The 'missing link' in conducting molecules, butadiene -- solved Researchers have solved a long-standing puzzle in the dynamics of the smallest linear polyene, butadiene

Linear polyenes are hydrocarbon chains with unusual optical and electrical properties. They have become a paradigm for studying photoisomerization -- when molecular structures rearrange from absorbing light -- because of their straightforward molecular structure, potential for electrical conductivity, and role in vision. Understanding how these molecules simultaneously rearrange through photoisomerization could advance materials science research by enabling artificial vision and producing wires from plastic, and new photovoltaic technologies.

Trans 1,3-butadiene, the smallest polyene, has challenged researchers over the past 40 years because of its complex excited-state electronic structure and its ultrafast (femtosecond, 10-15 s) dynamics. Butadiene remains the "missing link" between ethylene (C2H4,), which has only one double bond, and longer linear polyenes with three or more double bonds.

Now, an experimental team headed by Albert Stolow at the University of Ottawa and the National Research Council of Canada has solved trans 1,3-butadiene's electronic-structural dynamics. The researchers recently reported their findings in The Journal of Chemical Physics, from AIP Publishing.

Stolow's group developed an ultrafast laser spectroscopy called time-resolved photoelectron-photoion coincidence spectroscopy (TRPEPICO) to conduct this research. The method involves a femtosecond pump-probe process wherein an emitted photoelectron is measured as a function of time. The photoelectron spectrum and angular distribution is sensitive to the electronic and structural dynamics of molecules. Over the past 20 years, Stolow has applied his method to a broad range of problems, including the ultraviolet stability of DNA bases and intramolecular proton transfer.

"We've shown over many years that our approach works and have provided lots of examples," Stolow said. He previously studied under John C. Polanyi and Yuan T. Lee, two Nobel Prize winners who researched molecular collision dynamics.

"Many of us thought that if we could understand ethylene, the basic building block, we would be able to understand the longer linear polyenes," Stolow said. "But butadiene is the 'missing link.' It didn't seem to behave like either case."

Stolow's team discovered that trans 1,3-butadiene behaves, simultaneously, like both ethylene and longer polyenes. Specifically, there is an ultrafast competition between ethylenelike dynamics and polyenelike dynamics.

The research team's experimental results were independently modeled and confirmed computationally by Todd J. Martínez's research team. Martinez is a researcher and professor of chemistry at Stanford University, who specializes in molecular quantum dynamics. Michael S. Schuurman of the NRC, a theorist specializing in quantum dynamics, also helped confirm this work.

"This collaboration is key. We each independently came up with the same results," Stolow said. "Dramatic technical advances in both experiment and theory have allowed us to finally solve the long-standing puzzle of electronic dynamics in butadiene, the 'missing link' of polyene photophysics."

The elusive quantum mechanical phenomenon of entanglement has now been made a reality in objects almost macroscopic in size

This is an illustration of the 15-micrometre-wide drumheads prepared on silicon chips used in the experiment. The drumheads vibrate at a high ultrasound frequency, and the peculiar quantum state predicted by Einstein was created from the vibrations.

Perhaps the strangest prediction of quantum theory is entanglement, a phenomenon whereby two distant objects become intertwined in a manner that defies both classical physics and a "common-sense" understanding of reality. In 1935, Albert Einstein expressed his concern over this concept, referring to it as "spooky action at a distance."

Nowadays, entanglement is considered a cornerstone of quantum mechanics, and it is the key resource for a host of potentially transformative quantum technologies. Entanglement is, however, extremely fragile, and it has previously been observed only in microscopic systems such as light or atoms, and recently in superconducting electric circuits.

In work recently published in Nature, a team led by Prof. Mika Sillanpää at Aalto University in Finland has shown that entanglement of massive objects can be generated and detected.

The researchers managed to bring the motions of two individual vibrating drumheads -- fabricated from metallic aluminium on a silicon chip -- into an entangled quantum state. The objects in the experiment are truly massive and macroscopic compared to the atomic scale: the circular drumheads have a diametre similar to the width of a thin human hair.

The team also included scientists from the University of New South Wales Canberra in Australia, the University of Chicago, and the University of Jyväskylä in Finland. The approach taken in the experiment was based on a theoretical innovation developed by Dr. Matt Woolley at UNSW and Prof. Aashish Clerk, now at the University of Chicago.

'The vibrating bodies are made to interact via a superconducting microwave circuit. The electromagnetic fields in the circuit are used to absorb all thermal disturbances and to leave behind only the quantum mechanical vibrations,' says Mika Sillanpää, describing the experimental setup.

Eliminating all forms of noise is crucial for the experiments, which is why they have to be conducted at extremely low temperatures near absolute zero, at -273 °C. Remarkably, the experimental approach allows the unusual state of entanglement to persist for long periods of time, in this case up to half an hour.

'These measurements are challenging but extremely fascinating. In the future, we will attempt to teleport the mechanical vibrations. In quantum teleportation, properties of physical bodies can be transmitted across arbitrary distances using the channel of "spooky action at a distance",' explains Dr. Caspar Ockeloen-Korppi, the lead author on the work, who also performed the measurements.

The results demonstrate that it is now possible to have control over large mechanical objects in which exotic quantum states can be generated and stabilized. Not only does this achievement open doors for new kinds of quantum technologies and sensors, it can also enable studies of fundamental physics in, for example, the poorly understood interplay of gravity and quantum mechanics.

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A new material created by Oregon State University researchers is a key step toward the next generation of supercomputers.

Those "quantum computers" will be able to solve problems well beyond the reach of existing computers while working much faster and consuming vastly less energy.

Researchers in OSU's College of Science have developed an inorganic compound that adopts a crystal structure capable of sustaining a new state of matter known as quantum spin liquid, an important advance toward quantum computing.

In the new compound, lithium osmium oxide, osmium atoms form a honeycomb-like lattice, enforcing a phenomenon called "magnetic frustration" that could lead to quantum spin liquid as predicted by condensed matter physics theorists.

Corresponding author Mas Subramanian, Milton Harris Professor of Materials Science at OSU, explains that in a permanent magnet like a compass needle, the electrons spin in an aligned manner -- that is, they all rotate in the same direction.

"But in a frustrated magnet, the atomic arrangement is such that the electron spins cannot achieve an ordered alignment and instead are in a constantly fluctuating state, analogous to how ions would appear in a liquid," Subramanian said.

The lithium osmium oxide discovered at OSU shows no evidence for magnetic order even when frozen to nearly absolute zero, which suggests an underlying quantum spin liquid state is possible for the compound, he said.

"We are excited about this new development as it widens the search area for new quantum spin liquid materials that could revolutionize the way we process and store data," Subramanian said. "The quantum spin liquid phenomenon has so far been detected in very few inorganic materials, some containing iridium. Osmium is right next to iridium in the periodic table and has all the right characteristics to form compounds that can sustain the quantum spin liquid state."

Arthur Ramirez, condensed matter physicist at the University of California, Santa Cruz, one of the co-authors in the paper, noted that this compound is the first honeycomb-structured material to contain osmium and expects more to follow.

Ramirez also noted that this study demonstrates the importance of multidisciplinary collaboration involving materials chemists and condensed matter physicists engaged in synthesis, theory and measurements to tackle emerging science like quantum spin liquid.

The next step for Subramanian's team is exploring the chemistry needed to create various perfectly ordered crystal structures with osmium.

The National Science Foundation is funding the research through its DMREF program: Designing Materials to Revolutionize and Engineer our Future. Findings were published today in Scientific Reports.

The concept of quantum computing is based on the ability of subatomic particles to exist in more than one state at any time.

Classical computing relies on bits -- pieces of information that exist in one of two states, a 0 or a 1. In quantum computing, information is translated to quantum bits, or qubits, that can store much more information than a 0 or 1 because they can be in any "superposition" of those values.

Think of bits and qubits by visualizing a sphere. A bit can only be at either of the two poles on the sphere, whereas a qubit can be anywhere on the sphere. What that means is much more information storage potential and much less energy consumption.

A new material created by Oregon State University researchers is a key step toward the next generation of supercomputers.

A new source of intense terahertz (THz) radiation, which could offer a less harmful alternative to x-rays and has strong potential for use in industry, is being developed by scientists at the University of Strathclyde and Capital Normal University in Beijing.

Unlike visible light, THz radiation penetrates materials such as plastic, cardboard, wood and composite materials, making it an excellent replacement for harmful X-rays used in imaging, and security.

Although it is well known that THz electromagnetic waves can carry ultra-high bandwidth communications, far exceeding those of Wi-Fi, it is less well-known that it is a highly useful probe for detecting molecules and analysing semiconductors.

A research team led by Professor Dino Jaroszynski, of Strathclyde's Department of Physics, has shown experimentally that unprecedentedly high-charge bunches of relativistic electrons can be produced by a laser wakefield accelerator (LWFA). These are produced in addition to the usual high-energy, low-charge beams that are emitted.

The team showed that when an intense ultra-short laser pulse is focussed into helium gas, a plasma bubble moving at the nearly the speed of light is formed. These high-charge beams of electrons are distinct from the usual low-charge (picocoloumb), high-energy (100s MeV to GeV), femtosecond duration electron bunches that are commonly observed from the LWFA.

The research has been published in the New Journal of Physics.

Professor Jaroszynski, Director of the Scottish Centre for the Application Plasma-based Accelerators (SCAPA), who initiated the project, said: "This is an unprecedented efficiency at these THz energies. The increasing availability of intense THz sources will lead to completely new avenues in science and technology.

"New tools for scientists lead to new advances. The interaction of intense THz radiation with matter allows access to nonlinear processes, which enables the identification of normally hidden phenomena, and also unique control of matter, such as aligning molecules using high THz fields or distorting band structure in semiconductors.

"SCAPA provides an ideal environment for investigating these phenomena, which should lead to new advances in science. Our theoretical studies are the first steps in this exciting new direction."

Dr Enrico Brunetti, of Strathclyde's Department of Physics, carried out most of the simulations in the research. He said: "Since the charge of wide-angle beams increases linearly with laser intensity and plasma density, the energy of THz radiation will scale to milijoule-levels, which would make an intense source of THz radiation with peak powers in excess of GW, which is comparable with that of a far-infrared free-electron laser. An optical to terahertz conversion efficiency of the order of 1% can be reached."

Dr Xue Yang, a researcher in the project from Capital Normal University, said: "When electrons cross an interface between two media of different dielectric constant, transition radiation is emitted over a wide range of frequencies.

"Simulations show that wide-angle electron beams emitted by laser-wakefield accelerators can produce coherent terahertz radiation with 10s ?J to 100s ?J energy when passed through a thin metal foil or at the plasma-vacuum boundary of the accelerator."

THz radiation is far-infrared electromagnetic radiation that has a frequency between 0.1 THz and 10 THz (1 THz = 10^12 Hz), which fits between the mid-infrared and microwave spectra. The vibrational and rotational spectral fingerprints of large molecules coincide with the THz band, which makes THz spectroscopy a powerful tool for identifying hazardous substances, such as drugs and explosives. Moreover, THz radiation is important for biology and medicine because many biological macromolecules, such as DNA and proteins, have their collective motion at THz frequencies.

THz radiation can also be used to uncover the intricacies of semiconductors and nanostructures, and therefore are important tools for developing new electro-mechanical devices and solar cells.

Many different methods of generating THz radiation exist, including driving photocurrents in semiconductor antennas, excitation of quantum wells and optical rectification in electro-optic crystals. However, their maximum power is restricted because of damage to the optical materials at high powers. Plasma, in contrast, has no such limitation, as it is already broken

The new research shows that these high-charge -- nanocoloumb-, and relatively low energy (MeV), sub-picosecond duration electron bunches are emitted in a hollow cone with an opening angle of nearly 45 degrees to the laser beam axis. The researchers show that laser energy can be efficiently transferred to a very intense pulse of THz radiation.

The study was funded by the Engineering and Physical Sciences Research Council.

Topology meets superconductivity through innovative reverse-order sample preparation

A groundbreaking sample preparation technique has enabled researchers at the University of Illinois at Urbana-Champaign and the University of Tokyo to perform the most controlled and sensitive study to date of a topological insulator (TI) closely coupled to a superconductor (SC). The scientists observed the superconducting proximity effect -- induced superconductivity in the TI due to its proximity to the SC -- and measured its relationship to temperature and the thickness of the TI.

TIs with induced superconductivity are of paramount interest to physicists because they have the potential to host exotic physical phenomena, including the elusive Majorana fermion -- an elementary particle theorized to be its own antiparticle -- and to exhibit supersymmetry -- a phenomenon reaching beyond the standard model that would shed light on many outstanding problems in physics. Superconducting TIs also hold tremendous promise for technological applications, including topological quantum computation and spintronics.

Naturally occurring topological superconductors are rare, and those that have been investigated have exhibited extremely small superconducting gaps and very low transition temperatures, limiting their usefulness for uncovering the interesting physical properties and behaviors that have been theorized.

TIs have been used in engineering superconducting topological superconductors (TI/SC), by growing TIs on a superconducting substrate. Since their experimental discovery in 2007, TIs have intrigued condensed matter physicists, and a flurry of theoretical and experimental research taking place around the globe has explored the quantum-mechanical properties of this extraordinary class of materials. These 2D and 3D materials are insulating in their bulk, but conduct electricity on their edges or outer surfaces via special surface electronic states which are topologically protected, meaning they can't be easily destroyed by impurities or imperfections in the material.

But engineering such TI/SC systems via growing TI thin films on superconducting substrates has also proven challenging, given several obstacles, including lattice structure mismatch, chemical reactions and structural defects at the interface, and other as-yet poorly understood factors.

Now, a novel sample-growing technique developed at the U. of I. has overcome these obstacles. Developed by physics professor James Eckstein in collaboration with physics professor Tai-Chang Chiang, the new "flip-chip" TI/SC sample-growing technique allowed the scientists to produce layered thin-films of the well-studied TI bismuth selenide on top of the prototypical SC niobium -- despite their incompatible crystalline lattice structures and the highly reactive nature of niobium.

These two materials taken together are ideal for probing fundamental aspects of the TI/SC physics, according to Chiang: "This is arguably the simplest example of a TI/SC in terms of the electronic and chemical structures. And the SC we used has the highest transition temperature among all elements in the periodic table, which makes the physics more accessible. This is really ideal; it provides a simpler, more accessible basis for exploring the basics of topological superconductivity," Chiang comments.

The method allows for very precise control over sample thickness, and the scientists looked at a range of 3 to 10 TI layers, with 5 atomic layers per TI layer. The team's measurements showed that the proximity effect induces superconductivity into both the bulk states and the topological surface states of the TI films. Chiang stresses, what they saw gives new insights into superconducting pairing of the spin-polarized topological surface states.

"The results of this research are unambiguous. We see the signal clearly," Chiang sums up. "We investigated the superconducting gap as a function of TI film thickness and also as a function of temperature. The results are pretty simple: the gap disappears as you go above niobium's transition temperature. That's good -- it's simple. It shows the physics works. More interesting is the dependence on the thickness of the film. Not surprisingly, we see the superconducting gap reduces for increasing TI film thickness, but the reduction is surprisingly slow. This observation raises an intriguing question regarding how the pairing at the film surface is induced by coupling at the interface."

Chiang credits Eckstein with developing the ingenious sample preparation method. It involves assembling the sample in reverse order, on top of a sacrificial substrate of aluminum oxide, commonly known as the mineral sapphire. The scientists are able to control the specific number of layers of TI crystals grown, each of quintuple atomic thickness. Then a polycrystalline superconducting layer of niobium is sputter-deposited on top of the TI film. The sample is then flipped over and the sacrificial layer that had served as the substrate is dislodged by striking a "cleavage pin." The layers are cleaved precisely at the interface of the TI and aluminum oxide.

Eckstein explains, "The 'flip-chip' technique works because the layers aren't strongly bonded -- they are like a stack of paper, where there is strength in the stack, but you can pull apart the layers easily. Here, we have a triangular lattice of atoms, which comes in packages of five -- these layers are strongly bonded. The next five layers sit on top, but are weakly bonded to the first five. It turns out, the weakest link is right at the substrate-TI interface. When cleaved, this method gives a pure surface, with no contamination from air exposure."

The cleavage was performed in an ultrahigh vacuum, within a highly sensitive instrument at the Institute for Solid State Physics at the University of Tokyo capable of angle-resolved photoemission spectroscopy (ARPES) at a range of temperatures.

Chiang acknowledges, "The superconducting features occur at very small energy scales -- it requires a very high energy resolution and very low temperatures. This portion of the experiment was completed by our colleagues in the University of Tokyo, where they have the instruments with the sensitivity to get the resolution we need for this kind of study. We couldn't have done this without this international collaboration."

"This new sample preparation method opens up many new avenues in research, in terms of exotic physics, and, in the long term, in terms of possible useful applications -- potentially even including building a better superconductor. It will allow preparation of samples using a wide range of other TIs and SCs. It could also be useful in miniaturization of electronic devices, and in spintronic computing, which would require less energy in terms of heat dissipation," Chiang concludes.

Eckstein adds, "There is a lot of excitement about this. If we can make a superconducting TI, theoretical predictions tell us that we could find a new elementary excitation that would make an ideal topological quantum bit, or qubit. We're not there yet, and there are still many things to worry about. But it would be a qubit whose quantum mechanical wave function would be less susceptible to local perturbations that might cause dephasing, messing up calculations."

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Physics Nobel won for invention of blue LEDs

It may be going too far to say this year’s Nobel prize in physics will save the world – but it will certainly make it more efficient.

The prize has gone to three semiconductor physicists who invented the blue LED (light emitting diode). Their invention has transformed the way we light our world, watch movies and store data.

Isamu Akasaki and Hiroshi Amano at Nagoya University in Japan, and Shuji Nakamura at the University of California in Santa Barbara, share the prize of 8 million Swedish kronor (£0.7 million).

LEDs as we know them were invented in the late 1950s, but only came in one colour: red. Green followed later, but although both were fine for indicator lights and digital watches, researchers really wanted a white LED. These are replacing traditional incandescent bulbs, which lose much of their energy as heat.

“Lighting currently consumes 20 per cent of all electricity,” says Colin Humphreys at the University of Cambridge. “Switching to LEDs could save 50 per cent, or over £2 billion in the UK alone.”

Colour addition

The key to producing white light was to make a blue LED and shine its light through a thin layer of yellow-emitting phosphor. This combination of yellow and blue makes white light much more efficiently than incandescent bulbs.

But blue LEDs had stymied researchers until the Nobel trio made their breakthrough in the late 1980s. The reason is to do with the way the devices are made. LEDs are made from multiple layers of semiconductor sandwiched together, one with an excess of electrons and the next with an excess of positively charged holes. Applying a voltage to the device drives the electrons and holes together into the “filling layer”, where they combine to emit light. The colour of this light depends on the semiconductor material. Red LEDs, for example, are made from gallium arsenide crystals.

Researchers realised that gallium nitride with a smattering of indium had the perfect quantum properties to emit blue light. But no one was able to make a semiconductor sandwich with a gallium nitride filling. That’s because the crystal structure of the gallium nitride needs to match that of the surrounding layers to prevent defects from forming that wreck the light-emitting properties.

In 1986, Akasaki and Amano succeeded in finding a match. Their trick was to add an extra layer to the sandwich. They grew their gallium nitride on top of a sapphire covered with a layer of aluminium nitride. Meanwhile Nakamura found his own way to create a gallium nitride crystal by growing a thin layer at low temperatures, then subsequent layers at higher temperatures. Nakamura had to work on his idea in secret after bosses at the Japanese company he worked for, Nichia, stopped his research because of lack of progress. Nakamura later sued Nichia and won more than $7 million compensation.

Blu movies

The trio went on to turn their blue LEDs into blue lasers, found in Blu-ray players. Because the wavelength of blue light is shorter than that of red LEDs, the beam can be focused to a small spot. This lets you cram more information on to a disc and read it out, giving Blu-rays a better picture quality than regular DVDs.

As well as saving money, LED lights can reduce pressure on Earth’s resources. Lights made from LEDs last for 100,000 hours, 10 times longer than fluorescent lamps and 100 times longer than incandescent bulbs. “The electronic circuits that control them give out before the LED,” says Humphreys.

He predicts many more advances to come.

Smart lighting that adapts to its surroundings could reduce energy bills by another 5 per cent. And by changing the phosphor coating on a blue LED it should be possible to produce lighting that mimics sunlight, which plays a key role in the human body clock. “This could be the end of jet lag and could improve the health of shift workers, who have been shown to have a higher risk of cancer.”

Akasaki, Amano and Nakamura will receive their award at the Nobel ceremony in December.