Physicists succeed in bringing movement of photons and electrons under same laws

This visualisation shows layers of graphene used for membranes. Credit: University of Manchester

Scientists from ITMO, Sheffield University, and the University of Iceland proved that the movement of electrons and photons in two-dimensional materials with hexagonal symmetry, such as graphene, submits to the same laws. Now, the properties of electrons in solids can be modeled with the help of classical optical systems where this task can be solved easier. The article was published in Nature Photonics.

Graphene is the most famous two-dimensional material, and it is durable and has high conductivity. Andre Geim and Konstantin Novoselov got the 2010 Nobel Prize in Physics for its development. Despite being ‘light,’ it’s 300 times stronger than steel. Its unique properties have to do with its structure. The behavior of electrons in a material largely depends on the geometry of the substance’s crystal lattice. In the case of graphene, carbon atoms form hexagonal cells, thus electrons can behave as particles with zero effective mass, despite having mass in reality.

“This behavior of electrons in graphene is described by the laws of quantum mechanics, where the electron is not perceived as a particle that moves around an atom’s nucleus but as a material wave. Particular properties of waves of different physical nature depend only on a system’s symmetry. This makes it possible to create ‘photonic graphene.’ It resembles a thin transparent plate that looks like a honeycomb. If electrons can behave as particles with no mass in classical graphene, here, photons behave in a similar manner,” explains Alexey Yulin, researcher at ITMO’s Faculty of Physics and Engineering.

Scientists from Russia, England and Iceland set at the task to reproduce the dynamics of massless electrons that have spin in graphene using massless light that propagates in an optical system. Having created an optical counterpart of graphene, they’ve studied the effects that emerge when influencing it with photons: it’s excited by a focused laser emission that falls under a specific angle. A change in the incidence angle of light falling on a photonic system provided for the emergence of waves with the desired properties.

In the article, scientists studied an instance when they selectively excited massless photons in photonic graphene. The comparison of theory and experiment showed that the proposed mathematical model reproduces the experimental results. For comparison, they’ve also studied an instance when light in photonic graphene behaves as regular particles with a nonzero mass.

In the course of the experiment, the physicists discovered that the polarization effects are similar to spin effects that are well-known in solid state physics. The scientists also proved the possibility of describing these phenomena with the help of equations from the field of classical physics. Now the properties that are hard to measure or control in solids can be studied using photonic systems where these tasks can be solved relatively easily.

“Thanks to the processes that take place in regular graphene being similar to those in photonic systems, optical systems can be used to imitate the spin dynamics

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Researchers create nanoscale slalom course for electrons

Pitt researchers create nanoscale slalom course for electrons
Illustration of sketched serpentine nanowires created from lanthanum aluminate and strontium titanate. The side-to-side motion of the electrons as they travel gives them additional properties that can be used to make quantum devices. Credit: Jeremy Levy

A research team led by professors from the Department of Physics and Astronomy have created a serpentine path for electrons, imbuing them with new properties that could be useful in future quantum devices.

Jeremy Levy, a distinguished professor of condensed matter physics, and Patrick Irvin, research professor, are coauthors of the paper “Engineered spin-orbit interactions in LaAlO3/SrTiO3-based 1D serpentine electron waveguides,” published in Science Advances on November 25.

“We already know how to shoot electrons ballistically through one-dimensional nanowires made from these oxide materials,” explains Levy. “What is different here is that we have changed the environment for the electrons, forcing them to weave left and right as they travel. This motion changes the properties of the electrons, giving rise to new behavior.”

The work is led by a recent Ph.D. recipient, Dr. Megan Briggeman, whose thesis was devoted to the development of a platform for “quantum simulation” in one dimension. Briggeman is also the lead author on a related work published earlier this year in Science, where a new family of electronic phases was discovered in which electrons travel in packets of 2, 3, and more at a time.

Electrons behave very differently when forced to exist along a straight line (i.e., in one dimension). It is known, for example, that the spin and charge components of electrons can split apart and travel at different speeds through a 1D wire. These bizarre effects are fascinating and also important for the development of advanced quantum technologies such as quantum computers. Motion along a straight line is just one of a multitude of possibilities that can be created using this quantum simulation approach. This publication explores the consequences of making electrons weave side to side while they are racing down and otherwise linear path.

One recent proposal for topologically-protected quantum computation takes advantage of so-called “Majorana fermions”, particles which can exist in 1D quantum wires when certain ingredients are present. The LaAlO3/SrTiO3 system, it turns out, has most but not all of the required interactions. Missing is a sufficiently strong “spin-orbit interaction” that can produce the conditions for Majorana fermions. One of the main findings of this latest work from Levy is that spin-orbit interactions can in fact be engineered through the serpentine motion that electrons are forced to undertake.

In addition to identifying new engineered spin-orbit couplings, the periodic repetition of the serpentine path creates new ways for electrons to interact with one another. The experimental result of this is the existence of fractional conductances that deviate from those expected for single electrons.

These slalom paths are created using a nanoscale sketching technique analogous to an Etch A Sketch toy, but with a point size that is a trillion times smaller in area. These paths can

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Killer electrons in strumming sky lights

Killer electrons in strumming sky lights
Low-energy (blue) and high-energy (yellow) electrons form during the process that generates the pulsating aurora. The high-energy ‘relativistic’ electrons could cause localized destruction of the ozone. Credit: PsA project

Computer simulations explain how electrons with wide-ranging energies rain into Earth’s upper and middle atmosphere during a phenomenon known as the pulsating aurora. The findings, published in the journal Geophysical Research Letters, suggest that the higher-energy electrons resulting from this process could cause destruction of the part of the ozone in the mesosphere, about 60 kilometers above Earth’s surface. The study was a collaboration between scientists in Japan, including at Nagoya University, and colleagues in the US, including from NASA.

The northern and southern lights that people are typically aware of, called the aurora borealis and australis, look like colored curtains of reds, greens, and purples spreading across the night skies. But there is another kind of aurora that is less frequently seen. The pulsating aurora looks more like indistinct wisps of cloud strumming across the sky.

Scientists have only recently developed the technologies enabling them to understand how the pulsating aurora forms. Now, an international research team, led by Yoshizumi Miyoshi of Nagoya University’s Institute for Space-Earth Environmental Research, has developed a theory to explain the wide-energy electron precipitations of pulsating auroras and conducted computer simulations that validate their theory.

Their findings suggest that both low- and high-energy electrons originate simultaneously from interactions between chorus waves and electrons in the Earth’s magnetosphere.

Chorus waves are plasma waves generated near the magnetic equator. Once formed, they travel northwards and southwards, interacting with electrons in Earth’s magnetosphere. This interaction energizes the electrons, scattering them down into the upper atmosphere, where they release the light energy that appears as a pulsating aurora.

The electrons that result from these interactions range from lower-energy ones, of only a few hundred kiloelectron volts, to very high-energy ones, of several thousand kiloelectron volts, or ‘megaelectron’ volts.

Miyoshi and his team suggest that the high-energy electrons of pulsating auroras are ‘relativistic’ electrons, otherwise known as killer electrons, because of the damage they can cause when they penetrate satellites.

“Our theory indicates that so-called killer electrons that precipitate into the middle atmosphere are associated with the pulsating aurora, and could be involved in ozone destruction,” says Miyoshi.

The team next plans to test their theory by studying measurements taken during a space rocket mission called ‘loss through auroral microburst pulsations’ (LAMP), which is due to launch in December 2021. LAMP is a collaboration between NASA, the Japan Aerospace Exploration Agency (JAXA), Nagoya University, and other institutions. LAMP experiments will be able to observe the killer electrons associated with the pulsating aurora.

The paper, “Relativistic Electron Microbursts as High‐Energy Tail of Pulsating Aurora Electrons,” was published online in Geophysical Research Letters on October 13, 2020.

Pulsating aurora mysteries uncovered with help from NASA’s THEMIS mission

More information:
Y. Miyoshi et al. Relativistic Electron Microbursts as High‐Energy Tail of Pulsating Aurora Electrons, Geophysical Research Letters (2020). DOI: 10.1029/2020GL090360
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Direct observation of a single electron’s butterfly-shaped distribution in titanium oxide

Direct observation of a single electron's butterfly-shaped distribution in titanium oxide
Figure 1. (a) Distribution of a butterfly-shaped 3d electron orbital. (b) Valence electron density distribution around the titanium (Ti3+) ion at the centre of the titanium oxide (TiO6 ) octahedron obtained by the CDFS analysis developed by the research team for this project. Credit: Shunsuke Kitou

The functions and physical properties of solid materials, such as magnetic order and unconventional superconductivity, are greatly influenced by the orbital state of the outermost electrons (valence electrons) of the constituent atoms. In other words, it could be said that the minimal unit that determines a solid material’s physical properties consists of the orbitals occupied by the valence electrons. Moreover, an orbital can also be considered a minimal unit of ‘shape,’ so the orbital state in a solid can be deduced from observing the spatially anisotropic distribution of electrons (in other words, from how the electron distribution deviates from spherical symmetry).

The orbital states in elements are basic knowledge that can be found in quantum mechanics or quantum chemistry textbooks. For example, it is known that the 3d electrons in transition elements such as iron and nickel have characteristic butterfly-type or gourd-type shapes. However, until now, it has been extremely difficult to observe the real-space distribution of such electron orbitals directly.

Now, a research collaboration between Nagoya University, University of Wisconsin-Milwaukee, Japan’s RIKEN and Institute for Molecular Science, the University of Tokyo, and the Japan Synchrotron Radiation Research Institute (JASRI), has observed the spatial distribution of a single valence electron at the center of an octahedron-shaped titanium oxide molecule, using synchrotron X-ray diffraction.

To analyze the X-ray diffraction data from the titanium oxide sample, the team developed a Fourier synthesis method in which data from each titanium ion’s inner shell electrons—which do not contribute to the compound’s physical properties—are subtracted from the total electron distribution of each ion, leaving only the butterfly-shaped valence electron density distribution. The method is called core differential Fourier synthesis (CDFS).

Direct observation of a single electron's butterfly-shaped distribution in titanium oxide
Cross-sectional view of the valence electron density distribution of Ti3+ ion obtained by (a) the CDFS analysis and (b) the first-principles calculation. Credit: Shunsuke Kitou

Furthermore, a closer look at the butterfly-shaped electron density revealed that high density remained in the central region, in contrast with bare titanium in which electrons do not exist at the center because of the node of the 3d orbital. After careful data analysis, it was found that the electron density at the center consists of the valence electrons occupying the hybridized orbital generated by the bond between titanium and oxygen. First-principles calculations confirmed this non-trivial orbital picture and reproduced the results of the CDFS analysis very well. The image directly demonstrates the well-known Kugel-Khomskii model of the relationship between the magnetic and orbital-ordered states.

The CDFS method can determine the orbital states in materials regardless of the physical properties and can be applied to almost all elements and without the need for difficult experiments or analytical techniques: the method requires neither quantum-mechanical nor informatic models, so bias introduced by analysts is minimized. The

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