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Category: particle physics

Circa 2011

Scientists at M.I.T’s Media lab have created a camera that can capture the speed of light, taking a photo in less than two-trillionths of a second. Using multiple cameras, sensors, a pulse light source and mirrors, the researchers create slow motion movies of light moving through objects and liquids. They call the technique femto-photography. “We have built a virtual slow motion camera where we can see photons, or light particles through space,” said Associate Professor Ramesh Raskar in an video interview. “Photons travel about a million times photons travel a million times faster than bullets. So our camera can see photons, or bullets of light traveling through space.”

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Most of us are familiar with the four classical states of matter – solid, liquid, gas and plasma – but there’s a whole world of exotic states out there. Now, physicists at Radboud and Uppsala Universities have identified a new one named “self-induced spin glass,” which could be used to build new artificial intelligence platforms.

Magnetism usually arises when the electrons in the atoms of a material all spin in the same direction. But in a spin glass, the atomic magnets have no order, all spinning in random directions. The “glass” part of the name comes from the similarities to how atoms are arranged amorphously in a piece of regular old glass.

So far spin glasses have only been found in certain alloys, but now, researchers have discovered that the state occurs naturally in the pure element neodymium. To differentiate it from the alloy version, they’ve called the new state self-induced spin glass.

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Anyons – the particle-like collective excitations that can exist in some 2D materials – tend to bunch together in a two-dimensional conductor. This behaviour, which has now been observed by physicists at the Laboratory of Physics of the ENS (LPENS) and the Center for Nanoscience and Nanotechnologies (C2N) in Paris, France, is completely different to that of electrons, and experimental evidence for it is important both for fundamental physics and for the potential future development of devices based on these exotic quasiparticles.

The everyday three-dimensional world contains two types of elementary particles: fermions and bosons. Fermions, such as electrons, obey the Pauli exclusion principle, meaning that no two fermions can ever occupy the same quantum state. This tendency to flee from each other is at the heart of a wide range of phenomena, including the electronic structure of atoms, the stability of neutron stars and the difference between metals (which conduct electric current) and insulators (which don’t). Bosons such as photons, on the other hand, tend to bunch together – a gregarious behaviour that gives rise to superfluid and superconducting behaviours when many bosons exist in the same quantum state.

Within the framework of quantum mechanics, fermions also differ from bosons in that they have antisymmetric wavefunctions – meaning that a minus sign (that is, a phase φ equal to π) is introduced whenever two fermions are exchanged. Bosons, in contrast, have symmetric wavefunctions that remain the same when two bosons are exchanged (φ=0).

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There has been plenty of empirical evidence which shows that the single-particle picture holds to a good approximation in atomic nuclei. In this picture, protons and neutrons move independently inside a mean-field potential generated by an interaction among the nucleons. This leads to the concept of nuclear shells, similar to the electronic shells in atoms. In particular, the magic numbers due to closures of the nucleonic shells, corresponding to noble gases in elements, have been known to play an important role in nuclear physics. Here we propose a periodic table for atomic nuclei, in which the elements are arranged according to the known nucleonic shells. The nuclear periodic table clearly indicates that nuclei in the vicinity of the magic numbers can be understood in terms of a shell closure with one or two additional nucleons or nucleon holes, while nuclei far from the magic numbers are characterized by nuclear deformation.

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A few minutes into the life of the universe, colliding emissions of light energy created the first particles of matter and antimatter. We are familiar with the reverse process—matter generating energy—in everything from a campfire to an atomic bomb, but it has been difficult to recreate that critical transformation of light into matter.

Now, a new set of simulations by a research team led by UC San Diego’s Alexey Arefiev point the way toward making matter from light. The process starts by aiming a high-power laser at a target to generate a magnetic field as strong as that of a neutron star. This field generates that collide to produce—for the very briefest instant—pairs of matter and antimatter particles.

The study, published May 11 in Physical Review Applied offers a sort of recipe that experimentalists at the Extreme Light Infrastructure (ELI) high-power laser facilities in Eastern Europe could follow to produce real results in one to two years, said Arefiev, an associate professor of mechanical and aerospace engineering.

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Nanoparticle superlattice films that form at the solid-liquid interface are important for mesoscale materials but are challenging to analyze on the onset of formation at a solid-liquid interface. In a new report on Science Advances, E. Cepeda-Perez and a research team in materials, physics and chemistry in Germany studied the early stages of nanoparticle assembly at solid-liquid interfaces using liquid-phase electron microscopy. They observed oleylamine-stabilized gold nanoparticles to spontaneously form thin layers on a silicon nitride membrane window of the liquid enclosure. In the first monolayer, the assembly maintained dense packings of hexagonal symmetry independent of the nonpolar solvent type. The second layer displayed geometries ranging from dense packing in a hexagonal honeycomb structure to quasi-crystalline particle arrangements—based on the dielectric constant of the liquid. The complex structures made of weaker interactions remained preserved, while the surface remained immersed in liquid. By fine-tuning the properties of materials involved in nanoparticle superlattice formation, Cepeda-Perez et al. controlled the three-dimensional (3D) geometry of a superlattice, including quasi-crystals (a new state of matter).

Nanoparticles that are densely packed into two or three dimensions can form regular arrays of nanoparticle superlattices. For example, semiconductor particle superlattices can act as “meta” semiconductors when doped with particles to form new mesoscale materials, while plasmonic particles in dense superlattices can couple to form collective modes with angle-dependent and tunable wavelength responses. Large electric fields can occur between such particles for surface-enhanced Raman spectroscopy. Superlattices can be developed at liquid-liquid, gas-liquid and solid-liquid interfaces, where static and dynamic interactions between particle-substrate, particle-particle and particle-liquid interactions can dictate the structure of superlattices. However, it remains difficult to predict such structures in advance. For example, simulating the assembly of superlattices at multiple stages is not yet possible, with very little in-lab real-space data available for modeling.

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Just as a literature buff might explore a novel for recurring themes, physicists and mathematicians search for repeating structures present throughout nature.

For example, a certain geometrical structure of knots, which scientists call a Hopfion, manifests itself in unexpected corners of the universe, ranging from , to biology, to cosmology. Like the Fibonacci spiral and the golden ratio, the Hopfion pattern unites different scientific fields, and deeper understanding of its structure and influence will help scientists to develop transformative technologies.

In a recent theoretical study, scientists from the U.S. Department of Energy’s (DOE) Argonne National Laboratory, in collaboration with the University of Picardie in France and the Southern Federal University in Russia, discovered the presence of the Hopfion structure in nano-sized particles of ferroelectrics—materials with promising applications in microelectronics and computing.

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Acoustofluidics is the fusion of acoustics and fluid mechanics which provides a contact-free, rapid and effective manipulation of fluids and suspended particles. The applied acoustic wave can produce a non-zero time-averaged pressure field to exert an acoustic radiation force on particles suspended in a microfluidic channel. However, for particles below a critical size the viscous drag force dominates over the acoustic radiation forces due to the strong acoustic streaming resulting from the acoustic energy dissipation in the fluid. Thus, particle size acts as a key limiting factor in the use of acoustic fields for manipulation and sorting applications that would otherwise be useful in fields including sensing (plasmonic nanoparticles), biology (small bioparticle enrichment) and optics (micro-lenses).

Although acoustic nanoparticle manipulation has been demonstrated, terahertz (THz) or gigahertz (GHz) frequencies are usually required to create nanoscale wavelengths, in which the fabrication of very small feature sizes of SAW transducers is challenging. In addition, single nanoparticle positioning into discrete traps has not been demonstrated in nanoacoustic fields. Hence, there is a pressing need to develop a fast, precise and scalable method for individual nano- and submicron scale manipulation in acoustic fields using megahertz (MHz) frequencies.

An interdisciplinary research team led by Associate Professor Ye Ai from Singapore University of Technology and Design (SUTD) and Dr. David Collins from University of Melbourne, in collaboration with Professor Jongyoon Han from MIT and Associate Professor Hong Yee Low from SUTD, developed a novel acoustofluidic technology for massively multiplexed submicron particle trapping within nanocavities at the single-particle level.

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