## Physicists Say Graphene Could Create Mass 184

Posted
by
kdawson

from the boson-walks-into-a-church dept.

from the boson-walks-into-a-church dept.

eldavojohn writes

*"Graphene has gotten a lot of press lately. The Nobel prize-winning, fastest-spinning, nanobubble-enhanced silicon replacement is theorized to have a new, more outlandish property. As reported by Technology Review's Physics Blog, graphene should be able to create mass inside properly formed nanotubes. According to Abdulaziz Alhaidari's calculations, if one were to roll up graphene into a nanotube, this could compactifiy dimensions (from the sheet's two down to the tube's one), and thus 'the massless equations that describe the behavior of electrons and holes will change to include a term for mass. In effect, compactifying dimensions creates mass.' What once would require a massive high-energy particle accelerator can now be tested with carbon, electricity, and wires, according to the recent paper."*
## Re:Can anybody summarize TFA? (Score:5, Informative)

The /. title of this article is wrong, stupid and misleading.

The title of TFA is "Dynamical mass generation via space compactification in graphene", which is mostly incomprehensible.

The abstract sez "Fermions in a graphene sheet behave like massless particles. We show that by folding the sheet into a tube they acquire non-zero effective mass as they move along the tube axis. That is, changing the space topology of graphene from 2D to 1D (space compactification) changes the 2D massless problem into an effective massive 1D problem."

A plain english annotated translation is "Electrons in a graphene sheet behave like massless particles. We show that by folding the sheet into a tube they behave like massive particles as they move along the tube axis. That is, changing the shape of graphene from 2D to 1D changes the 2D massless problem into an effective 1D massive problem, which may be easier to solve or model or understand in certain respects.

Note electrons have the same real mass in both cases. Mass is not being created or destroyed.

## Re:Not 1-dimensional (Score:5, Informative)

## Re:Can anybody summarize TFA? (Score:3, Informative)

## Re:Can anybody summarize TFA? (Score:5, Informative)

All science predictions are math tricks. If the prediction holds up, our existing models are correct, otherwise, our existing models are broken. Creating mass from graphene is not a new theory, it is the _consequence_ of our existing theories that someone cleverly derived.

Point is, either way, Abdulaziz Alhaidari is now famous and has done the incredible. He's either famous for making a marvelous derivation of our existing theories, or he's famous for disproving our current models by explaining what our current models predict that would later be experimentally contradicted. Just as the Manhattan project was a test of atomic theory; if it worked, an amazing weapon was created; if it didn't work, it had profound ramifications on invalidating the the atomic theory of the day. Either it's a win for engineering, building something amazing, or a win for science, changing the models to more closely match reality.

## Re:It's called our circle of science! (Score:5, Informative)

The link to the paper just gives the executive summary, which actually conveys little information. Even wikipedia wasn't much help. If there's a physicist out there, I get the impression that somehow leptons are being converted to fermions? If so, how, and why do they? If not, could someone give a good explanation?

This is fascinating, but I can't find much explanation.

http://www.ncbi.nlm.nih.gov/pubmed/17358966 [nih.gov]

Magnetic confinement of massless Dirac fermions in graphene.

De Martino A, Dell'Anna L, Egger R.

Institut für Theoretische Physik, Heinrich-Heine-Universität, D-40225 Düsseldorf, Germany.

Abstract

Because of Klein tunneling, electrostatic potentials are unable to confine Dirac electrons. We show that it is possible to confine massless Dirac fermions in a monolayer graphene sheet by inhomogeneous magnetic fields. This allows one to design mesoscopic structures in graphene by magnetic barriers, e.g., quantum dots or quantum point contacts.

PMID: 17358966 [PubMed]

## Re:Can anybody summarize TFA? (Score:5, Informative)

Recall that this method of mass generation has been utilized exclusively in high energy physics, supergravity, string theory and related fields [9]. To the best of our knowledge, the present work constitutes the first successful application of this method in condensed matter physics. Another example of space compactification is found in a system consisting of a stack of graphene sheets with coupling between the layers making the massless 3D problem equivalent to an effective massive 2D problem [8].

In other words: "We applied an existing math trick to a new area of theoretical physics, and things look good so far."

You take that, feed it to the "Technology Review" blog, and you get:

The amazing properties of graphene now include the ability to create mass, according to a new prediction.

...which is not quite the same.

## Re:It's called our circle of science! (Score:5, Informative)

Actually, the abstract nails what the actual news here is.

You can't confine a Dirac electron electrostatically. They show that it can be done with magnetic fields. This is sort of cool because it has potential ramifications for incorporating nanotechnology into electronics.

After the wharrgarbl, it mutates into a headline about creating mass and using it to power FTL starships from video games.

## Re:Can anybody summarize TFA? (Score:3, Informative)

I asked a similar question, and this guy seems to have nailed it. [slashdot.org] I hope the mods notice his answer.

## Re:This phenomenon closely related to: (Score:4, Informative)

## Re:Can anybody summarize TFA? (Score:2, Informative)

The /. title of this article is wrong, stupid and misleading.

Seconded. Just to clarify, the only thing that's changing here is the dispersion relationship. In graphene the energy of carriers grows linearly with momentum due to strong spin-orbit coupling. In most materials the energy grows proportional to the momentum squared. People have known for a long time that you can do all sorts of things to graphene to change the dispersion relationship so that it acts like other materials. For a bit of a overview see http://www.lbl.gov/publicinfo/newscenter/pr/2008/ALS-graphene-electrons.html [lbl.gov]

## Re:Can anybody summarize TFA? (Score:5, Informative)

In graphene, electrons behave like massless particles traveling near the speed of light.

No, electrons do not.

"Charge carriers", which in the case of graphene are quasi-particles that result from the interaction of electrons with the more-or-less 2D medium, do.

The difference is tremendously important, althought admitedly your explaination is about a million times better than the gibberish in the headline and summary.

This is interesting and legitmate physics: charge-carrying quasi-particles in 2D graphene behave as massless particles in a 2+1D spacetime (according to the paper, at least.) If you role the sheet up the dynamics of the quasi-particles becomes that of massive particles in a 1+1D spacetime. This allows experimental realization of systems described by relativistic dynamics (the Dirac Equation) under much simpler circumstances than one might generally expect.

This is similar to the research on "solid state monopoles" which behave like Dirac monopoles over large distance scales. They allow the study of a wide range of phenomena that are otherwise inaccessible (and in the case of Dirac monopoles, entirely theoretical!)

No mass in the ordinary sense of the term is created in the situation the paper describes. If you weighted the system with a sufficiently sensitive balance you would not find that the apparatus weighted more when the graphene sheet was rolled up.

## Something Fishy (Score:5, Informative)

If there's a physicist out there, I get the impression that somehow leptons are being converted to fermions?

Leptons (e..g electron) are fermions. However there is something very fishy with this paper. For example 10^6 m/s is not relativistic. If you calculate the gamma factor (gamma=1 is what Newtonian physics assumes) you get 1.0000056. This means they are very non-relativistic and Schrodinger should work fine for them unless there is some subtle effect at play. Indeed to give electrons this energy you need to accelerate them through a potential of 2.8 volts so rather than needed a particle accelerator any one with a vacuum pump, a vacuum-tight container, some wire and two AA batteries can experience the fun of "relativistic" electrons.

What I suspect is happening is that the conditions on graphene have altered the electron behaviour so, rather than test anything fundamental, you are testing the properties of electrons on graphene. You cannot do real relativistic physics with this because if you get an unexpected result you have no idea whether it is because there is some new, unexpected physics at work or whether your approximation of the environment is simply wrong and you need to use a different model for it. Hence, while interesting, this is not the way to do real, relativistic physics: for that you need something that is truly relativistic, not just something which might, under certain conditions, act like something relativistic.

## Re:Well... (Score:1, Informative)

-1, incorrect. You never gain mass in your reference frame, you only gain mass in someone else's. It's always possible to accelerate in your reference frame, however, in someone else's, you seem to be accelerating less and less.

## Re:Well... (Score:2, Informative)

Except special relativity actually requires the true mass to increase when the speed increases. Mass/Energy is still conserved because energy is converted into mass (E=mc^2).

The effect described in the article is quantum mechanical in nature. In fact, the mass of the electrons is not changing. Only the "effective mass" is changing. Well, it turns out the effective mass is just an approximation we use to make the problem tractable. Basically, we look at the band structure in the material, and calculate an effective mass based on the curvature. If there is no curvature, such as in graphene, then there is no effective mass. However, the band structure for carbon nanotubes does have curvature, and therefor there is a finite effective mass. When the article talks about rolling graphene into nanotubes, they mean as a thought experiment. No one is actually sitting there with an atomic force microscope trying to roll a sheet of graphene into a carbon nanotube. So really, the paper brings nothing new. We already knew that there was zero effective mass in graphene, and finite effective mass in carbon nanotubes. Most likely, they are just suggesting a corollary to some other part of theoretical physics.