Tag Archives: T. Taniguchi

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.

Three teams observe graphene butterflies

It took me a few minutes to find the butterflies (visual pattern recognition is not one of my strengths) but here they are,

Caption: Graphene, combined with white graphene, forms stunning 'butterfly' images. Credit: The University of Manchester

Caption: Graphene, combined with white graphene, forms stunning ‘butterfly’ images.
Credit: The University of Manchester

The May 15, 2013 University of Manchester news release (on EurekAlert and on the University of Manchester news site) describes how the ‘butterflies’ are formed,

Writing in Nature, a large international team led Dr Roman Gorbachev from The University of Manchester shows that, when graphene placed on top of insulating boron nitride, or ‘white graphene’, the electronic properties of graphene change dramatically revealing a pattern resembling a butterfly.

The pattern is referred to as the elusive Hofstadter butterfly that has been known in theory for many decades but never before observed in experiments.

More of the science needs to be explained before moving on with the ‘butterflies’ (from the news release),

One of the most remarkable properties of graphene is its high conductivity – thousands of times higher than copper. This is due to a very special pattern created by electrons that carry electricity in graphene. The carriers are called Dirac fermions and mimic massless relativistic particles called neutrinos, studies of which usually require huge facilities such as at CERN. The possibility to address similar physics in a desk-top experiment is one of the most renowned features of graphene.

Now the Manchester scientists have found a way to create multiple clones of Dirac fermions. Graphene is placed on top of boron nitride so that graphene’s electrons can ‘feel’ individual boron and nitrogen atoms. Moving along this atomic ‘washboard’, electrons rearrange themselves once again producing multiple copies of the original Dirac fermions.

Here’s where the butterflies appear (from the news release),

The researchers can create even more clones by applying a magnetic field. The clones produce an intricate pattern; the Hofstadter butterfly. It was first predicted by mathematician Douglas Hofstadter in 1976 and, despite many dedicated experimental efforts, no more than a blurred glimpse was reported before.

In addition to the described fundamental interest, the Manchester study proves that it is possible to modify properties of atomically-thin materials by placing them on top of each other. This can be useful, for example, for graphene applications such as ultra-fast photodetectors and transistors, providing a way to tweak its incredible properties.

Coincidentally, another team has also observed the Hofstadter butterfly on a graphene substrate. From the May 16, 2013 news item on Azonano,

Two research teams at the National High Magnetic Field Laboratory (MagLab) broke through a nearly 40-year barrier recently when they observed a never-before-seen energy pattern.

“The observation of the ‘Hofstadter butterfly’ marks a real landmark in condensed matter physics and high magnetic field research,” said Greg Boebinger, director of the MagLab. “It opens a new experimental direction in materials research.”

This groundbreaking research demanded the ability to measure samples of materials at very low temperatures and very high magnetic fields, up to 35 tesla. Both of those conditions are available at the MagLab, making it an international destination for scientific exploration.

The unique periodic structure used to observe the butterfly pattern was composed of boron nitride (BN) and graphene.

The May 15, 2013 Florida State University news release by Kristin Roberts, which originated the news item, describes the two teams using the MagLab facilities for their ‘butterfly’ observations,

One research team was led by Columbia University’s Philip Kim and included researchers from City University of New York, the University of Central Florida, Tohoku University and the National Institute for Materials Science in Japan. The team’s work will be published today in the Advanced Online Publication of the journal Nature. Similar results were discovered at the MagLab by a group led by Pablo Jarillo-Herrero and Raymond Ashoori at MIT, as well as scientists from Tohoku University and the National Institute for Materials Science in Japan. Their work is expected to be published soon.

For those who just can’t get enough graphene butterflies here are citations for and links to both recently published papers (the Jarillo-Herrero/Ashoori team will be publishing their work soon).

Cloning of Dirac fermions in graphene superlattices by L. A. Ponomarenko, R. V. Gorbachev, G. L. Yu,D. C. Elias, R. Jalil, A. A. Patel, A. Mishchenko, A. S. Mayorov, C. R. Woods, J. R. Wallbank, M. Mucha-Kruczynski, B. A. Piot, M. Potemski, I. V. Grigorieva, K. S. Novoselov, F. Guinea, V. I. Fal’ko & A. K. Geim. Nature doi:10.1038/nature12187 Published online   

and,

Hofstadter’s butterfly and the fractal quantum Hall effect in moiré superlattices by C. R. Dean, L. Wang, P. Maher, C. Forsythe, F. Ghahari, Y. Gao, J. Katoch, M. Ishigami, P. Moon, M. Koshino, T. Taniguchi, K. Watanabe, K. L. Shepard, J. Hone & P. Kim. Nature (2013) doi:10.1038/nature12186 Published online 15 May 2013

Both papers are behind paywalls.