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| An image of the interference pattern made by wave-like electron
pairs in a high-temperature, iron-based superconductor as measured
using a phase-sensitive scanning tunneling microscope. |
| Copyright : © 2010 Tetsuo Hanaguri |
Achieving superconductivity at room temperature has represented one
of the holy grails of physics for decades. A practical material
with zero electrical resistance would not only represent a major
advance in physics, but also revolutionize technologies from power
grids to electric motors. However, the mechanism behind so-called
‘high-temperature’ superconductors, which are
superconducting above approximately -240 Celsius, has been unclear,
and the highest temperature at which superconductivity has been
observed remains at a frigid -108 Celsius.
Now, the mechanism responsible for superconductivity in an
important class of high-temperature superconducting materials,
discovered in 2008, has been revealed by Tetsuo Hanaguri and
colleagues at the RIKEN Advanced Science Institute, the Japan
Science and Technology Agency (JST), The University of
Electro-Communications in Tokyo, and The University of
Tokyo1.
Pairing up
The researchers studied the mechanism behind a key property of all
superconductors: electron pairing. In an ordinary material,
electrons travel independently and their motion is regularly
disrupted, or scattered, by defects and by vibrations (or phonons)
of the atomic lattice they are traveling through. This leads to
electrical resistance, so that any flowing current must be
‘pushed’ along by an applied voltage. In
superconductors, electrons travel in pairs, rather than
individually, making them less prone to scattering. A minimum
amount of energy called the ‘superconducting gap’
energy must then be expended to break an electron pair. Since this
energy is unavailable at low temperatures, the motion of the
electron pairs remains unperturbed, and the material’s
resistance is zero. This means a current can flow perpetually
without any applied voltage.
Hanaguri and colleagues focused on understanding how electron
pairing occurs in iron-based superconductors, one of the two major
classes of high-temperature superconductors. In conventional,
low-temperature superconductors, electrons are paired because
phonons create attractions between them, overcoming the natural
repulsion the electrons have as a result of their identical
negative charges. In iron-based superconductors, however,
superconductivity is associated with a particular ordering of the
atomic magnets found in the materials. This generated speculation
among physicists that these tiny magnets, or spins, may be involved
in the pairing mechanism. The work by Hanaguri and colleagues
provides strong evidence that these spins are indeed responsible
for electron pairing in iron-based superconductors.
Out of phase
The researchers leveraged their expertise with scanning tunneling
microscopes (STMs) to gather this evidence. Traditionally used to
map the shapes of nanostructures and atoms, these microscopes
measure the current between a sharp nanoscale tip and a surface
just beneath it. They can also be used to measure the momentum of
electrons traveling across a surface. Just before the discovery of
iron-based superconductors, Hanaguri had developed a method at
RIKEN in Hidenori Takagi’s laboratory to use STMs to measure
the phase of electrons, and this capability was the key to their
work on superconductors.
Hanaguri and colleagues were able to measure the interference
pattern of electron pairs by purposefully scattering them from
magnetic vortices that they created in the superconductor Fe(Se,Te)
using an applied magnetic field. Electron pairs behave like waves
at very small scales so, like all waves, they have a phase. For
example, two water waves traveling across a pond at the same speed
have different phases if one wave is slightly behind the other. If
they collide, they make an interference pattern that is affected by
the phase difference between them. Similarly, the interference
pattern made by electron pairs is affected by the phase difference
between those pairs.
The researchers measured and interpreted these interference
patterns to understand iron-based superconductors. After initial
measurements on high-quality crystals grown by their collaborator
Seiji Niitaka, they began the task of data interpretation.
Unfortunately, they made an early mistake with the coordinate
system that stymied their progress until Kazuhiko Kuroki from The
University of Electro-Communications realized the error at a
presentation. Kuroki later joined the collaboration and helped
interpret the measured interference patterns.
The team found that the patterns could be explained by assuming
that the phase of an electron pair, and its associated
superconducting gap, depends on the momentum of the pair (Fig. 2).
This telltale sign of spin-mediated electron pairing had been
predicted theoretically but never realized experimentally. By
confirming the role of spins in iron-based superconductors, the
team’s data lay the foundation for an understanding of
superconductivity that is not based on lattice vibrations unlike
more conventional superconductors.
Past and future
Hanaguri says his group was in a lucky position at the outset.
“My ‘aha!’ moment came when I realized that the
phase-sensitive STM technique that I had already developed could be
applied to iron superconductors, which had just been
discovered.” He also counts openness as a key to the success
of the work: had Hanaguri not comprehensively described his
preliminary results at a conference, Kuroki would not have
identified his mistake. “My policy is that all the data,
techniques and plans that I have must be as open as
possible,” Hanaguri says.
Hanaguri also notes that the phase-sensitive scanning tunneling
microscope developed by his team yielded a significant result in
only its first years of operation, and can be expected to produce
important results in other realms of physics, including magnetism.
Ultimately, Hanaguri would be most satisfied by finding something
completely new. “Our equipment is capable of studying matter
under extreme conditions, and it is under extreme conditions that
many new physical phenomena have been discovered,” he
explains. “To discover a new phenomenon would be much more
exciting than the elucidation of an existing phenomenon’s
mechanism.”
About the Researcher
Tetsuo Hanaguri
Tetsuo Hanaguri was born in Tokyo, Japan, in 1965. He graduated
from the Department of Applied Physics at Tohoku University in
1988, and received his PhD in applied physics from the same
university in 1993. He then worked as a research associate and
associate professor at The University of Tokyo until he joined
RIKEN. Since 2004, he has held the position of senior research
scientist in the Takagi Magnetic Materials Laboratory at RIKEN. He
works in the field of experimental condensed-matter physics at low
temperatures, and his current research focus is on spectroscopic
imaging scanning tunneling microscopy of complex electron systems
including superconductors and topological insulators. He is also
interested in measurement science and technology and enjoys
building scientific apparatus.
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