Wednesday, May 9, 2018

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."

Sunday, May 6, 2018

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.

Tiny, Lens-Free Camera Could Hide in Clothes, Glasses

A tiny, paper-thin camera that has no lens could turn conventional photography on its head, according to new research.
The device, a square that measures just 0.04 inches by 0.05 inches (1 by 1.2 millimeters), has the potential to switch its "aperture" among wide angle, fish eye and zoom instantaneously. And because the device is so thin, just a few microns thick, it could be embedded anywhere. (For comparison, the average width of a human hair is about 100 microns.)
The entire backside of your phone could be a camera," said Ali Hajimiri, a professor of electrical engineering and medical engineering at the California Institute of Technology (Caltech) and the principal investigator of the research paper, describing the new camera. [Photo Future: 7 High-Tech Ways to Share Images]
It could be embedded in a watch or in a pair of eyeglasses or in fabric, Hajimiri told Live Science. It could even be designed to launch into space as a small package and then unfurl into very large, thin sheets that image the universe at resolutions never before possible, he added.
"There's no fundamental limit on how much you could increase the resolution," Hajimiri said. "You could do gigapixels if you wanted.” (A gigapixel image has 1 billion pixels, or 1,000 times more than an image from a 1-megapixel digital camera.)
Hajimiri and his colleagues presented their innovation, called an optical phased array, at the Optical Society's (OSA) Conference on Lasers and Electro-Optics, which was held in March. The research was also published online in the OSA Technical Digest.
The proof-of-concept device is a flat sheet with an array of 64 light receivers that can be thought of as tiny antennas tuned to receive light waves, Hajimiri said. Each receiver in the array is individually controlled by a computer program.
In fractions of a second, the light receivers can be manipulated to create an image of an object on the far right side of the view or on the far left or anywhere in between. And this can be done without pointing the device at the objects, which would be necessary with a camera.
"The beauty of this thing is that we create images without any mechanical movement," he said.
Hajimiri called this feature a "synthetic aperture." To test how well it worked, the researchers laid the thin arrayover a silicon computer chip. In experiments, the synthetic aperture collected light waves, and then other components on the chip converted the light waves to electrical signals that were sent to a sensor.
The resulting image looks like a checkerboard with illuminated squares, but this basic low-resolution image is just first step, Hajimiri said. The device's ability to manipulate incoming light waves is so precise and fast that, theoretically, it could capture hundreds of different kinds of images in any kind of light, including infrared, in a matter of seconds, he said.
"You can make an extremely powerful and large camera," Hajimiri said.
Achieving a high-power view with a conventional camera requires that the lens be very big, so that it can collect enough light. This is why professional photographers on the sidelines of sporting events wield huge camera lenses.
But bigger lenses require more glass, and that can introduce light and color flaws in the image. The researchers' optical phased array doesn't have that problem, or any added bulk, Hajimiri said.
For the next stage of their research, Hajimiri and his colleagues are working to make the device larger, with more light receivers in the array.
"Essentially, there's no limit on how much you could increase the resolution," he said. "It's just a question of how large you can make the phased array."


More Than a View: Windows Double as Solar Panels

A tech startup on a mission to make modern commercial and housing estates energy neutral has outfitted the headquarters of a Dutch bank with the world's first commercial, fully transparent solar-power-generating windows.
The windows have solar cells installed in the edges at a specific angle that allows the incoming solar light to be efficiently transformed into electricity.
"Large commercial estates consume a lot of energy," said Ferdinand Grapperhaus, co-founder and CEO of the startup, called Physee. "If you want to make these buildings energy neutral, you never have enough roof surface. Therefore, activating the buildings' facades will significantly contribute to making the buildings energy neutral."
The windows could generate 8 to 10 watts of power, according to Grapperhaus.
"This enables the user to charge a phone per every square meter [11 square feet] two times a day," he told Live Science.
The first installation of Physee's PowerWindows was unveiled in June in Eindhoven, in the south of the Netherlands. The headquarters of Rabobank, the Netherlands' biggest bank, has been fitted with 323 square feet (30 square m) of the PowerWindows. The bank's employees will be able to plug their smartphones into the windows using USB ports to charge their batteries, according to Physee.
Other buildings in the Netherlands are already lined up to receive the innovative solar technology, which has won Physee a place on the World Economic Forum's Technology Pioneers 2017 list.
At the end of June, the headquarters of the Amsterdam-based charity the Postcode Lottery were fitted with the PowerWindows. After that, Physee will move forward with its first large-scale project: a 19,000-square-foot (1,800 square m) installation in a large, newly built residential complex in Amsterdam, the Bold tower.
"I believe that every new type of glass needs power," Grapperhaus said. "Either for the glass to be tinted electrically or heated or inside windows there are these solar blinds, which are electrical and can go up and down but also more and more you can see video glass."
Grapperhaus said that the cost of the wiring that brings power from the grid to such windows is considerable in large commercial estates, and investing in power-generating windows would therefore make commercial sense.
Physee is already working on the next-generation technology that would triple the efficiency of the PowerWindows. The surface of the second generation of PowerWindows will be coated with a special material that transforms oncoming visible light into near-infrared light, which is then transported toward the solar cells in the edges of the windows.
"It works similarly to a [glow-in-the-dark star]," Grapperhaus said. "The difference is that the glow star emits the green wavelength, but the coating on our windows emits light in near-infrared wavelength."
The coating is based on the rare-earth metal thulium. Grapperhaus, together with his friend Willem Kesteloo, discovered the ability of thulium to transform a broad spectrum of light into near-infrared light in 2014, during their studies at the Delft University of Technology.
"Over time, our efficiency will improve further due to the development of better solar cells but also because of the economies of scale," Grapperhaus said. "Right now, we are looking for iconic projects all over the world to show that a large glass building can be made energy neutral in an aesthetic way."
Physee was among 30 early stage technology pioneers highlighted for 2017 and selected by the World Economic Forum for their potential to change the world. The list, announced June 14, consisted of firms developing various technologies, including artificial intelligence, cybersecurity solutions and biotechnology.
Physee's presence on the list shows that the world is starting to take climate change seriously, Grapperhaus said.
"Ten years ago, sustainability was something that wasn't taken very seriously — not by venture capitalists, not by many governments and neither by large corporations," Grapperhaus said. "What I have seen over the last three years is that corporations are becoming more and more responsible, governments are becoming more and more supportive, and venture capitalists are becoming more and more interested" in sustainability.

Hybrid Driving-Flying Robots Could Go Beyond the Flying Car

Whether they're swooping in to deliver packages or spotting victims in disaster zones, swarms of flying robots could have a range of important applications in the future, a new study found. The robots can transition from driving to flying without colliding with each other and could offer benefits beyond the traditional flying-car concepts of sci-fi lore, the study said.
The ability to both fly and walk is common in nature. For instance, many birds, insects and other animals can do both.
Robots with similar versatility could fly over impediments on the ground or drive under overhead obstacles. But currently, robots that are good at one mode of transportation are usually bad at others, study lead author Brandon Araki, a roboticist at the Massachusetts Institute of Technology's Computer Science and Artificial Intelligence Laboratory, and his colleagues said in their new study. [The 6 Strangest Robots Ever Created]

The researchers previously developed a robot named the "flying monkey" that could run and fly, as well as grasp items. However, the researchers had to program the paths the flying monkey would take; in other words, it could not find safe routes by itself.
Now, these scientists have developed flying cars that can both fly and drive through a simulated city-like setting that has parking spots, landing pads and no-fly zones. Moreover, these drones can move autonomously without colliding with each other, the researchers said. "Our vehicles can find their own safe paths," Araki told Live Science.
The researchers took eight four-rotor "quadcopter" drones and put two small motors with wheels on the bottom of each drone, to make them capable of driving. In simulations, the robots could fly for about 295 feet (90 meters) or drive for 826 feet (252 meters) before their batteries ran out.
The roboticists developed algorithms that ensured the robots did not collide with one another. In tests in a miniature town made using everyday materials such as pieces of fabric for roads and cardboard boxes for buildings, all drones successfully navigated from a starting point to an ending point on collision-free paths.
Adding the driving apparatus to each drone added weight and so slightly reduced battery life, decreasing the maximum distances the drones could fly by about 14 percent, the researchers said. Still, the scientists noted that driving remained more efficient than flying, offsetting the relatively small loss in efficiency in flying due to the added weight.
"The most important implication of our research is that vehicles that combine flying and driving have the potential to be both much more efficient and much more useful than vehicles that can only drive or only fly," Araki said.
The scientists cautioned that fleets of automated flying taxis are likely not coming anytime soon. "Our current system of drones certainly isn't robust enough to actually carry people right now," Araki said. Still, these experiments with quadcopters help explore "various ideas related to flying cars," he said.
The scientists detailed their findings on June 1 at the Institute of Electrical and Electronics Engineers' International Conference on Robotics and Automation in Singapore.

Atomic Fingerprinting' Tech Could End Counterfeit Goods

In the sophisticated world of counterfeiting, it can often be difficult to tell fakes from the real deal. But now, scientists have developed a new method that can stamp things with "atomic fingerprints" to keep phony products at bay.
"There is no bigger crime than counterfeit crime," said Robert Young, a professor of physics at Lancaster University in the United Kingdom and chief technology officer of the tech startup Quantum Base. [Faux Real: A Gallery of Forgeries]
Earlier this month, Young and his colleagues announced a relatively simple technique for confirming the authenticity of an object — an advance that could put a dent in the counterfeit industry, where fakes, forgeries and imitations cost the global economy half a trillion dollars in lost revenue each year, according to the most recent data from the Organization for Economic Co-operation and Development, headquartered in Paris. 

The unique pattern is created by intentionally fabricating flaws into an atom-thin layer of material, such as graphene oxide. Flaws may include removing a carbon atom, or adding extra oxygen atoms, or creating a ridge of atoms, according to the researchers. Once the flaw is set, the material is incorporated into an ink and then, using an inkjet printer, printed onto a hologram, which can be added as a label to any product.
To confirm the presence of the atomic pattern, a person would use a smartphone camera and its built-in flash to photograph the label. The flash excites the atoms, which produce a unique color based on the pattern. A corresponding app can instantly analyze the image and confirm whether the label is authentic or not, the researchers said.
"I'm really satisfied by how simple it is," Young told Live Science.
Solving such an extensive problem like counterfeiting requires a solution that can be adopted by a large number of people, Young added. A technique that's easy to incorporate and easy to analyze could ensure that it's widely adopted much faster, he said.
Young and his team are working with a company that prints 10 billion holograms per year and said that the first application could be in the automotive industry, where parts are already spray-painted with labels. By piggybacking onto existing manufacturing applications, the researchers can prove that the method works, according to Young.
"We're expecting the first products in market in the first quarter of next year, in 2018," he said.
From there, the researchers would like to branch out to other industries, including pharmaceuticals, where $200 billion a year is lost from counterfeit drugs, Young said. And what's worse, this illegal medicine can sometimes lead to death.
"Thirty percent of counterfeit pharmaceuticals don't contain the correct active ingredient," Young said. "People buy these things, believe they're real, but they're not being treated for the disease."
Young said that eventually, the atomic fingerprints his team has developed could be laminated directly onto individual pills.
"This is genuinely a really exciting application," he said.

Into the brave new age of irrationality

Much has been written and said about the assault on liberal arts under way in India since the new political era dawned. But the real assault is on science and rationality. And it has not been difficult to mount this attack.
For long, India has as a nation proudly claimed to be a society of belief. And Indians like to assert that faith is a ‘way of life’ here. Terms such as modernity, rational thinking and scientific analysis are often frowned upon, and misdiagnosed as disrespect to Indian culture.

Freshly minted spokesmodel

In recent years, we have entered a new era. I call it the Era of Irrationality. The new Chief Minister of Tripura, Biplab Kumar Deb, is the freshly minted spokesmodel of this bold, new era.
There appears to be a relay race among people in public positions, each one making an astonishingly ridiculous claim and then passing on the baton. Mr. Deb’s claim that the Internet existed in the times of the Mahabharata is the latest. But there have been several other persons before that: Ganesh was the first example of plastic surgery, Darwin’s theory of evolution is hokum because nobody has seen monkeys turning into humans, and that Stephen Hawking had said that Vedas have a theory superior to Einstein’s E = mc2.
Such statements have made us the laughing stock of the global scientific community. But more importantly, they also undermine the significant scientific achievements we have made post-Independence.
We cannot even dismiss these as random remarks by the fringe, the babas and the sadhus. These claims are often made by public officials (it’s another matter that the babas and sadhus are now occupying several public offices). The assault on rationality is a consequence of a concerted strategy of political forces. As rational thinking thins, the same political forces fatten.
We Indians have never really adopted the scientific temper, irrespective of our education. It’s evident from our obsession with crackpot sciences such as astrology and palmistry in our daily lives. However, in the past four years, the belief in pseudo-sciences has gained a political fig leaf as have tall, unverifiable claims on science.
The cultivation of scientific temper involves asking questions and demanding empirical evidence. It has no place for blind faith. The ruling political dispensation is uncomfortable with questioning Indians. But at the same time, it also wants to come across as a dispensation that champions a 21st century modern India. Therein lies a catch-22 situation.
So, they have devised a devious strategy to invest in the culture of blind belief. They already have a willing constituency. Ludicrous statements like those mentioned above — made by leaders in positions of power with alarming frequency — go on to legitimise and boost the Era of Irrationality.
An unscientific society makes the job of an incompetent ruler a lot easier. No questions are asked; not even basic ones. The ruler has to just make a claim and the believers will worship him. Rather than conforming, a truly rational community often questions disparity, exploitation, persecution on the basis of caste, religion or gender. It demands answers and accountability for such violations, which are often based on irrational whims. Hence rationality must be on top of the casualty list followed quickly by the minorities, Dalits, women, liberals. For the ‘Irrationality project’ to succeed, the ruler needs a willing suspension of disbelief on a mass scale.

Science v. technology

The vigour with which the government is making an assault on the scientific temper only confirms that it is actually frightened of it. This is the reason why authoritarian regimes are often intolerant of those who champion the spirit of science, but encourage scientists who will launch satellites and develop nuclear weapons — even as they break coconuts, chant hymns and press “Enter” with their fingers laden with auspicious stones.
These ‘techno-scientists’ are what I call ‘the DJs of the scientific community’. And they are often the establishment’s yes-men and yes-women.
The founders of the Constitution were aware of this. Hence the words “scientific temper” and “the spirit of inquiry and reform” find place in the Constitution, along with “secular” (belatedly), “equality” and “rights”. To dismantle secularism, dilute equality and pushback rights, it is imperative to destroy a scientific temperament.
The indoctrination against the scientific temper begins very early in our lives. It starts in our families and communities where young minds are aggressively discouraged from questioning authority and asking questions. An upper caste child for example may be forced to follow customs, which among others include practising and subscribing to the age-old caste system. The same methodology is used to impose fixed gender, sexual and religious identities. As a result, we are hardwired to be casteist, majoritarian and misogynist.
The final step in the ‘Irrationality project’ is to inject with regularity, preposterous, over-the-top claims about the nation’s past. It effectively blurs vision of the present.
The world is busy studying string theory, the god particle in a cyclotron, quantum mechanics. But we are busy expanding our chest size with claims of a fantastic yore.


Finding funds: On COP28 and the ‘loss and damage’ fund....

A healthy loss and damage (L&D) fund, a three-decade-old demand, is a fundamental expression of climate justice. The L&D fund is a c...