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."