Physical Review Letters

Physicists Just Found a New Way to Bend a Fundamental Rule of Light Waves

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One of the more well-known rules in physics is that light can only ever go one speed, so long as nothing stands in its way.

But new research has found there could be an interesting exception to this rule, where the mixing of light waves could bring them to a complete standstill.

 

The discovery hints at new ways of wrangling not just photons but nearly any kind of wave, which could be useful in technology that relies on information sent and stored using light.

Delaying light’s journey isn’t itself all that hard. Put a bunch of atoms in their way, and photons will take their time slipping in and out of the forest of particles.

Chill those particles right down so they lose their individual identities, and light can be set into slow motion and even stopped completely as it passes through the cloud.

More recently, it’s been shown that light’s pathway can be affected by changing its angular momentum, effectively twisting it so it takes longer than its usual 299,792,458 metres per second to get from A to B.

A small group of physicists from the Israel Institute of Technology and the Institute for Pure and Applied Mathematics (IMPA) in Brazil have now come up with another method, showing it’s theoretically possible to weave waves of light together in such a way that they stop dead in their tracks.

 The trick relies on tuning the light waves so they meet at what’s called an exceptional point – mathematical jargon that describes how the features of different waves match one another at a given coordinate.

Exceptional points were little more than mathematical concepts until fairly recently, when researchers demonstrated experimentally that they could be created by confining microwaves within a narrow grid.

When we talk about light waves, most of us imagine ripples of varying heights and length.

Light is of course defined by qualities such as wavelength and frequency, but it also has numerous other properties that form repeating patterns as photons traverse space.

These patterns can be tweaked by constraining their properties using things called waveguides, so two light waves can coalesce within the same space. This combination of properties described as an exceptional point gives rise to some interesting behaviours.

Last year researchers applied these points to the development of sensors that could respond to the smallest disruptions.

Now physicists have shown using mathematical models that it’s possible to use a kind of waveguide that balances the wave’s energy to produce an exceptional point where light stands still.

 By toggling the set-up in such a way that the waves can gain or lose energy, the light waves can be made to coalesce and freeze, or speed up and resume their journey out the other side.

If we’re to be particularly pedantic, we shouldn’t imagine it as photons standing still waiting for the go signal. No fundamental laws are being broken.

Just as light passing through a medium or being twisted is still technically moving at light speed, the photons in these waves are caught in a figurative electromagnetic whirlwind based on their interactions at the exceptional point.

For now, this achievement is still just by the numbers – a light-stopping device based on exceptional points hasn’t been built.

But if it does, we’d have another way to manipulate waves of light. The researchers also speculate that the same concepts technically apply to any kind of wave, including sound.

Given photons are quickly becoming the new electrons in information technology, we need all the tools we can find to get a firm grip on these speedy little particles.

Researchers design first battery-powered invisibility cloak.

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Researchers at The University of Texas at Austin have proposed the first design of a cloaking device that uses an external source of energy to significantly broaden its bandwidth of operation.

Andrea Alù, associate professor at the Cockrell School of Engineering, and his team have proposed a design for an active cloak that draws energy from a battery, allowing objects to become undetectable to radio sensors over a greater range of frequencies.

The team’s paper, “Broadening the Cloaking Bandwidth with Non-Foster Metasurfaces,” was published Dec. 3 in Physical Review Letters. Alù, researcher Pai-Yen Chen and postdoctoral research fellow Christos Argyropoulos co-authored the paper. Both Chen and Argyropoulos were at UT Austin at the time this research was conducted. The proposed active cloak will have a number of applications beyond camouflaging, such as improving cellular and radio communications, and biomedical sensing.

Cloaks have so far been realized with so-called passive technology, which means that they are not designed to draw energy from an external source. They are typically based on metamaterials (advanced artificial materials) or metasurfaces (a flexible, ultrathin metamaterial) that can suppress the scattering of light that bounces off an object, making an object less visible. When the scattered fields from the cloak and the object interfere, they cancel each other out, and the overall effect is transparency to radio-wave detectors. They can suppress 100 times or more the detectability at specific design frequencies. Although the proposed design works for radio waves, active cloaks could one day be designed to make detection by the human eye more difficult.

“Many cloaking designs are good at suppressing the visibility under certain conditions, but they are inherently limited to work for specific colors of light or specific frequencies of operation,” said Alù, David & Doris Lybarger Endowed Faculty Fellow in the Department of Electrical and Computer Engineering. In this paper, on the contrary, “we prove that cloaks can become broadband, pushing this technology far beyond current limits of passive cloaks. I believe that our design helps us understand the fundamental challenges of suppressing the scattering of various objects at multiple wavelengths and shows a realistic path to overcome them.”

The proposed active cloak uses a battery, circuits and amplifiers to boost signals, which makes possible the reduction of scattering over a greater range of frequencies. This design, which covers a very broad frequency range, will provide the most broadband and robust performance of a cloak to date. Additionally, the proposed active technology can be thinner and less conspicuous than conventional cloaks.

In a related paper, published in Physical Review X in October, Alù and his graduate student Francesco Monticone proved that existing passive cloaking solutions are fundamentally limited in the bandwidth of operation and cannot provide broadband cloaking. When viewed at certain frequencies, passively cloaked objects may indeed become transparent, but if illuminated with white light, which is composed of many colors, they are bound to become more visible with the cloak than without. The October paper proves that all available cloaking techniques based on passive cloaks are constrained by Foster’s theorem, which limits their overall ability to cancel the scattering across a broad frequency spectrum.

In contrast, an active cloak based on active metasurfaces, such as the one designed by Alù’s team, can break Foster’s theorem limitations. The team started with a passive metasurface made from an array of metal square patches and loaded it with properly positioned operational amplifiers that use the energy drawn from a battery to broaden the bandwidth.

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“In our case, by introducing these suitable amplifiers along the cloaking surface, we can break the fundamental limits of passive cloaks and realize a ‘non-Foster’ surface reactance that decreases, rather than increases, with frequency, significantly broadening the of operation,” Alù said.

The researchers are continuing to work both on the theory and design behind their non-Foster active cloak, and they plan to build a prototype.

Alù and his team are working to use active cloaks to improve wireless communications by suppressing the disturbance that neighboring antennas produce on transmitting and receiving antennas. They have also proposed to use these cloaks to improve biomedical sensing, near-field imaging and energy harvesting devices.

Accidental discovery dramatically improves electrical conductivity.

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Quite by accident, Washington State University researchers have achieved a 400-fold increase in the electrical conductivity of a crystal simply by exposing it to light. The effect, which lasted for days after the light was turned off, could dramatically improve the performance of devices like computer chips.

Strontium Titanate

WSU doctoral student Marianne Tarun chanced upon the discovery when she noticed that the conductivity of some strontium titanate shot up after it was left out one day. At first, she and her fellow researchers thought the sample was contaminated, but a series of experiments showed the effect was from light.

“It came by accident,” said Tarun. “It’s not something we expected. That makes it very exciting to share.”

The phenomenon they witnessed—”persistent photoconductivity“—is a far cry from superconductivity, the complete lack of  pursued by other physicists, usually using temperatures near absolute zero. But the fact that they’ve achieved this at room temperature makes the phenomenon more immediately practical.

And while other researchers have created persistent photoconductivity in other materials, this is the most dramatic display of the phenomenon.

The research, which was funded by the National Science Foundation, appears this month in the journal Physical Review Letters.

“The discovery of this effect at  opens up new possibilities for practical devices,” said Matthew McCluskey, co-author of the paper and chair of WSU’s physics department. “In standard computer memory, information is stored on the surface of a computer chip or hard drive. A device using persistent photoconductivity, however, could store information throughout the entire volume of a crystal.”

This approach, called holographic memory, “could lead to huge increases in information capacity,” McCluskey said.

Strontium titanate and other oxides, which contain oxygen and two or more other elements, often display a dizzying variety of electronic phenomena, from the high resistance used for insulation to superconductivity’s lack of resistance.

“These diverse properties provide a fascinating playground for scientists but applications so far have been limited,” said McCluskey.

McCluskey, Tarun and physicist Farida Selim, now at Bowling Green State University, exposed a sample of  to light for 10 minutes. Its improved conductivity lasted for days. They theorize that the light frees electrons in the material, letting it carry more current.

New invisibility cloak type designed

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A new “broadband” invisibility cloak which hides objects over a wide range of frequencies has been devised.

Despite the hype about Harry Potter-style cloaks, our best current designs can only conceal objects at specific wavelengths of light or microwaves.

At other frequencies, invisibility cloaks actually make things more visible, not less, US physicists found.

Their solution is a new ultrathin, electronic system, which they describe in Physical Review Letters.

“Start Quote

If you want to make an object transparent at all angles and over broad bandwidths, this is a good solution”

Andrea Alu University of Texas

“Our active cloak is a completely new concept and design, aimed at beating the limits of [current cloaks] and we show that it indeed does,” said Prof Andrea Alu, from the University of Texas at Austin.

“If you want to make an object transparent at all angles and over broad bandwidths, this is a good solution.

“We are looking into realising this technology at the moment, but we are still at the early stages.”

Passive vs Active

While the popular image of an invisibility cloak is the magical robe worn by Harry Potter, there is another kind which is not so far-fetched.

The first working model – which concealed a small copper cylinder by bending microwaves around it – was first demonstrated in 2006.

Left: uncloaked sphere. Right: Same sphere covered with a plasmonic cloak
The sphere on the right is “cloaked” but actually scatters more radiation than when bare (left)

It was built with a thin shell of metamaterials – artificial composites whose structures allow properties which do not exist in nature.

Cloaking materials could have applications in the military, microscopy, biomedical sensing, and energy harvesting devices.

The trouble with current designs is they only work at limited bandwidths. Even this “perfect” 3D cloak demonstrated last year could only hide objects from microwaves.

At other frequencies the cloak acts as a beacon – making the hidden object more obvious – as Prof Alu and his team have now demonstrated in a new study in Physical Review X.

They looked at three popular types of “passive” cloaks – which do not require electricity – a plasmonic cloak, a mantle cloak, and a transformation-optics cloak.

Other ways to disappear

optical camouflage, keio university
  • Optical camouflage technology: A modified background image is projected onto a cloak of retro-reflective material (the kind used to make projector screens); the wearer becomes invisible to anyone standing at the projection source
  • The “mirage effect”: Electric current is passed through submerged carbon nanotubes to create very high local temperatures, this causes light to bounce off them, hiding objects behind
  • Adaptive heat cloaking: A camera records background temperatures, these are displayed by sheets of hexagonal pixels which change temperature very quickly, camouflaging even moving vehicles from heat-sensitive cameras
  • Calcite crystal prism: Calcite crystals send the two polarisations of light in different directions. By gluing prism-shaped crystals together in a specific geometry, polarised light can be directed around small objects, effectively cloaking them

All three types scattered more waves than the bare object they were trying to hide – when tested over the whole range of the electromagnetic spectrum.

“If you suppress scattering in one range, you need to pay the price, with interest, in some other range,” Prof Alu told BBC News.

“For example, you might make a cloak that makes an object invisible to red light. But if you were illuminated by white light (containing all colours) you would actually look bright blue, and therefore stand out more.”

A cloak that allows complete invisibility is “impossible” with current passive designs, the study concluded.

“When you add material around an object to cloak it, you can’t avoid the fact that you are adding matter, and that this matter still responds to electromagnetic waves,” Prof Alu explained.

Instead, he said, a much more promising avenue is “active” cloaking technology – designs which rely on electrical power to make objects “vanish”.

Active cloaks can be thinner and less conspicuous than passive cloaks.

Alu’s team have proposed a new design which uses amplifiers to coat the surface of the object in an electric current.

This ultrathin cloak would hide an object from detection at a frequency range “orders of magnitude broader” than any available passive cloaking technology, they wrote.

Nothing’s perfect

Prof David Smith of Duke University, one of the team who created the first cloak in 2006, said the new design was one of the most detailed he had yet seen.

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This does not necessarily preclude the Harry Potter cloak”

Professor David Smith Duke University

“It’s an interesting implementation but as presented is probably a bit limited to certain types of objects,” he told BBC News.

“There are limitations even on active materials. It will be interesting to see if it can be experimentally realised.”

Prof Smith points out that even an “imperfect” invisibility cloak might be perfectly sufficient to build useful devices with real-world applications.

For example, a radio-frequency cloak could improve wireless communications – by helping them bypass obstacles and reducing interference from neighbouring antennas.

“To most people, making an object ‘invisible’ means making it transparent to visible wavelengths. And the visible spectrum is a tiny, tiny sliver of the overall electromagnetic spectrum,” he told BBC News.

“So, this finding does not necessarily preclude the Harry Potter cloak, nor does it preclude any other narrow bandwidth application of cloaking.”

Quantum ‘sealed envelope’ system enables ‘perfectly secure’ information storage.

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A breakthrough in quantum cryptography demonstrates that information can be encrypted and then decrypted with complete security using the combined power of quantum theory and relativity – allowing the sender to dictate the unveiling of coded information without any possibility of intrusion or manipulation.

Scientists sent encrypted data between pairs of sites in Geneva and Singapore, kept “perfectly secure” for fifteen milliseconds – putting into practice what cryptographers call a ‘bit ‘ protocol, based on theoretical work by study co-author Dr Adrian Kent, from Cambridge’s Department of Applied Mathematics and Theoretical Physics.

Researchers describe it as the first step towards impregnable information networks controlled by “the combined power of Einstein’s relativity and ” which might one day, for example, revolutionise financial trading and other markets across the world.

‘Bit commitment’ is a mathematical version of a securely sealed envelope. Data are delivered from party A to party B in a locked state that cannot be changed once sent and can only be revealed when party A provides the key – with security guaranteed, even if either of the parties tries to cheat.

The technique could one day be used for everything from global financial trading to secure voting and even long-distance gambling, although researchers point out that this is the “very first step into new territory”.

This is a significant breakthrough in the world of ‘quantum cryptography’ – one that was once believed to be impossible. The results are published in the journal Physical Review Letters.

“This is the first time perfectly secure bit commitment – relying on the laws of and nothing else – has been demonstrated,” said Adrian Kent.

“It is immensely satisfying to see these theoretical ideas at last made practical thanks to the ingenuity of all the theorists and experimenters in this collaboration.”

Any signal between Geneva and Singapore takes at least fifteen milliseconds – with a millisecond equal to a thousandth of a second. This blink-of-an-eye is long enough with current technology to allow data to be handed over encrypted at both sites, and later decrypted – with security “unconditionally guaranteed” by the laws of physics, say the team.

The researchers have exploited two different areas of physics: Einstein’s special relativity – which interprets uniform motion between two objects moving at relative speeds – combined with the power of quantum theory, the new physics of the subatomic world that Einstein famously dismissed as “spooky”.

Completely secure ‘bit commitment’ using quantum theory alone is known to be impossible, say researchers, and the “extra control” provided by relativity is crucial.

Professor Gilles Brassard FRS of the Universit’e de Montr’eal, one of the co-inventors of quantum cryptography who was not involved in this study, spoke of the “vision” he had fifteen years ago – when trying to combine quantum ‘bit commitment’ with relativity to “save” the theory – in which Einstein and early quantum physicist Niels Bohr “rise from their graves and shake hands at last”:

“Alas, my idea at the time was flawed. I am so thrilled to see this dream finally come true, not only in theory but also as a beautiful experiment!” he said.

Bit commitment is a building block – what researchers call a “primitive” – that can be put together in lots of ways to achieve increasingly complex tasks, they say. “I see this as the first step towards a global network of information controlled by the combined power of and quantum theory,” Kent said.

One possible future use of relativistic could be global stock markets and other trading networks. It might be a way of leveling the technological ‘arms race’ in which traders acquire and exploit information as fast as possible, the team suggest, although they stress at such an early stage these suggestions are speculative.

The new study builds on previous experiments that, while successful, had to assume limitations in the technology of one or both parties – and consequently not entirely “safe or satisfactory” says Kent, “since you never really know what technology is out there”.

Nanodiamond production in ambient conditions opens door for flexible electronics, implants and more.

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Instead of having to use tons of crushing force and volcanic heat to forge diamonds, researchers at Case Western Reserve University have developed a way to cheaply make nanodiamonds on a lab bench at atmospheric pressure and near room temperature.

The are formed directly from a gas and require no surface to grow on.

The discovery holds promise for many uses in technology and industry, such as coating plastics with ultrafine diamond powder and making flexible electronics, implants, drug-delivery devices and more products that take advantage of diamond’s exceptional properties.

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Their investigation is published today in the scientific journal Nature Communications. The findings build on a tradition of diamond research at Case Western Reserve.

Beyond its applications, the discovery may offer some insight into our universe: an explanation of how nanodiamonds seen in space and found in meteorites may be formed.

“This is not a complex process: ethanol vapor at and pressure is converted to diamond,” said Mohan Sankaran, associate professor of chemical engineering at Case Western Reserve and leader of the project. “We flow the gas through a plasma, add hydrogen and out come diamond nanoparticles. We can put this together and make them in almost any lab.”

The process for making these small “forever stones” won’t melt plastic so it is well suited for certain high-tech applications. Diamond, renowned for being hard, has excellent optical properties and the highest velocity of sound and thermal conductivity of any material.

Unlike the other form of carbon, graphite, diamond is a semiconductor, similar to silicon, which is the dominant material in the electronics industry, and gallium arsenide, which is used in lasers and other optical devices.

While the process is simple, finding the right concentrations and flows—what the researchers call the “sweet spot”—took time.

The other researchers involved were postdoctoral researcher Ajay Kumar, PhD student Pin Ann Lin, and undergraduate student Albert Xue, of Case Western Reserve; and physics professor Yoke Khin Yap and graduate student Boyi Hao, of Michigan Technical University.

Sankaran and John Angus, professor emeritus of chemical engineering, came up with the idea of growing nanodiamonds with no heat or pressure about eight years ago. Angus’ research in the 1960s and 1970s led him and others to devise a way to grow diamond films at low pressure and high temperature, a process known as chemical vapor deposition that is now used to make coatings on computer disks and razor blades. Sankaran’s specialty, meanwhile, is making nanoparticles using cool microplasmas.

It usually requires high pressures and high temperatures to convert graphite to diamond or a combination of hydrogen gas and a heated substrate to grow diamond rather than graphite.

“But at the nanoscale, surface energy makes diamond more stable than graphite,” Sankaran explained. “We thought if we could nucleate carbon clusters in the gas phase that were less than 5 nanometers, they would be diamond instead of graphite even at normal pressure and temperature.”

After several ups and downs with the effort, the process came together when Kumar joined Sankaran’s lab. The engineers produced diamond much like they’d produce carbon soot.

They first create a plasma, which is a state of matter similar to a gas but a portion is becoming charged, or ionized. A spark is an example of a plasma, but it’s hot and uncontrollable.

To get to cooler and safer temperatures, they ionized argon gas as it was pumped out of a tube a hair-width in diameter, creating a microplasma. They pumped ethanol—the source of carbon—through the microplasma, where, similar to burning a fuel, carbon breaks free from other molecules in the , and yields particles of 2 to 3 nanometers, small enough that they turn into diamond.

In less than a microsecond, they add hydrogen. The element removes carbon that hasn’t turned to diamond while simultaneously stabilizing the diamond particle surface.

The diamond formed is not the large perfect crystals used to make jewelry, but is a powder of diamond particles. Sankaran and Kumar are now consistently making high-quality diamonds averaging 2 nanometers in diameter.

The researchers spent about a year of testing to verify they were producing diamonds and that the process could be replicated, Kumar said. The team did different tests themselves and brought in Yap’s lab to analyze the nanoparticles by Raman spectroscopy.

Currently, nanodiamonds are made by detonating an explosive in a reactor vessel to provide heat and pressure. The diamond particles must then be removed and purified from contaminating elements massed around them. The process is quick and cheap but the nanodiamonds aggregate and are of varying size and purity.

The new research offers promising implications. Nanodiamonds, for instance, are being tested to carry drugs to tumors. Because diamond is not recognized as an invader by the immune system, it does not evoke resistance, the main reason why chemotherapy fails.

Sankaran said his nanodiamonds may offer an alternative to diamonds made by detonation methods because they are purer and smaller.

The group’s process produces three kinds of diamonds: about half are cubic, the same structure as gem , a small percentage are a form suspected of having hydrogen trapped inside and about half are lonsdaleite, a hexagonal form found in but rarely found on Earth.

A recent paper in the journal Physical Review Letters suggests that when interstellar dust collides, such high pressure is involved that the graphitic turns into londsdaleite nanodiamonds.

Sankaran and Kumar contend that an alternative with no high requirement, such as their method, should be considered, too.

“Maybe we’re making diamond in the way diamond is sometimes made in outer space,” Sankaran proposed. “Ethanol and plasmas exist in outer space, and our nanodiamonds are similar in size and structure to those found in space.”

The group is now investigating whether it can fine-tune the process to control which form of diamond is made, analyzing the structures and determining if each has different properties. Lonsdaleite, for instance, is harder than cubic diamond.

The researchers have made a kind of nanodiamond spray paint. “We can do this in a single step, by spraying the nanodiamonds as they are produced out of the plasma and purified with hydrogen, to coat a surface,” Kumar said.

And they are working on scaling up the process for industrial use.

“Will they be able to scale up? That’s always a crap shoot,” Angus said. “But I think it can be done, and at very high rates and cheaply. Ultimately, it may take some years to get there, but there is no theoretical reason it can’t be done.”

If the scaled-up process is as simple and cheap as the lab process, industry will find many applications for the product, Sankaran said.

New particle might make quantum condensation at room temperature possible.

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Researchers from FOM Institute AMOLF, Philips Research, and the Autonomous University of Madrid have identified a new type of particle that might make quantum condensation possible at room temperature. The particles, so called PEPs, could be used for fundamental studies on quantum mechanics and applications in lasers and LEDs. The researchers published their results on 18 October in Physical Review Letters.

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In quantum condensation (also known as Bose-Einstein condensation) microscopic with different energy levels collapse into a single macroscopic quantum state. In that state, particles can no longer be distinguished. They lose their individuality and so the matter can be considered to be one ‘superparticle’.

Quantum condensation was predicted in the 1920s by Bose and Einstein, who theorised that particles will form a condensate at very low temperatures. The first experimental demonstration of the quantum condensate followed in the 1990s, when a gas of atoms was cooled to just a few billionths of a degree above absolute zero (-273°C). The need for such an extremely low temperature is related to the mass of the particles: the heavier the particles, the lower the temperature at which condensation occurs. This motivated an ongoing search for that may condense at higher temperatures than atoms. The eventual goal is to find particles that form a condensate at .

PEPs

The researchers have created a particle that is a potential candidate for fulfilling the quest: the extremely light plasmon-exciton-polariton (PEP). This particle is hybrid between light and . It consists of photons (light particles), plasmons (particles composed of electrons oscillating in metallic nanoparticles) and excitons (charged particles in ).

The researchers made PEPs using an array of metallic nanoparticles coated with molecules that emit light. This system generates PEPs when it is loaded with energy. Through a careful design of the coupling between plasmons, excitons and photons, the researchers created PEPs with a mass a trillion times smaller than the mass of atoms.

Because of their small mass, these PEPs are suitable candidates for quantum condensation even at room temperature. However, due to losses in the system (such as absorption in the metal) PEPs have a short lifespan, which makes keeping them around long enough to condense a challenge.

First steps

The researchers have shown the first steps towards condensation of PEPs, demonstrating that PEPs cool down as their density increases. However, in the current system cooling down is limited by properties of the organic molecules used in the experiments, which lead to a saturation of the PEP density before sets in. The researchers envisage that it should be possible to overcome these challenges in the future.

Applications

To a large extent, PEPs are composed of photons. Therefore, their decay results in the emission of light. This emitted light has unique properties, which could constitute the basis of new optical devices. In view of recent advances from AMOLF and Philips Research towards improving white LEDs with similar systems, the researchers suggest that from a Bose-Einstein condensate might illuminate our living rooms in the future.

Materials Prediction Scores a Hit

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 Figure 1

 The energetics of predicting materials. A schematic free energy landscape for different crystallographic configurations is given by the blue line. Note the small difference in energy between various structures compared with the total energy of a crystal demanding high computational accuracy. The application of pressure as done by Gou et al. will modify the energy landscape (red curve), potentially stabilizing new structures. The ground state (such as superconductivity, magnetism, or other forms of order) for a given structure is determined at even lower energy scales, as depicted in the inset. The addition of strong electronic correlations in some materials will further modify the landscape over large energy scales up to 10eV, making predictions even more challenging.

Had the great American philosopher Yogi Berra been a condensed matter physicist, he might have said “It’s difficult to make predictions, especially about superconductivity.” Predictions about a material’s structure and even more so its function have been goals of materials research for a long time, but the track record for predicting that a given compound will superconduct is notoriously bad [1]. Fortunately, advances in the fidelity and resolution of electronic structure calculations are beginning to change this trend [2]. In fact, the White House’s Materials Genome Initiative [3] represents a recognition that with recent advances in computational capability and materials models, such breakthroughs are possible and, in fact, likely probable. In a paper in Physical Review Letters, Huiyang Gou at the University of Bayreuth, Germany, and colleagues [4] describe a success story in the search for predictability. They report the observation of superconductivity in iron tetraboride (FeB4) at approximately 3 kelvin (K). Not only did they find superconductivity where electronic structure calculations told them to look, they used high-pressure synthesis techniques to discover a compound that wasn’t readily apparent in nature. Further, the resulting compound, orthorhombic FeB4, turns out to be very mechanically hard as well as superconducting, thus possessing two desirable traits.

Most attempts to predict superconductivity invoke the physicist Bernd Matthias [5]. In the 1950s–1970s Matthias articulated a number of empirical “rules” that anticipated a large number of superconducting materials based on crystal structure and the number of valence electrons per atom. However, these rules were clearly based on intuition and not predictive theory. The experimental discovery that cuprates, magnesium diboride (MgB2), and more recently, iron pnictides all superconduct drove home the reality that serendipity was still the best materials discovery engine. However, that reality is beginning to evolve.

Why is it so difficult to predict new superconducting materials? One issue is the difficulty predicting the structural stability of a compound, that is, whether the binding energy between atoms is large enough to keep them stuck together in a particular configuration. Electronic structure calculations provide the total energy for a crystal, which is on the order of 105 electron volts per atom (eV/atom) (see Fig. 1). However, the stability with respect to competing phases is typically as small as 10-2 eV/atom, thus demanding incredibly high accuracy of the calculations. Furthermore, calculations are typically performed at T=0K in ideal crystals, while the thermal energy at which the crystals are synthesized and the energy scale created by defects can easily shift the relative total energies of competing phases by similar amounts. Another factor is that superconductivity is a very low-energy instability of the electronic structure. For a superconductor with a transition temperature Tc of 3K, as discovered by Gou et al., this amounts to an energy scale of 10-4eV. Few predictive models (yet) have accuracy at the parts per billion level covered by these energy scales.

Advanced electronic structure calculations for predictions have increased effectiveness due to the relative accessibility and availability of high-pressure techniques. Recent discoveries demonstrate that surprises still exist at high pressure [6]. We now know that a dozen or so additional elements superconduct at elevated pressure even though they are normal materials under ambient conditions, including calcium at 220 gigapascals (with Tc=29K, the highest Tc for an elemental superconductor). More broadly, materials science has been transformed by our ability to apply sufficient pressure to tune structural energetics on this scale to make new states of matter available.

In 2010, Kolmogorov, a coauthor of the present study, and colleagues predicted additional phases in the iron–boron (Fe-B) binary phase diagram that had yet to be observed [7]. They used a high-throughput search method coupled to an evolutionary algorithm to identify new structures for which superconductivity was theoretically evaluated. Subsequently, Bialon et al. suggested that the stability of iron tetraboride (FeB4) would be enhanced under pressure, and predicted the material’s hardness [8]. In the present manuscript, Gou et al. confirmed that FeB4 can be synthesized under pressure, and furthermore, that it possess the two novel predicted properties: superconductivity and high incompressibility. In addition, though not definitive, Gou et al. obtained preliminary data that superconductivity is phonon mediated like other conventional superconductors.

While the paper by Gou et al. gives promise that theory may finally be able to guide experimentalists where to look for conventional superconductors, it’s important to remember that the predicted Tc was 5 times too large in a structure that couldn’t be synthesized at ambient pressure. Further, the situation remains much more challenging for unconventional superconductors such as the cuprates, pnictides, heavy fermion materials, and organics. The biggest issue is that strong electronic correlations alter the electronic structure in these materials over an energy scale of order 1–10eV. While modern electronic structure calculations such as dynamical mean-field theory are making progress in understanding these effects, we currently lack the ability to reliably identify an additional superconducting instability on this strongly correlated background. How these electronic correlations modify the ability to compute structural stability of compounds also remains an open question. Given that superconductivity emerges in strongly correlated systems in ways we least expect it [9], future searches would be aided by guidance on where to find such correlations and competing electronic instabilities.

Gou et al. provide an encouraging step in the quest for materials by design, but one can also hope that this is a harbinger of even more and better things to come. Leveraging advanced computational capabilities and associated materials algorithms, together with synthetic techniques that allow broader access to phase space, including metastable materials, holds the exciting potential of delivering on the vision of the Materials Genome Initiative. We look forward to this, bearing in mind the quote attributed to Yogi Berra: “It’s difficult to make predictions, especially about the future.”

Acknowledgment

Our work in this area has been supported by the Department of Energy’s Office of Basic Energy Sciences Division of Materials Science and Engineering.

References

  1. I. I. Mazin, “Superconductivity Gets an Iron Boost,” Nature 464, 183 (2010).
  2. R. Akashi and R. Arita, “Development of Density-Functional Theory for a Plasmon-Assisted Superconducting State: Application to Lithium Under High Pressures,” Phys. Rev. Lett. 111, 057006 (2013).
  3. Materials Genome Initiative for Global Competitiveness, http://www.whitehouse.gov/blog/2011/06/24/materials-genome-initiative-renaissance-american-manufacturing.
  4. H. Gou et al., “Discovery of a Superhard Iron Tetraboride Superconductor,” Phys. Rev. Lett. 111, 157002 (2013).
  5. B. T. Matthias, T. H. Geballe, and V. B. Compton, “Superconductivity,” Rev. Mod. Phys. 35, 1 (1963).
  6. M. Sakata, Y. Nakamoto, K. Shimizu, T. Matsuoka, and Y. Ohishi, “Superconducting state of Ca-VII below a critical temperature of 29 K at a pressure of 216 GPa,” Phys. Rev. B 83, 220512(R) (2011).
  7. A. N. Kolmogorov, S. Shah, E. R. Margine, A. F. Bialon, T. Hammerschmidt, and R. Drautz, “New Superconducting and Semiconducting Fe-B Compounds Predicted with an Ab Initio Evolutionary Search,” Phys. Rev. Lett. 105, 217003 (2010).
  8. A. F. Bialon, T. Hammerschmidt, R. Drautz, S. Shah, E. R. Margine, and A. N. Kolmogorov, “Possible Routes for Synthesis of New Boron-Rich Fe–B and Fe1-xCrxB4 Compounds,” Appl. Phys. Lett. 98, 081901 (2011).
  9. Z. Fisk, H. R. Ott, and J. D. Thompson, “Superconducting materials: What the record tells us,” Philos. Mag. 89, 2111 (2009).

Evidence for new periodic table element boosted.

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Scientists have presented new evidence for the existence of an unconfirmed element with atomic number 115.

Th_69501617_a1500360-periodic_tablee element is highly radioactive and exists for less than a second before decaying into lighter atoms.

First proposed by Russian scientists in 2004, the super-heavy element has yet to be verified by the governing body of chemistry and physics.

The new evidence by a Swedish team is published in the journal Physical Review Letters.

“This was a very successful experiment and is one of the most important in the field in recent years”, said Dirk Rudolph, professor at the division of atomic physics at Lund University, who led the research.

After the discovery of element 115, independent confirmation to measure the exact proton number was required, Prof Rudolph told BBC News.

He said the finding “goes beyond the standard measurement” which had been observed previously. A new isotope of a potential new element was produced, which transformed into other particles via a radioactive process named alpha decay.

The researchers also gained access to data that they say gives them a deeper insight into the structure and properties of super-heavy atomic nuclei.

The team bombarded a thin film of the element americium with calcium ions, which allowed them to measure photons in connection with the new element’s alpha decay.

Certain energies of the photons (light particles) agreed with the expected energies for X-ray radiation, which acts as a “fingerprint” of a given element.

The experiment was conducted at the GSI research facility in Germany, where scientists have previously discovered six other new elements.

The potential new element will now be reviewed by a committee which consists of members of the international unions of pure and applied physics and chemistry.

They will decide whether to recommend further experiments before the discovery of the new element is acknowledged.