Scientists reveal effects of quantum 'traffic jam' in high-temperature superconductors

Aug 27, 2008
Scientists reveal effects of quantum 'traffic jam' in high-temperature superconductors
This image shows two states of a cuprate high-temperature superconductor simultaneously: Each circle represents the two electrons of a Cooper pair, which exist at relatively low energy and can carry current with no resistance. In this image, the superconducting Cooper-pair state is superimposed on a dashed pattern that indicates the static positions of electrons caught in a quantum "traffic jam" at higher energy - when the material acts as a Mott-insulator incapable of carrying current.

( -- Scientists at the U.S. Department of Energy's Brookhaven National Laboratory, in collaboration with colleagues at Cornell University, Tokyo University, the University of California, Berkeley, and the University of Colorado, have uncovered the first experimental evidence for why the transition temperature of high-temperature superconductors -- the temperature at which these materials carry electrical current with no resistance -- cannot simply be elevated by increasing the electrons' binding energy. The research -- to be published in the August 28, 2008, issue of Nature -- demonstrates how, as electron-pair binding energy increases, the electrons' tendency to get caught in a quantum mechanical "traffic jam" overwhelms the interactions needed for the material to act as a superconductor -- a freely flowing fluid of electron pairs.

"We've made movies to show this traffic jam as a function of energy. At some energies, the traffic is moving and at others the electron traffic is completely blocked," said physicist J.C. Seamus Davis of Brookhaven National Laboratory and Cornell University, lead author on the paper. Davis will be giving a Pagels Memorial Public Lecture to announce these results at the Aspen Center for Physics on August 27.

Understanding the detailed mechanism for how quantum traffic jams (technically referred to as "Mottness" after the late Sir Neville Mott of Cambridge, UK) impact superconductivity in cuprates may point scientists toward new materials that can be made to act as superconductors at significantly higher temperatures suitable for real-world applications such as zero-loss energy generation and transmission systems and more powerful computers.

The idea that increasing binding energy could elevate a superconductor's transition temperature stems from the mechanism underlying conventional superconductors' ability to carry current with no resistance. In those materials, which operate close to absolute zero (0 kelvin, or -273 degrees Celsius), electrons carry current by forming so-called Cooper pairs. The more strongly bound those electron pairs, the higher the transition temperature of the superconductor.

But unlike those metallic superconductors, the newer forms of high-temperature superconductors, first discovered some 20 years ago, originate from non-metallic, Mott-insulating materials. Elevating these materials' pair-binding energy only appears to push the transition temperature farther down, closer to absolute zero rather than toward the desired goal of room temperature or above.

"It has been a frustrating and embarrassing problem to explain why this is the case," Davis said. Davis's research now offers an explanation.

In the insulating "parent" materials from which high-temperature superconductors arise, which are typically made of materials containing copper and oxygen, each copper atom has one "free" electron. These electrons, however, are stuck in a Mott insulating state -- the quantum traffic jam -- and cannot move around. By removing a few of the electrons — a process called "hole doping" -- the remaining electrons can start to flow from one copper atom to the next. In essence, this turns the material from an insulator to a metallic state, but one with the startling property that it superconducts -- it carries electrical current effortlessly without any losses of energy.

"It's like taking some cars off the highway during rush hour. All of a sudden, the traffic starts to move," said Davis.

The proposed mechanism for how these materials carry the current depends on magnetic interactions between the electrons causing them to form superconducting Cooper pairs. Davis's research, which used "quasiparticle interference imaging" with a scanning tunneling microscope to study the electronic structure of a cuprate superconductor, indicates that those magnetic interactions get stronger as you remove holes from the system. So, even as the binding energy, or ability of electrons to link up in pairs, gets higher, the "Mottness," or quantum traffic-jam effect, increases even more rapidly and diminishes the ability of the supercurrent to flow.

"In essence, the research shows that what is believed to be required to increase the superconductivity in these systems — stronger magnetic interactions -- also pushes the system closer to the 'quantum traffic-jam' status, where lack of holes locks the electrons into positions from which they cannot move. It's like gassing up the cars and then jamming them all onto the highway at once. There's lots of energy, but no ability to go anywhere," Davis said.

With this evidence pointing the scientists to a more precise theoretical understanding of the problem, they can now begin to explore solutions. "We need to look for materials with such strong pairing but which don't exhibit this Mottness or 'quantum traffic-jam' effect," Davis said.

Scientists at Brookhaven are now investigating promising new materials in which the basic elements are iron and arsenic instead of copper and oxygen. "Our hope is that they will have less 'traffic-jam' effect while having stronger electron pairing," Davis said. Techniques developed for the current study should allow them to find out.

Provided by Brookhaven National Laboratory

Explore further: With neutrons, scientists can now look for dark energy in the lab

add to favorites email to friend print save as pdf

Related Stories

Making 'bucky-balls' in spin-out's sights

6 hours ago

( —A new Oxford spin-out firm is targeting the difficult challenge of manufacturing fullerenes, known as 'bucky-balls' because of their spherical shape, a type of carbon nanomaterial which, like ...

A beautiful, peculiar molecule

5 hours ago

"Carbon is peculiar," said Nobel laureate Sir Harold Kroto. "More peculiar than you think." He was speaking to a standing-room-only audience that filled the Raytheon Amphitheater on Monday afternoon for the ...

Unlocking secrets of new solar material

4 hours ago

( —A new solar material that has the same crystal structure as a mineral first found in the Ural Mountains in 1839 is shooting up the efficiency charts faster than almost anything researchers have ...

Recommended for you

How to test the twin paradox without using a spaceship

6 hours ago

Forget about anti-ageing creams and hair treatments. If you want to stay young, get a fast spaceship. That is what Einstein's Theory of Relativity predicted a century ago, and it is commonly known as "twin ...

User comments : 8

Adjust slider to filter visible comments by rank

Display comments: newest first

3 / 5 (4) Aug 28, 2008
A metallic state can NEVER superconduct because its charge-carriers can ALWAYS be accelerated. The concomitant kinetic energy has to dissipate resistively: Even if this does NOT happen within the material, it will have to dissipate outside the material. This will still register as a resistance of the material just as the electrons moving through a vacuum-diode registers a "vacuum-resitance". A superconductor is an insulator which consists of localised states. The "orbitals" of these states are able to convey a supercurrent by cyclic movement when allowed by Heisenberg's uncertainty relationship to borrow enough energy to effect movement from one anchor-position to the next. During this movement the concomitant kinetic energy is on loan: Therefore, no kinetic energy is generated which requires dissipation: AND therefore superconduction is possible. The same mechanism (Heisenberg uncertainty) is responsible for superconduction in all materials.
2.3 / 5 (3) Aug 28, 2008
Thank you! Wonderful incisive question! Keep up your clarity of thinking and you will win the Nobel Prize!!
The original superconductors are all semi-metals. REAL metals, with spherical Fermi-surfaces, do NOT superconduct. Semi-metals have gaps in their valence-electron spectra. Wigner already predicted in 1938 that thes metals will undergo a metal-insulator transition at low temperatures by forming a "Wigner-crystal". He was a genius; because it is exactly this metal-insulator transition which causes superconduction.
2.3 / 5 (3) Aug 28, 2008
Dear Alizee,
I just noticed what BCS without the C implies. Thanks for noticing that it is BS!!
2.3 / 5 (3) Aug 29, 2008
I know BCS theory has nothing to do with a metal-insulator transition. This is so because BCS theory cannot explain the most fundamental aspect of superconduction: namely the cancellation of an applied conservative electric field between two contacts when a supercurrent starts to flow. A conservative electric field can ONLY be cancelled by dielectric polarisation. BCS does not give such a dielectric mechanism. In fact, it is highly unlikely that Cooper pairs even exist: BCS without C!! Wigner did not predict a ferromagnet. He predicted an array of Gaussian "zero-point" obitals. There is no reason why their spins should align. Furthermore, not all the valence electrons form such orbitals. They are only formed when the valence electrons cannot screen all the positive charges: i.e. in materials that have Kohn-anomalies. Superconduction only kicks in when the distances between adjacent orbitals become smaller than a critical coherence-distance. In the Alkali-metals the pressure deforms the Fermi-surface making the formation of a Wigner array possible. (By the way, you cannot compare the behaviour of a metal under pressure direcvtly with that of a semiconductor). Furthermore pressure decreases the inter-orbital distances thus making the onset of superconduction possible. It is for the same reason that pressure also increases the critical temperature for certain ceramic superconductors. I hope this is of help.
2.3 / 5 (3) Aug 29, 2008
Another point: Each otrbital can exist of two electrons and thus have zero spin.
2.3 / 5 (3) Aug 30, 2008
Really!? So the voltage also becomes zero when a current flows through a semiconductor? Please be more specific! Furthermore, in a superconductor the electric-field must also be cancelled at the position of each and every charge-carrier WHILE a supercurrent is flowing. If not, the voltage will not fall to zero.

Your explanation is just hand-waving and utterly wrong! The only way in which the electric-field can go to zero is when the kinetic-energy with which the charge-carriers move "disappears into thin air" after it has been used to effect motion. If this kinetic-energy does not "disappear" it will have to dissipate and a resistance will then be measured.This is why the kinetic-energy must be "on loan" by borrowing it from the "vacuum-energy", and then giving it back. In other words, superconduction is driven by quantum-fluctuations. This is the only reason why superconduction is at all possible! After all, superconduction is nothing else than "perpetual motion": Thermodynamics tells us that "perpetual motion" is only possible if energy from a source is turned into work and the total amount of work is then again turned into energy and given back to the source. In the case of superconduction the source is "the vacuum". There is no other microscopic mechanism that can explain superconduction.
2.3 / 5 (3) Aug 30, 2008
BCS theory has nothing to do with metal-insulator transition (can you show us some evidence of it, please?).

It can - it considers the presence of superconductive gap, which generates the zone of volume charge by the same way, like the semiconductor gap in semiconductors.

So suddenly there is an insulating gap involved: I thought that BCS had NOTHING to do with a metal-insulator transition. Unfortunately however, the "gap" caused by Cooper pairs is equal to ZERO at the onset of superconduction. So how is the field then cancelled by Cooper-pairs when superconsduction initiates? BCS=BS

3 / 5 (2) Aug 31, 2008
Dear Alizee,

Instead of trying to vote me down, how about trying to argue physics? It seems that your knowledge of physics is a bit rusty, if you ever did know some of it!

More news stories

Progress in the fight against quantum dissipation

( —Scientists at Yale have confirmed a 50-year-old, previously untested theoretical prediction in physics and improved the energy storage time of a quantum switch by several orders of magnitude. ...