System Bits: Oct. 27

Uncovering a novel phase of matter
In a finding that could have implications for high-temperature superconductivity, a team of physicists led by David Hsieh, assistant professor of physics at Caltech has discovered an unusual phase of matter that is characterized by an unusual ordering of electrons.

The researchers said this finding offers possibilities for new electronic device functionalities and could hold the solution to a long-standing mystery in condensed matter physics having to do with high-temperature superconductivity.

The discovery was made while testing a laser-based measurement technique recently developed to look for what is called multipolar order.

To understand multipolar order, the researchers suggested considering a crystal with electrons moving around throughout its interior. Under certain conditions, it can be energetically favorable for these electrical charges to pile up in a regular, repeating fashion inside the crystal, forming what is called a charge-ordered phase. The building block of this type of order, namely charge, is a scalar quantity—i.e, it can be described by just a numerical value, or magnitude.

Along with charge, they said, electrons have a degree of freedom known as spin. When spins line up parallel to each other (in a crystal, for example), they form a ferromagnet. And because spin has both a magnitude and a direction, a spin-ordered phase is described by a vector.

David Hsieh directs the path of a laser beam through an apparatus, which is used to measure spontaneous self-organization of electrons inside a crystal. (Source: Caltech)

Over the last several decades, physicists have developed sophisticated techniques to look for both of these types of phases. The Caltech team questioned what would happen if the electrons in a material are not ordered in one of those ways? As in, what if the order were described not by a scalar or vector but by something with more dimensionality, like a matrix? This could happen, for example, if the building block of the ordered phase was a pair of oppositely pointing spins—one pointing north and one pointing south—described by what is known as a magnetic quadrupole. Such examples of multipolar-ordered phases of matter are difficult to detect using traditional experimental probes.

The new phase that the Hsieh group identified is precisely this type of multipolar order.  

Using nanoquakes to understand monolayer films
A research project resulting from collaboration between students and researchers at UC Riverside and the University of Augsburg, Germany has found a new and exciting way to elucidate the properties of novel 2D semiconductors with materials that have unique properties that promise better integration of optical communication with traditional silicon-based devices.

The researchers fabricated a single-atomic-layer-thin film of molybdenum disulfide (MoS2) on a substrate of lithium niobate (LiNbO3), which is used in many electronic devices dealing with high-frequency signals such as cell phones or radar installations. Applying electrical pulses to LiNbO3, the researchers created very high frequency sound waves – “surface acoustic waves” – that run along the surface of LiNbO3, akin to earthquake tremors on land. Cell phones, for example, use resonances of these surface waves to filter electric signals in a manner similar to a wine glass resonating when a voice hits it at exactly the right pitch.

Specifically, the research team used the surface waves of LiNbO3 to listen to how the illumination of LiNbO3 by laser light changes the electric properties of MoS2.

The team learned to ‘hear’ the LiNbO3 sound waves and infer how much current the laser light allowed to flow in the MoS2. They also fabricated transistor structures onto the MoS2 films and proved that their analysis was correct

They believe this could lead to remote, wireless sensing applications.

Nanoscale light manipulation
Before realizing the promise of photonic devices, which use light to transport large amounts of information quickly, it must be possible to manipulate light at the nanoscale, according to researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) who say they have done just that.

A Harvard team of researchers has designed what they say is the first on-chip metamaterial with a refractive index of zero, meaning that the phase of light can travel infinitely fast. This new metamaterial was developed in the lab of Eric Mazur, the Balkanski Professor of Physics and Applied Physics and Area Dean for Applied Physics at SEAS.

New zero-index material made of silicon pillar arrays embedded in a polymer matrix and clad in gold film creates a constant phase of light, which stretches out in infinitely long wavelengths. (Source: Harvard SEAS)  

Mazur explained that light doesn’t typically like to be squeezed or manipulated but this metamaterial permits light to be manipulated from one chip to another, to squeeze, bend, twist and reduce diameter of a beam from the macroscale to the nanoscale.

This on-chip metamaterial opens the door to exploring the physics of zero index and its applications in integrated optics, Mazur added.