Metamaterials experts show a way to reduce electrons' effective mass to nearly zero

Dec 18, 2012

The field of metamaterials involves augmenting materials with specially designed patterns, enabling those materials to manipulate electromagnetic waves and fields in previously impossible ways. Now, researchers from the University of Pennsylvania have come up with a theory for moving this phenomenon onto the quantum scale, laying out blueprints for materials where electrons have nearly zero effective mass.

Such materials could make for faster circuits with novel properties.

The work was conducted by Nader Engheta, the H. Nedwill Ramsey Professor of Electrical and Systems Engineering in Penn's School of Engineering and , and Mario G. Silveirinha, who was a visiting scholar at the Engineering School when their collaboration began. He is currently an associate professor at the University of Coimbra, Portugal.

Their paper was published in the journal Physical Review B: Rapid Communications.

Their idea was born out of the similarities and analogies between the mathematics that govern —Maxwell's Equations—and those that govern the of electrons—Schrödinger's Equations.

On the electromagnetic side, inspiration came from work the two researchers had done on that manipulate permittivity, a trait of materials related to their reaction to electric fields. They theorized that, by alternating between thin layers of materials with positive and negative permittivity, they could construct a bulk metamaterial with an effective permittivity at or near zero. Critically, this property is only achieved when an electromagnetic wave passes through the layers head on, against the grain of the stack. This directional dependence, known as , has practical applications.

The researchers saw parallels between this phenomenon and the electron demonstrated in Leo Esaki's -winning work on superlattices in the 1970s: semiconductors constructed out of alternating layers of materials, much like the permittivity-altering metamaterial.

A semiconductor's qualities stem from the lattice-like pattern its constituent atoms are arranged in; an electron must navigate the electric potentials of all of these atoms, moving faster or slower depending on how directly it can pass by them. Esaki and his colleagues showed that, by making a superlattice out of layers of different materials, they could produce a composite material that had different properties than either of the components.

Though the actual mass of electrons is fixed, Engheta and Silveirinha thought the same principle could be applied to the effective mass of the electron. Engineers have been tailoring materials to alter the effective mass of electrons for decades; existing semiconductors that give electrons a negative effective mass were a prerequisite for the team's new theory.

"Imagine you have a ball inside a fluid," Engheta said. "You can calculate how fast the ball falls as a combination of the force of gravity and the reaction of the fluid, or you can say that the ball has an effectively different mass in the fluid than it does normally. The effective mass can even be negative, which we see in the case of a bubble. The bubble looks like it has negative mass, because it's moving against gravity, but it is really the fluid moving down around it."

Like the optical metamaterial with alternating bands of positive and negative permittivity, Engheta and Silveirinha theorized, a material with alternating bands of positive and negative effective electron mass would allow the overall structure's effective electron mass to approach zero.

And like the optical metamaterial, the electron's effective mass in this case would be anisotropic. While travelling against the grain of the alternating materials, its effective mass would be near-zero, and thus it would travel very fast. But trying to move the electron along the grain would result in a very high effective mass, making it very difficult for it to move at all. "In the direction the electrons are collimated, we see an effective mass of zero," Engheta said. "This is like what we see with graphene, where electrons have an effective mass of zero but only along its plane.

"But a plane of graphene is only one atom thick, whereas here we would see that property in a bulk material. It's essentially like the material has wires running through it, even though there is no wire surface."

As with graphene, the properties of this composite material would be dependent on structure at the smallest scale; a few stray atoms could significantly degrade the material's overall performance. A single uniform layer of atoms is ideal in both cases, and, while deposition techniques are improving, working at the scale of a few nanometers still represents a physical challenge. The team hopes to address this challenge in future studies.

"While physics prevents us from having infinite velocity, having that give electrons near-zero effective mass will let us move them much faster," Engheta said.

Explore further: A new multi-bit 'spin' for MRAM storage

More information: prb.aps.org/abstract/PRB/v86/i16/e161104

Related Stories

Graphene mini-lab

Oct 31, 2012

A team of physicists from Europe and South Africa showed that electrons moving randomly in graphene can mimic the dynamics of particles such as cosmic rays, despite travelling at a fraction of their speed, in a paper about ...

Two graphene layers may be better than one

Apr 27, 2011

(PhysOrg.com) -- Researchers at the National Institute of Standards and Technology have shown that the electronic properties of two layers of graphene vary on the nanometer scale. The surprising new results ...

New technique controls graphite to graphene transition

Jul 02, 2012

(Phys.org) -- University of Arkansas physicists have found a way to systematically study and control the transition of graphite, the “lead” found in pencils, to graphene, one of the strongest, lightest ...

Recommended for you

IHEP in China has ambitions for Higgs factory

8 hours ago

Who will lay claim to having the world's largest particle smasher?. Could China become the collider capital of the world? Questions tease answers, following a news story in Nature on Tuesday. Proposals for ...

The physics of lead guitar playing

9 hours ago

String bends, tapping, vibrato and whammy bars are all techniques that add to the distinctiveness of a lead guitarist's sound, whether it's Clapton, Hendrix, or BB King.

The birth of topological spintronics

10 hours ago

The discovery of a new material combination that could lead to a more efficient approach to computer memory and logic will be described in the journal Nature on July 24, 2014. The research, led by Penn S ...

The electric slide dance of DNA knots

14 hours ago

DNA has the nasty habit of getting tangled and forming knots. Scientists study these knots to understand their function and learn how to disentangle them (e.g. useful for gene sequencing techniques). Cristian ...

User comments : 7

Adjust slider to filter visible comments by rank

Display comments: newest first

Doug_Huffman
1 / 5 (2) Dec 18, 2012
At 511 eV it's damn near zero already. 9.109 382 15(45)×10^−31 kg
axemaster
not rated yet Dec 18, 2012
I believe you meant to say 511 keV.
The Alchemist
2.6 / 5 (5) Dec 19, 2012
Are they saying superconductivity, or something like it?
ValeriaT
1 / 5 (4) Dec 19, 2012
Yes, it's the same principle. The relative permitivity of superconductive stripes is negative so the permitivity of the bulk superconductor is zero (the external electric field always results in zero electric intensity within the material). These guys are reinventing the wheel - and they even don't know about it...;-P
Q-Star
1.8 / 5 (5) Dec 19, 2012
Yes, it's the same principle. The relative permitivity of superconductive stripes,,,,

Permit me to point out that you misspelled permittivity, twice.

so the permitivity of the bulk superconductor is zero (the external electric field always results in zero electric intensity within the material). These guys are reinventing the wheel - and they even don't know about it...;-P


As the magnet wheels turn, the power will flow. In a way that mimics the flat surface of water waves when paddled. In much the same way as the non moving electron permits aether wave incoherence.
ValeriaT
1 / 5 (1) Dec 22, 2012
Because the electrons within graphene do behave like the material with negative permitivity too, it's possible, that the above mechanism could explain the recently observed superconductivity of graphite soaked with water, because the water is the material of relatively high relative permitivity. If it's so, then the mixing of graphite with even higher polar fluid, like the sulphoxide, dimethylformamide or even HMTPA could improve this effect significantly (not to say, these fluids are far less volatile, so that the resulting material would be more stable.)
ValeriaT
1 / 5 (2) Dec 22, 2012
BTW Note the negative electrical resistance in carbon fiber composites, observed before years - and completely ignored with mainstream physics, BTW. It seems for me, if the mainstream physics trolls would find a piece of levitating room temperature superconductor, which would produce the light and heat spontaneously in addition - then they would ignore it too, because they couldn't apply any of their marvelous theories to it. We all know, how the contemporary science is (not) working today: no theory means no publication accepted in high-impact journals, which means no grants and money for further research - so no actual interest about subject.