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Jac Londe

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started by Jac Londe on 02 Nov 15
  • Jac Londe
     

    As it turns out, the new phase that the Hsieh group identified is precisely this type of multipolar order.


    To detect multipolar order, Hsieh's group utilized an effect called
    optical harmonic generation, which is exhibited by all solids but is
    usually extremely weak. Typically, when you look at an object
    illuminated by a single frequency of light, all of the light that you
    see reflected from the object is at that frequency. When you shine a red
    laser pointer at a wall, for example, your eye detects red light.
    However, for all materials, there is a tiny amount of light bouncing off
    at integer multiples of the incoming frequency. So with the red laser
    pointer, there will also be some blue light bouncing off of the wall.
    You just do not see it because it is such a small percentage of the
    total light. These multiples are called optical harmonics.


    The Hsieh group's experiment exploited the fact that changes in the
    symmetry of a crystal will affect the strength of each harmonic
    differently. Since the emergence of multipolar ordering changes the
    symmetry of the crystal in a very specific way-a way that can be largely
    invisible to conventional probes-their idea was that the optical
    harmonic response of a crystal could serve as a fingerprint of
    multipolar order.


    "We found that light reflected at the second harmonic frequency
    revealed a set of symmetries completely different from those of the
    known crystal structure, whereas this effect was completely absent for
    light reflected at the fundamental frequency," says Hsieh. "This is a
    very clear fingerprint of a specific type of multipolar order."


    The specific compound that the researchers studied was strontium-iridium oxide (Sr2IrO4),
    a member of the class of synthetic compounds broadly known as iridates.
    Over the past few years, there has been a lot of interest in Sr2IrO4
    owing to certain features it shares with copper-oxide-based compounds,
    or cuprates. Cuprates are the only family of materials known to exhibit
    superconductivity at high temperatures-exceeding 100 Kelvin (-173
    degrees Celsius). Structurally, iridates and cuprates are very similar.
    And like the cuprates, iridates are electrically insulating
    antiferromagnets that become increasingly metallic as electrons are
    added to or removed from them through a process called chemical doping. A
    high enough level of doping will transform cuprates into
    high-temperature superconductors, and as cuprates evolve from being
    insulators to superconductors, they first transition through a
    mysterious phase known as the pseudogap, where an additional amount of
    energy is required to strip electrons out of the material. For decades,
    scientists have debated the origin of the pseudogap and its relationship
    to superconductivity-whether it is a necessary precursor to
    superconductivity or a competing phase with a distinct set of symmetry
    properties. If that relationship were better understood, scientists
    believe, it might be possible to develop materials that superconduct at
    temperatures approaching room temperature.


    Recently, a pseudogap phase also has been observed in Sr2IrO4-and
    Hsieh's group has found that the multipolar order they have identified
    exists over a doping and temperature window where the pseudogap is
    present. The researchers are still investigating whether the two overlap
    exactly, but Hsieh says the work suggests a connection between
    multipolar order and pseudogap phenomena.


    "There is also very recent work by other groups showing signatures of
    superconductivity in Sr2IrO4 of the same variety as that found in
    cuprates," he says. "Given the highly similar phenomenology of the
    iridates and cuprates, perhaps iridates will help us resolve some of the
    longstanding debates about the relationship between the pseudogap and
    high-temperature superconductivity."


    Hsieh says the finding emphasizes the importance of developing new
    tools to try to uncover new phenomena. "This was really enabled by a
    simultaneous technique advancement," he says.


    Furthermore, he adds, these multipolar orders might exist in many more materials. "Sr2IrO4
    is the first thing we looked at, so these orders could very well be
    lurking in other materials as well, and that's exactly what we are
    pursuing next."



    Read more at: http://phys.org/news/2015-10-physicists-uncover-phase.html#jCp

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