Lifeboat Foundation

Topological materials that possess certain atomic-level symmetries, including topological insulators and topological semi-metals, have elicited fascination among many condensed matter scientists because of their complex electronic properties. Now, researchers in Japan have demonstrated that a normal semiconductor can be transformed into a topological semi-metal by light irradiation. Further, they showed how spin-dependent responses could appear when illuminated with circularly-polarized laser light. Published in Physical Review B, this work explores the possibility of creating topological semi-metals and manifesting new physical properties by light control, which may open up a rich physical frontier for topological properties.

Most ordinary substances are either electrical conductors, like metals, or insulators, like plastic. In contrast, topological insulators can exhibit unusual behavior in which electrical currents flow along the surface of the sample, but not inside the interior. This characteristic behavior is strongly connected to topological properties inherent in the electronic state. Furthermore, a novel phase called a topological semi-metal provides a new playground for exploring the role of topology in condensed matter. However, the underlying physics of these systems is still being pondered.

Researchers at the University of Tsukuba studied the dynamics of excitations in zinc arsenide (Zn₃As₂) when irradiated with a laser with circular polarization. Zinc arsenide is normally thought of as a narrow-gap semiconductor, which means that electrons are not free to move around on their own but can be easily propelled by energy from an external light source. Under the right conditions, the material can show a special topological state called a “Floquet-Weyl semi-metal,” which is a topological semi-metal coupled with light. In this case, the electrical current can be carried in the form of quasiparticles called Weyl fermions. Because these quasiparticles travel as if they have zero mass and resist becoming scattered, Weyl fermions can move easily through the material.

Today, an international team of researchers led by Séamus Davis, Professor of Physics at the University of Oxford and University College Cork, has announced results that reveal the atomic mechanism behind high-temperature superconductors. The findings are published in PNAS.

Superconductors are materials that can conduct electricity with zero resistance, so that an electric current can persist indefinitely. These are already used in various applications, including MRI scanners and high-speed maglev trains, however superconductivity typically requires extremely low temperatures, limiting their widespread use. A major goal within physics research is to develop super conductors that work at ambient temperatures, which could revolutionize energy transport and storage.

Certain copper oxide materials demonstrate superconductivity at higher temperatures than conventional superconductors, however the mechanism behind this has remained unknown since their discovery in 1987.

Many of us are all too familiar with how strain in work relationships can impact performance, but new research shows that materials in electricity-producing fuel cells may be sensitive to strain on an entirely different level.

Researchers from Kyushu University report that strain caused by just a 2% reduction in the distance between atoms when deposited on a surface leads to a whopping 99.999% decrease in the speed at which the materials conduct hydrogen ions, greatly reducing the performance of solid oxide fuel cells.

Developing methods to reduce this strain will help bring high-performance fuel cells for clean energy production to a wider number of households in the future.

Magnetorotational instability—a process that might explain the dynamics of astrophysical accretion disks—has finally been observed in the laboratory.

What do black holes, forming stars, and a tank of liquid metal in Princeton, New Jersey, have in common? The first two might and the third one definitely does play host to an important process in magnetized-fluid dynamics called magnetorotational instability (MRI). MRI has been well studied theoretically and computationally, and related processes have been seen experimentally [1]. But until now, there has not been an unambiguous laboratory confirmation of its existence. Yin Wang and his colleagues at Princeton University have demonstrated MRI in an ingenious liquid-metal experiment—the culmination of more than 20 years of work [2].

The team’s discovery is significant because MRI has long been suspected of being at the heart of accretion [3]. Accretion, in which material spirals inward in a flattened disk around a black hole or a young star, is a major source of the light coming from those objects. For accretion to occur, the material in the disk must lose its angular momentum. However, angular momentum is conserved: much like the trash we generate in our daily lives, it does not cease to exist when it is not wanted. Instead, angular momentum must be passed from the inner parts of the disk to the outer parts. What drives this angular-momentum transport has long been a mystery.

Scientists from the Faculty of Pure and Applied Sciences at The University of Tsukuba created scanning tunneling microscopy (STM) “snapshots” with a delay between frames much shorter than previously possible. By using ultrafast laser methods, they improved the time resolution from picoseconds to tens of femtoseconds, which may greatly enhance the ability of condensed matter scientists to study extremely rapid processes.

One picosecond, which is a mere trillionth of a second, is much shorter than the blink of an eye. For most applications, a movie camera that could record frames in a picosecond would be much faster than necessary. However, for scientists trying to understand the ultrafast dynamics of materials using STM, such as the rearrangement of atoms during a phase transition or the brief excitation of electrons, it can be painfully slow.

Now, a team of researchers at the University of Tsukuba designed an STM system based on a pump-probe method that can be used over a wide range of delay times as short as 30 femtoseconds (ACS Photonics, “Subcycle mid-infrared electric-field-driven scanning tunneling microscopy with a time resolution higher than 30 fs”).

They will charge in minutes and last much longer.

Two-dimensional (2D) semiconductors, like transition-metal dichalcogenides, have become a competitive alternative to traditional semiconducting materials in the post-Moore era, and caused worldwide interest. However, before they can be used in practical applications, some key obstacles must be resolved. One of them is the large electrical contact resistances at the metal-semiconductor interfaces. Researchers have proposed a brand-new contact resistance lowering strategy of 2D semiconductors with a good feasibility, a wide generality and a high stability.

Two-dimensional van der Waals materials have been the focus of work by numerous research groups for some time. Standing just a few atomic layers thick, these structures are produced in the laboratory by combining atom-thick layers of different materials (in a process referred to as “atomic Lego”).

Interactions between the stacked layers allow the heterostructures to exhibit properties that the individual constituents lack.

Continue reading “Coupling of electron-hole pairs” »

The adoption rate of fuel cells has increased owing to the rising need for clean energy.

In a research that could jump-start the work on a range of technologies, including fuel cells, which are key to storing solar and wind energy, MIT researchers have found a simple way to significantly increase the lifetimes of fuel cells and other devices – changing the pH of the system.

Fuel/electrolysis cells made of materials known as solid metal oxides are in interest for several reasons. In electrolysis mode, they are very efficient at converting electricity from a renewable source into a storable fuel like hydrogen or methane. This storable fuel can be used in the fuel cell mode to generate electricity when the sun is not shining, or the wind isn’t blowing.