Painting quantum electronics with beams of light

A team of scientists from the University of Chicago and Penn State University has accidentally discovered a new way of using light to draw and erase quantum-mechanical circuits in a unique class of materials called topological insulators.

In contrast to using advanced nanofabrication facilities based on chemical processing of materials, this flexible technique allows for rewritable “optical fabrication” of devices. This finding is likely to spawn new developments in emerging technologies such as low-power electronics based on the spin of electrons or ultrafast quantum computers. The research was published Oct. 9 in the American Association for the Advancement of Science’s new online journal Science Advances.

“This observation came as a complete surprise,” said David D. Awschalom, the Liew Family Professor and deputy director in the Institute of Molecular Engineering at UChicago, who was one of two lead researchers on the project. “It’s one of those rare moments in experimental science where a seemingly random event—turning on the room lights—generated unexpected effects with potentially important impacts in science and technology.” 

The electrons in topological insulators have unique quantum properties that many scientists believe will be useful for developing spin-based electronics and quantum computers. However, making even the simplest experimental circuits with these materials has proved difficult because traditional semiconductor engineering techniques tend to destroy their fragile quantum properties. Even a brief exposure to air can reduce their quality.

In Science Advances, the researchers report the discovery of an optical effect that allows them to “tune” the energy of electrons in these materials using light, and without ever having to touch the material itself. They have used it to draw and erase p-n junctions—one of the central components of a transistor—in a topological insulator for the first time.

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Like many advances in science, the path to this discovery had an unexpected twist.

“To be honest, we were trying to study something completely different,” said Andrew Yeats, a graduate student in Awschalom’s laboratory and the paper’s lead author. “There was a slow drift in our measurements that we traced to a particular type of fluorescent lights in our lab. At first we were glad to be rid of it, and then it struck us—our room lights were doing something that people work very hard to do in these materials.”

The researchers went back to Bulley & Andrews Construction, the contractor that renovated the lab space, for more information about the lights. “I’ve never had a client so obsessed with the overhead lighting,” said Frank Floss, superintendent for Bulley & Andrews. “I could have never imagined how important it would turn out to be.”  

The researchers found that the surface of strontium titanate, the substrate material on which they had grown their samples, becomes electrically polarized when exposed to ultraviolet light, and their room lights happened to emit at just the right wavelength. The electric field from the polarized strontium titanate was leaking into the topological insulator layer, changing its electronic properties.

Awschalom and his colleagues found that by intentionally focusing beams of light on their samples, they could draw electronic structures that persisted long after the light was removed.

“It’s like having a sort of quantum Etch A Sketch in our lab,” he said. They also found that bright red light counteracted the effect of the ultraviolet light, allowing them to both write and erase.

“Instead of spending weeks in the cleanroom and potentially contaminating our materials,” said Awschalom, “now we can sketch and measure devices for our experiments in real time. When we’re done, we just erase it and make something else. We can do this in less than a second.”

To test whether the new technique might interfere with the unique properties of topological insulators, the team measured their samples in high magnetic fields. They found promising signatures of an effect called weak anti-localization, which arises from quantum interference between the different simultaneous paths that electrons can take through a material when they behave as waves.

“One exciting aspect of this work is that it’s noninvasive,” said Prof. Nitin Samarth, the George A. and Margaret M. Downsbrough Department Head of Physics at Penn State, and a lead researcher on the project. “Since the electrical polarization occurs in an adjacent material, and the effect persists in the dark, the topological insulator remains relatively undisturbed. With these fragile quantum materials, sometimes you have to use a light touch.”

To better understand the physics behind the effect, the researchers conducted a number of control measurements. They showed that the optical effect is not unique to topological insulators, but that it can act on other materials grown on strontium titanate as well.

“In a way, the most exciting aspect of this work is that it should be applicable to a wide range of nanoscale materials such as complex oxides, graphene and transition metal dichalcogenides,” said Awschalom. “It’s not just that it’s faster and easier. This effect could allow electrical tuning of materials in a wide range of optical, magnetic and spectroscopic experiments where electrical contacts are extremely difficult or simply impossible.”

University of Chicago

Physicists determine the three-dimensional positions of individual atoms for the first time

Atoms are the building blocks of all matter on Earth, and the patterns in which they are arranged dictate how strong, conductive or flexible a material will be. Now, scientists at UCLA have used a powerful microscope to image the three-dimensional positions of individual atoms to a precision of 19 trillionths of a meter, which is several times smaller than a hydrogen atom.

Their observations make it possible, for the first time, to infer the macroscopic properties of materials based on their structural arrangements of atoms, which will guide how scientists and engineers build aircraft components, for example. The research, led by Jianwei (John) Miao, a UCLA professor of physics and astronomy and a member of UCLA’s California NanoSystems Institute, is published Sept. 21 in the online edition of the journal Nature Materials.

For more than 100 years, researchers have inferred how atoms are arranged in three-dimensional space using a technique called X-ray crystallography, which involves measuring how light waves scatter off of a crystal. However, X-ray crystallography only yields information about the average positions of many billions of atoms in the crystal, and not about individual atoms’ precise coordinates.

“It’s like taking an average of people on Earth,” Miao said. “Most people have a head, two eyes, a nose and two ears. But an image of the average person will still look different from you and me.”

Because X-ray crystallography doesn’t reveal the structure of a material on a per-atom basis, the technique can’t identify tiny imperfections in materials such as the absence of a single atom. These imperfections, known as point defects, can weaken materials, which can be dangerous when the materials are components of machines like jet engines.

“Point defects are very important to modern science and technology,” Miao said.

Miao and his team used a technique known as scanning transmission electron microscopy, in which a beam of electrons smaller than the size of a hydrogen atom is scanned over a sample and measures how many electrons interact with the atoms at each scan position. The method reveals the atomic structure of materials because different arrangements of atoms cause electrons to interact in different ways.

However, scanning transmission electron microscopes only produce two-dimensional images. So creating a 3-D picture requires scientists to scan the sample once, tilt it by a few degrees and re-scan it — repeating the process until the desired spatial resolution is achieved — before combining the data from each scan using a computer algorithm. The downside of this technique is that the repeated electron beam radiation can progressively damage the sample.

Using a scanning transmission electron microscope at the Lawrence Berkeley National Laboratory’s Molecular Foundry, Miao and his colleagues analyzed a small piece of tungsten, an element used in incandescent light bulbs. As the sample was tilted 62 times, the researchers were able to slowly assemble a 3-D model of 3,769 atoms in the tip of the tungsten sample.

The experiment was time consuming because the researchers had to wait several minutes after each tilt for the setup to stabilize.

“Our measurements are so precise, and any vibrations — like a person walking by — can affect what we measure,” said Peter Ercius, a staff scientist at Lawrence Berkeley National Laboratory and an author of the paper.

The researchers compared the images from the first and last scans to verify that the tungsten had not been damaged by the radiation, thanks to the electron beam energy being kept below the radiation damage threshold of tungsten.

Miao and his team showed that the atoms in the tip of the tungsten sample were arranged in nine layers, the sixth of which contained a point defect. The researchers believe the defect was either a hole in an otherwise filled layer of atoms or one or more interloping atoms of a lighter element such as carbon.

Regardless of the nature of the point defect, the researchers’ ability to detect its presence is significant, demonstrating for the first time that the coordinates of individual atoms and point defects can be recorded in three dimensions.

“We made a big breakthrough,” Miao said.

Miao and his team plan to build on their results by studying how atoms are arranged in materials that possess magnetism or energy storage functions, which will help inform our understanding of the properties of these important materials at the most fundamental scale.

“I think this work will create a paradigm shift in how materials are characterized in the 21st century,” he said. “Point defects strongly influence a material’s properties and are discussed in many physics and materials science textbooks. Our results are the first experimental determination of a point defect inside a material in three dimensions.”

The study’s co-authors include Rui Xu, Chien-Chun Chen, Li Wu, Mary Scott, Matthias Bartels, Yongsoo Yang and Michael Sawaya, all of UCLA; as well as Colin Ophus of Lawrence Berkeley National Laboratory; Wolfgang Theis of the University of Birmingham; Hadi Ramezani-Dakhel and Hendrik Heinz of the University of Akron; and Laurence Marks of Northwestern University.

UCLA

Nanotechnology World Association

Physicists discover spiral vortex patterns from electron waves

University of Nebraska-Lincoln physicists have made a compelling discovery that graces the Sept. 11 cover of the journal Physical Review Letters.

In their new study, physicists Anthony Starace and Jean Marcel Ngoko Djiokap report an unusual pattern of wave interference produced when an electron is ejected, or ionized, from its orbit around a helium atom.

Like all sub-atomic particles, electrons occupy a realm governed by quantum mechanics. This means that their position, velocity and other properties are probabilistic, existing within a range of possible values. Electrons can also exhibit the behavior of waves that, like ripples in a pond, often gain or lose amplitude as they cross paths.

By firing two time-delayed, ultrashort laser pulses at a helium atom, the researchers found that the distribution of momentum values for these intersecting electron waves can take the form of a two-armed vortex that resembles a spiral galaxy.

Though the spiral had been observed in waves of light, the team’s study is the first to produce the pattern with electrons. In doing so, it also dramatically demonstrates the wave-like property of matter, Starace said.

Starace called the pattern an “excellent diagnostic tool” for characterizing electron-manipulating laser pulses, which occur on such fast time scales that physicists have sought multiple ways to measure their durations and intensities. The team’s pulses lasted just 124 attoseconds, which compares to one second as one second compares to roughly 256 million years.

“You get this pattern, and you have all the time in the world to make measurements of it,” said Starace, a George Holmes University Professor of physics. “You don’t have to do this on the fly.

“Having a way to know what you’re inputting into an unknown situation is important. If you’re a doctor (examining) a patient, you have to know the properties of (your instruments) so that you can determine a patient’s health. Similarly, we physicists are developing new tools. One has to completely understand those tools before one can put them to use.”

Like all light, laser pulses feature electric fields that normally point in many directions. Polarizing a laser pulse aligns these fields along one direction, while circularly polarizing a pulse aligns and then essentially rotates the fields around an axis.

The team’s first pulse of circularly polarized light rotated in one direction, with the second rotating the opposite way. These orientations dictate whether the resulting spiral pattern appears to swirl left or right, Ngoko Djiokap said. The time delay between the pulses determines the number of arms in the spiral, he said, whereas the duration of the pulses corresponds to the width of the arms.

“If you use (longer) pulses to probe the electrons, you are going to compress the spiral pattern that you want to (analyze),” said Ngoko Djiokap, research assistant professor of physics. “Using attosecond pulses, the pattern is clearly visible.”

Offering their peers new tools, said Starace, should help inform future investigations involving ultrafast laser physics.

“Attosecond science is still a new field,” Starace said. “We’re forging ahead and predicting what kinds of experiments can be done.”

Starace and Ngoko Djiokap authored their study with researchers from the University of Rochester, Aarhus University in Denmark and Voronezh State University in Russia.

University of Nebraska–Lincoln

Nanotechnology world Association

Molecular Graphene Heralds New Era of ‘Designer Electrons’

A version of molecular graphene in which the electrons respond as if they're experiencing a very high magnetic field (red areas) when none is actually present. Scientists from Stanford and SLAC National Accelerator Laboratory calculated the positions where carbon atoms in graphene should be to make its electrons believe they were being exposed to a magnetic field of 60 Tesla, more than 30 percent higher than the strongest continuous magnetic field ever achieved on Earth. (A 1 Tesla magnetic field is about 20,000 times stronger than the Earth's.) The researchers then used a scanning tunneling microscope to place carbon monoxide molecules (black circles) at precisely those positions. The electrons responded by behaving exactly as expected — as if they were exposed to a real field, but no magnetic field was turned on in the laboratory. Image credit: Hari Manoharan / Stanford University.

Researchers from Stanford University and the U.S. Department of Energy’s SLAC National Accelerator Laboratory have created the first-ever system of “designer electrons” – exotic variants of ordinary electrons with tunable properties that may ultimately lead to new types of materials and devices.

“The behavior of electrons in materials is at the heart of essentially all of today’s technologies,” said Hari Manoharan, associate professor of physics at Stanford and a member of SLAC’s Stanford Institute for Materials and Energy Sciences, who led the research. “We’re now able to tune the fundamental properties of electrons so they behave in ways rarely seen in ordinary materials.”

Their first examples, reported today in Nature, were hand-crafted, honeycomb-shaped structures inspired by graphene, a pure form of carbon that has been widely heralded for its potential in future electronics. Initially, the electrons in this structure had graphene-like properties; for example, unlike ordinary electrons, they had no mass and traveled as if they were moving at the speed of light in a vacuum. But researchers were then able to tune these electrons in ways that are difficult to do in real graphene.

To make the structure, which Manoharan calls molecular graphene, the scientists use a scanning tunneling microscope to place individual carbon monoxide molecules on a perfectly smooth copper surface. The carbon monoxide repels the free-flowing electrons on the copper surface and forces them into a honeycomb pattern, where they behave like graphene electrons.

To tune the electrons’ properties, the researchers repositioned the carbon monoxide molecules on the surface; this changed the symmetry of the electron flow. In some configurations, electrons acted as if they had been exposed to a magnetic or electric field. In others, researchers were able to finely tune the density of electrons on the surface by introducing defects or impurities. By writing complex patterns that mimicked changes in carbon-carbon bond lengths and strengths in graphene, the researchers were able to restore the electrons’ mass in small, selected areas.

“One of the wildest things we did was to make the electrons think they are in a huge magnetic field when, in fact, no real field had been applied,”Manoharan said. Guided by the theory developed by co-author Francisco Guinea of Spain, the Stanford team calculated the positions where carbon atoms in graphene should be to make its electrons believe they were being exposed to magnetic fields ranging from zero to 60 Tesla, more than 30 percent higher than the strongest continuous magnetic field ever achieved on Earth. The researchers then moved carbon monoxide molecules to steer the electrons into precisely those positions, and the electrons responded by behaving exactly as predicted – as if they had been exposed to a real field.

“Our new approach is a powerful new test bed for physics,” Manoharan said. “Molecular graphene is just the first in a series of possible designer structures. We expect that our research will ultimately identify new nanoscale materials with useful electronic properties.”

Additional authors included Kenjiro K. Gomes, Warren Mar and Wonhee Ko of the Stanford Institute for Material and Energy Sciences. Francisco Guinea is a researcher at the Madrid Materials Science Institute. The research was supported by the U.S. Department of Energy’s Office of Basic Energy Sciences, the National Science Foundation and the Spanish Ministry of Science & Innovation.

SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the U.S. Department of Energy Office of Science. To learn more, please visit www.slac.stanford.edu.