Physicists working at the California Institute of Technology (Caltech) have discovered a new phase of matter with a highly unusual arrangement of electrons that could see the creation of innovative electronic devices with novel functionalities never before considered. Not quantifiable as a conventional metal, an insulator, or a type of magnet, this previously unknown state may also help answer a range of fundamental questions in the field of "high-temperature" superconductivity.
A chance discovery made while testing a laser-based measurement technique that the Caltech team recently developed, the novel phase was neither predicted nor sought after. Rather, it just appeared in their instrument readings while they were looking into a phenomenon known as "multipolar order."
In multipolar order research, scientists start with the known observations of electrons in regard to their position and spin state within a crystal. In regard to position, it is known that electrons subject to certain conditions will line up in an ordered, repetitive way inside a material, to create what is dubbed a charge-ordered phase. This type of ordered electron arrangement is a scalar quantity that may be described as a numerical value or magnitude of charge.
In the second attribute, when the spins of these same electrons line up parallel to one another, the minute forces produced by each electron are collectively applied to the material in which they reside to create a ferromagnet. This type of electron arrangement is characterized by spin magnitude and direction, so this spin-ordered phase is described by a vector.
However, with scientists being the curious lot that they are, these standard, ordered states led them to hypothesize about other possible states of matter. That is, what if electrons could be lined-up in different ways to those known? This led to the idea of matter that was not simply attributed with scalar and vector properties, but with other aspects of dimension, such as interaction in a matrix.
One such hypothesis, for example, is the idea that the constituent components of the matter in question may be constructed with pairs of electrons with opposing spins. That is, one with a north/south polarization and one with a south/north polarization, and illustrated mathematically as a quadrupole (a distribution of such electric charges arranged closely together and with alternating polarity).
In order to look for such multipolar order in materials, the research team took advantage of a phenomenon known as optical harmonic generation. In essence, this is the phenomenon that occurs with all electromagnetic frequencies (in this case, light) where whole number multiples of the main frequency are generated. This is because, even though a particular color can be generated as a sine wave at a fundamental frequency using ordinary light, it also nearly always contains other much lower levels of light at different frequencies (colors).
The researchers used this property in their experiments to look for harmonic strength changes in response to changes in the the symmetry of a crystal material under observation. As multipolar ordering alters that symmetry in very specific ways that can be detected relatively easily, the team settled on the notion that the optical harmonic response of a material could act as a type of fingerprint identifying 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," said assistant professor David Hsieh of the Caltech research team responsible for the discovery. "This is a very clear fingerprint of a specific type of multipolar order."
Specifically, the compound analyzed was strontium-iridium oxide (Sr2IrO4), an amalgam belonging to a class of synthetic compounds generally known as iridates. Given the similarity to copper-oxide-based compounds (cuprates), Sr2IrO4 has been the subject of much interest in the physics community in recent years. This is because cuprates represent the only known group of materials to display superconductivity at "high" temperatures; that is, exceeding 100° Kelvin (-173° C, -279.4° F).
As iridates and cuprates are similar, they also display properties as electrically insulating antiferromagnets that increase towards being highly metallic as electrons are added or removed in what is known as "doping." A large enough doping level of electrons alters cuprates sufficiently to allow them to become high-temperature superconductors, and on the way to reaching this state, cuprates must move from an insulator phase to a superconductor one. In this transition they must first pass through a phase called the pseudogap where, for a reason not yet fully understood, extra energy must be applied before further electrons can be stripped from the material.
In recent studies, a similar pseudogap phase has also been seen in Sr2IrO4, and the Caltech researchers have discovered that the multipolar order they have categorized appears at doping amounts and temperature levels where the pseudogap is found. Still in the throes of researching if the two phenomena overlap precisely, the experiments so far point to a connection between them.
"There is also very recent work by other groups showing signatures of superconductivity in Sr2IrO4 of the same variety as that found in cuprates," said Hsieh. "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."
If the new phase state discovered in the material can be better understood and exploited, it may lead to the creation of superconductors able to operate at higher temperatures – perhaps even approaching room temperature –, semiconductors with novel electron flow paths, and electronic devices endowed with as-yet-undiscovered properties.
The results of this research were recently published in the journal Nature Physics.
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