A Conscious Coupling Of Magnetic And Electric Materials

Scientists have successfully paired ferroelectric and ferrimagnetic materials so that their alignment can be controlled with a small electric field at near room temperatures, an achievement that could open doors to ultra low-power microprocessors, storage devices and next-generation electronics.

The work, co-led by researchers at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and Cornell University, is described in a study to be published Sept. 22 in the journal Nature.

The researchers engineered thin, atomically precise films of hexagonal lutetium iron oxide (LuFeO3), a material known to be a robust ferroelectric, but not strongly magnetic. Lutetium iron oxide consists of alternating single monolayers of lutetium oxide and single monolayers of iron oxide, and differs from a strong ferrimagnetic oxide that consists of alternating monolayers of lutetium oxide with double monolayers of iron oxide (LuFe2O4).

The researchers found that by carefully adding one extra monolayer of iron oxide to every 10 atomic repeats of the single-single monolayer pattern, they could dramatically change the material’s properties and produce a strongly ferrimagnetic layer near room temperature. They then tested the new material to show that the ferrimagnetic atoms followed the alignment of their ferroelectric neighbors when switched by an electric field.

They did this at temperatures ranging from 200-300 kelvins (minus 100 to 80 degrees Fahrenheit), relatively balmy compared with other such multiferroics that typically work at much lower temperatures.

“Developing materials that can work at room temperature makes them viable candidates for today’s electronics,” said study co-lead author Julia Mundy, a University of California Presidential Postdoctoral Fellow and an affiliate at Berkeley Lab. “The multiferroic we created takes us a major step toward that goal.”

Researchers have increasingly sought alternatives to semiconductor-based electronics over the past decade as the increases in speed and density of microprocessors come at the expense of greater demands on electricity and hotter circuits.

“If you look at this in a broad sense, about 5 percent of our total global energy consumption is spent on electronics,” said co-senior author Ramamoorthy Ramesh, Berkeley Lab’s Associate Laboratory Director for Energy Technologies and a UC Berkeley professor of materials science and engineering and of physics. “It’s the fastest growing consumer of energy worldwide. The Internet of Things is leading to the installation of electronic devices everywhere. The world’s energy consumed by microelectronics is projected to be 40-50 percent by 2030 if we continue at the current pace and if there are no major advances in the field that lead to lower energy consumption.”

A major path to reducing energy consumption involves ferroic materials. Key advantages of ferroelectrics include their reversible polarization in response to low-power electric fields, and their ability to hold their polarized state without the need for continuous power. Common examples of ferroelectric materials include transit cards and, more recently, memory chips.

Ferromagnets and ferrimagnets have similar features, responding to magnetic fields, and are used in hard drives and sensors.

Pairing ferroelectric and ferrimagnetic materials into one multiferroic film would capture the advantages of both systems, enabling a wider range of memory applications with minimal power requirements. It has been an uneasy marriage, however, because the forces needed to align one type of material fail to work for the other. Polarizing the ferroelectric material would have no effect on the ferrimagnetic one.

Mundy began to tackle this challenge of creating a viable multiferroic while she was a Cornell University graduate student in the lab of Darrell Schlom, a professor of materials science and engineering and a leading expert in molecular-beam epitaxy. The ultra-precise technique – something Schlom likens to atomic spray painting – allowed the researchers to design and assemble the two different materials atom by atom, layer after layer. They intentionally seated a lutetium iron oxide with alternating iron oxide double layers (LuFe2O4) next to lutetium iron oxide with alternating iron oxide single layers (LuFeO3), and that positioning made all the difference in nudging the ferrimagnetic atoms to move in conjunction with the ferroelectric ones.

To show that this coupling was working at the atomic level, the researchers took the multiferroic film created at Cornell to Berkeley Lab’s Advanced Light Source (ALS). There, they had the equipment and expertise to test the material and capture images of the result using photoemission electron microscopy.

Working with staff scientists Andreas Scholl and Elke Arenholz at the ALS, they used a 5-volt probe from an atomic force microscope to switch the polarization of the ferroelectric material up and down, creating a geometric pattern of concentric squares. They then showed that the ferrimagnetic regions within the layered sample displayed the same pattern, even though no magnetic field was used. The direction was controlled by the electric field generated by the probe.

“It was when our collaborators at Berkeley Lab demonstrated electrical control of magnetism in the material that we made that things got super exciting!” said Schlom at Cornell. “Room-temperature multiferroics are rare. Including our new material, a total of four are known, but only one room-temperature multiferroic was known in which magnetism could be controlled electrically. Our work shows that an entirely different mechanism is active in this new material, giving us hope for even better — higher temperature and stronger — manifestations for the future.”

The researchers next plan to explore strategies for lowering the voltage threshold for influencing the direction of polarization. This includes experimenting with different substrates for building new materials.

“We want to show that this works at half a volt as well as at 5 volts,” said Ramesh. “We also want to make a working device with the multiferroic.”

Hena Das, a visiting scientist at Berkeley Lab and associate specialist at UC Berkeley, is another co-author on the study. Das started the work as a postdoctoral researcher at Cornell University and is the lead theorist on the study.

The Department of Energy’s Office of Science helped support this work. The ALS is a DOE Office of Science User Facility.

Quantum Computing A Step Closer To Reality

Physicists at The Australian National University (ANU) have brought quantum computing a step closer to reality by stopping light in a new experiment.

Lead researcher Jesse Everett said controlling the movement of light was critical to developing future quantum computers, which could solve problems too complex for today’s most advanced computers.

“Optical quantum computing is still a long way off, but our successful experiment to stop light gets us further along the road,” said Mr Everett from the Research School of Physics and Engineering (RSPE) and ARC Centre of Excellence for Quantum Computation and Communication Technology at ANU.

He said quantum computers based on light – photons – could connect easily with communication technology such as optic fibres and had potential applications in fields such as medicine, defence, telecommunications and financial services.

The research team’s experiment – which created a light trap by shining infrared lasers into ultra-cold atomic vapour – was inspired by Mr Everett’s discovery of the potential to stop light in a computer simulation.

“It’s clear that the light is trapped, there are photons circulating around the atoms,” Mr Everett said.

“The atoms absorbed some of the trapped light, but a substantial proportion of the photons were frozen inside the atomic cloud.”

Mr Everett likened the team’s experiment at ANU to a scene from Star Wars: The Force Awakens when the character Kylo Ren used the Force to stop a laser blast mid-air.

“It’s pretty amazing to look at a sci-fi movie and say we actually did something that’s a bit like that,” he said.

Associate Professor Ben Buchler, who leads the ANU research team, said the light-trap experiment demonstrated incredible control of a very complex system.

“Our method allows us to manipulate the interaction of light and atoms with great precision,” said Associate Professor Buchler from RSPE and ARC Centre of Excellence for Quantum Computation and Communication Technology at ANU.

Co-researcher Dr Geoff Campbell from ANU said photons mostly passed by each other at the speed of light without any interactions, while atoms interacted with each other readily.

“Corralling a crowd of photons in a cloud of ultra-cold atoms creates more opportunities for them to interact,” said Dr Campbell from RSPE and ARC Centre of Excellence for Quantum Computation and Communication Technology at ANU.

“We’re working towards a single photon changing the phase of a second photon. We could use that process to make a quantum logic gate, the building block of a quantum computer,” Dr Campbell said.

The research was supported by funding from the ARC Centre of Excellence for Quantum Computation and Communication Technology, which involves ANU, University of New South Wales, University of Melbourne, University of Queensland, Griffith University, University of Sydney, Australian Defence Force Academy, along with 12 international university and industry partners.

The results from the experiment are published in Nature Physics.

You can watch a video interview with Associate Professor Ben Buchler and Dr Geoff Campbell about their successful light-trap experiment and its implications for quantum computing on the ANU YouTube channel.

Secure Passwords Can Be Sent Through Your Body, Instead Of Air

Sending a password or secret code over airborne radio waves like WiFi or Bluetooth means anyone can eavesdrop, making those transmissions vulnerable to hackers who can attempt to break the encrypted code.

Now, University of Washington computer scientists and electrical engineers have devised a way to send secure passwords through the human body — using benign, low-frequency transmissions generated by fingerprint sensors and touchpads on consumer devices.

“Fingerprint sensors have so far been used as an input device. What is cool is that we’ve shown for the first time that fingerprint sensors can be re-purposed to send out information that is confined to the body,” said senior author Shyam Gollakota, UW assistant professor of computer science and engineering.

These “on-body” transmissions offer a more secure way to transmit authenticating information between devices that touch parts of your body — such as a smart door lock or wearable medical device — and a phone or device that confirms your identity by asking you to type in a password.

This new technique, which leverages the signals already generated by fingerprint sensors on smartphones and laptop touchpads to transmit data in new ways, is described in a paper presented in September at the 2016 Association for Computing Machinery’s International Joint Conference on Pervasive and Ubiquitous Computing (UbiComp 2016) in Germany.

“Let’s say I want to open a door using an electronic smart lock,” said co-lead author Merhdad Hessar, a UW electrical engineering doctoral student. “I can touch the doorknob and touch the fingerprint sensor on my phone and transmit my secret credentials through my body to open the door, without leaking that personal information over the air.”

The research team tested the technique on iPhone and other fingerprint sensors, as well as Lenovo laptop trackpads and the Adafruit capacitive touchpad. In tests with 10 different subjects, they were able to generate usable on-body transmissions on people of different heights, weights and body types. The system also worked when subjects were in motion — including while they walked and moved their arms.

“We showed that it works in different postures like standing, sitting and sleeping,” said co-lead author Vikram Iyer, a UW electrical engineering doctoral student. “We can also get a strong signal throughout your body. The receivers can be anywhere — on your leg, chest, hands — and still work.”

The research team from the UW’s Networks and Mobile Systems Lab systematically analyzed smartphone sensors to understand which of them generates low-frequency transmissions below 30 megahertz that travel well through the human body but don’t propagate over the air.

The researchers found that fingerprint sensors and touchpads generate signals in the 2 to 10 megahertz range and employ capacitive coupling to sense where your finger is in space, and to identify the ridges and valleys that form unique fingerprint patterns.

Normally, sensors use these signals to receive input about your finger. But the UW engineers devised a way to use these signals as output that corresponds to data contained in a password or access code. When entered on a smartphone, data that authenticates your identity can travel securely through your body to a receiver embedded in a device that needs to confirm who you are.

Their process employs a sequence of finger scans to encode and transmit data. Performing a finger scan correlates to a 1-bit of digital data and not performing the scan correlates to a 0-bit.

The technology could also be useful for secure key transmissions to medical devices such as glucose monitors or insulin pumps, which seek to confirm someone’s identity before sending or sharing data.

The team achieved bit rates of 50 bits per second on laptop touchpads and 25 bits per second with fingerprint sensors — fast enough to send a simple password or numerical code through the body and to a receiver within seconds.

This represents only a first step, the researchers say. Data can be transmitted through the body even faster if fingerprint sensor manufacturers provide more access to their software.

The research was funded by the Intel Science and Technology Center for Pervasive Computing, a Google faculty award and the National Science Foundation.

Room-Temp Superconductors Could Be Possible

Superconductors are the Holy Grail of energy efficiency.

These mind-boggling materials allow electric current to flow freely without resistance. But that generally only happens at temperatures within a few degrees of absolute zero (minus 459 degrees Fahrenheit), making them difficult to deploy today. However, if we’re able to harness the powers of superconductivity at room temperature, we could transform how energy is produced, stored, distributed and used around the globe.

In a recent breakthrough, scientists at the Department of Energy’s Brookhaven National Laboratory got one step closer to understanding how to make that possible. The research, led by physicist Ivan Bozovic, involves a class of compounds called cuprates, which contain layers of copper and oxygen atoms.

Under the right conditions — which, right now, include ultra-chilly temperatures — electrical current flows freely through these cuprate superconductors without encountering any “roadblocks” along the way. That means none of the electrical energy they’re carrying gets converted to heat. If you’ve ever rested your laptop on your lap, you’ve felt the heat lost by a non-superconducting material.

Creating the right conditions for superconductivity in cuprates also involves adding other chemical elements such as strontium. Somehow, adding those atoms and chilling the material causes electrons — which normally repel one another — to pair up and effortlessly move together through the material. What makes cuprates so special is that they can achieve this “magical” state of matter at temperatures a hundred degrees or more above those required by standard superconductors. That makes them very promising for real-world, energy-saving applications.

Think about it: If scientists can figure out why cuprates become superconducting at such unusually high temperatures, they may be able to engineer materials that become superconducting at room temperature.

These materials wouldn’t require any cooling, so they’d be relatively easy and inexpensive to incorporate into our everyday lives. Picture power grids that never lose energy, more affordable mag-lev train systems, cheaper medical imaging machines like MRI scanners, and smaller yet powerful supercomputers.

To figure out the mystery of “high-temperature” superconductivity in the cuprates, scientists need to understand how the electrons in these materials behave. Bozovic’s team has now solved part of the mystery by determining what exactly controls the temperature at which cuprates become superconducting.

The standard theory of superconductivity says that this temperature is controlled by the strength of the electron-pairing interaction, but Bozovic’s team has discovered otherwise. After 10 years of preparing and analyzing more than 2,000 samples of a cuprate with varying amounts of strontium, they found that the number of electron pairs within a given area (say, per cubic centimeter), or the density of electron pairs, controls the superconducting transition temperature. In other words, it’s not the forces between objects that matter here, but the density of objects–in this case, electron pairs.

Researchers Bring Theorized Mechanism Of Conduction To Life

Humans have harnessed large portions of the electromagnetic spectrum for diverse technologies, from X-rays to radios, but a chunk of that spectrum has remained largely out of reach. This is known as the terahertz gap, located between radio waves and infrared radiation, two parts of the spectrum we use in everyday technologies including cell phones, TV remotes and toasters.

A theory developed by the late Stanford professor and Nobel laureate Felix Bloch suggested that a specially structured material that allowed electrons to oscillate in a particular way might be able to conduct these sought-after terahertz signals.

Now, decades after Bloch’s theory, Stanford physicists may have developed materials that enable these theorized oscillations, someday allowing for improvements in technologies from solar cells to airport scanners. The group published their findings in the Sept. 29 issue of Science.

Innovations in superlattice materials

Researchers have long thought that materials with repeating spatial patterns on the nanoscale might allow for Bloch’s oscillations, but technology is only just catching up to theory. Such a material requires that electrons travel long distances without deflection, where even the smallest imperfection in the medium through which the electrons flow can put them off their original path, like a stream trying to wind over and around rocks and fallen trees.

Burgeoning research in the field of two-dimensional materials and superlattices could make this type of material a reality. Superlattices are semiconductors made by layering ultra-thin materials whose atoms are arranged in a periodic lattice pattern.

For this study, the researchers created a two-dimensional superlattice by sandwiching a sheet of atomically thin graphene in between two sheets of electrically insulating boron nitride. The atoms in the graphene and boron nitride have slightly different spacing, so when they are stacked on top of each other they create a special wave interference pattern called a moiré pattern.

New uses for electrons

Protected from air and contaminants by boron nitride above and below, electrons in the graphene flow along smooth paths without deflection, exactly as theory suggested would be needed to conduct terahertz signals. The researchers were able to send electrons through the graphene sheet, collect them on the other side and use them to thus infer the activity of the electrons along the way.

Usually, when a voltage is applied across a crystal, electrons are continuously accelerated in the direction of the electric field until they are deflected. In this moiré superlattice, researchers showed that the electrons can be confined to narrower bands of energy, said physics Professor David Goldhaber-Gordon, co-author of the study. Combined with very long times between deflections, this should lead the electrons to oscillate in place and emit radiation in the terahertz frequency range. This is a foundational success on the path toward creating controlled emission and sensing of terahertz frequencies.

In addition to bringing Bloch’s theory closer to reality, the researchers found a completely surprising change in the electronic structure of their superlattice material.

“In semiconductors, like silicon, we can tune how many electrons are packed into this material,” said Goldhaber-Gordon. “If we put in extra, they behave as though they are negatively charged. If we take some out, the current that moves through the system behaves as if it’s instead composed of positive charges, even though we know it’s all electrons.”

But this superlattice brings a new twist: Adding even more electrons produces particles of positive charge, and taking out even more returns to negative charge.

Future applications of this reversal in the character of the electrons could come in the form of more efficient p-n junctions, which are crucial building blocks to most semiconductor electronic devices such as solar cells, LEDs and transistors. Normally, if one shines light on a p-n junction, sending out one electron for every photon absorbed is considered excellent performance. But these new junctions could emit several electrons per photon, harvesting the energy of the light more effectively.

Terahertz and Stanford, past and future

While this new research hasn’t yet created a Bloch oscillator, the scientists have achieved the first step by showing that the momentum and velocity of an electron can be preserved over long times and distances within this superlattice, said Menyoung Lee, co-author of the study who conducted the research as a graduate student in the Goldhaber-Gordon Group.

“We apply the very first original lessons of solid-state physics that Felix Bloch figured out a long time ago, and it turns out we can use that to drive unique conduction phenomena in novel engineered materials,” Lee said.

Terahertz frequency technology could eventually be an improvement on today’s technologies. When U.S. airports scan passengers at security checkpoints today, they use microwaves, which penetrate nonmetal materials to reveal concealed metal objects. Goldhaber-Gordon explained that terahertz has similar transmission properties but shorter wavelength, potentially revealing even nonmetal concealed objects at high resolution. He added that terahertz scanners could also be used to detect defects such as hidden cavities in objects on a manufacturing assembly line.

The clean electronic conduction demonstrated in this work also furthered understanding of the ways in which electrons interact and flow, and Goldhaber-Gordon said his lab plans to use these insights to work on creating extremely narrow beams of electrons to send through superlattices. He called this new field “electron optics in 2-D materials” because these beams travel in straight lines and obey laws of refraction similarly to beams of light.

“This is going to be an area that opens up a lot of new possibilities,” said Goldhaber-Gordon, “and we’re just at the start of exploring what we can do.”

Additional authors of this paper, “Ballistic Miniband Conduction in a Graphene Superlattice,” are Patrick Gallagher of Stanford University, John R. Wallbank and Vladimir I. Fal’ko of University of Manchester, and Kenji Watanabe and Takashi Taniguchi of the National Institute for Materials Science in Tsukuba, Japan.

Funding came from the Air Force Office of Scientific Research and the Gordon and Betty Moore Foundation, and was performed in part in the Stanford Nano Shared Facilities.