2D MXenes...

Helper two-dimensional metal-carbide layers could improve perovskite solar cell stability and help make these complex solar cells a viable green energy option. Credit: iStock Milos-Muller

Topics: Condensed Matter Physics, Green Tech, Materials Science, Metamaterials, Nanotechnology, Solar Power

With the reality of climate change looming, the importance of realistic green energy sources is higher than ever. Solar cells are one promising avenue, as they can convert readily available visible and ultraviolet energy into usable electricity. In particular, perovskite materials sandwiched between other support layers have demonstrated impressive power conversion efficiencies. Current challenges reside in optimizing perovskite/support layer interfaces, which can directly impact power conversion and cell degradation. Researchers Antonio Agresti et al. under the direction of Aldo Di Carlo at the University of Rome Tor Vergata in Italy have investigated how cells containing two-dimensional titanium-carbide MXene support layers could improve perovskite solar cell performance.

To obtain good power conversion within a perovskite solar cell, all layers and layer interfaces within the cell must have good compatibility. Typical cells contain the active perovskite material sandwiched between two charge transport layers, which are then adjacent to their corresponding electrodes. Support layers may also be added. Charge mobility, energy barriers, interface energy alignment, and interfacial vacancies all impact compatibility and subsequent cell performance and stability. Thus, engineering well-suited interfaces with the cell is paramount to cell success and long-term stability, an important criterion for potential commercialization.

Two-dimensional buffer materials could help to modify and promote useful interface interactions. MXenes, a growing class of two-dimensional transitional metal carbides, nitrides, and carbonitrides, have shown impressive electronic properties that are easily tuned via surface modification. For example, the band gap of an MXene can be modified by changing the surface termination group from an oxygen atom to a hydroxide molecule. Additionally, MXene composition impacts the overall material performance. This type of fine-tuning allows impressive control over MXene properties and makes them ideal for interface adjustments.

Two-dimensional MXenes improve perovskite solar cell efficiency
Amanda Carr, Physics World

#P4TC: MXenes...August 24, 2015

Super State...

Super state: three independent groups have caught sight of supersolidity. (Courtesy: iStock/3quarks)

Topics: Bose-Einstein Condensate, Condensed Matter Physics, Electromagnetism, Quantum Mechanics

Atomic systems that behave very much like supersolids have been created independently by teams of physicists in Italy, Germany have Austria. The teams have shown that dipolar quantum gases trapped by magnetic fields can spontaneously separate into arrays of coherent droplets, providing a system closer to the original conception of a supersolid.

The supersolid phase is a counterintuitive quantum state of matter that has both crystalline order and frictionless flow at very low temperatures. The phenomenon is related to superfluidity and was predicted 50 years ago by Soviet physicists Alexander Andreev and Ilya Lifschitz. However, supersolidity has proved frustratingly difficult to observe.

In a superfluid, the energy required to create a density modulation generally increases as the modulation’s wavelength gets shorter. At one characteristic wavelength, however, the energy takes a sudden dip – much as waves pass more easily through a crystal when the wavelength equals the separation between the atoms. If the superfluid were cold enough, Andreev and Lifschitz reasoned, the energy required would drop to zero at this wavelength. The superfluid would then spontaneously separate into tiny droplets, effectively forming an ordered crystal.

Supersolid behavior spotted in dipolar quantum gases, Tim Wogan, Physics World

Quantum Droplets...

For ultracold experiments, atoms are held in vacuum by a glass cell, shown here. The atoms are then cooled to ultralow temperatures to create a Bose–Einstein condensate. (Photo courtesy of Wolfram Scheible.)

Topics: Bose-Einstein Condensate, Condensed Matter Physics, van der Waals

In his PhD thesis from 1873, Johannes van der Waals devised a theoretical framework to describe the gas and liquid phases of a molecular ensemble and the phase transition from one to the other. That work resulted in the celebrated equation of state bearing his name. To this day, the van der Waals theory is still the prevailing picture in most physicists’ minds to explain the emergence of the liquid state. It asserts that the liquid state arises at high densities from an equilibrium between attractive interatomic forces and short-range repulsion. Now, a new type of liquid has emerged in ultracold, extremely dilute atomic systems for which the van der Waals model does not predict a liquid phase.

Using the tools of laser cooling and trapping, experimenters can reach the ultracold regime to create atomic quantum gases.1 Quantum interference effects between atoms are an important part of the statistical descriptions of those systems. However, if a monatomic ensemble is simply cooled, any chemical species will form a liquid instead of a gas due to van der Waals forces and the system will never reach the quantum regime. So to see quantum effects, the classical liquid state must be avoided. That requires extremely low densities that keep the distances between atoms much larger than the range of attractive forces that would bind the liquid. But keeping the atoms far apart traps them in a dilute, metastable state. A whole new mechanism is needed for atoms in such dilute conditions to form a liquid phase.

Ultradilute Quantum Droplets, Igor Ferrier-Barbut, Physics Today
Igor Ferrier-Barbut is a researcher at the Institut d’Optique and CNRS in Palaiseau, France.

Strange Metals...

Topics: Condensed Matter Physics, Materials Science, Quantum Mechanics, Superconductors

A ubiquitous quantum phenomenon has been detected in a large class of superconducting materials, fueling a growing belief among physicists that an unknown organizing principle governs the collective behavior of particles and determines how they spread energy and information. Understanding this organizing principle could be a key into “quantum strangeness at its deepest level,” said Subir Sachdev, a theorist at Harvard University who was not involved with the new experiments.

The findings, reported today in Nature Physics by a team working at the University of Sherbrooke in Canada and the National Laboratory for Intense Magnetic Fields (LNCMI) in France, indicate that electrons inside a variety of ceramic crystals called “cuprates” seem to dissipate energy as quickly as possible, apparently bumping up against a fundamental quantum speed limit. And past studies, especially a 2013 paper in Science, found that other exotic superconducting compounds — strontium ruthenates, pnictides, tetramethyltetrathiafulvalenes and more — also burn energy at what appears to be a maximum allowed rate.

Universal Quantum Phenomenon Found in Strange Metals, Natalie Wolchover, Quanta Magazine

Excitons and Bilayer Graphene...

The band structure of a bandgap-opened bilayer graphene is shown in the upper left corner, where the trigonal warping effect results in three pockets near the edge of conduction and valence bands. Infrared light illuminates bilayer graphene and creates an exciton (a bound state of an electron and an electron hole), located mostly in the top and bottom layer of carbon atoms respectively. Courtesy: Long Ju and Enrique Sahagún Alonso (Scixel)

Topics: Condensed Matter Physics, Graphene, Particle Physics, Nanotechnology

Researchers in the US have succeeded in observing excitons in bilayer graphene for the first time using photocurrent spectroscopy and modified Fourier transform infrared spectroscopy techniques. The new result could help in the development of next-generation optoelectronics instruments, such as tunable infrared detectors, light-emitting diodes and lasers for molecular spectroscopy, thermal imaging and astronomy applications.

“The excitons we observed can be tuned using an electrical field, have a high quality factor, strongly absorb light and lie in the technologically important mid-infrared to terahertz wavelength range,” explains team member and lead author of the study Long Ju, who is at Cornell University. “No other conventional semiconductor contains such excitons.”

Graphene is a sheet of carbon atoms just one atom thick arranged in a honeycomb lattice. It is a semi-metal and does not contain a bandgap in its pristine state. Bilayer graphene is different, however, in that a large and tunable bandgap can be induced in it using an applied electric field – something that cannot be done for single-layer graphene.

Researchers theorize that bilayer graphene also supports tunable excitons (electron-hole pairs) but these had never been actually observed in an experiment until now.

Excitons seen in bilayer graphene, Belle Dumé, Nanotechweb.org

B-Doped Q-Carbon Superconductors...

FIG. 1.
SIMS profiles of (a) as-deposited boron and carbon layers with the inset showing a schematic of the alternating layers of amorphous carbon and boron deposited on c-sapphire using the pulsed laser deposition technique and (b) pulsed laser annealed (B-doped Q-carbon) thin films with the inset showing a schematic of B-doped amorphous Q-carbon formed on c-sapphire.

Topics: Condensed Matter Physics, Materials Science, Solid State Physics, Superconductors

ABSTRACT

Following a brief report on high-temperature superconductivity in B-doped Q-carbon [Bhaumik et al., ACS Nano 11(6), 5351–5357 (2017)], we present detailed structure-property correlations to understand the origin of superconductivity in strongly bonded lightweight materials and methods to further enhance the superconducting transition temperature (T_c). Nanosecond melting of carbon in a super undercooled state and rapid quenching result in a strongly bonded unique phase of B-doped Q-carbon. The temperature-dependent resistivity and magnetic susceptibility measurements demonstrate type II superconductivity in this material with a transition temperature of 36.0 ± 0.5 K and an upper critical field of 5.4 T at ∼0 K. It has also been shown that in B-doped Q-carbon, the upper critical magnetic field (H_c2(T)) follows H_c2(0) [1-(T/Tc)^2.1] temperature dependence and is consistent with the Bardeen–Cooper–Schrieffer formalism. In the present study, B-doped Q-carbon thin films are formed on sapphire substrates by employing pulsed laser annealing (PLA) using a nanosecond excimer laser. This process involves the rapid quenching of highly undercooled melt of homogenously mixed B and C. Through the structure-property correlation measurements in B-doped Q-carbon, we estimate a higher electronic density of states near the Fermi level. Higher density of states near the Fermi-level along with higher Debye temperature and phonon frequency are responsible for the enhanced T_c. As a result of rapid melting and quenching, we can achieve 17.0 ± 1.0 or higher atomic % of B in the electrically active sites of Q-carbon which leads to the formation of shallow electronic states near the valence band maximum. From the critical current density versus field moments, the value of critical current density (J_c (2T)) in B-doped Q-carbon at 21 K is calculated as 4.3 × 10⁷ A cm⁻², which indicates that this novel material can be used for the persistent mode of operation in MRI and nuclear magnetic resonance applications. This discovery of high-temperature superconductivity in B-doped amorphous Q-carbon shows that the non-equilibrium synthesis technique using the super undercooling process can be used to fabricate materials with greatly enhanced physical properties.

A novel high-temperature carbon-based superconductor: B-doped Q-carbon
Anagh Bhaumik¹, Ritesh Sachan^1,2, and Jagdish Narayan^1,a)
Journal of Applied Physics 122, 045301 (2017); doi: http://dx.doi.org/10.1063/1.4994787

HTS...

Image Source: Aventurine

Topics: Bose-Einstein Condensate, Condensed Matter Physics, Materials Science, Superconductors

The perfect performance of superconductors could revolutionize everything from grid-scale power infrastructure to consumer electronics, if only they could be coerced into operating above frigid temperatures. Even so-called high-temperature superconductors (HTS) must be chilled to hundreds of degrees Fahrenheit below zero.

Now, scientists from the U.S. Department of Energy's (DOE) Brookhaven National Laboratory and Yale University have discovered new, surprising behavior by electrons in a HTS material. The results, published July 27 in the journal Nature, describe the symmetry-breaking flow of electrons through copper-oxide (cuprate) superconductors. The behavior may be linked to the ever-elusive mechanism behind HTS.

"Our discovery challenges a cornerstone of condensed matter physics," said lead author and Brookhaven Lab physicist Jie Wu. "These electrons seem to spontaneously 'choose' their own paths through the material—a phenomenon in direct opposition to expectations."

Strange electrons break the crystal symmetry of high-temperature superconductors, Brookhaven National Laboratory, Phys.org

Selenide Vibrations...

In this illustration, an infrared laser beam (orange) triggers atomic vibrations in a thin layer of iron selenide, which are then recorded by ultrafast X-ray laser pulses (white) to create an ultrafast movie. The motion of the selenium atoms (red) changes the energy of the electron orbitals of the iron atoms (blue). (Courtesy: Greg Stewart/SLAC National Accelerator Laboratory)

Topics: Condensed Matter Physics, Materials Science, Solid State Physics, Superconductors

Two important breakthroughs in the understanding of iron-selenide superconductors have been made by two independent research groups. One team has shown that the electrons responsible for superconductivity in the material probably come from a specific atomic orbital. The other team, meanwhile, has measured the interaction between electrons and atomic vibrations in iron selenide, which is believed to be involved in its superconductivity.

The research could shed light on the mystery of why some materials based on iron selenide are superconductors at relatively high temperatures, which has puzzled physicists for more than a decade. While bulk iron selenide is a superconductor below 8.5K, this transition temperature can reach as high as 75K when an ultrathin trilayer of the material is grown on certain substrates.

Experiments shed new light on iron superconductors, Hamish Johnston, Physics World

Crystal Cavitation...

Time sequence showing the growth of a cavitation bubble. The large circle is an obstruction to the flow of a liquid crystal, which is moving from left to right. The cavitation bubble is forming at the right side of the obstruction. (Courtesy: Max Planck Institute for Dynamics and Self-Organization)

Topics: Condensed Matter Physics, Fluid Mechanics, Materials Science

The formation and subsequent collapse of bubbles has been seen for the first time in a flowing liquid crystal. This process is called cavitation and occurs when the pressure drop in a flowing fluid is large enough to allow some of the fluid to vaporize and create a bubble. Cavitation is of great interest in hydrodynamics because the collapsing bubbles can dissipate large amounts of energy in small regions and cause significant damage to machinery such as propellers.

The discovery was made by Tillmann Stieger and colleagues at the Max Planck Institute for Dynamics and Self-Organization in Göttingen, the Technical University of Berlin and the ETH Zürich. Liquid crystals are fluids that are made of rod-like molecules that tend to align under certain conditions. In its experiments, the team pumped liquid-crystal fluids through tiny channels just 0.1 mm wide. The channels contained obstructions, which increase the speed of the flow and encourage cavitation (see image).

Bubble cavitation spotted in liquid crystals, Hamish Johnston, Physics World

Ultra Cold and Fermi-Hubbard...

Raw fermionic microscope image (left) and processed image showing that spin-up atoms occupy alternating lattice sites as expected in an antiferromagnet. The spin-down atoms have been removed from the image. (Courtesy: A Mazurenko et al. / Nature)

Topics: Condensed Matter Physics, Optical Physics, Quantum Mechanics, Solid State Physics, Superconductors

New insights into a popular and potentially useful model of how electrons behave in solids have been provided by an experiment involving ultracold atoms. Markus Greiner and colleagues at Harvard University in the US studied the behaviour of lithium-6 atoms that are held in an optical lattice and interact according to rules set out by the Fermi-Hubbard model.

They found that the system becomes magnetic at low temperatures – and that the magnetism disappears when the density of atoms is reduced. The team can now use its atomic simulator to explore regimes of the Fermi-Hubbard model that could harbour very interesting physics including high-temperature superconductivity.

The electronic properties of solid materials arise from quantum-mechanical interactions between large numbers of electrons. It is notoriously difficult to calculate these properties, so physicists rely on simple models to simplify the mathematics – but even models have significant computational challenges. One such scheme is the Fermi-Hubbard model, which represents electrons as Fermi–Dirac particles (fermions) that hop between fixed sites on a lattice and only interact with each other when they occupy the same lattice site.

Physics World: Ultracold atoms shed light on the Fermi-Hubbard model
Hamish Johnston

Musing Hydrogen...

Harvard physicists say they photographed hydrogen in three different forms, from left to right: transparent hydrogen, black hydrogen, and finally, shiny metallic hydrogen. (R. Dias and I.F. Silvera) Source: Forbes.com

Topics: Condensed Matter Physics, Materials Science, Research, Theoretical Physics

Disclaimer: I was just as initially psyched by the reports of Metallic Hydrogen at Harvard. It's been theoretical since I was an undergrad (and that was a long time ago). Then, I remembered all the hype over cold fusion and calmed down. The key to my skepticism is repetition: another lab (several, in fact) will have to repeat the experiment to within an acceptable degree of error to one another. It will have to face grueling peer review that won't be kind, or for the squeamish. It's through this process we can distinguish science from malarkey.

Two physicists say that they have crushed hydrogen under such immense pressures that the gas became a shiny metal — a feat that physicists have been trying to accomplish for more than 80 years.

But other researchers have serious doubts about the claim, the latest in a field with a long history of failed attempts.

Ranga Dias and Isaac Silvera, both physicists at Harvard University in Cambridge, Massachusetts, first posted a report of their results on the arXiv preprint server last October [1], which attracted immediate criticism. A peer-reviewed version of the report was published on 26 January in Science [2], but sceptics say that it includes little new information.

Five experts told Nature’s news team that they do not yet believe the claim, and need more evidence. “I don’t think the paper is convincing at all,” says Paul Loubeyre, a physicist at France’s Atomic Energy Commission in Bruyères-le-Châtel.

Silvera and Dias say that they wanted to publish their first observation before making further tests on their fragile material.

Nature: Physicists doubt bold report of metallic hydrogen, Davide Castelvecchi

Flexible Ferroelectrics...

A scanning electron microscopy image of flexible haloimidazole crystals, which were found to show both ferroelectric and piezoelectric properties. (Image by Seungbum Hong/Argonne National Laboratory.)

Topics: Condensed Matter Physics, Nanotechnology, Materials Science, Metamaterials, Solid State Physics

Until recently, “flexible ferroelectrics” could have been thought of as the same type of oxymoronic phrase. However, thanks to a new discovery by the U.S. Department of Energy’s (DOE) Argonne National Laboratory in collaboration with researchers at Northwestern University, scientists have pioneered a new class of materials with advanced functionalities that moves the idea from the realm of irony into reality.

Ferroelectrics are a useful type of material that is found in capacitors that are used in sensors, as well as computer memory and RFID cards. Their special properties originate from the fact that they contain charged regions polarized in a specific orientation, which can be controlled with an external electric field. But they’ve also had a big drawback as engineers try to use them in new inventions.

“Ferroelectric materials are known for being quite brittle, and so it has always been a big challenge to make them mechanically flexible,” said Argonne nanoscientist Seungbum Hong, who helped to lead the research. “Because ferroelectricity and this kind of flexibility are relatively rare properties to see on their own, to have both ferroelectricity and flexibility in this new material is basically unprecedented.”

Argonne National Laboratory: Flexible ferroelectrics bring two material worlds together

Diamonds Are Forever...

Image Source: Entrepreneur.com, a similar photo is also at the link below.

Topics: Condensed Matter Physics, Nuclear Physics, Nuclear Power

Well, not exactly forever, but at least past our lifetimes.

Scientists from the University of Bristol Cabot Institute are hitting two birds with one stone, thanks to their lab-made diamond that can generate electricity and is made from upcycled radioactive waste.

In nuclear power plants, radioactive uranium is split in a process called nuclear fission. When the atoms are split, heat is generated, and that heat then vaporizes water into steam that turns electricity-generating turbines.

A severe downside of this process is the creation of dangerous radioactive waste, which ultimately deposits in the graphite core that it is housed in. Today, this nuclear contamination is safely stored away until it stops being radioactive…and with a half-life of 5,730 years, that takes quite a while.

The scientists found a way to heat the radioactive graphite to release most of the radioactivity in a gaseous form. The gas is subjected to high temperature and low pressures that turn it into a man-made diamond.

Futurism:
Diamond Batteries Made of Nuclear Waste Can Generate Power For Thousands of Years
Author: Jess Vilvestre, Editor: Patrick Caughill

H-Theorem...

Image Source: Hyper Physics

Topics: Condensed Matter Physics, Quantum Mechanics, Theoretical Physics, Thermodynamics

For more than a century and a half of physics, the Second Law of Thermodynamics, which states that entropy always increases, has been as close to inviolable as any law we know. In this universe, chaos reigns supreme.

But researchers with the U.S. Department of Energy's (DOE's) Argonne National Laboratory announced recently that they may have discovered a little loophole in this famous maxim.

Their research, published in Nature Scientific Reports, lays out a possible avenue to a situation where the Second Law is violated on the microscopic level.

The Second Law is underpinned by what is called the H-theorem, which says that if you open a door between two rooms, one hot and one cold, they will eventually settle into lukewarm equilibrium; the hot room will never end up hotter.

Recent advancements in a field called quantum information theory offered a mathematical construction in which entropy increases.

"What we did was formulate how these beautiful abstract mathematical theories could be connected to our crude reality," said Valerii Vinokur, an Argonne Distinguished Fellow and corresponding author on the study.

The scientists took quantum information theory, which is based on abstract mathematical systems, and applied it to condensed matter physics, a well-explored field with many known laws and experiments.

Argonne National Laboratory:
Argonne researchers posit way to locally circumvent Second Law of Thermodynamics
Louise Lerner

Supersolids...

Electron snapshot: the probable position of the remaining helium electron after photoionization. (Courtesy: M Ossiander/ TUM, M Schultze/ MPQ)

Topics: Applied Physics, Bose-Einstein Condensate, Condensed Matter Physics, Materials Science

Fresh evidence for a new state of matter called a supersolid has been put forth by two independent teams of physicists. Supersolidity has been a controversial concept whereby some atoms in a solid material are able to form a superfluid at very low temperatures – allowing them to flow ghost-like through the solid without any resistance. While initial observations of supersolidity in solid helium-4 in the 2000s have since been explained in terms of more mundane physics, some physicists believe that supersolids should exist – at least in principle. Now, Wolfgang Ketterle and colleagues at the Massachusetts Institute of Technology in the US and Tilman Esslinger and colleagues of ETH Zürich in Switzerland have created supersolid analogues using ultracold atoms. Both systems comprise Bose–Einstein condensates (BEC), which are already superfluids. The teams used different optical techniques to make the atoms arrange themselves into crystalline structures of high and low density resembling a solid. They then showed that the atoms can flow freely through such crystals, while the regions of high and low density do not move. While these experiments involve dilute gases, rather than actual solids, both studies show that the supersolid state of matter is possible. Both experiments are described in preprints on arXiv.

Physics World: Have supersolids been seen at last? Hamish Johnston

Unconventional Superconductors...

A crystal sample of one of the iron-based unconventional superconductors studied by Ames Laboratory scientists. Their systematic investigation ofthis class of superconductors may lead to the creation of new materials with unique superconducting properties. Credit: U.S. Department of Energy, Ames Laboratory

Topics: Applied Physics, Condensed Matter Physics, Solid State Physics, Superconductors

Researchers at the U.S. Department of Energy's Ames Laboratory and partner institutions conducted a systematic investigation into the properties of the newest family of unconventional superconducting materials, iron-based compounds. The study may help the scientific community discover new superconducting materials with unique properties.

Researchers combined innovative crystal growth, highly sensitive magnetic measurements, and the controlled introduction of disorder through electron bombardment to create and study an entire range of compositions within a class of iron-based superconductors. They found that the key fundamental properties—transition temperature and magnetic field penetration depth—of these complex superconductors were dependent on composition and the degree of disorder in the material structure.

"This was a systematic approach to more fully understand the behavior of unconventional superconductors," said Ruslan Prozorov, Ames Laboratory faculty scientist and professor in the Department of Physics and Astronomy at Iowa State University. "We found that some proposed models of unconventional superconductivity in these iron-based compounds were compatible with our results, and this study further limited the possible theoretical mechanisms of superconductivity."Researchers combined innovative crystal growth, highly sensitive magnetic measurements, and the controlled introduction of disorder through electron bombardment to create and study an entire range of compositions within a class of iron-based superconductors. They found that the key fundamental properties—transition temperature and magnetic field penetration depth—of these complex superconductors were dependent on composition and the degree of disorder in the material structure.

Phys.org:
Scientists gain insight on mechanism of unconventional superconductivity
Ruslan Prozorov

Shrinking Hard Drive...

Photomontage of a diamond and a hard-drive head. (Courtesy: I Jakobi)

Topics: Computer Engineering, Condensed Matter Physics, Materials Science

A nitrogen-vacancy (NV) defect in diamond has been used to create a magnetometer that can measure the broadband magnetic fields generated by hard-disk write heads. The work was done by researchers in Germany and the UK who have shown that a single NV can detect the oscillating and static magnetic fields associated with write heads at nanometre resolution. The new work could help further miniaturize hard-disk drives, thereby increasing their data-storage capacity.

"The hard-disk industry currently has no established sensors that can resolve the magnetic field of write heads on the scale of 5–10 nm," explains team-member Ingmar Jakobi of the University of Stuttgart. "This is a serious impediment for developing these devices, which will carry the biggest share of data in the ever-growing digital world."

The NV defect in diamond offers a solution to this problem because it comprises a single electron spin that is highly isolated from its immediate surroundings. This spin is essentially a tiny magnet that could be used to detect changes in a magnetic field over nanometre distances.

Physics World:
Diamond magnetometer could help shrink computer hard drives, Belle Dumé

Ultracapacitors...

FastCAP Systems' ultracapacitors (pictured) can withstand extreme temperatures and harsh environments, opening up new uses for the devices across a wide range of industries, including oil and gas, aerospace and defense, and electric vehicles. Credit: FastCAP Systems

Topics: Condensed Matter Physics, Materials Science, Solid State Physics

Devices called ultracapacitors have recently become attractive forms of energy storage: They recharge in seconds, have very long lifespans, work with close to 100 percent efficiency, and are much lighter and less volatile than batteries. But they suffer from low energy-storage capacity and other drawbacks, meaning they mostly serve as backup power sources for things like electric cars, renewable energy technologies, and consumer devices.

But MIT spinout FastCAP Systems is developing ultracapacitors, and ultracapacitor-based systems, that offer greater energy density and other advancements. This technology has opened up new uses for the devices across a wide range of industries, including some that operate in extreme environments.

Based on MIT research, FastCAP's ultracapacitors store up to 10 times the energy and achieve 10 times the power density of commercial counterparts. They're also the only commercial ultracapacitors capable of withstanding temperatures reaching as high as 300 degrees Celsius and as low as minus 110 C, allowing them to endure conditions found in drilling wells and outer space. Most recently, the company developed a AA-battery-sized ultracapacitor with the perks of its bigger models, so clients can put the devices in places where ultracapacitors couldn't fit before.

Phys.org: New applications for ultracapacitors, Rob Matheson

Na vs Li...

Electrochemical characteristics of Na₂Ti₃O₇ and VOPO₄ electrodes in the "half-cell format" vs. Na+/Na. Courtesy: G Yu

Topics: Condensed Matter Physics, Nanotechnology, Semiconductor Technology, Solid State Physics

Researchers at the University of Texas at Austin in the US and Nanjing University of Aeronautics and Astronautics in China have developed a high-energy sodium-ion battery based on sodium titanate nanotubes and vanadyl phosphate layered nanosheet materials. The new device, which works over a wide temperature range of between –20 to +55°C, has a high operating voltage of close to 2.9 V and delivers a large reversible capacity of 114 mA h/g. It also boasts a high energy density of 220 Wh/kg, which makes it competitive with state-of-the-art lithium-ion batteries.

Sodium-ion batteries are similar to their lithium-ion cousins since they store energy in the same way. They consist of two electrodes – anode and cathode – separated by an electrolyte. When the battery is being charged with electrical energy, metal ions move from the cathode through the electrolyte to the anode, where they are absorbed into the bulk of the anode material. Sodium-based devices are in principle more attractive though since sodium is highly abundant on Earth (its Clarke’s number is 2.64) and is therefore much cheaper than lithium. Sodium is also more environmentally friendly than lithium.

However, the radius of the sodium ion is significantly larger than that of the lithium ion. This makes it difficult to find a host electrolyte material that allows ions to be rapidly absorbed and removed. What is more, sodium-ion batteries made thus far suffer from a relatively low working potential, large capacity decay during cycling (which leads to a limited battery life) and poor safety.

Nanotechweb: Sodium-ion device could compete with lithium-ion batteries, Belle Dumé

Exciton Condensate...

Figure 1: A Coulomb drag experiment measures the interactions between charges in two closely spaced layers. The experiment entails running a current through the “drive” layer (here, the top layer) and measuring the resulting flow of charge in the “drag” layer (the bottom layer). The panels indicate three (of many) possible drag scenarios associated with two sheets of bilayer graphene (grey). At left, exciton pairs form between holes (red) in the drive layer and electrons (green) in the drag layer, giving rise to a large drag effect. At center, holes drag electrons in the same direction (positive drag) because of momentum transfer between the charges in different sheets. At right, holes drag electrons in the opposite direction (negative drag), an observation in bilayer graphene that is yet to be explained.

Topics: Atomic Physics, Bose-Einstein Condensate, Condensed Matter Physics, Quantum Mechanics

Superfluids (fluids with zero viscosity) and superconductors (materials with zero resistance) have a common ingredient: bosons. These particles obey Bose-Einstein statistics, allowing a collection of them at low temperatures to collapse into a single quantum-mechanical state, or Bose-Einstein condensate. Bosons in superconductors consist of two paired electrons, but the pairing is weak and only occurs at low temperatures. In a quest to build devices that carry electricity with low dissipation at higher temperatures, researchers have therefore explored the possibility of engineering electrical condensates [1] out of strongly bound pairs of electrons and holes, or excitons. Now, two research groups have, independently, fabricated and characterized a graphene-based device that is thought to be a promising platform for realizing an exciton condensate [2, 3]. Neither group has yet found evidence for such a condensate—the ultimate goal of such experiments. But their measurements lay the groundwork for future searches.

Excitons form in semiconductors and insulators. The binding energy between the exciton’s electron and hole can be quite strong, greatly exceeding their thermal energy at room temperature. Unfortunately, excitons recombine quickly, too fast to allow a condensate to form. Although excitons coupled to light confined within a cavity can form hybrid particles (exciton-polaritons) that do live long enough to condense [4], such condensates require a continuous input of light.

APS Viewpoint: Chasing the Exciton Condensate
Michael S. Fuhrer, Alex R. Hamilton