.
\nAchieving superconductivity at room temperature has represented one of
\n the holy grails of physics for decades. A practical material with zero electrical
\nresistance would not only represent a major advance in physics, but also
\n revolutionize technologies from power grids to electric motors. However, the
\n mechanism behind so-called \u2018high-temperature\u2019 superconductors, which are
\nsuperconducting above approximately -240 Celsius, has been unclear, and the
\nhighest temperature at which superconductivity has been observed remains at a
\nfrigid -108 Celsius.
\n.
\nNow, the mechanism responsible for superconductivity in an important class of
\nhigh-temperature superconducting materials, discovered in 2008, has been revealed by
\nTetsuo Hanaguri and colleagues at the RIKEN Advanced Science Institute, the Japan
\nScience and Technology Agency (JST), The University of Electro-Communications in
\nTokyo, and The University of Tokyo.
\n.<\/p>\n
Pairing up<\/strong> The researchers leveraged their expertise with scanning tunneling microscopes (STMs) to . . 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 \u2018high-temperature\u2019 superconductors, which are superconducting above approximately … <\/p>\n
\n.
\nThe researchers studied the mechanism behind a key property of all superconductors:
\n electron pairing. In an ordinary material, electrons travel independently and their motion
\nis regularly disrupted, or scattered, by defects and by vibrations (or phonons) of the atomic
\nlattice they are traveling through. This leads to electrical resistance, so that any flowing current
\nmust be \u2018pushed\u2019 along by an applied voltage. In superconductors, electrons travel in pairs,
\nrather than individually, making them less prone to scattering. A minimum amount of energy
\ncalled the \u2018superconducting gap\u2019 energy must then be expended to break an electron pair.
\nSince this energy is unavailable at low temperatures, the motion of the electron pairs remains
\nunperturbed, and the material\u2019s resistance is zero. This means a current can flow perpetually
\nwithout any applied voltage.
\n.
\nHanaguri and colleagues focused on understanding how electron pairing occurs in iron-based
\nsuperconductors, one of the two major classes of high-temperature superconductors. In conventional,
\nlow-temperature superconductors, electrons are paired because phonons create attractions
\nbetween them, overcoming the natural repulsion the electrons have as a result of their identical
\nnegative charges. In iron-based superconductors, however, superconductivity is associated with
\na particular ordering of the atomic magnets found in the materials. This generated speculation among
\nphysicists that these tiny magnets, or spins, may be involved in the pairing mechanism. The work by
\n Hanaguri and colleagues provides strong evidence that these spins are indeed responsible for electron
\npairing in iron-based superconductors.
\n.
\nOut of phase<\/strong>
\n.<\/p>\n
\ngather this evidence. Traditionally used to map the shapes of nanostructures and atoms, these
\n microscopes measure the current between a sharp nanoscale tip and a surface just beneath it.
\nThey can also be used to measure the momentum of electrons traveling across a surface. Just
\nbefore the discovery of iron-based superconductors, Hanaguri had developed a method at RIKEN
\nin Hidenori Takagi\u2019s laboratory to use STMs to measure the phase of electrons, and this capability
\nwas the key to their work on superconductors.
\n.
\nHanaguri and colleagues were able to measure the interference pattern of electron pairs by purposefully
\n scattering them from magnetic vortices that they created in the superconductor Fe(Se,Te) using an
\napplied magnetic field. Electron pairs behave like waves at very small scales so, like all waves, they
\nhave a phase. For example, two water waves traveling across a pond at the same speed have different
\nphases if one wave is slightly behind the other. If they collide, they make an interference pattern that is
\naffected by the phase difference between them. Similarly, the interference pattern made by electron pairs
\n is affected by the phase difference between those pairs.
\n.
\nThe researchers measured and interpreted these interference patterns to understand iron-based
\nsuperconductors. After initial measurements on high-quality crystals grown by their collaborator
\nSeiji Niitaka, they began the task of data interpretation. Unfortunately, they made an early mistake
\nwith the coordinate system that stymied their progress until Kazuhiko Kuroki from The University
\nof Electro-Communications realized the error at a presentation. Kuroki later joined the collaboration
\nand helped interpret the measured interference patterns.
\n.
\nThe team found that the patterns could be explained by assuming that the phase of an
\nelectron pair, and its associated superconducting gap, depends on the momentum of the pair.
\nThis telltale sign of spin-mediated electron pairing had been predicted theoretically but
\nnever realized experimentally. By confirming the role of spins in iron-based superconductors,
\nthe team\u2019s data lay the foundation for an understanding of superconductivity that is not
\nbased on lattice vibrations unlike more conventional superconductors
\n.
\nPast and future<\/strong><\/p>\n
\nThe research group was in a lucky position at the outset. \u201cMy \u2018aha!\u2019 moment came when
\nI realized that the phase-sensitive STM technique that I had already developed could be
\napplied to iron superconductors, which had just been discovered.\u201d He also counts openness
\nas a key to the success of the work: had Hanaguri not comprehensively described his
\npreliminary results at a conference, Kuroki would not have identified his mistake. \u201cMy policy
\n is that all the data, techniques and plans that I have must be as open as possible,\u201d
\nHanaguri says.
\n.
\nHanaguri also notes that the phase-sensitive scanning tunneling microscope developed by his
\n team yielded a significant result in only its first years of operation, and can be expected to produce
\n important results in other realms of physics, including magnetism. Ultimately, Hanaguri would be most
\nsatisfied by finding something completely new. \u201cOur equipment is capable of studying matter under
\nextreme conditions, and it is under extreme conditions that many new physical phenomena have been
\ndiscovered,\u201d he explains. \u201cTo discover a new phenomenon would be much more exciting than the
\nelucidation of an existing phenomenon\u2019s
\n.<\/p>\n","protected":false},"excerpt":{"rendered":"