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Exotic physics phenomenon is observed for first time

Exotic physics phenomenon is observed for first time

An exotic physical phenomenon, involving optical waves, synthetic magnetic fields, and time reversal, has been directly observed for the first time, following decades of attempts. The new finding could lead to realizations of what are known as topological phases, and eventually to advances toward fault-tolerant quantum computers, the researchers say.

The new finding involves the non-Abelian Aharonov-Bohm Effect and is reported today in the journal Science by MIT graduate student Yi Yang, MIT visiting scholar Chao Peng (a professor at Peking University), MIT graduate student Di Zhu, Professor Hrvoje Buljan at University of Zagreb in Croatia, Francis Wright Davis Professor of Physics John Joannopoulos at MIT, Professor Bo Zhen at the University of Pennsylvania, and MIT professor of physics Marin Soljačić.

The finding relates to gauge fields, which describe transformations that particles undergo. Gauge fields fall into two classes, known as Abelian and non-Abelian. The Aharonov-Bohm Effect, named after the theorists who predicted it in 1959, confirmed that gauge fields — beyond being a pure mathematical aid — have physical consequences.

But the observations only worked in Abelian systems, or those in which gauge fields are commutative — that is, they take place the same way both forward and backward in time.  In 1975, Tai-Tsun Wu and Chen-Ning Yang generalized the effect to the non-Abelian regime as a thought experiment. Nevertheless, it remained unclear whether it would even be possible to ever observe the effect in a non-Abelian system. Physicists lacked ways of creating the effect in the lab, and also lacked ways of detecting the effect even if it could be produced. Now, both of those puzzles have been solved, and the observations carried out successfully.

The effect has to do with one of the strange and counterintuitive aspects of modern physics, the fact that virtually all fundamental physical phenomena are time-invariant. That means that the details of the way particles and forces interact can run either forward or backward in time, and a movie of how the events unfold can be run in either direction, so there’s no way to tell which is the real version. But a few exotic phenomena violate this time symmetry.

Creating the Abelian version of the Aharonov-Bohm effects requires breaking the time-reversal symmetry, a challenging task in itself, Soljačić says. But to achieve the non-Abelian version of the effect requires breaking this time-reversal multiple times, and in different ways, making it an even greater challenge.

To produce the effect, the researchers use photon polarization. Then, they produced two different kinds of time-reversal breaking. They used fiber optics to produce two types of gauge fields that affected the geometric phases of the optical waves, first by sending them through a crystal biased by powerful magnetic fields, and second by modulating them with time-varying electrical signals, both of which break the time-reversal symmetry. They were then able to produce interference patterns that revealed the differences in how the light was affected when sent through the fiber-optic system in opposite directions, clockwise or counterclockwise. Without the breaking of time-reversal invariance, the beams should have been identical, but instead, their interference patterns revealed specific sets of differences as predicted, demonstrating the details of the elusive effect.

The original, Abelian version of the Aharonov-Bohm effect “has been observed with a series of experimental efforts, but the non-Abelian effect has not been observed until now,” Yang says. The finding “allows us to do many things,” he says, opening the door to a wide variety of potential experiments, including classical and quantum physical regimes, to explore variations of the effect.

The experimental approach devised by this team “might inspire the realization of exotic topological phases in quantum simulations using photons, polaritons, quantum gases, and superconducting qubits,” Soljačić says. For photonics itself, this could be useful in a variety of optoelectronic applications, he says. In addition, the non-Abelian gauge fields that the group was able to synthesize produced a non-Abelian Berry phase, and “combined with interactions, it may potentially one day serve as a platform for fault-tolerant topological quantum computation,” he says.

At this point, the experiment is primarily of interest for fundamental physics research, with the aim of gaining a better understanding of some basic underpinnings of modern physical theory. The many possible practical applications “will require additional breakthroughs going forward,” Soljačić says.

For one thing, for quantum computation, the experiment would need to be scaled up from one single device to likely a whole lattice of them. And instead of the beams of laser light used in their experiment, it would require working with a source of single individual photons. But even in its present form, the system could be used to explore questions in topological physics, which is a very active area of current research, Soljačić says.

“The non-Abelian Berry phase is a theoretical gem that is the doorway to understanding many intriguing ideas in contemporary physics,” says Ashvin Vishwanath, a professor of physics at Harvard University, who was not associated with this work. “I am glad to see it getting the experimental attention it deserves in the current work, which reports a well-controlled and characterized realization. I expect this work to stimulate progress both directly as a building block for more complex architectures, and also indirectly in inspiring other realizations.”

Journal Reference: Synthesis and observation of non-Abelian gauge fields in real space

Materials provided by Massachusetts Institute of Technology

World Magnetic Declination

For the first time in several centuries, compasses in Greenwich will point at true north

Compasses in Greenwich will be pointing towards the true north for the first time. This coincidence of magnetism and time has not occurred for almost 360 years. It is set to occur in the coming fortnight and it is a reminder of the fact that magnetic north pole of Earth is constantly wandering, unlike Earth’s geographic north pole. 

The angular difference between the geographic and magnetic meridian at any place is known as magnetic declination. Although this difference does not affect the normal citizens in a significant way, this disparity can last for a long time. For almost a hundred years in the United Kingdom, the compass needles have been pointing to west of actual north since the magnetic declination has been negative. 

This is not permanent. The agonic, an invisible line which joins the north and south magnetic poles of Earth which also represents the zero declination has been shifting in the western direction at a rate of nearly 20 kilometres or 12 miles every year

If this rate continues, it will be passing through Greenwich in this month which is the site of the Greenwich Royal Observatory thus creating a historic occurrence. Ciaran Beggan, geomagnetism researcher at British Geological Survey said that in the month of September, the agonic will meet zero longitudes at Greenwich. Since the creation of the Observatory, this is the first time that geomagnetic and geographic coordinate systems will be coinciding at this place. 

The Royal Observatory was founded at the decree of King Charles II in 1675. It is a coincidence that compasses in Greenwich due to zero declination also pointed towards true north. From that time, the agonic has been shifting as Earth’s magnetic north pole has been varying in its position due to changes in the molten outer core. 

Beggan said that the agonic will continue to pass across the United Kingdom for the next 15-20 years. So this phenomenon is expected to continue beyond circumstantial synchronicity of this September. By 2040, compasses will most probably point east of true north. Beyond this scientists cannot provide any prediction as it is difficult to estimate the future magnetic movements. Beggan said that currently, it is not possible to predict how the magnetic field will change in the time span of several decades. For 360 more years in the United Kingdom, the compass may point towards the east of the true north direction. 

light sound waves

Researchers demonstrate storage and release of mechanical waves without loss of energy

In several technologies which are used today, light and sound waves are the fundamentals for transporting energy and signals. However, until now there has been no method to store a wave for a long period of time and then redirect it to a specific location when needed. This would provide the opportunity to manipulate waves for several purposes such as quantum computing, storing information, energy harvesting and many more.

A team of scientists led by Andrea Alù, founding director of Photonics Initiative, Advanced Science Research Center, CUNY and Massimo Ruzzene, Aeronautics Engineering professor at Georgia Tech has demonstrated experimentally that it is possible to capture a wave and store it efficiently while redirecting it later to a specific location. The work appears in Science Advances journal.

Alù said that the experiment demonstrates new opportunities can be unlocked in wave scattering and propagation through unconventional scattering methods. Researchers found ways to change the basic interaction between waves and particles. On striking an obstacle, a light or sound wave can go through two processes, partial absorption or reflection and scattering. In absorption, the wave is immediately converted to different forms of energy including heat. For those who cannot absorb waves, they are reflected and scattered.

In this experiment, the aim of the researchers was to find some technique to mimic the process of absorption in which the wave would not be converted to any other form instead stored in the material. This is known as coherent virtual absorption and it was introduced by ASRC two years ago.

For proving the theory, it was necessary to tailor the time evolution of waves so that on contacting non-absorbing materials, they would not be scattered, transmitted or reflected. This would prevent the wave from escaping and store it inside the material efficiently. Then it could be released on demand. In the course of the experiment, two mechanical waves were propagated in opposite directions along a carbon steel waveguide that had a cavity. Time variations of every wave were controlled so that the cavity would retain all the energy. The excitation of one of the waves was stopped which enabled the researchers to control the stored energy and send it towards a specific direction.

The experiment was performed using elastic waves which traveled inside a solid material. It can also be replicated for light and radiowaves thus opening the doors to exciting opportunities such as efficient harvesting of energy, wireless power transfer and greater control on wave propagation.

Research Paper: Coherent virtual absorption of elastodynamic waves

Blueberry pancake

Researchers explain the unique movement of Pancake when swirled

When a glass filled with wine is swirled clockwise, the wine will also spin in the clockwise direction, however, while making a blueberry pancake, if it is swirled in the clockwise direction, the pancake spins in an anticlockwise direction.

The same is the case with a glass of beads. A few beads will rotate clockwise when the glass is swirled clockwise. However, a lot of beads in a glass when swirled clockwise will rotate counterclockwise.

Lisa Lee, a graduate student of Applied Physics at School of Engineering and Applied Sciences, Harvard said she was surprised at the behaviour of these exact same objects under the same situations.

The research team set about to understand the physics behind these actions and it turns out that friction is responsible for this. Beads are a part of a class of material called granular media, which means a collection of macroscopic particles such as sand or snow. The work appears in the Physical Review E journal.

Wine rotates clockwise when moved clockwise due to wine being a liquid-like granular media in low friction, while pancakes rotate in an anticlockwise direction when rotated clockwise which is similar to granular media under high friction.

Macroscopic particles are very interesting as they can move like a liquid or a solid depending on the conditions. Sand flows like a liquid in an hourglass but acts like a solid to support your weight on beaches. The object transition from liquid to solid has been an open question for decades.

Lee and the researchers found out that smaller groups of beads will have lower effective friction than larger pairs of beads which results in the transition from liquid to solid. When one particle rolls in one direction it experiences little friction however if many particles which are in contact with each other roll in the same direction, then they experience a large amount of friction which causes the group to solidify and thus change the behavior.

Using computer simulations, Lee and co-authors, John Paul Ryan and Miranda Holmes-Cerfon showed that in the absence of friction, the particles never solidified, no matter the quantity in which they were present. The rougher the particles were, the quicker was the transition from liquid to solid.

Shmuel Rubinstein, Associate Professor of Applied Physics at SEAS and senior author of the study said that this is an interesting case of system-size behaviors emerging from local interactions. The emergence of coherent circulations is an exciting subject for study like the case of 2D turbulence and active spinners. It is quite interesting that daily objects such as marbles and dishes can demonstrate similar physics.

Journal Reference: Physical Review E.

MIT engineers build advanced microprocessor out of carbon nanotubes

MIT engineers build advanced microprocessor out of carbon nanotubes

After years of tackling numerous design and manufacturing challenges, MIT researchers have built a modern microprocessor from carbon nanotube transistors, which are widely seen as a faster, greener alternative to their traditional silicon counterparts.

The microprocessor, described today in the journal Nature, can be built using traditional silicon-chip fabrication processes, representing a major step toward making carbon nanotube microprocessors more practical.

Silicon transistors — critical microprocessor components that switch between 1 and 0 bits to carry out computations — have carried the computer industry for decades. As predicted by Moore’s Law, industry has been able to shrink down and cram more transistors onto chips every couple of years to help carry out increasingly complex computations. But experts now foresee a time when silicon transistors will stop shrinking, and become increasingly inefficient.

Making carbon nanotube field-effect transistors (CNFET) has become a major goal for building next-generation computers. Research indicates CNFETs have properties that promise around 10 times the energy efficiency and far greater speeds compared to silicon. But when fabricated at scale, the transistors often come with many defects that affect performance, so they remain impractical.

The MIT researchers have invented new techniques to dramatically limit defects and enable full functional control in fabricating CNFETs, using processes in traditional silicon chip foundries. They demonstrated a 16-bit microprocessor with more than 14,000 CNFETs that performs the same tasks as commercial microprocessors. The Nature paper describes the microprocessor design and includes more than 70 pages detailing the manufacturing methodology.

The microprocessor is based on the RISC-V open-source chip architecture that has a set of instructions that a microprocessor can execute. The researchers’ microprocessor was able to execute the full set of instructions accurately. It also executed a modified version of the classic “Hello, World!” program, printing out, “Hello, World! I am RV16XNano, made from CNTs.”

“This is by far the most advanced chip made from any emerging nanotechnology that is promising for high-performance and energy-efficient computing,” says co-author Max M. Shulaker, the Emanuel E Landsman Career Development Assistant Professor of Electrical Engineering and Computer Science (EECS) and a member of the Microsystems Technology Laboratories. “There are limits to silicon. If we want to continue to have gains in computing, carbon nanotubes represent one of the most promising ways to overcome those limits. [The paper] completely re-invents how we build chips with carbon nanotubes.”

Joining Shulaker on the paper are: first author and postdoc Gage Hills, graduate students Christian Lau, Andrew Wright, Mindy D. Bishop, Tathagata Srimani, Pritpal Kanhaiya, Rebecca Ho, and Aya Amer, all of EECS; Arvind, the Johnson Professor of Computer Science and Engineering and a researcher in the Computer Science and Artificial Intelligence Laboratory; Anantha Chandrakasan, the dean of the School of Engineering and the Vannevar Bush Professor of Electrical Engineering and Computer Science; and Samuel Fuller, Yosi Stein, and Denis Murphy, all of Analog Devices.

Fighting the “bane” of CNFETs

The microprocessor builds on a previous iteration designed by Shulaker and other researchers six years ago that had only 178 CNFETs and ran on a single bit of data. Since then, Shulaker and his MIT colleagues have tackled three specific challenges in producing the devices: material defects, manufacturing defects, and functional issues. Hills did the bulk of the microprocessor design, while Lau handled most of the manufacturing.

For years, the defects intrinsic to carbon nanotubes have been a “bane of the field,” Shulaker says. Ideally, CNFETs need semiconducting properties to switch their conductivity on an off, corresponding to the bits 1 and 0. But unavoidably, a small portion of carbon nanotubes will be metallic, and will slow or stop the transistor from switching. To be robust to those failures, advanced circuits will need carbon nanotubes at around 99.999999 percent purity, which is virtually impossible to produce today.

The researchers came up with a technique called DREAM (an acronym for “designing resiliency against metallic CNTs”), which positions metallic CNFETs in a way that they won’t disrupt computing. In doing so, they relaxed that stringent purity requirement by around four orders of magnitude — or 10,000 times — meaning they only need carbon nanotubes at about 99.99 percent purity, which is currently possible.

Designing circuits basically requires a library of different logic gates attached to transistors that can be combined to, say, create adders and multipliers — like combining letters in the alphabet to create words. The researchers realized that the metallic carbon nanotubes impacted different pairings of these gates differently. A single metallic carbon nanotube in gate A, for instance, may break the connection between A and B. But several metallic carbon nanotubes in gates B may not impact any of its connections.

In chip design, there are many ways to implement code onto a circuit. The researchers ran simulations to find all the different gate combinations that would be robust and wouldn’t be robust to any metallic carbon nanotubes. They then customized a chip-design program to automatically learn the combinations least likely to be affected by metallic carbon nanotubes. When designing a new chip, the program will only utilize the robust combinations and ignore the vulnerable combinations.

“The ‘DREAM’ pun is very much intended, because it’s the dream solution,” Shulaker says. “This allows us to buy carbon nanotubes off the shelf, drop them onto a wafer, and just build our circuit like normal, without doing anything else special.”

Exfoliating and tuning

CNFET fabrication starts with depositing carbon nanotubes in a solution onto a wafer with predesigned transistor architectures. However, some carbon nanotubes inevitably stick randomly together to form big bundles — like strands of spaghetti formed into little balls — that form big particle contamination on the chip.

To cleanse that contamination, the researchers created RINSE (for “removal of incubated nanotubes through selective exfoliation”). The wafer gets pretreated with an agent that promotes carbon nanotube adhesion. Then, the wafer is coated with a certain polymer and dipped in a special solvent. That washes away the polymer, which only carries away the big bundles, while the single carbon nanotubes remain stuck to the wafer. The technique leads to about a 250-times reduction in particle density on the chip compared to similar methods.

Lastly, the researchers tackled common functional issues with CNFETs. Binary computing requires two types of transistors: “N” types, which turn on with a 1 bit and off with a 0 bit, and “P” types, which do the opposite. Traditionally, making the two types out of carbon nanotubes has been challenging, often yielding transistors that vary in performance. For this solution, the researchers developed a technique called MIXED (for “metal interface engineering crossed with electrostatic doping”), which precisely tunes transistors for function and optimization.

In this technique, they attach certain metals to each transistor — platinum or titanium — which allows them to fix that transistor as P or N. Then, they coat the CNFETs in an oxide compound through atomic-layer deposition, which allows them to tune the transistors’ characteristics for specific applications. Servers, for instance, often require transistors that act very fast but use up energy and power. Wearables and medical implants, on the other hand, may use slower, low-power transistors.

The main goal is to get the chips out into the real world. To that end, the researchers have now started implementing their manufacturing techniques into a silicon chip foundry through a program by Defense Advanced Research Projects Agency, which supported the research. Although no one can say when chips made entirely from carbon nanotubes will hit the shelves, Shulaker says it could be fewer than five years. “We think it’s no longer a question of if, but when,” he says.

After years of tackling numerous design and manufacturing challenges, MIT researchers have built a modern microprocessor from carbon nanotube transistors, which are widely seen as a faster, greener alternative to their traditional silicon counterparts.

The microprocessor, described today in the journal Nature, can be built using traditional silicon-chip fabrication processes, representing a major step toward making carbon nanotube microprocessors more practical.

Silicon transistors — critical microprocessor components that switch between 1 and 0 bits to carry out computations — have carried the computer industry for decades. As predicted by Moore’s Law, industry has been able to shrink down and cram more transistors onto chips every couple of years to help carry out increasingly complex computations. But experts now foresee a time when silicon transistors will stop shrinking, and become increasingly inefficient.

Making carbon nanotube field-effect transistors (CNFET) has become a major goal for building next-generation computers. Research indicates CNFETs have properties that promise around 10 times the energy efficiency and far greater speeds compared to silicon. But when fabricated at scale, the transistors often come with many defects that affect performance, so they remain impractical.

The MIT researchers have invented new techniques to dramatically limit defects and enable full functional control in fabricating CNFETs, using processes in traditional silicon chip foundries. They demonstrated a 16-bit microprocessor with more than 14,000 CNFETs that performs the same tasks as commercial microprocessors. The Nature paper describes the microprocessor design and includes more than 70 pages detailing the manufacturing methodology.

The microprocessor is based on the RISC-V open-source chip architecture that has a set of instructions that a microprocessor can execute. The researchers’ microprocessor was able to execute the full set of instructions accurately. It also executed a modified version of the classic “Hello, World!” program, printing out, “Hello, World! I am RV16XNano, made from CNTs.”

“This is by far the most advanced chip made from any emerging nanotechnology that is promising for high-performance and energy-efficient computing,” says co-author Max M. Shulaker, the Emanuel E Landsman Career Development Assistant Professor of Electrical Engineering and Computer Science (EECS) and a member of the Microsystems Technology Laboratories. “There are limits to silicon. If we want to continue to have gains in computing, carbon nanotubes represent one of the most promising ways to overcome those limits. [The paper] completely re-invents how we build chips with carbon nanotubes.”

Joining Shulaker on the paper are: first author and postdoc Gage Hills, graduate students Christian Lau, Andrew Wright, Mindy D. Bishop, Tathagata Srimani, Pritpal Kanhaiya, Rebecca Ho, and Aya Amer, all of EECS; Arvind, the Johnson Professor of Computer Science and Engineering and a researcher in the Computer Science and Artificial Intelligence Laboratory; Anantha Chandrakasan, the dean of the School of Engineering and the Vannevar Bush Professor of Electrical Engineering and Computer Science; and Samuel Fuller, Yosi Stein, and Denis Murphy, all of Analog Devices.

Fighting the “bane” of CNFETs

The microprocessor builds on a previous iteration designed by Shulaker and other researchers six years ago that had only 178 CNFETs and ran on a single bit of data. Since then, Shulaker and his MIT colleagues have tackled three specific challenges in producing the devices: material defects, manufacturing defects, and functional issues. Hills did the bulk of the microprocessor design, while Lau handled most of the manufacturing.

For years, the defects intrinsic to carbon nanotubes have been a “bane of the field,” Shulaker says. Ideally, CNFETs need semiconducting properties to switch their conductivity on an off, corresponding to the bits 1 and 0. But unavoidably, a small portion of carbon nanotubes will be metallic, and will slow or stop the transistor from switching. To be robust to those failures, advanced circuits will need carbon nanotubes at around 99.999999 percent purity, which is virtually impossible to produce today.

The researchers came up with a technique called DREAM (an acronym for “designing resiliency against metallic CNTs”), which positions metallic CNFETs in a way that they won’t disrupt computing. In doing so, they relaxed that stringent purity requirement by around four orders of magnitude — or 10,000 times — meaning they only need carbon nanotubes at about 99.99 percent purity, which is currently possible.

Designing circuits basically requires a library of different logic gates attached to transistors that can be combined to, say, create adders and multipliers — like combining letters in the alphabet to create words. The researchers realized that the metallic carbon nanotubes impacted different pairings of these gates differently. A single metallic carbon nanotube in gate A, for instance, may break the connection between A and B. But several metallic carbon nanotubes in gates B may not impact any of its connections.

In chip design, there are many ways to implement code onto a circuit. The researchers ran simulations to find all the different gate combinations that would be robust and wouldn’t be robust to any metallic carbon nanotubes. They then customized a chip-design program to automatically learn the combinations least likely to be affected by metallic carbon nanotubes. When designing a new chip, the program will only utilize the robust combinations and ignore the vulnerable combinations.

“The ‘DREAM’ pun is very much intended, because it’s the dream solution,” Shulaker says. “This allows us to buy carbon nanotubes off the shelf, drop them onto a wafer, and just build our circuit like normal, without doing anything else special.”

Exfoliating and tuning

CNFET fabrication starts with depositing carbon nanotubes in a solution onto a wafer with predesigned transistor architectures. However, some carbon nanotubes inevitably stick randomly together to form big bundles — like strands of spaghetti formed into little balls — that form big particle contamination on the chip.

To cleanse that contamination, the researchers created RINSE (for “removal of incubated nanotubes through selective exfoliation”). The wafer gets pretreated with an agent that promotes carbon nanotube adhesion. Then, the wafer is coated with a certain polymer and dipped in a special solvent. That washes away the polymer, which only carries away the big bundles, while the single carbon nanotubes remain stuck to the wafer. The technique leads to about a 250-times reduction in particle density on the chip compared to similar methods.

Lastly, the researchers tackled common functional issues with CNFETs. Binary computing requires two types of transistors: “N” types, which turn on with a 1 bit and off with a 0 bit, and “P” types, which do the opposite. Traditionally, making the two types out of carbon nanotubes has been challenging, often yielding transistors that vary in performance. For this solution, the researchers developed a technique called MIXED (for “metal interface engineering crossed with electrostatic doping”), which precisely tunes transistors for function and optimization.

In this technique, they attach certain metals to each transistor — platinum or titanium — which allows them to fix that transistor as P or N. Then, they coat the CNFETs in an oxide compound through atomic-layer deposition, which allows them to tune the transistors’ characteristics for specific applications. Servers, for instance, often require transistors that act very fast but use up energy and power. Wearables and medical implants, on the other hand, may use slower, low-power transistors.

The main goal is to get the chips out into the real world. To that end, the researchers have now started implementing their manufacturing techniques into a silicon chip foundry through a program by Defense Advanced Research Projects Agency, which supported the research. Although no one can say when chips made entirely from carbon nanotubes will hit the shelves, Shulaker says it could be fewer than five years. “We think it’s no longer a question of if, but when,” he says.

Materials provided by Massachusetts Institute of Technology

Large scale structure of light distribution

Scientists conduct experiment to define constraints on the force of dark energy

It is now well established through observations that the universe is expanding which has been speeding up over time. However, the cause of the expansion is not yet known. It is assumed to be a mysterious energy acting opposite to gravity causing this expansion.

This energy is termed as “dark energy” and it is estimated that it makes up to 72 percent of the constituents in the universe. A new experiment has confirmed that it is not a new force. The study appears in the Physical Review Letters journal. Ed Copeland, a physicist from the University of Nottingham said that the experiment related to cosmology and atomic physics helped in ruling out several models which have been proposed for the explanation of dark energy and will eliminate many more models.

There are four fundamental forces in the Universe which are gravity, electromagnetic force, strong nuclear force and weak nuclear force. Some anomalies do not fit into the current model of how the Universe functions. So they are clubbed under a fifth hypothetical force.

These anomalies have helped in putting constraints on the fifth force and helped in constructing alternatives to general relativity for explaining the increasing rate of expansion of the Universe. Some of the alternatives are chameleon theory and symmetron theory.

If another force present in the Universe was responsible for almost 75 percent of its mass-energy density then it should be easy to detect. But if it acts opposite to gravity i.e. a repulsive force then it should be weakest in the area where gravity is the strongest force. This should hold true for objects with significant mass such as a planet.

An experiment was designed to observe this anti-gravitational force. Within a sealed vacuum chamber, a large aluminium sphere and small weights – atoms of rubidium-87 were placed and atom interferometry was used for measuring the acceleration of the smaller weight towards the larger one. If a fifth force was present then the atom should have been veered off its path as it moved towards the sphere. But no such deviation was observed which clearly explains that no such anti-gravity force was present in the chamber. It does not mean that there is no fifth force at all. However, the force can be defined by several constraints which also means that similar constraints can be placed on chameleon and symmetron theories.

There may not be always a straightforward answer to fundamental questions of this nature so the correct questions are asked to narrow down on the correct answer. Answering this question would give significant answers about the evolution of the universe.

Journal Reference: Physical Review Letters

facet slac

New generation of X-Ray lasers and particle colliders to be produced by Trojan Horse technique

Researchers have developed “super microscopes” to study nature on its primary level which can solve atomic and sub-atomic mysteries. They can investigate the smallest dimensions of matter with electron rays although it won’t work with visible light. It can be done either by directly using them in particle colliders or converting their energy to bright X-rays in X-ray lasers. Particle accelerators first produce electrons at source, then excite them through accelerator cavities.

Scientists from the SLAC National Accelerator Laboratory together with an international group of researchers have shown potentially much brighter plasma-based electron source suitable for use in powerful accelerators. Trojan horse technique is the process of releasing a beam of electrons from neutral atoms inside the plasma.

Bernhard Hidding from the University of Strathclyde in Glasgow, Scotland, the principal investigator of the study said that the Trojan horse method is experimentally proven and can be used for future electron sources. The research has been published in Nature Physics.

Currently, electrons are produced by shining laser light onto a metallic photocathode which pulls electrons out of the metal. Then the electrons are accelerated inside metal cavities where more energy is withdrawn from radiofrequency field resulting in a high-power electron beam. The ray results in the generation of very bright X-ray light in X-ray lasers such as Linac Coherent Light Source (LCLS) of SLAC.

Accelerators for high-energy beams become very large and costlier because the metal cavities can sustain only a specific energy gain over a certain distance before it collapses. Researchers at SLAC tried to make compact accelerators as they experimented substituting metal cavities with plasma which decreases the length of accelerators by 100 to 1000 times along with higher acceleration gradients.

Co-author Mark Hogan from SLAC mentioned that plasma acceleration can be very powerful but enhanced beams suitable for future use have not been produced yet which makes improving beam quality a top priority along with developing new forms of electron sources. The Trojan horse method could make electron beams 100 to 10,000 times brighter than the most powerful beams currently according to past estimation by researchers.

James Rosenzweig, co-author and principal investigator for Trojan horse project at the University of California, Los Angeles said that it is possible to build X-ray lasers with same power over few meters rather than a kilometer by uniting high acceleration and beam creation in plasma. The scientists conducted their experiment at Facility for Advanced Accelerator Experimental Tests (FACET) facility that produces high energetic electron pulses for research on future accelerator technologies, including plasma acceleration.

The researchers beamed laser light having enough energy to remove electrons off hydrogen into a mixture of hydrogen and helium gas turning neutral hydrogen into plasma which was not possible with helium where electrons stayed inside plasma being neutral. Researchers produced a plasma wake by sending one of FACET’s electron beam through the plasma similar to a motorboat generates a stir when gliding through the water, where the trailing electrons gain a high amount of energy. A lot of research needs to be done for making compact X-Ray lasers a reality with a need to improve their quality and quantity.

Vitaly Yakimenko, director of SLAC’s FACET Division said that these advancements can be done after FACET upgradation and FACET-II completion as it will be the only place in the world to generate beams with enormous intensity and energy because the experiment depends on the ability to use a powerful electron beam to produce the plasma stir.

Researchers destroyed the helium when the trailing electrons passed by, with a focused laser flash. Electrons after being pulled out of the helium atoms by a light pulse having enough energy were then accelerated in the stir. UCLA’s Aihua Deng, one of the lead authors said that it is important as well as challenging for the synchronization between the electron beam and laser flash running through the plasma with the speed of light to last merely a few millionths of a billionth second.

The generated electron will trouble the formation of plasma stir if the flash comes too early and plasma would have moved on and the electrons would not get accelerated if it comes too late. Oliver Karger, the other lead author said that the electrons knocked from helium gets quickly accelerated in the forward direction keeping the beam tightly packaged making this method transformative.

Research Paper: https://www.nature.com/articles/s41567-019-0610-9

New type of electrolyte could enhance supercapacitor performance

New type of electrolyte could enhance supercapacitor performance

Supercapacitors, electrical devices that store and release energy, need a layer of electrolyte — an electrically conductive material that can be solid, liquid, or somewhere in between. Now, researchers at MIT and several other institutions have developed a novel class of liquids that may open up new possibilities for improving the efficiency and stability of such devices while reducing their flammability.

“This proof-of-concept work represents a new paradigm for electrochemical energy storage,” the researchers say in their paper describing the finding, which appears today in the journal Nature Materials.

For decades, researchers have been aware of a class of materials known as ionic liquids — essentially, liquid salts — but this team has now added to these liquids a compound that is similar to a surfactant, like those used to disperse oil spills. With the addition of this material, the ionic liquids “have very new and strange properties,” including becoming highly viscous, says MIT postdoc Xianwen Mao PhD ’14, the lead author of the paper.

“It’s hard to imagine that this viscous liquid could be used for energy storage,” Mao says, “but what we find is that once we raise the temperature, it can store more energy, and more than many other electrolytes.”

That’s not entirely surprising, he says, since with other ionic liquids, as temperature increases, “the viscosity decreases and the energy-storage capacity increases.” But in this case, although the viscosity stays higher than that of other known electrolytes, the capacity increases very quickly with increasing temperature. That ends up giving the material an overall energy density — a measure of its ability to store electricity in a given volume — that exceeds those of many conventional electrolytes, and with greater stability and safety.

The key to its effectiveness is the way the molecules within the liquid automatically line themselves up, ending up in a layered configuration on the metal electrode surface. The molecules, which have a kind of tail on one end, line up with the heads facing outward toward the electrode or away from it, and the tails all cluster in the middle, forming a kind of sandwich. This is described as a self-assembled nanostructure.

“The reason why it’s behaving so differently” from conventional electrolytes is because of the way the molecules intrinsically assemble themselves into an ordered, layered structure where they come in contact with another material, such as the electrode inside a supercapacitor, says T. Alan Hatton, a professor of chemical engineering at MIT and the paper’s senior author. “It forms a very interesting, sandwich-like, double-layer structure.”

This highly ordered structure helps to prevent a phenomenon called “overscreening” that can occur with other ionic liquids, in which the first layer of ions (electrically charged atoms or molecules) that collect on an electrode surface contains more ions than there are corresponding charges on the surface. This can cause a more scattered distribution of ions, or a thicker ion multilayer, and thus a loss of efficiency in energy storage; “whereas with our case, because of the way everything is structured, charges are concentrated within the surface layer,” Hatton says.

The new class of materials, which the researchers call SAILs, for surface-active ionic liquids, could have a variety of applications for high-temperature energy storage, for example for use in hot environments such as in oil drilling or in chemical plants, according to Mao. “Our electrolyte is very safe at high temperatures, and even performs better,” he says. In contrast, some electrolytes used in lithium-ion batteries are quite flammable.

The material could help to improve performance of supercapacitors, Mao says. Such devices can be used to store electrical charge and are sometimes used to supplement battery systems in electric vehicles to provide an extra boost of power. Using the new material instead of a conventional electrolyte in a supercapacitor could increase its energy density by a factor of four or five, Mao says. Using the new electrolyte, future supercapacitors may even be able to store more energy than batteries, he says, potentially even replacing batteries in applications such as electric vehicles, personal electronics, or grid-level energy storage facilities.

The material could also be useful for a variety of emerging separation processes, Mao says. “A lot of newly developed separation processes require electrical control,” in various chemical processing and refining applications and in carbon dioxide capture, for example, as well as resource recovery from waste streams. These ionic liquids, being highly conductive, could be well-suited to many such applications, he says.

The material they initially developed is just an example of a variety of possible SAIL compounds. “The possibilities are almost unlimited,” Mao says. The team will continue to work on different variations and on optimizing its parameters for particular uses. “It might take a few months or years,” he says, “but working on a new class of materials is very exciting to do. There are many possibilities for further optimization.”

The research team included Paul Brown, Yinying Ren, Agilio Padua, and Margarida Costa Gomes at MIT; Ctirad Cervinka at École Normale Supérieure de Lyon, in France; Gavin Hazell and Julian Eastoe at the University of Bristol, in the U.K.; Hua Li and Rob Atkin at the University of Western Australia; and Isabelle Grillo at the Institut Max-von-Laue-Paul-Langevin in Grenoble, France. The researchers dedicate their paper to the memory of Grillo, who recently passed away.

“It is a very exciting result that surface-active ionic liquids (SAILs) with amphiphilic structures can self-assemble on electrode surfaces and enhance charge storage performance at electrified surfaces,” says Yi Cui, a professor of materials science and engineering at Stanford University, who was not associated with this research. “The authors have studied and understood the mechanism. The work here might have a great impact on the design of high energy density supercapacitors, and could also help improve battery performance,” he says.

Nicholas Abbott, the Tisch University Professor at Cornell University, who also was not involved in this work, says “The paper describes a very clever advance in interfacial charge storage, elegantly demonstrating how knowledge of molecular self-assembly at interfaces can be leveraged to address a contemporary technological challenge.”

Materials provided by Massachusetts Institute of Technology

Spherical aberration disk

Physicists came up with a solution to the 2000 years old ‘unsolvable’ optical problem

  • A group of physicists from National Autonomous University, Mexico and Tec de Monterrey has found the solution to the Wasserman-Wolf problem(Basically it is the problem of spherical aberration).
  • González-Acuña, Chaparro-Romo and Gutiérrez-Vega managed to come up with a lengthy mathematical equation involving work of several months which provides an analytical solution to eliminate the problem of spherical aberration.
  • The solution involves fixing the shape of a second aspherical surface in addition to a first surface.

A group of physicists from National Autonomous University, Mexico and Tec de Monterrey has found the solution to the Wasserman-Wolf problem. It is a 2000 year old optical problem. The scientists Rafael González-Acuña, Julio Gutiérrez-Vega and Héctor Chaparro-Romo described the mathematics of the puzzle along with its applications and efficiency results. The paper has been published in the Applied Optics journal

Diocles, a Greek scientist had identified the problem with optical lenses 2000 years ago. He observed that when objects are viewed with devices equipped with optical lenses, the edges appeared to be fuzzier than the centre. He explained this effect attributing the reason for the lenses being spherical, upon which incident light cannot be focused due to refractive differences. Imperfections in the material and shape of the lens also contribute to the light missing the target.

This is now known as a spherical aberration which was considered unsolvable as even physicists such as Isaac Newton and Gottfried Leibniz could not crack it.

Wasserman and Wolf described the problem analytically in 1949 and termed it as Wasserman-Wolf problem. They tried to solve the problem with the approach of using two aspheric adjacent surfaces for correcting the aberrations. Lensmakers and researchers have come up with a variety of approaches since then for producing sharp uniform images by eliminating aberration. There have been developments in lens manufacturing and design from several companies which primarily involved aspherical lenses. But they have been generally very expensive and not so convenient to manufacture although leading to improvement in images. 

But González-Acuña, Chaparro-Romo and Gutiérrez-Vega managed to come up with a lengthy mathematical equation involving work of several months which provides an analytical solution to eliminate the problem of spherical aberration. The equation looks incomprehensible to the common man but it is applicable for a lens of any size. The solution involves fixing the shape of a second aspherical surface in addition to a first surface. The solution is independent of the material, size and application of the lens and provides the exact specifications for the lens to be optically perfect. 

This breakthrough will solve the headache of the photographers who were desperate for perfect images but could not get them irrespective of the money spent. It will also help in scientific observations where better images can lead to accurate results. The normal users will also be benefited from this solution as they can get images of higher quality in their smartphones and cameras. 

Journal Reference: Applied Optics journal

Caltech researchers take a direct image of magic-angle twisted graphene sheets

Caltech researchers take a direct image of magic-angle twisted graphene sheets

Just over a year after researchers at MIT stunned the physics world with the discovery of the “magic angle” for stacked sheets of graphene, researchers at Caltech have directly observed and studied this material using a scanning tunnelling microscope that can image electronic properties at atomic-length scales.

Understanding the “magic angle”—a specific orientation between the stacked graphene that yields special electric properties—could pave the way to realizing the dream of room-temperature superconductors, which could transmit enormous electric currents while producing zero heat.

But first: what is the magic angle? Say you take two sheets of graphene—single-atom-thick lattices of carbon atoms—and lay one atop the other to create a bilayer material, then twist one of the sheets of graphene to shift their orientation to one another. As the orientation shifts, the electronic properties of the bilayer material will change with it. In early 2018, researchers at MIT discovered that, at a certain orientation (about 1.1 degrees of relative twist), the bilayer material, surprisingly, becomes superconducting and moreover, the superconducting properties can be controlled with the electric fields. Their discovery launched a new field of research into magic angle-oriented graphene, known as “twistronics.”

Engineers and physicists at Caltech have built upon that discovery by generating an image of the atomic structure and electronic properties of magic angle-twisted graphene, yielding new insight into the phenomenon by offering a more direct way of studying it. A paper on their work was published in the journal Nature Physics on August 5.

“This pulls back the shroud on twistronics,” says Caltech’s Stevan Nadj-Perge, corresponding author of the paper and assistant professor of applied physics and materials science in the Division of Engineering and Applied Science.

Research on the magic angle requires an extreme level of precision to get the two sheets of graphene aligned at just the right angle. Old techniques for doing so necessitated embedding the graphene in an insulating material, which had the unfortunate side effect of preventing direct study of the sample. Instead, researchers had to use indirect methods of probing the graphene sample—for example, by taking measurements of how electrons flow through it. Nadj-Perge and his colleagues developed a new method of creating samples of magic angle-twisted graphene that can be used to align the two sheets of graphene very precisely while leaving it exposed for direct observation.

Using this technique, the researchers could learn more about the electronic properties of the material at the magic angle as well as study how these properties change as the twist angle moves away from the magic value. Their work provided several key insights that will guide future theoretical modeling and experiments, including the observation that the electronic correlation plays an important role near the charge-neutrality point—the angle at which the bilayer is electronically neutral.

“Previously, it was thought that correlation effects do not play a major role in charge neutrality,” Nadj-Perge says. “Closer, more detailed examination of samples like this could help us to explain why the exotic electronic effects near the magic angle exist. Once we know that, we could help pave the way for useful applications of it, perhaps even leading to room-temperature superconductivity one day.”

The paper is titled “Electronic correlations in twisted bilayer graphene near the magic angle.”Co-authors of the paper include Gil Refael, Taylor W. Lawrence Professor of Theoretical Physics; Jason Alicea, professor of theoretical physics; Caltech graduate students Youngjoon Choi, Harpreet Arora, Robert Polski, and Yiran Zhang; Caltech postdoctoral scholars Jeannette Kemmer, Yang Peng, Alex Thomson, Hechen Ren; as well as Felix von Oppen of Freie Universität Berlin in Germany; and Kenji Watanabe and Takashi Taniguchi of the National Institute for Materials Science in Japan. This research was funded by the National Science Foundation, the Kavli Nanoscience Institute, the Institute for Quantum Information and Matter at Caltech, the Walter Burke Institute for Theoretical Physics at Caltech, the Deutsche Forschungsgemeinschaft, the Kwanjeong Educational Foundation, and the Transregional Collaborative Research Center 183.