System Bits: July 28

Massless particles; smartphone device for medical diagnostics; spintronic device.

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Massless particles for faster electronics
Princeton University researchers along with an international team have finally proved a massless particle that had been theorized for 85 years. They say this particle could give rise to faster and more efficient electronics because of its unusual ability to behave as matter and antimatter inside a crystal.

An international team led by Princeton University scientists has discovered Weyl fermions, elusive massless particles theorized 85 years ago that could give rise to faster and more efficient electronics because of their unusual ability to behave as matter and antimatter inside a crystal. The team included numerous researchers from Princeton's Department of Physics, including (from left to right) graduate students Ilya Belopolski and Daniel Sanchez; Guang Bian, a postdoctoral research associate; corresponding author M. Zahid Hasan, a Princeton professor of physics who led the research team; and associate research scholar Hao Zheng. (Source: Princeton University)

An international team led by Princeton University scientists has discovered Weyl fermions, elusive massless particles theorized 85 years ago that could give rise to faster and more efficient electronics because of their unusual ability to behave as matter and antimatter inside a crystal. The team included numerous researchers from Princeton’s Department of Physics, including (from left to right) graduate students Ilya Belopolski and Daniel Sanchez; Guang Bian, a postdoctoral research associate; corresponding author M. Zahid Hasan, a Princeton professor of physics who led the research team; and associate research scholar Hao Zheng. (Source: Princeton University)

Specifically, the researchers are reporting the first observation of Weyl fermions, which, if applied to next-generation electronics, could allow for a nearly free and efficient flow of electricity in electronics, and thus greater power, especially for computers.

The researchers reminded that Weyl fermions were proposed by the mathematician and physicist Hermann Weyl in 1929, and have been long sought by scientists because they have been regarded as possible building blocks of other subatomic particles, and are even more basic than the ubiquitous, negative-charge carrying electron (when electrons are moving inside a crystal). Their basic nature means they could provide a much more stable and efficient transport of particles than electrons, which are the principle particle behind modern electronics. Unlike electrons, Weyl fermions are massless and possess a high degree of mobility; the particle’s spin is both in the same direction as its motion — which is known as being right-handed — and in the opposite direction in which it moves, or left-handed.

M. Zahid Hasan, a Princeton professor of physics, led the research team.

A detector image (top) signals the existence of Weyl fermions. The plus and minus signs note whether the particle's spin is in the same direction as its motion — which is known as being right-handed — or in the opposite direction in which it moves, or left-handed. This dual ability allows Weyl fermions to have high mobility. A schematic (bottom) shows how Weyl fermions also can behave like monopole and antimonopole particles when inside a crystal, meaning that they have opposite magnetic-like charges can nonetheless move independently of one another, which also allows for a high degree of mobility. (Source: Princeton Department of Physics)

A detector image (top) signals the existence of Weyl fermions. The plus and minus signs note whether the particle’s spin is in the same direction as its motion — which is known as being right-handed — or in the opposite direction in which it moves, or left-handed. This dual ability allows Weyl fermions to have high mobility. A schematic (bottom) shows how Weyl fermions also can behave like monopole and antimonopole particles when inside a crystal, meaning that they have opposite magnetic-like charges can nonetheless move independently of one another, which also allows for a high degree of mobility. (Source: Princeton Department of Physics)

The researchers also said that Weyl fermions can be used to create massless electrons that move very quickly with no backscattering, wherein electrons are lost when they collide with an obstruction. In electronics, backscattering hinders efficiency and generates heat. Weyl electrons simply move through and around roadblocks, Hasan noted. “It’s like they have their own GPS and steer themselves without scattering. They will move and move only in one direction since they are either right-handed or left-handed and never come to an end because they just tunnel through. These are very fast electrons that behave like unidirectional light beams and can be used for new types of quantum computing.”

Smartphone-based device for reading medical tests
Enzyme-linked immunosorbant assay (ELISA) is a diagnostic tool that identifies antigens such as viruses and bacteria in blood samples that can detect a number of diseases, including HIV, West Nile virus and hepatitis B, and is widely used in hospitals. It can also be used to identify potential allergens in food, among other applications. Now, a team of researchers from the California NanoSystems Institute at UCLA has developed a mobile phone-based device that can read ELISA plates in the field with the same level of accuracy as the large machines normally found in clinical laboratories.

The new ELISA platform is created with a 3D printer and attaches to a smartphone. (Source: UCLA)

The new ELISA platform is created with a 3D printer and attaches to a smartphone. (Source: UCLA)

The research was led by Aydogan Ozcan, associate director of the California NanoSystems Institute, along with Dino Di Carlo, professor of bioengineering, and Omai Garner, associate director of clinical microbiology for the UCLA Health System. UCLA undergraduate Brandon Berg was the study’s first author, and two other undergraduates also contributed to the research.

Chiral property of silicon has photonic applications
According to University of Pennsylvania researchers, by encoding information in photons via their spin, “photonic” computers could be orders of magnitude faster and efficient than their current-day counterparts. Likewise, encoding information in the spin of electrons, rather than just their quantity, could make “spintronic” computers with similar advantages.

To this end, these researchers have discovered a property of silicon that combines aspects of all of these desirable qualities and have demonstrated a silicon-based photonic device that is sensitive to the spin of the photons in a laser shined on one of its electrodes. Light that is polarized clockwise causes current to flow in one direction, while counter-clockwise polarized light makes it flow in the other direction.

They said this property was hiding in plain sight; it is a function of the geometric relationship between the pattern of atoms on the surface of silicon nanowires and how electrodes placed on those wires intersect them.

By shining lasers with different polarizations on their device, the researchers could make current flow in different directions. (Source: University of Pennsylvania)

By shining lasers with different polarizations on their device, the researchers could make current flow in different directions. (Source: University of Pennsylvania)

The interaction between the semiconducting silicon and the metallic electrodes produces an electric field at an angle that breaks the mirror symmetry that silicon typically exhibits. This chiral property is what sends electrons in one direction or the other down the nanowire depending on the polarity of the light that hits the electrodes.