If we are to see the promised benefits of high-temperature superconductors, such as low-loss motors and generators or maglev trains, we will need superconductors that can carry very large currents. Decades-old experiments have shown that the current density a superconductor can carry may be increased by bombarding the material with high-energy ion beams. But to engineer such materials on a large scale, we need to know what’s actually happening in the lab samples. The emerging technology of the scanning tunneling microscope (STM) has finally provided a picture at the atomic level that may lead to a theory to guide future engineers. Researchers at Cornell and Brookhaven and Argonne national laboratories have found that irradiation of the material creates nanometer-sized defects that trap swirling eddies in the flow of electrons, keeping them out of the way so more current can flow. They reported their discovery in the May 22 issue of the journal ("Imaging atomic-scale effects of high-energy ion irradiation on superconductivity and vortex pinning in Fe(Se,Te)"). “No previous experiment has ever imaged the effects of such damage on the atomic scale electronic structure of any material,” said J.C. Séamus Davis, the James Gilbert White Distinguished Professor in the Physical Sciences and director of the Center for Emergent Superconductivity at Brookhaven. “It is an eye-opener that for the first time we can see what happens.” The STM scans a sample with a probe so sharp that its tip is a single atom, moving across a surface in steps smaller that the diameter of an atom. Measuring current flow between the tip and the surface as the probe is moved up and down gives a topographic map of the surface, while varying the voltage reveals the energy of electrons under the probe, in effect measuring how much positive charge is needed to pull an electron loose. The specially built STM allows scanning at cryonic temperatures and under a high magnetic field such as might be found in motors and other practical devices. The researchers scanned a sample of a compound of iron, selenium and tellurium that becomes a superconductor at 19 degrees above absolute zero – and in some forms much higher – that had been irradiated by intense beams of gold ions from a particle accelerator – the only way to generate a beam powerful enough to punch through a solid material. In a magnetic field, moving electrons are pushed off at right angles to their path. Electrons carrying current in a superconductor can be thought of as a fluid, and the magnetic field causes bunches of electrons to turn sideways and create “vortices” – eddies in the river of electrons – that block current-carrying electrons. As expected, the STM scan found “columnar defects” – holes a few nanometers in diameter punched down through the crystal lattice by the gold projectiles. And in those columns they found the characteristic signature of vortices, the many swirling electrons producing bright spots on the STM images. They also found small defects scattered around the large columns, apparently created by what amounted to flying debris from the ion strike, and these too trapped a few vortices. “As long as the vortices are fixed in a location [where] they can't cause havoc,” Davis said. Understanding the geometry of these defects, the researchers suggest, could lead to engineering new materials with such geometry built in.
read more "Trapping vortices key to high-current superconductors"
Nanospiked bacteria are the brightest hard X-ray emitters
In a step that overturns traditional assumptions and practice, researchers at the Tata Institute of Fundamental Research, Mumbai and Institute for Plasma Research, Gandhi Nagar have fashioned bacteria to emit intense, hard x-ray radiation ("Enhanced x-ray emission from nano-particle doped bacteria"). 
This image shows a 10,000 fold enhanced X-ray emission from nanoparticle doped bacteria, from plasma generated by intense ultra short infrared pulses. (Image: Tata Institute of Fundamental Research) When one thinks of hard x-rays and bacteria it is usually that the bacteria are at the receiving end of the x-ray source - being imaged, irradiated for some modification or simply assessed for radiation damage. One hardly thinks of using bacteria as a source of x-rays, far from turning them into the brightest among such sources. The experiment consists of a femtosecond, infrared, high intensity laser irradiating a glass slide coated with E. coli bacterial cells, turning the cell material into a hot, dense plasma. Laser driven plasmas have been known to be very useful table top x-ray sources and efforts are constantly being made to improve their brightness. One such effort, an important one, has been to create plasmas on a nanostructured surface where the nanostructure amplifies the incident intensity by electromagnetic local field enhancement. The present advance has been made possible by the insight the researchers had when they realized that natural micro and nanostructures in the bacteria can be readily used for such intensity enhancement leading to hotter, brighter plasma. They showed that the bacterial cells increased the x-ray flux by a factor of 100 in the 50 - 300 keV x-ray region. Further they grow the bacterial cells in a silver chloride solution whereby the silver atoms aggregated as nanoparticles inside the cell. They could then use these bacteria spiked with nanoparticles to boost the emission another 100 times, leading to an overall enhancement of 10,000 times from the flux emitted by plain glass slides without the bacterial coating. This is the highest conversion of laser light to hard x-rays ever achieved. This lateral stride could potentially lead to biologically inspired plasma physics and high energy density science with myriad applications among novel particle sources, creation of extreme excited states and related areas.
read more "Nanospiked bacteria are the brightest hard X-ray emitters"
This image shows a 10,000 fold enhanced X-ray emission from nanoparticle doped bacteria, from plasma generated by intense ultra short infrared pulses. (Image: Tata Institute of Fundamental Research) When one thinks of hard x-rays and bacteria it is usually that the bacteria are at the receiving end of the x-ray source - being imaged, irradiated for some modification or simply assessed for radiation damage. One hardly thinks of using bacteria as a source of x-rays, far from turning them into the brightest among such sources. The experiment consists of a femtosecond, infrared, high intensity laser irradiating a glass slide coated with E. coli bacterial cells, turning the cell material into a hot, dense plasma. Laser driven plasmas have been known to be very useful table top x-ray sources and efforts are constantly being made to improve their brightness. One such effort, an important one, has been to create plasmas on a nanostructured surface where the nanostructure amplifies the incident intensity by electromagnetic local field enhancement. The present advance has been made possible by the insight the researchers had when they realized that natural micro and nanostructures in the bacteria can be readily used for such intensity enhancement leading to hotter, brighter plasma. They showed that the bacterial cells increased the x-ray flux by a factor of 100 in the 50 - 300 keV x-ray region. Further they grow the bacterial cells in a silver chloride solution whereby the silver atoms aggregated as nanoparticles inside the cell. They could then use these bacteria spiked with nanoparticles to boost the emission another 100 times, leading to an overall enhancement of 10,000 times from the flux emitted by plain glass slides without the bacterial coating. This is the highest conversion of laser light to hard x-rays ever achieved. This lateral stride could potentially lead to biologically inspired plasma physics and high energy density science with myriad applications among novel particle sources, creation of extreme excited states and related areas.
Chemists design a quantum-dot spectrometer small enough for a smartphone
Instruments that measure the properties of light, known as spectrometers, are widely used in physical, chemical, and biological research. These devices are usually too large to be portable, but MIT scientists have now shown they can create spectrometers small enough to fit inside a smartphone camera, using tiny semiconductor nanoparticles called quantum dots. Such devices could be used to diagnose diseases, especially skin conditions, or to detect environmental pollutants and food conditions, says Jie Bao, a former MIT postdoc and the lead author of a paper describing the quantum dot spectrometers in the July 2 issue of ("A colloidal quantum dot spectrometer"). 
In this illustration, the Quantum Dot (QD) spectrometer device is printing QD filters — a key fabrication step. Other spectrometer approaches have complicated systems in order to create the optical structures needed. Here in the QD spectrometer approach, the optical structure — QD filters — are generated by printing liquid droplets. This approach is unique and advantageous in terms of flexibility, simplicity, and cost reduction. (Image: Mary O’Reilly) This work also represents a new application for quantum dots, which have been used primarily for labeling cells and biological molecules, as well as in computer and television screens. “Using quantum dots for spectrometers is such a straightforward application compared to everything else that we’ve tried to do, and I think that’s very appealing,” says Moungi Bawendi, the Lester Wolfe Professor of Chemistry at MIT and the paper’s senior author. Shrinking spectrometers The earliest spectrometers consisted of prisms that separate light into its constituent wavelengths, while current models use optical equipment such as diffraction gratings to achieve the same effect. Spectrometers are used in a wide variety of applications, such as studying atomic processes and energy levels in physics, or analyzing tissue samples for biomedical research and diagnostics. Replacing that bulky optical equipment with quantum dots allowed the MIT team to shrink spectrometers to about the size of a U.S. quarter, and to take advantage of some of the inherent useful properties of quantum dots. Quantum dots, a type of nanocrystals discovered in the early 1980s, are made by combining metals such as lead or cadmium with other elements including sulfur, selenium, or arsenic. By controlling the ratio of these starting materials, the temperature, and the reaction time, scientists can generate a nearly unlimited number of dots with differences in an electronic property known as bandgap, which determines the wavelengths of light that each dot will absorb. However, most of the existing applications for quantum dots don’t take advantage of this huge range of light absorbance. Instead, most applications, such as labeling cells or new types of TV screens, exploit quantum dots’ fluorescence — a property that is much more difficult to control, Bawendi says. “It’s very hard to make something that fluoresces very brightly,” he says. “You’ve got to protect the dots, you’ve got to do all this engineering.” Scientists are also working on solar cells based on quantum dots, which rely on the dots’ ability to convert light into electrons. However, this phenomenon is not well understood, and is difficult to manipulate. On the other hand, quantum dots’ absorption properties are well known and very stable. “If we can rely on these properties, it is possible to create applications that will have a greater impact in the relative short term,” Bao says. Broad spectrum The new quantum dot spectrometer deploys hundreds of quantum dot materials that each filter a specific set of wavelengths of light. The quantum dot filters are printed into a thin film and placed on top of a photodetector such as the charge-coupled devices (CCDs) found in cellphone cameras. The researchers created an algorithm that analyzes the percentage of photons absorbed by each filter, then recombines the information from each one to calculate the intensity and wavelength of the original rays of light. The more quantum dot materials there are, the more wavelengths can be covered and the higher resolution can be obtained. In this case, the researchers used about 200 types of quantum dots spread over a range of about 300 nanometers. With more dots, such spectrometers could be designed to cover an even wider range of light frequencies. “Bawendi and Bao showed a beautiful way to exploit the controlled optical absorption of semiconductor quantum dots for miniature spectrometers. They demonstrate a spectrometer that is not only small, but also with high throughput and high spectral resolution, which has never been achieved before,” says Feng Wang, an associate professor of physics at the University of California at Berkeley who was not involved in the research. If incorporated into small handheld devices, this type of spectrometer could be used to diagnose skin conditions or analyze urine samples, Bao says. They could also be used to track vital signs such as pulse and oxygen level, or to measure exposure to different frequencies of ultraviolet light, which vary greatly in their ability to damage skin. “The central component of such spectrometers — the quantum dot filter array — is fabricated with solution-based processing and printing, thus enabling significant potential cost reduction,” Bao adds.
read more "Chemists design a quantum-dot spectrometer small enough for a smartphone"
In this illustration, the Quantum Dot (QD) spectrometer device is printing QD filters — a key fabrication step. Other spectrometer approaches have complicated systems in order to create the optical structures needed. Here in the QD spectrometer approach, the optical structure — QD filters — are generated by printing liquid droplets. This approach is unique and advantageous in terms of flexibility, simplicity, and cost reduction. (Image: Mary O’Reilly) This work also represents a new application for quantum dots, which have been used primarily for labeling cells and biological molecules, as well as in computer and television screens. “Using quantum dots for spectrometers is such a straightforward application compared to everything else that we’ve tried to do, and I think that’s very appealing,” says Moungi Bawendi, the Lester Wolfe Professor of Chemistry at MIT and the paper’s senior author. Shrinking spectrometers The earliest spectrometers consisted of prisms that separate light into its constituent wavelengths, while current models use optical equipment such as diffraction gratings to achieve the same effect. Spectrometers are used in a wide variety of applications, such as studying atomic processes and energy levels in physics, or analyzing tissue samples for biomedical research and diagnostics. Replacing that bulky optical equipment with quantum dots allowed the MIT team to shrink spectrometers to about the size of a U.S. quarter, and to take advantage of some of the inherent useful properties of quantum dots. Quantum dots, a type of nanocrystals discovered in the early 1980s, are made by combining metals such as lead or cadmium with other elements including sulfur, selenium, or arsenic. By controlling the ratio of these starting materials, the temperature, and the reaction time, scientists can generate a nearly unlimited number of dots with differences in an electronic property known as bandgap, which determines the wavelengths of light that each dot will absorb. However, most of the existing applications for quantum dots don’t take advantage of this huge range of light absorbance. Instead, most applications, such as labeling cells or new types of TV screens, exploit quantum dots’ fluorescence — a property that is much more difficult to control, Bawendi says. “It’s very hard to make something that fluoresces very brightly,” he says. “You’ve got to protect the dots, you’ve got to do all this engineering.” Scientists are also working on solar cells based on quantum dots, which rely on the dots’ ability to convert light into electrons. However, this phenomenon is not well understood, and is difficult to manipulate. On the other hand, quantum dots’ absorption properties are well known and very stable. “If we can rely on these properties, it is possible to create applications that will have a greater impact in the relative short term,” Bao says. Broad spectrum The new quantum dot spectrometer deploys hundreds of quantum dot materials that each filter a specific set of wavelengths of light. The quantum dot filters are printed into a thin film and placed on top of a photodetector such as the charge-coupled devices (CCDs) found in cellphone cameras. The researchers created an algorithm that analyzes the percentage of photons absorbed by each filter, then recombines the information from each one to calculate the intensity and wavelength of the original rays of light. The more quantum dot materials there are, the more wavelengths can be covered and the higher resolution can be obtained. In this case, the researchers used about 200 types of quantum dots spread over a range of about 300 nanometers. With more dots, such spectrometers could be designed to cover an even wider range of light frequencies. “Bawendi and Bao showed a beautiful way to exploit the controlled optical absorption of semiconductor quantum dots for miniature spectrometers. They demonstrate a spectrometer that is not only small, but also with high throughput and high spectral resolution, which has never been achieved before,” says Feng Wang, an associate professor of physics at the University of California at Berkeley who was not involved in the research. If incorporated into small handheld devices, this type of spectrometer could be used to diagnose skin conditions or analyze urine samples, Bao says. They could also be used to track vital signs such as pulse and oxygen level, or to measure exposure to different frequencies of ultraviolet light, which vary greatly in their ability to damage skin. “The central component of such spectrometers — the quantum dot filter array — is fabricated with solution-based processing and printing, thus enabling significant potential cost reduction,” Bao adds.
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