Tag Archives: conductivity

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.

From flubber to thubber

Flubber (flying rubber) is an imaginary material that provided a plot point for two Disney science fiction comedies, The Absent-Minded Professor in 1961 which was remade in 1997 as Flubber. By contrast, ‘thubber’ (thermally conductive rubber) is a real life new material developed at Carnegie Mellon University (US).

A Feb. 13, 2017 news item on phys.org makes the announcement (Note: A link has been removed),

Carmel Majidi and Jonathan Malen of Carnegie Mellon University have developed a thermally conductive rubber material that represents a breakthrough for creating soft, stretchable machines and electronics. The findings were published in Proceedings of the National Academy of Sciences this week.

The new material, nicknamed “thubber,” is an electrically insulating composite that exhibits an unprecedented combination of metal-like thermal conductivity, elasticity similar to soft, biological tissue, and can stretch over six times its initial length.

A Feb.13, 2017 Carnegie Mellon University news release (also on EurekAlert), which originated the news item, provides more detail (Note A link has been removed),

“Our combination of high thermal conductivity and elasticity is especially critical for rapid heat dissipation in applications such as wearable computing and soft robotics, which require mechanical compliance and stretchable functionality,” said Majidi, an associate professor of mechanical engineering.

Applications could extend to industries like athletic wear and sports medicine—think of lighted clothing for runners and heated garments for injury therapy. Advanced manufacturing, energy, and transportation are other areas where stretchable electronic material could have an impact.

“Until now, high power devices have had to be affixed to rigid, inflexible mounts that were the only technology able to dissipate heat efficiently,” said Malen, an associate professor of mechanical engineering. “Now, we can create stretchable mounts for LED lights or computer processors that enable high performance without overheating in applications that demand flexibility, such as light-up fabrics and iPads that fold into your wallet.”

The key ingredient in “thubber” is a suspension of non-toxic, liquid metal microdroplets. The liquid state allows the metal to deform with the surrounding rubber at room temperature. When the rubber is pre-stretched, the droplets form elongated pathways that are efficient for heat travel. Despite the amount of metal, the material is also electrically insulating.

To demonstrate these findings, the team mounted an LED light onto a strip of the material to create a safety lamp worn around a jogger’s leg. The “thubber” dissipated the heat from the LED, which would have otherwise burned the jogger. The researchers also created a soft robotic fish that swims with a “thubber” tail, without using conventional motors or gears.

“As the field of flexible electronics grows, there will be a greater need for materials like ours,” said Majidi. “We can also see it used for artificial muscles that power bio-inspired robots.”

Majidi and Malen acknowledge the efforts of lead authors Michael Bartlett, Navid Kazem, and Matthew Powell-Palm in performing this multidisciplinary work. They also acknowledge funding from the Air Force, NASA, and the Army Research Office.

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

High thermal conductivity in soft elastomers with elongated liquid metal inclusions by Michael D. Bartlett, Navid Kazem, Matthew J. Powell-Palm, Xiaonan Huang, Wenhuan Sun, Jonathan A. Malen, and Carmel Majidi.  Proceedings of the National Academy of Sciences of the United States of America (PNAS, Proceedings of the National Academy of Sciences) doi: 10.1073/pnas.1616377114

This paper is open access.

Disorderly conduct amongst electrons

An Oct. 7, 2016 news item on Nanowerk highlights some research from A*STAR (Singapore’s Agency for Science and Technology Research), Note: A link has been removed,

Solid materials whose atoms are arranged in a well-ordered crystalline structure are usually better conductors of electricity than randomly structured, or amorphous, solids. Recently, however, A*STAR researchers found that iron-tellurium (FeTe) breaks this rule, displaying higher conductivity, and optical reflectivity, in the amorphous phase.

A recent study, published in the journal Acta Materialia (“Unravelling the anomalous electrical and optical phase-change characteristics in FeTe”), describes their efforts to understand why FeTe’s behavior is counterintuitive to expectations.

Iron-tellurium conducts electricity best when in a disordered amorphous phase. ©KTSDESIGN/Science Photo Library/Getty Courtesy: A*STAR

Iron-tellurium conducts electricity best when in a disordered amorphous phase. ©KTSDESIGN/Science Photo Library/Getty Courtesy: A*STAR

An Oct. 7, 2016 A*STAR press release, which originated the news item, explains more,

FeTe is a phase-change material, with the ability to rapidly switch its state from crystalline to amorphous and back again when it is heated or cooled, a property which makes it useful for data storage and memory applications. Conventional phase-change materials such as germanium-antimony-tellurium (GST), commonly used in rewritable DVDs, display higher optical reflectivity and electrical conductivity in their crystalline state because the highly-ordered structuring of atoms in the crystal results in more electron vacancies, or holes, that act as charge carriers.

“FeTe behaves differently from other phase-change materials,” explains Kewu Bai at the A*STAR Institute of High Performance Computing, who worked on the project with scientists from the National University of Singapore. “We hypothesized that these unusual characteristics may be connected with the behavior of ‘lone-pair’ electrons. This refers to a pair of electrons from any one atom that are not involved in the bonding of materials.”

The team prepared thin films of FeTe at room temperature to produce amorphous structures, and at 220 degrees Celsuis to acquire crystalline samples, and showed that the films could be flipped between the two states using a fast pulsing laser. They analyzed the molecular structure of the different films using X-ray spectroscopy, electron microscopy and first-principle calculations to investigate these unusual properties of FeTe.

The researchers confirmed the existence of lone-pair electrons in both the amorphous and crystalline phases. In the crystalline phase, where Te and Fe atoms were strongly bonded in a regular lattice, electrons were engaged in strong hybridization, meaning their orbitals overlapped and caused their electrons to localize. Thus, lone-pair electrons were incorporated as part of the integral structure.

In contrast, when FeTe entered its amorphous phase, some Te atoms were orientated so that their lone-pair electrons delocalized from the atoms, resulting in holes that acted as charge carriers.

“We are hopeful that FeTe could prove to be useful material for phase-change memory,” says Bai. “It could also act as an effective thermo-electric material, generating electric current in response to temperature.”

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

Unravelling the anomalous electrical and optical phase-change characteristics in FeTe by H.W. Ho, P.S. Branicio, W.D. Song, K. Bai, Teck L. Tan, R. Ji, Y. Yang, P. Yang, Y.H. Du, M.B. Sullivan. Acta Materialia Volume 112, 15 June 2016, Pages 67–76  http://dx.doi.org/10.1016/j.actamat.2016.04.017

This paper is behind a paywall.