Tag Archives: disorder

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.

Chimera state: simultaneous synchrony and asynchrony

It turns out there’s more than one kind of chimera. (I published a Sept. 7, 2016 post about chimeras that are animal/human hybrids and a US public consultation on the matter.)

The chimera being investigated by researchers at the University of New Mexico (US) is of an altogether different kind. From a Nov. 15, 2016 American Institute of Physics (AIP) news release (also on EurekAlert),

Order and disorder might seem dichotomous conditions of a functioning system, yet both states can, in fact, exist simultaneously and durably within a system of oscillators, in what’s called a chimera state. Taking its name from a composite creature in Greek mythology, this exotic state still holds a lot of mystery, but its fundamental nature offers potential in understanding governing dynamics across many scientific fields. A research team at the University of New Mexico has recently advanced this understanding with work that will be published this week in the journal Chaos, from AIP Publishing.

“A system of oscillators” may sound obscure, but it actually describes, in a very general but fundamental way, all sorts of physical systems.

“Lots of biological systems can be thought of as populations of oscillators. The heartbeat is just oscillating heart cells that a wave propagates on. And neurons in the brain are oscillators as well, and have been treated with these methods,” said Karen Blaha, a post-doctoral researcher at the University of New Mexico working on the project. “But doing experiments on those systems is really, really hard. The cells can die, and if you can manipulate them in a way that you can measure the data, they may not be behaving as they do naturally.”

For this reason, the team, led by Francesco Sorrentino, a mechanical engineering professor at the University of New Mexico, built on previous work done to understand chimera states with mechanical oscillators, in this case a collection of metronomes, resting on coupled platforms.

“The ultimate goal is that these systems are better behaved than the biological systems that we hope eventually they might be good proxies for,” Blaha said.

The team built a system of three coupled platforms, each supporting up to 15 ticking metronomes whose motions were individually tracked. A chimera state in this system consisted of in-phase, or synchronous, motion of a subset of the metronomes, and asynchronous motion of the others. By varying characteristics of the system, such as the strength of coupling between the platforms or the number of metronomes, they could deduce which factors led to more perfect chimera states.

Of particular interest in this experiment was the effect symmetries of the system had on the emergence of chimera states. Sorrentino and his team looked at, for example, the effect of having the same versus different coupling strengths of the outer platforms to the center platform.

“It puts together a new ingredient that kind of makes the whole thing more complex. Basically we are wondering how this type of mixed behavior can occur in systems that have symmetries. And our work is experimental so we see this chimera state in systems with symmetries,” Sorrentino said.

In addition to developing a method for better understanding these important, complex systems, Sorrentino views the effort to be a powerful educational tool. The tabletop scale and visual nature of the measurements and effects offer students more direct involvement with the concepts being investigated.

“It’s a full experience for the student [and] we have a broad authorship,” Sorrentino said, highlighting the collaboration between undergraduates, graduate students and senior researchers. “It’s really a team effort.”

Future work by the diverse team will investigating other symmetries, as well as varying factors such as coupling method. They also plan to add methods of controlling the system and synchrony. “We are working in several directions. Definitely the symmetries are something we will keep in mind and try to generalize to more complex situations,” Sorrentino said.

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

Symmetry effects on naturally arising chimera states in mechanical oscillator networks by Karen Blaha, Ryan J. Burrus, Jorge L. Orozco-Mora, Elvia Ruiz-Beltrán, Abu B. Siddique, V. D. Hatamipour, and Francesco Sorrentino. Chaos 26, 116307 (2016); http://dx.doi.org/10.1063/1.4965993

This paper appears to be open access.