UMD CMNS Physics S1 Color

Shaking Bosons into Fermions

Particles can be classified as bosons or fermions. A defining characteristic of a boson is its ability to pile into a single quantum state with other bosons. Fermions are not allowed to do this. One broad impact of fermionic anti-social behavior is that it allows for carbon-based life forms, like us, to exist. If the universe were solely made from bosons, life would certainly not look like it does. Recently, JQI theorists* have proposed an elegant method for achieving transmutation--that is, making bosons act like fermions. This work was published in the journal Physical Review Letters. Read More

Direct Evidence of the Topological Kondo Insulator

Researchers from the Center for Nanophysics and Advanced Materials, find evidence of one-dimensional surface transport with a quantized conductance value of e2/h originating from the chiral edge channels of ferromagnetic domain walls, providing strong evidence that topologically non-trivial surface states exist in SmB6. Read More

UMD Discovery Could Enable Portable Particle Accelerators

Conventional particle accelerators are typically big machines that occupy a lot of space. Even at more modest energies, such as that used for cancer therapy and medical imaging, accelerators need large rooms to accommodate the required hardware, power supplies and radiation shielding.

A new discovery by physicists at the University of Maryland could hold the key to the construction of inexpensive, broadly useful, and portable particle accelerators in the very near future. The team has accelerated electron beams to nearly the speed of light using record-low laser energies, thus relieving a major engineering bottleneck in the development of compact particle accelerators. The work appears in the November 6, 2015 issue of the journal Physical Review Letters.

“We have accelerated high-charge electron beams to more than 10 million electron volts using only millijoules of laser pulse energy. This is the energy consumed by a typical household lightbulb in one-thousandth of a second.” said Howard Milchberg, professor of Physics and Electrical and Computer Engineering at UMD and senior author of the study. “Because the laser energy requirement is so low, our result opens the way for laser-driven particle accelerators that can be moved around on a cart.”

This schematic illustrates the laser-driven electron accelerator experiment at the University of Maryland. The three images at the top directly depict three key phases of the process. At left, a laser pulse is directed into a dense jet of hydrogen gas, where it ionizes the gas to form a plasma and initiates an effect called relativistic self-focusing. (See left inset.) Electrons within the plasma are rapidly accelerated to nearly the speed of light, which produces a brief, intense flash of visible light. (See middle inset.) The accelerated ultra-short bunch of electrons continues to gain energy and then exits the plasma, where it produces intense radiation that can be used for ultra-fast, high-energy imaging applications. (See right inset.) Image credit: Howard Milchberg/George Hine (Click image to download hi-res version.)

This schematic illustrates the laser-driven electron accelerator experiment at the University of Maryland. The three images at the top directly depict three key phases of the process. At left, a laser pulse is directed into a dense jet of hydrogen gas, where it ionizes the gas to form a plasma and initiates an effect called relativistic self-focusing. (See left inset.) Electrons within the plasma are rapidly accelerated to nearly the speed of light, which produces a brief, intense flash of visible light. (See middle inset.) The accelerated ultra-short bunch of electrons continues to gain energy and then exits the plasma, where it produces intense radiation that can be used for ultra-fast, high-energy imaging applications. (See right inset.) Image credit: Howard Milchberg/George Hine (Click image to download hi-res version.)

“As an unexpected bonus, the accelerator generates an intense flash of optical light so short that we believe it represents only one-half of a wavelength cycle,” Milchberg added. These ultrashort light flashes could lead to the development of optical strobe lights that can capture the motion of electrons as they swarm across their atomic orbits—a potentially important development for materials science and nanotechnology.

The UMD team began with a technique known as laser-driven plasma wakefield acceleration and pushed it to the extreme. Generally speaking, the approach works by shooting a laser pulse into plasma, which is a gas (in this case, hydrogen) that has been fully ionized to remove all the electrons from the gas atoms. An intense laser pulse can create a plasma wake that follows the laser, much like the water wake that trails a speedboat. A bunch of electrons following the initial laser pulse can “surf” the waves of this wake, accelerating to nearly the speed of light in millionths of a meter.

“Unless your laser pulse can induce the plasma wake in the first place—and it takes a very intense pulse to do that—you’re out of luck,” Milchberg explained. Prior efforts needed much bigger laser energies to accomplish this effect. So Milchberg and his team tried a different approach, instead forcing the plasma itself to transform a weak laser pulse into a very intense one.

When a laser pulse passes through plasma, the laser causes the electrons to wiggle back and forth in the laser field. The electrons in the center experience the most intense part of the beam, so they wiggle the fastest. As they do they become more massive, as dictated by Einstein’s law of relativity, which says that faster objects must increase in mass. The result is that the center of the beam—where the electrons become heaviest—slows down compared to the outer parts of the beam. This causes the beam to self-focus, gaining intensity as it collapses, finally generating a strong plasma wake. This effect is known as relativistic self-focusing, and becomes more pronounced as the plasma density increases.

The UMD team took advantage of this self-focusing effect, drastically increasing the density of the plasma to as much as 20 times that used in typical experiments. In the process, they dramatically reduced the laser pulse energy needed to initiate relativistic self-focusing and thereby generate a strong plasma wake.

“If you increase the plasma density enough, even a pipsqueak of a laser pulse can generate strong relativistic effects,” Milchberg added.

“From a practical standpoint, the key difference here is the footprint of the accelerator. What once required a room full of equipment and a very powerful laser could eventually be done with a small machine on a movable cart, with a standard wall-socket plug,” said Andrew Goers, a graduate student in Physics at UMD and the study’s lead author. “We started with a very powerful laser and found that we were able to keep dialing the energy back. Eventually we got down to about 1 percent of the laser’s peak energy, but we were still seeing an effect. We were blown away by this.”

The UMD laser-driven accelerator produces a beam of electrons and radiation, including gamma rays, which can be used for safe medical imaging and other applications without the need for significant levels of radiation shielding outside the beam path. The secondary effect of bright, extremely brief flashes of light is the result of the initial accelerations of electrons within the plasma wake, as they are accelerated from rest to almost the speed of light in less than 1 millionth of a meter.

“Such a violent acceleration means they radiate like crazy,” Goers said. “As much as 3 percent of the initial laser radiation is emitted in the flash in a millionth of a billionth of a second.”

In terms of sheer acceleration, laser-driven particle accelerators have a long way to go before they are ready for applications in high-energy physics, where facilities such as Fermilab and CERN reign supreme. But for more immediate applications, such as ultra-fast medical and scientific imaging, the main barriers to laser-driven acceleration are cost, complexity, and portability.

“We may have found a solution to overcome all three of these barriers,” Milchberg said.

Nanostructured Surfaces Give Cells a Ticket to Ride

Biologists have known for centuries that cells change shape to move, divide, and interact with other cells. But it has been harder to figure out exactly how cells do this. For the first time, a team of biophysicists at the University of Maryland and the National Institutes of Health has determined why the texture of external surfaces plays a huge role in cells’ ability to react to their environment. Much like a car requires pavement with just the right level of roughness, cells propel themselves forward more efficiently over surfaces covered in nanoscale patterns.

The results, which appear September 28, 2015, in the online early edition of the Proceedings of the National Academy of Sciences, describe new nanoscale-patterned surfaces that can guide cells to move in specific directions, over nearly limitless distances. The textured surfaces are covered with tiny sawtooth-shaped features oriented in the same direction. Each sawtooth is much smaller than a cell and has a gently sloping surface on one edge, with a steep drop on the opposite edge. Similar surfaces could one day help in the development of new nanotechnology applications, such as highly effective wound dressings and stents that can accelerate the healing process.

The researchers studied movement in the single-celled amoeba Dictyostelium discoideum, as well as in human white blood cells. Both cell types responded to the sawtooth-patterned surface, suggesting that many species might share this need for textured surfaces on which to move.

“The fact that this mode of guidance works in both primitive and highly evolved cells suggests that cell guidance is important across all living things,” said co-author John Fourkas, Millard Alexander Professor in the Department of Chemistry and Biochemistry and the Institute for Physical Science and Technology at UMD.

The team looked at a protein called actin, which forms chains that make up a cell’s flexible internal skeleton. Among their findings, the team determined that nanoscale features, such as the sawtooth patterns described above, can guide the disassembly and reassembly of these actin chains, which in turn can propel the cell in a preferred direction of motion.

“Imagine a cell as a mosh pit that has a large rubber band surrounding the group of dancers,” said study co-author Wolfgang Losert, a professor of physics at UMD with joint appointments in the Institute for Research in Electronics and Applied Physics and the Institute for Physical Science and Technology. “The dancers represent the actin proteins and the rubber band represents the cell membrane. The dancers in the pit shove against each other, which in turn pushes against the rubber band, changing the shape of the mosh pit.”

Cells use their actin chains to move away from toxic chemicals, or toward nutrients and other beneficial signals. The chemical signals that normally cause cells to move are only effective if they are close enough and strong enough for the cell to react. To continue the mosh pit analogy, these chemical signals will attract the cell in the same way that loudspeakers attract a mosh pit full of dancers.

“One way to get the mosh pit to move in a given direction is to put the speakers where you want the dancers to go,” said Fourkas. “This scenario is similar to the chemical signaling that directs motion in biological systems. This is a powerful mechanism, but it can only work over a limited distance. If the speakers are too far away, the dancers in the mosh pit can’t hear the music.”

The new sawtooth surfaces provide a foothold for the cell’s internal skeleton, allowing the cell to move in one direction, over distances limited only by the size of the sawtooth surface.

“Imagine that the dance floor is composed of rows of sawteeth about the size of someone’s foot. It is easy to move up the gently sloped side of the sawtooth, but it is hard to move in the other direction against the vertical edge of the sawtooth,” Losert explained. “As soon as someone starts moving along a row of sawteeth, the people nearby will start to do the same. In this manner, features on the floor that are much smaller than the mosh pit can still cause the group of dancers to move in a preferred direction.”

The sawtooth surface passively prevents the cell from moving in the opposite direction. But Losert, Fourkas and their colleagues also demonstrated that a cell’s actin chains actively engage with each sawtooth, thus guiding motion in a specific direction. Each sawtooth is much smaller than the cell, so as soon as a cell clears one sawtooth, the cell’s actin fibers will already be engaged with the next several sawteeth down the line. This allows the cell to cover nearly limitless distances, so long as it has uninterrupted access to the sawtooth surface.

“This discovery shows that we can design patterned surfaces that can control cell behavior through manipulation of actin chains,” Losert added. “This capability opens the door to measuring and engineering cell behaviors through similar patterned surfaces, which will have many potential biomedical applications.”

Rabindra Mohapatra Explores Left-right Symmetric Models

New Scientist Magazine has writen an article on left-right symmetric models, a model suggested by Rabi Mohapatra, Jogesh Pati and Goran Senjanovic in 1974-75, and mentioned in a recent paper by Mohapatra and Bhupal Dev. Read More