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 Tokyo.
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.
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
The research 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 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