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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.
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 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.”
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.”
