Featured Research

from universities, journals, and other organizations

New tool to attack the mysteries of high-temperature superconductivity

Date:
May 31, 2012
Source:
DOE/Lawrence Berkeley National Laboratory
Summary:
Using ultrafast lasers, scientists have tackled the long-standing mystery of how Cooper pairs form in high-temperature superconductors. With pump and probe pulses spaced just trillionths of a second apart, the researchers used photoemission spectroscopy to map rapid changes in electronic states across the superconducting transition, revealing relationships of energy and momentum never seen before in these promising, but stubborn, complex materials.

Part of the momentum map of Bi2212 derived from ultrafast laser ARPES shows that after initial excitation by a pump probe, how fast quasiparticles recombine into Cooper pairs greatly depends on their position in momentum space. (Only one of the four corners of the Fermi surface momentum map is shown, as insets in left panels.) Near the central nodes the quasiparticles recombine very slowly. Far from the nodes, they recombine quickly.
Credit: Lanzara Group, Lawrence Berkeley National Laboratory and University of California at Berkeley

Superconductivity, in which electric current flows without resistance, promises huge energy savings -- from low-voltage electric grids with no transmission losses, superefficient motors and generators, and myriad other schemes. But such everyday applications still lie in the future, because conventional superconductivity in metals can't do the job.

Related Articles


Although they play important roles in science, industry, and medicine, conventional superconductors must be maintained at temperatures a few degrees above absolute zero, which is tricky and expensive. Wider uses will depend on higher-temperature superconductors that can function well above absolute zero. Yet known high-temperature (high-Tc) superconductors are complex materials whose electronic structures, despite decades of work, are still far from clear.

Now a team of scientists at the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California at Berkeley, led by Alessandra Lanzara in collaboration with Joseph Orenstein and Dung-Hai Lee of the Lab's Materials Sciences Division (MSD), has used a new and uniquely powerful tool to attack some of the biggest obstacles to understanding the electronic states of high-temperature superconductors -- and how they may eventually be put to practical use. The team reports their research using ultrafast laser ARPES (ultrafast angle-resolved photoemission spectroscopy) in the June 1, 2012, issue of the journal Science.

Pairing off the electrons

Cooper pairs of electrons are the hallmark of superconductivity, forming a sea of correlated charge carriers that barely interact with their crystalline surroundings. The formation of these pairs in conventional superconductors is well described by the Bardeen Cooper Schrieffer (BCS) theory. With high-Tc superconductors, however, the situation is not straightforward.

"The mechanism binding Cooper pairs together in high-Tc superconductors is one of the great mysteries in materials science," says Christopher Smallwood, a member of Lanzara's group and first author of the Science paper. "What we've done with ultrafast laser ARPES is to start with a high-Tc superconductor called Bi2212 and cool it to well below the critical temperature where it becomes superconducting."

The researchers fired an infrared laser pulse at the sample, temporarily cracking some of the Cooper pairs open into their constituent parts, called quasiparticles. As these states decayed, recombining back into Cooper pairs, the researchers used ARPES to measure their changing energy and momentum.

"The relaxation process takes just a few trillionths of a second from start to finish, and in the end, we were able to assemble and watch an extremely slow-motion movie of Cooper-pair formation -- which showed that the quasiparticles tend to recombine faster or slower depending both on their momentum and on the intensity of the pump pulse," Smallwood says. "It's an exciting development, because these trends may be directly connected to the mechanism holding Cooper pairs together."

A Cooper pair has less energy than two independent electrons, leaving an energy gap between the sea of Cooper pairs and the usual lowest energy of the charge carriers in the material. Maps of this superconducting gap can be calculated -- or, remarkably, they can be drawn directly by the charge carriers themselves.

In an ARPES experiment, the momenta and angles of the electrons that are knocked loose by a sufficiently energetic beam of light are used to map out the material's momentum space on a flat detector screen. The momentum-space map shows the material's band structure, the energy levels accessible to its charge carriers.

Long used to probe the electronic structures of materials, ARPES is usually associated with synchrotron light sources like Berkeley Lab's Advanced Light Source (ALS), which produces extremely bright beams of x-rays. Laser ARPES is much simpler but limited in energy.

"We're stuck with 5.9 electron-volt photon energy and we can't tune it much, like we could at the ALS," Smallwood says. "But by happenstance this energy is great for looking at high-Tc superconductors, and the low photon energy gives us better momentum resolution."

Most high-Tc superconductors, including Bi2212, resemble cuprate ceramics, rich in copper and oxygen. In almost all conventional metal superconductors the superconducting gap is uniform, but in the cuprates it varies greatly. For some momenta the gap is large, but at four special points in momentum space it drops all the way to zero. The existence of such "nodes" in the gap is a distinguishing characteristic of cuprate high-Tc superconductors.

Ultrafast lasers open new vistas

"This is where ultrafast laser ARPES, which is only about five years old, really comes into play to give us results not accessible by other means," Smallwood says. "The laser we use is a titanium-sapphire laser that can emit femtosecond-scale pulses." (A femtosecond is a quadrillionth of a second.)

The same beam pulse that creates the infrared pump pulse is split to form the more energetic ultraviolet probe pulse, by passing part of it through frequency doubling crystals. The time delay between pump and probe can be adjusted with femtosecond precision, using a motorized mirror to change the distance the probe pulse travels before it reaches the sample. The tiny sample can be tilted to any desired angle, which determines what part of the band structure is being examined by ARPES.

In this way the research team discovered the relation between the initial excitation energy, the quasiparticles' position in momentum space, and how quickly the quasiparticles decay. Greater initial excitation energy gives faster recombination into Cooper pairs, but so does crystal momentum far from the nodes. Quasiparticles with momentum that places them near the nodes on the Fermi surface decay very slowly.

When additional ultrafast all-optical techniques, using infrared for both pump and probe pulses, were applied to the same sample, the results were in good agreement with ARPES.

"It's exciting that now we are able to measure these components of recombination distinctly and see what each contributes," says Smallwood. "It gives us a new handle on ways to assess some of the candidate ideas about how Cooper pairs form, such as the suggestion that the energy and momenta of quasiparticles far from a node may resonate with waves of spin density or charge density to form Cooper pairs. We've shown the way to measure this and other ideas to see if they play a significant role in the transition to high-temperature superconductivity."


Story Source:

The above story is based on materials provided by DOE/Lawrence Berkeley National Laboratory. Note: Materials may be edited for content and length.


Journal References:

  1. J. Graf, C. Jozwiak, C. L. Smallwood, H. Eisaki, R. A. Kaindl, D-H. Lee, A. Lanzara. Nodal quasiparticle meltdown in ultrahigh-resolution pump–probe angle-resolved photoemission. Nature Physics, 2011; 7 (10): 805 DOI: 10.1038/nphys2027
  2. C. L. Smallwood, J. P. Hinton, C. Jozwiak, W. Zhang, J. D. Koralek, H. Eisaki, D.-H. Lee, J. Orenstein, A. Lanzara. Tracking Cooper Pairs in a Cuprate Superconductor by Ultrafast Angle-Resolved Photoemission. Science, 2012; 336 (6085): 1137 DOI: 10.1126/science.1217423

Cite This Page:

DOE/Lawrence Berkeley National Laboratory. "New tool to attack the mysteries of high-temperature superconductivity." ScienceDaily. ScienceDaily, 31 May 2012. <www.sciencedaily.com/releases/2012/05/120531145718.htm>.
DOE/Lawrence Berkeley National Laboratory. (2012, May 31). New tool to attack the mysteries of high-temperature superconductivity. ScienceDaily. Retrieved October 25, 2014 from www.sciencedaily.com/releases/2012/05/120531145718.htm
DOE/Lawrence Berkeley National Laboratory. "New tool to attack the mysteries of high-temperature superconductivity." ScienceDaily. www.sciencedaily.com/releases/2012/05/120531145718.htm (accessed October 25, 2014).

Share This



More Matter & Energy News

Saturday, October 25, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

IKEA Desk Converts From Standing to Sitting With One Button

IKEA Desk Converts From Standing to Sitting With One Button

Buzz60 (Oct. 24, 2014) IKEA is out with a new convertible desk that can convert from a sitting desk to a standing one with just the push of a button. Jen Markham explains. Video provided by Buzz60
Powered by NewsLook.com
Ebola Protective Suits Being Made in China

Ebola Protective Suits Being Made in China

AFP (Oct. 24, 2014) A factory in China is busy making Ebola protective suits for healthcare workers and others fighting the spread of the virus. Duration: 00:38 Video provided by AFP
Powered by NewsLook.com
Real-Life Transformer Robot Walks, Then Folds Into a Car

Real-Life Transformer Robot Walks, Then Folds Into a Car

Buzz60 (Oct. 24, 2014) Brave Robotics and Asratec teamed with original Transformers toy company Tomy to create a functional 5-foot-tall humanoid robot that can march and fold itself into a 3-foot-long sports car. Jen Markham has the story. Video provided by Buzz60
Powered by NewsLook.com
Police Testing New Gunfire Tracking Technology

Police Testing New Gunfire Tracking Technology

AP (Oct. 24, 2014) A California-based startup has designed new law enforcement technology that aims to automatically alert dispatch when an officer's gun is unholstered and fired. Two law enforcement agencies are currently testing the technology. (Oct. 24) Video provided by AP
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.

Save/Print:
Share:

Breaking News:

Strange & Offbeat Stories


Space & Time

Matter & Energy

Computers & Math

In Other News

... from NewsDaily.com

Science News

Health News

Environment News

Technology News



Save/Print:
Share:

Free Subscriptions


Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

Get Social & Mobile


Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

Have Feedback?


Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
Mobile: iPhone Android Web
Follow: Facebook Twitter Google+
Subscribe: RSS Feeds Email Newsletters
Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins