090546 or Appl. Phys. Lett. 114, 141902 (2019); https://doi.org/10.1063/1.5090546. Published Online: 12 April 2019

This paper is behind a paywall.

Café Scientifique (Vancouver, Canada) April 25, 2017 talk: No Small Feat: Seeing Atoms and Molecules

I thought I’d been knocked off the list but finally I have a notice for an upcoming Café Scientifique talk that arrived and before the event, at that.  From an April 12, 2017 notice (received via email),

Our next café will happen on TUESDAY APRIL 25TH, 7:30PM in the back
room at YAGGER’S DOWNTOWN (433 W Pender). Our speaker for the
evening will be DR. SARAH BURKE, an Assistant Professor in the
Department of Physics and Astronomy/ Department of Chemistry at UBC [University of British Columbia]. The title of her talk is:

NO SMALL FEAT: SEEING ATOMS AND MOLECULES

From solar cells to superconductivity, the properties of materials and
the devices we make from them arise from the atomic scale structure of
the atoms that make up the material, their electrons, and how they all
interact.  Seeing this takes a microscope, but not like the one you may
have had as a kid or used in a university lab, which are limited to
seeing objects on the scale of the wavelength of visible light: still
thousands of times bigger than the size of an atom.  Scanning probe
microscopes operate more like a nanoscale record player, scanning a very
sharp tip over a surface and measuring interactions between the tip and
surface to create atomically resolved images.  These techniques show us
where atoms and electrons live at surfaces, on nanostructures, and in
molecules.  I will describe how these techniques give us a powerful
glimpse into a tiny world.

I have a little more about Sarah Burke from her webpage in the UBC Physics and Astronomy webspace,

Building an understanding of important electronic and optoelectronic processes in nanoscale materials from the atomic scale up will pave the way for next generation materials and technologies.

My research interests broadly encompass the study of electronic processes where nanoscale structure influences or reveals the underlying physics. Using scanning probe microscopy (SPM) techniques, my group investigates materials for organic electronics and optoelectronics, graphene and other carbon-based nanomaterials, and other materials where a nanoscale view offers the potential for new understanding. We also work to expand the SPM toolbox; developing new methods in order to probe different aspects of materials, and working to understand leading edge techniques.

For the really curious, you can find more information about her research group, UBC Laboratory for Atomic Imaging Research (LAIR) here.

Superconducting graphene from Saint Jean Carbon (a Canadian company)

An announcement from Saint Jean Carbon helps to paint a picture of one Canadian graphene research and commercialization effort. From an Oct. 26, 2015 news item on Azonano,

Saint Jean Carbon Inc., a carbon sciences company engaged in the development of natural graphite properties and related carbon products is pleased to announce that it has completed an initial phase of research and development (R&D) work on the development of superconducting graphene.

An Oct. 22, 2015 Saint Jean Carbon news release, (also on Marketwired) which originated the news item, explains the company’s interest in superconducting graphene,

The result of the work has produced graphene that possibly may have magnetic properties; Magnetic properties are what is needed if the material is used in superconducting applications. This is believed to be a first. The encouraging result is just the very first step with many more tests to complete. Hopefully, this puts the project on the path towards the development of a low-temperature superconductor that leverages key properties of graphene.

Superconductivity is defined as a quantum mechanical phenomenon that offers the potential for zero electrical resistance. The ability to operate with no electrical resistance at or near room temperature holds significant potential in a wide range of product and technology applications. This include high-performance smart grids, electric power transmission, transformers, power storage devices, electric motors used in vehicle propulsion as in maglev trains, magnetic levitation devices, spintronic devices and superconducting magnetic refrigeration. Solving this puzzle; would have enormous technological importance.

The work has been based on the identification of the growing understanding of the magnetic properties (the ability to repel magnetic fields) of graphene. These properties could play a crucial role in enhancing superconductivity and therefore make it a good candidate for continued efforts to realize its potential. To truly understand the magnetic properties, the material has been sent to a third party for full magnetometer temperature testing; this is believed to be the only way to get accurate nano material measurements. The tests are very complex and time consuming but will provide us with absolute definitive measurements and a clear path forward for possible applications. Upon completion of the tests (estimated to be completed by October 28th 2015), the company will release the results. [emphases mine] Elements of the research work have relied on a patented (nanoparticle ultrasound separation) system designed to isolate and create large quantities of graphene cost effectively.

Company management must feel quite confident about the results of their testing to issue this ‘preview’ news release which goes on to highlight the advantages of using Canadian graphite for producing graphene,

The base graphite used in the research program was very pure, which minimized the need for costly and environmentally harsh purification. In addition, the graphene that was produced has excellent electrical/thermal connectivity; large high surface area, very good wettability, and had some promise of magnetic properties.

The production method has been initially shown to be less aggressive and significantly more cost effective than other processes such as the Hummers Method. This should further improve the overall ability to produce base material for many other needed applications for graphene today. The process may greatly shorten the time to market, and we are encouraged that there are already real needs for the material in all sorts of applications including polymers, epoxies and other coatings. The company plans to work with industry partners to develop a solution based application that can be developed today and be in use in a short time frame.

The next phase of the joint research effort is to prepare a bench scale system capable of producing larger quantities of high purity graphene samples for potential industry partners. Mr. Ogilvie commented, “We believe our working relationship with the university teams is an excellent opportunity to leverage Saint Jean’s graphite experience and assets while simultaneously expanding our focus on critical new carbon-based opportunities such as graphene superconductors. As one of the next steps in our go-forward plan is to quickly advance the product applications by working with a number of companies and potential strategic partners. Given the potential of graphene in everything from quantum computing to energy storage, Saint Jean has been encouraged by the breadth and depth of these preliminary discussions. As the work unfolds we look forward to keeping our shareholders actively informed on our continued efforts and results.” Dr. Don MacIntyre, the Company’s geologist, P. Geo., and Qualified Person, reviewed and approved the technical and scientific information in this release.

While the company’s executive offices are in Ontario with a second office in Alberta (company contact page), the graphite mines are in Québec (from  the news release),

About Saint Jean

Saint Jean is a publicly traded carbon sciences company with interest graphite mining claims on five 100% Company owned properties located in the province of Quebec in Canada. The five properties include the Walker property, a past producing mine, the Wallingford property, the St. Jovite property, East Miller and Clot property. For information on Saint Jean’s other properties and the latest news please go to the website: www.saintjeancarbon.com

Saint Jean Carbon’s chief executive officer (CEO) has an interesting carbon background (from the Management page),

Mr. Ogilvie brings a wealth of knowledge to the graphite sector. Mr. Ogilvie has been extensively involved in several start-ups, including emerging graphite companies, for over 33 years. He most recently served as Chief Executive Officer and Director for both Mega Graphite Inc. and Canada Carbon. Prior to this, in 2007 Mr. Ogilvie led a private investment group in the redevelopment and turnaround of Industrial Minerals Inc. (now known as Northern Graphite [emphasis mine] Corporation (NGC-TSX.V), a junior mining company that is presently developing one of the largest large-flake natural graphite deposits in the world. Mr. Ogilvie has direct experience in the development of technologies related to the production of graphite ores and the operation of global graphite markets for base and high purity graphite products.

Northern Graphite was last mentioned here in a March 9, 2015 post (scroll down about 50% of the way) featuring a report about the worldwide graphite market. In a Feb. 6, 2012 post, the first one about Northern Graphite, the focus is on the flakes.

Final comment: It seems like quite the month for Canadian graphene efforts of all stripes; I wrote an October 19, 2015 post featuring a new international graphene foundation (GO Foundation for graphene commercialization) being launched in Canada.