MIT

An international team of researchers, including ones from ICFO, MIT, RWTH Aachen University and others, has used terahertz light to explore exotic phenomena within magic-angle twisted bilayer graphene. This approach revealed previously unseen behaviors and provided direct insights into the quantum geometry of electronic wavefunctions —the fundamental framework underlying these phenomena.

It has been less than ten years since scientists placed two graphene layers on top of each other, twisted them exactly 1.1º and observed the emergence of exotic phenomena like superconductivity and topological phases of matter. The unlocked new physics attracted great attention among the community, and the whole system soon became known as “magic-angle twisted bilayer graphene”. This magic-angle has been extensively studied ever since with most efforts focused on understanding how electron interactions lead to such exotic collective quantum phases. However, electrons at the single-particle level have also been predicted to exhibit intriguing quantum behaviors underscored by the geometry of their wavefunctions – their quantum geometry. Yet, observing these behaviors has proven challenging.

Researchers from MIT and Japan's National Institute for Materials Science have directly measured superfluid stiffness for the first time in “magic-angle” graphene — two or more atomically thin sheets of graphene twisted with respect to each other at just the right angle to enable a host of exceptional properties, including unconventional superconductivity. The term “superfluid stiffness,” or the ease with which a current of electron pairs can flow, is a key measure of a material’s superconductivity.

This superconductivity makes magic-angle graphene a promising building block for future quantum-computing devices, but exactly how the material superconducts is not well-understood. Knowing the material’s superfluid stiffness will help scientists identify the mechanism of superconductivity in magic-angle graphene. The team’s measurements suggest that magic-angle graphene’s superconductivity is primarily governed by quantum geometry, which refers to the conceptual “shape” of quantum states that can exist in a given material.

Researchers from MIT, Princeton University, SLAC National Accelerator Laboratory and Japan's National Institute for Materials Science have created a new ultrathin 2D material with unusual magnetic properties that initially surprised the researchers before they went on to solve the complicated puzzle behind those properties’ emergence. As a result, the work introduces a new platform for studying how materials behave at the most fundamental level — the world of quantum physics.

The scientists, led by MIT's Pablo Jarillo-Herrero, worked with three layers of graphene. Each layer was twisted on top of the next at the same angle, creating a helical structure reminiscent of a DNA helix.

MIT researchers have gained new understanding of what leads electrons to split into fractions of themselves. Their solution sheds light on the conditions that give rise to exotic electronic states in graphene and other two-dimensional systems.

The recent work attempts to make sense of a discovery that was reported earlier this year by a different group of physicists at MIT, led by Assistant Professor Long Ju. Ju’s team found that electrons appear to exhibit “fractional charge” in pentalayer graphene — a configuration of five graphene layers that are stacked atop a similarly structured sheet of boron nitride.

Researchers at the Massachusetts Institute of Technology (MIT), University of Texas at Dallas and Japan's National Institute for Materials Science have reported the quantum anomalous Hall effect (QAHE), a topological phenomenon that features quantized Hall resistance at zero magnetic field, in a rhombohedral pentalayer graphene-monolayer tungsten disulfide (WS2) heterostructure. 

This achievement can also be described as a 'five-lane superhighway' for electrons, that could allow ultra-efficient electronics and more. The team explained that its discovery could have direct implications for low-power electronic devices because no energy is lost during the propagation of electrons, which is not the case in regular materials where the electrons are scattered.

Researchers from the University of Göttingen, Japan's National Institute for Materials Science and Massachusetts Institute of Technology (MIT) have demonstrated experimentally that electrons in naturally occurring double-layer graphene move like particles without any mass, in the same way that light travels. Furthermore, they have shown that the current can be "switched" on and off, which has potential for developing tiny, energy-efficient transistors. 

Among its many unusual properties, graphene is known for its extraordinarily high electrical conductivity due to the high and constant velocity of electrons travelling through this material. This unique feature has made scientists try to use graphene for faster and more energy-efficient transistors. The challenge has been that to make a transistor, the material needs to be controlled to have a highly insulating state in addition to its highly conductive state. In graphene, however, such a "switch" in the speed of the carrier cannot be easily achieved. In fact, graphene usually has no insulating state, which has limited graphene's potential a transistor.

Researchers at the University of Massachusetts and Massachusetts Institute of Technology (MIT) have successfully built a tissue-like bioelectronic mesh system integrated with an array of graphene sensors that can simultaneously measure both the electrical signal and the physical movement of cells in lab-grown human cardiac tissue.

A bioelectronic mesh, studded with graphene sensors (red), can measure the electrical signal and movement of cardiac tissue (purple and green) at the same time. Image credit: UMass Amherst
 

The tissue-like mesh can grow along with the cardiac cells, allowing researchers to observe how the heart’s mechanical and electrical functions change during the developmental process. The new device can be extremely useful for those studying cardiac disease as well as those studying the potentially toxic side-effects of many common drug therapies.

Researchers at MIT and Japan's National Institute for Materials Science (NIMS) have observed an exotic electronic state in a material made of five layers of graphene, that could enable new forms of quantum computing. 

Generally speaking, the electron is the basic unit of electricity, as it carries a single negative charge. At least, that's the case in most materials in nature. But in very special states of matter, electrons can splinter into fractions of their whole. This phenomenon, known as “fractional charge,” is extremely rare, and if it can be corralled and controlled, the exotic electronic state could help to build resilient, fault-tolerant quantum computers. To date, this effect, known to physicists as the “fractional quantum Hall effect,” has been observed a handful of times, and mostly under very high, carefully maintained magnetic fields. Now, the scientists have also seen the effect in a material that did not require such powerful magnetic manipulation. They found that when five sheets of graphene are stacked like steps on a staircase, the resulting structure inherently provides just the right conditions for electrons to pass through as fractions of their total charge, with no need for any external magnetic field.

Researchers at Northwestern University, MIT, Harvard University, CIFAR Azrieli Global Scholars Program and Japan's National Institute for Materials Science have developed a graphene-based synaptic transistor capable of higher-level thinking.

The device simultaneously processes and stores information just like the human brain. In new experiments, the researchers demonstrated that the transistor goes beyond simple machine-learning tasks to categorize data and is capable of performing associative learning.

Researchers at MIT, Harvard and Japan's National Institute for Materials Science have reported a surprising property in graphene: When stacked in five layers, in a rhombohedral pattern, graphene displays a rare, “multiferroic” state, in which the material exhibits both unconventional magnetism and an exotic type of electronic behavior, which the team has named "ferro-valleytricity".

“Graphene is a fascinating material,” said Long Ju, assistant professor of physics at MIT. “Every layer you add gives you essentially a new material. And now this is the first time we see ferro-valleytricity, and unconventional magnetism, in five layers of graphene. But we don’t see this property in one, two, three, or four layers”. The discovery could promote ultra-low-power, high-capacity data storage devices for classical and quantum computers.