Lună: septembrie 2017

Strângerea memoriei: Materialele inovatoare de tip prototip sunt viitorul stocării datelor(Straining the memory: Prototype strain engineered materials are the future of data storage)

Când atomii de la interfață sunt dezordonați, materialul are o rezistență electrică ridicată, în timp ce interfața comandată are o rezistență electrică scăzută. Deoarece numai interfața comută, un subset de straturi din material poate rămâne neschimbat și cristalin. Aceasta înseamnă că interfața poate fi proiectată de straturile fără comutare – întreaga structură nu trebuie să treacă într-o stare dezordonată. Acest lucru face superlattica foarte diferită de aliajele de memorie cu schimbări de fază nestructurate, cum ar fi aliajul Ge 2 Sb 2 Te 5 .

Posted: Sep 20, 2017

Straining the memory: Prototype strain engineered materials are the future of data storage

(Nanowerk News) Researchers from the Singapore University of Technology and Design (SUTD) and Shanghai Institute of Microsystems and Information Technology have nano-engineered a superlattice data storage material. Data is recorded at the interfaces of the superlattice layers.
When the atoms at the interface are disordered, the material has a high electrical resistance while the ordered interface has a low electrical resistance. Since only the interface switches, a subset of layers within the material can remain unchanged and crystalline. This means that the interface can be engineered by the non-switching layers – the entire structure need not switch into a disordered state. This makes the superlattice very different to unstructured phase change memory alloys, such as the Ge2Sb2Te5 alloy.
comparison of the electrical switching current and switching times for the strained superlattices interfacial phase change memory with other state-of-the-art phase change memory materials
A comparison of the electrical switching current and switching times for the strained superlattices interfacial phase change memory with other state-of-the-art phase change memory materials. (Image: Zhou et al.) (click on image to enlarge)
The authors showed in a paper published in Nano Futures („Avalanche atomic switching in strain engineered Sb2Te3–GeTe interfacial phase-change memory cells”) that fast switching in these nanostructured materials is due to avalanche atomic switching at the interface.
The first atom that switches requires a large amount of energy, but subsequent atoms require less energy. As more atoms switch, the energy required for subsequent atoms to switch is lowered. This leads to an exponential increase in the switching probability with the number of atoms switching.
Zhou et al showed that the energy for the first atom to switch can be engineered by straining the layer interfaces. The research team created prototype memory devices, which exploit this effect that outperformed state-of-the-art phase change memory devices. The switching voltage, current, and switching time are substantially reduced while the electrical resistance changes by a factor of 500. Thus these prototype devices are faster and more efficient than current competing technologies.
One of the members of the research team, Assistant Professor Robert Simpson, said: „The superlattices devices are remarkably energy efficient. We foresee this technology impacting new 3D memory architectures, such as Intel’s 3D x-point. We are now building on the success of these data storage materials by optimising similar phase change materials for switchable nano photonics applications.”
Source: Singapore University of Technology and Design

Grafenul și alte nanomateriale de carbon pot înlocui metalele rare(Graphene and other carbon nanomaterials can replace scarce metals)

Nanomaterialele de carbon constau exclusiv sau în principal din carbon și sunt materiale puternice cu o bună conductivitate. Mai multe metale rare au proprietăți similare. Metalele se găsesc, de exemplu, în cabluri, ecrane subțiri, ignifuge, protecție la coroziune și condensatoare.
Rickard Arvidsson și Björn Sandén de la Universitatea de Tehnologie din Chalmers au investigat dacă nanoparticulele de carbon, grafena, fullerenele și nanotuburile de carbon au potențialul de a înlocui 14 metale rare în principalele domenii de aplicare (vezi tabelul din imaginea atașată). Ei au găsit soluții potențiale bazate pe tehnologie pentru a înlocui metalele cu nanomateriale de carbon pentru toate aplicațiile, cu excepția aurului din bijuterii. Metalele pe care le avem cel mai aproape de a putea înlocui sunt indiu, galiu, beriliu și argint.

Posted: Sep 19, 2017

Graphene and other carbon nanomaterials can replace scarce metals

(Nanowerk News) Scarce metals are found in a wide range of everyday objects around us. They are complicated to extract, difficult to recycle and so rare that several of them have become „conflict minerals” which can promote conflicts and oppression. A survey at Chalmers University of Technology now shows that there are potential technology-based solutions that can replace many of the metals with carbon nanomaterials, such as graphene (Journal of Cleaner Production, „Carbon nanomaterials as potential substitutes for scarce metals”).
They can be found in your computer, in your mobile phone, in almost all other electronic equipment and in many of the plastics around you. Society is highly dependent on scarce metals, and this dependence has many disadvantages.
Scarce metals such as tin, silver, tungsten and indium are both rare and difficult to extract since the workable concentrations are very small. This ensures the metals are highly sought after – and their extraction is a breeding ground for conflicts, such as in the Democratic Republic of the Congo where they fund armed conflicts.
In addition, they are difficult to recycle profitably since they are often present in small quantities in various components such as electronics.
Graphene and Other Carbon Nanomaterials Can Replace Scarce Metals
Scarce metals are found in a wide range of everyday objects around us. A survey at Chalmers University of Technology now shows that there are potential technology-based solutions that can replace many of the metals with carbon nanomaterials, such as graphene. (Image: Jonas Sandwall) (click on image to enlarge)
Rickard Arvidsson and Björn Sandén, researchers in environmental systems analysis at Chalmers University of Technology, have now examined an alternative solution: substituting carbon nanomaterials for the scarce metals. These substances – the best known of which is graphene – are strong materials with good conductivity, like scarce metals.
„Now technology development has allowed us to make greater use of the common element carbon,” says Sandén. „Today there are many new carbon nanomaterials with similar properties to metals. It’s a welcome new track, and it’s important to invest in both the recycling and substitution of scarce metals from now on.”
The Chalmers researchers have studied the main applications of 14 different metals, and by reviewing patents and scientific literature have investigated the potential for replacing them by carbon nanomaterials. The results provide a unique overview of research and technology development in the field.
According to Arvidsson and Sandén the summary shows that a shift away from the use of scarce metals to carbon nanomaterials is already taking place.
„There are potential technology-based solutions for replacing 13 out of the 14 metals by carbon nanomaterials in their most common applications. The technology development is at different stages for different metals and applications, but in some cases such as indium and gallium, the results are very promising,” Arvidsson says.
„This offers hope,” says Sandén. „In the debate on resource constraints, circular economy and society’s handling of materials, the focus has long been on recycling and reuse. Substitution is a potential alternative that has not been explored to the same extent and as the resource issues become more pressing, we now have more tools to work with.”
The research findings were recently published in the Journal of Cleaner Production. Arvidsson and Sandén stress that there are significant potential benefits from reducing the use of scarce metals, and they hope to be able to strengthen the case for more research and development in the field.
„Imagine being able to replace scarce metals with carbon,” Sandén says. „Extracting the carbon from biomass would create a natural cycle.”
„Since carbon is such a common and readily available material, it would also be possible to reduce the conflicts and geopolitical problems associated with these metals,” Arvidsson says.
At the same time they point out that more research is needed in the field in order to deal with any new problems that may arise if the scarce metals are replaced.
„Carbon nanomaterials are only a relatively recent discovery, and so far knowledge is limited about their environmental impact from a life-cycle perspective. But generally there seems to be a potential for a low environmental impact,” Arvidsson says.

Facts
Carbon nanomaterials consist solely or mainly of carbon, and are strong materials with good conductivity. Several scarce metals have similar properties. The metals are found, for example, in cables, thin screens, flame-retardants, corrosion protection and capacitors.
Rickard Arvidsson and Björn Sandén at Chalmers University of Technology have investigated whether the carbon nanomaterials graphene, fullerenes and carbon nanotubes have the potential to replace 14 scarce metals in their main areas of application (see table in attached image). They found potential technology-based solutions to replace the metals with carbon nanomaterials for all applications except for gold in jewellery. The metals which we are closest to being able to substitute are indium, gallium, beryllium and silver.
Source: Chalmers University of Technology

 

O celulă solară pe care o poți pune în spălare(A solar cell you can put in the wash)

au dezvoltat un nou tip de dispozitiv fotovoltaic ultra-subțire, acoperit pe ambele părți cu folii stretchable și impermeabile, care pot continua să furnizeze energie electrică din lumina soarelui chiar și după ce sunt înmuiate în apă sau întinse și comprimate. Ar putea deschide calea către celulele solare care să poată fi alimentate de dispozitive precum monitoarele de sănătate încorporate în îmbrăcăminte.sperăm că aceste fotovoltaice organice lavabile, ușor și elastice va deschide o nouă cale pentru utilizarea ca sistem de sursă de alimentare pe termen lung pentru senzori care pot fi purtați și alte dispozitive

Posted: Sep 18, 2017

A solar cell you can put in the wash

(Nanowerk News) Scientists from RIKEN and the University of Tokyo have developed a new type of ultra-thin photovoltaic device, coated on both sides with stretchable and waterproof films, which can continue to provide electricity from sunlight even after being soaked in water or being stretched and compressed.
The work, published in Nature Energy („Stretchable and waterproof elastomer-coated organic photovoltaics for washable electronic textile applications”), could open the way to wearable solar cells, which will provide power to devices such as health monitors incorporated into clothing. One of the requirements of the Internet of Things–referring to a world where devices of all sorts are connected to the Internet–is the development of power sources for a host of devices, including devices that can be worn on the body.
According to Takao Someya, the leader of the research group, these could include sensors that record heartbeats and body temperature, for example, providing early warning of medical problems. In the past, attempts have been made to create photovoltaics that could be incorporated into textiles, but typically they lacked at least one of the important properties–long-term stability in both air and water, energy efficiency, and robustness including resistance to deformation–that are key to successful devices.
For the present work, the members of the research group developed extremely thin and flexible organic photovoltaic cells, based on a material called PNTz4T, which they had developed in earlier work. They deposited the device in an inverse architecture, which they had previously developed, onto a 1-um-thick parylene film. The ultra-thin device was then placed onto acrylic-based elastomer and the top side of the device was coated with an identical elastomer, giving it a coating on both sides to prevent water infiltration.. The elastomer, while allowing light to enter, prevented water and air from leaking into the cells, making them more long-lasting than previous experiments.
The researchers then subjected the device to a variety of tests, finding first that it had a strong energy efficiency of 7.9 percent, producing a current of 7.86 milliwatts per square centimeter, as the current density was 13.8 milliamperes per square centimeter at 0.57 volts, based on a simulated sunlight of 100 milliwatts per square centimeter. To test its resistance to water, they soaked it in water for two hours, and found that the efficiency decreased by just 5.4 percent. And to test the durability, they subjected it to compression, and found that after compressing by nearly half for twenty cycles while placing drops of water on it, it still had 80 percent of the original efficiency.
According to Kenjiro Fukuda of the RIKEN Center for Emergent Matter Science, „We were very gratified to find that our device has great environmental stability while simultaneously having a good efficiency and mechanical robustness. We very much hope that these washable, lightweight and stretchable organic photovoltaics will open a new avenue for use as a long-term power source system for wearable sensors and other devices.”
Source: RIKEN

O nouă abordare a impulsurilor ultrascurte de lumină(A new approach to ultrafast light pulses)

Deoarece reacția materialului este atât de puternic influențată de apropierea exactă a substratului metalic din apropiere, astfel de sisteme ar putea fi de asemenea folosite pentru instrumente de măsurare foarte precise. „Interacțiunea este redusă ca o funcție a mărimii decalajului, astfel încât aceasta ar putea fi acum utilizată dacă vrem să măsuram apropierea unei suprafețe”, spune Fang.
Pe măsură ce echipa își continuă studiile cu privire la aceste materiale, următorul pas este de a studia efectele pe care ar putea avea modelul suprafeței metalice, deoarece testele au utilizat până acum numai suprafețe plane.

Posted: Sep 18, 2017

A new approach to ultrafast light pulses

(Nanowerk News) Two-dimensional materials called molecular aggregates are very effective light emitters that work on a different principle than typical organic light-emitting diodes (OLEDs) or quantum dots. But their potential as components for new kinds of optoelectronic devices has been limited by their relatively slow response time. Now, researchers at MIT, the University of California at Berkeley, and Northeastern University have found a way to overcome that limitation, potentially opening up a variety of applications for these materials.
The findings are described in the journal Proceedings of the National Academy of Sciences(„Ultrafast fluorescent decay induced by metal-mediated dipole–dipole interaction in two-dimensional molecular aggregates”), in a paper by MIT associate professor of mechanical engineering Nicholas X. Fang, postdocs Qing Hu and Dafei Jin, and five others.
light strikes a molecular lattice deposited on a metal substrate
In this image, light strikes a molecular lattice deposited on a metal substrate. The molecules can quickly exchange energy with the metal below, a mechanism that leads to a much faster response time for the emission of fluorescent light from the lattice. (Image: MIT)
The key to enhancing the response time of these 2-D molecular aggregates (2DMA), Fang and his team found, is to couple that material with a thin layer of a metal such as silver. The interaction between the 2DMA and the metal that is just a few nanometers away boosts the speed of the material’s light pulses more than tenfold.
These 2DMA materials exhibit a number of unusual properties and have been used to create exotic forms of matter, known as Bose-Einstein condensates, at room temperature, while other approaches required extreme cooling. They have also been applied in technologies such as solar cells and light-harvesting organic antennas. But the new work for the first time identifies the strong influence that a very close sheet of metal can have on the way these materials emit light.
In order for these materials to be useful in devices such as photonic chips – which are like semiconductor chips but carry out their operations using light instead of electrons – „the challenge is to be able to switch them on and off quickly,” which had not been possible before, Fang says.
With the metal substrate nearby, the response time for the light emission dropped from 60 picoseconds (trillionths of a second) to just 2 picoseconds, Fang says: „This is pretty exciting, because we observed this effect even when the material is 5 to 10 nanometers away from the surface,” with a spacing layer of polymer in between. That’s enough of a separation that fabricating such paired materials in quantity should not be an overly demanding process. „This is something we think could be adapted to roll-to-roll printing,” he says.
If used for signal processing, such as sending data by light rather than radio waves, Fang says, this advance could lead to a data transmission rate of about 40 gigahertz, which is eight times faster than such devices can currently deliver. This is „a very promising step, but it’s still very early” as far as translating that into practical, manufacturable devices, he cautions.
The team studied only one of the many kinds of molecular aggregates that have been developed, so there may still be opportunities to find even better variations. „This is actually a very rich family of luminous materials,” Fang says.
Because the responsiveness of the material is so strongly influenced by the exact proximity of the nearby metal substrate, such systems could also be used for very precise measuring tools. „The interaction is reduced as a function of the gap size, so it could now be used if we want to measure the proximity of a surface,” Fang says.
As the team continues its studies of these materials, one next step is to study the effects that patterning of the metal surface might have, since the tests so far only used flat surfaces. Other questions to be addressed include determining the useful lifetimes of these materials and how they might be extended.
Fang says a first prototype of a device using this system might be produced „within a year or so.”
Source: By David L. Chandler, Massachusetts Institute of Technology

Uniforme de ultimă generație dezvoltate pentru primii respondenți(Next-Generation Uniforms Developed for First Responders)

proiectul pentru uniforme avansate a fost extins;Uniforma AMBER este o soluție de generare a cuiburilor care integrează tehnologii, inclusiv rezistența la flacără, protecția împotriva patogenului din sânge, repellența la apă, confort îmbunătățit, tampoane integrate pentru genunchi și vizibilitate ridicată printre alte caracteristici critice

Next Generation

Nanocoatingul protejează clădirile împotriva poluării(Nanocoating protects buildings against pollution)

soluție
Proprietățile fotocatalitice ale anatazei, una dintre cele trei forme naturale de dioxid de titan, au fost descoperite în Japonia la sfârșitul anilor ’60. Sub influența radiației UV în lumina soarelui, se pot descompune poluanți organici, cum ar fi bacterii, ciuperci și nicotină, și unele materiale anorganice în dioxid de carbon. Efectul catalitic este cauzat de nanostructura cristalelor sale.
Aplicată în aer liber, acest material accesibil și disponibil pe scară largă ar putea reprezenta o soluție eficientă de autocurățare pentru clădiri. Acest lucru se datorează reacției chimice, care lasă un reziduu pe fațadele clădirilor, un reziduu apoi spălat atunci când plouă. Aplicarea lui la monumentele din zonele urbane poate salva patrimoniul nostru cultural, care este amenințat de poluanți.
Cu toate acestea, „vopselele fotocatalitice și aditivii au fost mult timp o provocare pentru industria de acoperire, deoarece acțiunea catalitică afectează durabilitatea lianților de rășină și oxidează componentele vopselei”, explică Ioannis Arabatzis, fondator și director general al NanoPhos, cu sediul în orașul grecesc din Lavrio, într-una din țările în care se află unele dintre cele mai importante monumente ale istoriei omenirii. Compania greacă testează o vopsea numită Kirei, inspirată de un cuvânt japonez care înseamnă atât curat, cât și frumos.

Posted: Sep 15, 2017

Nanocoating protects buildings against pollution

(Nanowerk News) The organic pollution decomposing properties of titanium dioxide (TiO2) have been known for about half a century. However, practical applications have been few and hard to develop, but now a Greek paint producer claims to have found a solution
The photocatalytic properties of anatase, one of the three naturally occurring forms of titanium dioxide, were discovered in Japan in the late 1960s. Under the influence of the UV-radiation in sunlight, it can decompose organic pollutants such as bacteria, fungi and nicotine, and some inorganic materials into carbon dioxide. The catalytic effect is caused by the nanostructure of its crystals.
Applied outdoors, this affordable and widely available material could represent an efficient self-cleaning solution for buildings. This is due to the chemical reaction, which leaves a residue on building façades, a residue then washed away when it rains. Applying it to monuments in urban areas may save our cultural heritage, which is threatened by pollutants.
However, “photocatalytic paints and additives have long been a challenge for the coating industry, because the catalytic action affects the durability of resin binders and oxidizes the paint components,” explains Ioannis Arabatzis, founder and managing director of NanoPhos, based in the Greek town of Lavrio, in one of the countries home to some of the most important monuments of human history. The Greek company is testing a paint called Kirei, inspired by a Japanese word meaning both clean and beautiful.
According to Arabatzis, it’s an innovative product because it combines the self-cleaning action of photocatalytic nanoparticles and the reflective properties of cool wall paints. “When applied on exterior surfaces this paint can reflect more than 94% of the incident InfraRed radiation (IR), saving energy and reducing costs for heating and cooling”, he says. “The reflection values are enhanced by the self-cleaning ability. Compared to conventional paints, they remain unchanged for longer.”
The development of Kirei has been included in the European project BRESAER, which is studying a sustainable and adaptable “envelope system” to renovate buildings. The new paint was tested and subjected to quality controls following ISO standard procedures at the company’s own facilities and in other independent laboratories. “The lab results from testing in artificial, accelerated weathering conditions are reliable,” Arabatzis claims. “There was no sign of discolouration, chalking, cracking or any other paint defect during 2,000 hours of exposure to the simulated environmental conditions. We expect the coating’s service lifetime to be at least ten years.”
Many studies are being conducted to exploit the properties of titanium dioxide. Jan Duyzer, researcher at the Netherlands Organisation for Applied Scientific Research (TNO) in Utrecht, focused on depollution: “There is no doubt about the ability of anatase to decrease the levels of nitrogen oxides in the air. But in real situations, there are many differences in pollution, wind, light, and temperature. We were commissioned by the Dutch government specifically to find a way to take nitrogen oxides out of the air on roads and in traffic tunnels. We used anatase coated panels. Our results were disappointing, so the government decided to discontinue the research. Furthermore, we still don’t know what caused the difference between lab and life. Our best current hypothesis is that the total surface of the coated panels is very small compared to the large volumes of polluted air passing over them,” he tells youris.com.
“In laboratory conditions the air is blown over the photocatalytic surface with a certain degree of turbulence. This results in the NOx-particles and the photocatalytic material coming into full contact with one another,” says engineer Anne Beeldens, visiting professor at KU Leuven, Belgium. Her experience with photocatalytic TiO2 is also limited to nitrogen dioxide (NOx) pollution.
“In real applications, the air stream at the contact surface becomes laminar. This results in a lower velocity of the air at the surface and a lower depollution rate. Additionally, not all the air will be in contact with the photocatalytic surfaces. To ensure a good working application, the photocatalytic material needs to be positioned so that all the air is in contact with the surface and flows over it in a turbulent manner. This would allow as much of the NOx as possible to be in contact with photocatalytic material. In view of this, a good working application could lead to a reduction of 5 to 10 percent of NOx in the air, which is significant compared to other measures to reduce pollutants.”
The depollution capacity of TiO2 is undisputed, but most applications and tests have only involved specific kinds of substances. More research and measurements are required if we are to benefit more from the precious features of this material.
Source: By Koen Mortelmans, Youris

Fizicienii prezic o semi-metalicitate nemetalică( Physicists predict nonmetallic half-metallicity )

Spre deosebire de un metal obișnuit, într-un material semi-metalic, electronii cu o singură orientare în rotație – de exemplu, spin-up – participă la curentul electric. În ceea ce privește electronii spin-down, energia lor este prea mare și, prin urmare, nu pot purta curent de încărcare. Aceasta înseamnă că, atunci când curentul trece printr-o jumătate de metal, este generat și un curent polarizat în spin. Dar tronica spin-valley încearcă să manipuleze nu numai o populație de electroni spin-polarizată în actualul dar și așa-numitul index de vale.Pentru un material cu o undă de densitate pentru a deveni o jumătate de metal, este nevoie de un tratament special cunoscut sub numele de dopaj. Aceasta implică încorporarea electronilor sau a găurilor în izolator. Alexandru Rozhkov, co-autor al lucrării și cercetător la Departamentul de Probleme de Fizică și Energetică al MIPT, explică faptul că un sistem poate fi dopat prin supunerea lui la un câmp electric extern sau la modificări chimice de vrac sau de suprafață: „Pentru fiecare sistem , trebuie selectat un tip adecvat de atomi de dopaj – cum ar fi azot, fosfor sau alt element. Prin înlocuirea atomilor din sistemul gazdă cu impuritățile care dau sau acceptă electroni de conducere, este indusă o schimbare a proprietăților materialului original. ”

Posted: Sep 14, 2017

Physicists predict nonmetallic half-metallicity

(Nanowerk News) A team of researchers from the Moscow Institute of Physics and Technology (MIPT) and the Institute for Theoretical and Applied Electrodynamics (ITAE) of the Russian Academy of Sciences (RAS), in collaboration with a colleague from RIKEN (an institute for physical and chemical research in Japan), has provided theoretical proof of the existence of a new class of materials. It was suggested that such systems be referred to as ‘spin-valley half-metals.’
The paper was published in the journal Physical Review Letters („Spin-Valley Half-Metal as a Prospective Material for Spin Valleytronics”). The discovery will find use in implantable electronics, alongside devices based on graphene, nanotubes, and a number of other promising materials.
The microscopic mechanism proposed by the researchers differs significantly from the usual half-metal model based on a strong electron-electron interaction. This might give rise to a new direction in search for ‘nonmetallic’ half-metals, i.e., those that do not contain atoms of transition metals, such as nickel, manganese, and lanthanum. These would come in handy in implantable devices and systems. The authors use the term ‘spin-valley-tronics’ to refer to the possible alternative to traditional electronics.
Since the invention of the electron tube, the world has seen electronics exhaust most of its possibilities. It would be highly challenging to continue increasing the number of transistors or microprocessor clock rate. That is why researchers around the globe are exploring new possibilities. One of them is spintronics which makes use of electron spins and already has some important practical applications. Around the turn of the century, the use of giant magnetoresistance materials in magnetic field sensors (used to read data in hard disk drives) has enabled the storage of much larger amounts of data on HDDs.
Researchers all over the world are working on new spintronic devices, and half-metals are believed to have great potential. They were first predicted based on computer simulations and later proved to exist experimentally. Unlike a usual metal, in a half-metallic material, electrons of only one spin orientation — e.g., spin up — participate in the electric current. As for spin-down electrons, their energy is too high, and therefore they cannot carry charge current. This means that when the current is passed through a half-metal, a spin-polarized current is generated as well. But spin-valley-tronics seeks to manipulate not only a spin-polarized population of electrons in the current but also the so-called valley index.
electronic spin
Fig. 1. The term ‘spin’ refers to the intrinsic angular momentum of a particle. The spin of a particle has both a magnitude and a direction. In the case of the electron, the magnitude is 1/2 times Planck’s constant, and the direction is either up or down. (Image: MIPT)
The term ‘valley’ is borrowed from semiconductor physics. Mathematically, the excitation energy in a solid is expressed by E (κ, n), where κ is the momentum of the electron and n is the zone index, i.e., a discrete quantum property of the state of the electron. This function may look rather odd, and in the case of several minimums with comparable excitation energies, one can speak of the presence of multiple ‘valleys’.
With certain reserve, one could say that electrons whose states correspond to one of the valleys do not interact with electrons from another valley. Such an ensemble of electrons can carry not only spin and charge, but also a distinct value called the valley index.
electron and hole energy is plotted against momentum
In the figure, electron and hole energy is plotted against momentum. (The four black curves correspond to two projections of electron spin and two projections of hole spin.) The thick curve corresponds to electron states capable of conducting an electric current (forming a Fermi surface). Depending on the mutual orientation of the spins of electrons (blue arrows) and holes (red arrows) on the Fermi surface, the system is either an ordinary half-metal (c) or a spin-valley half-metal (d). (Image: MIPT)
The valley index can be used to transfer information with the help of valley currents – in this regard, the valley index is quite similar to spin. Research in this direction is currently being carried out by several groups, which implies that the ideas proposed in the publication are not pure abstractions. The idea of using the valley index is certainly not a new one, but what the researchers did is, they theoretically proved the existence of a novel class of materials for use in spin-valley-tronics.

So what’s new?
The half-metals available to the researchers all contain atoms of transition metals: nickel, manganese, or lanthanum, etc. The team of researchers, which included physicists from Russia and Japan, has demonstrated a theoretical mechanism for achieving half-metallicity that requires no transition metal atoms. This possibility would have a number of useful applications, including in implantable devices.
The physicists suggest that ‘nonmetallic half-metals’ be obtained from a special class of dielectric materials called charge- or spin-density wave insulators. The term charge- or spin-density wave refers to a state with periodic microscopic regions with nonzero average charge (spin) in the material. Theorists describe such systems as a quantum condensate of electron-hole pairs. For a pair of this kind to form, two valleys are required: One provides electrons, the other provides holes. It is the presence of two valleys in the original system that gives rise to spin-valley half-metallicity.
In semiconductor physics, a ‘hole’ is a quasiparticle that is considered to have a positive charge.
For a material with a density wave to become a half-metal, it requires a special treatment known as doping. This involves the incorporation of electrons or holes into the insulator. Alexander Rozhkov, a co-author of the paper and a researcher at MIPT’s Department of Problems of Physics and Energetics, explains that a system can be doped by subjecting it to an external electric field or chemical modifications of bulk or surface: ‘For each system, a suitable type of doping atoms – such as nitrogen, phosphorus, or some other element – needs to be selected. By replacing atoms of the host system with impurities donating or accepting conduction electrons, a change in the properties of the original material is induced.’
The possibility of doping materials with density waves has been discussed in the literature for a long time. Thus, among articles cited in the newly published paper are those dating back to 1970s. The systems dealt with by the researchers have been shown to have various phases, including spatially inhomogeneous (for example, states with the so-called electronic phase separation, and the phases with domain walls, often called ‘stripes’). Now, the unexpected discovery of two new phases – regular and spin-valley half-metallicity – has been made.
Source: Moscow Institute of Physics and Technology

Read more: Physicists predict nonmetallic half-metallicity

Faza de invizibilitate(The beam of invisibility)

Un material complet opac este iradiat de sus, cu un model specific de undă – cu efectul că undele luminoase din stânga pot trece acum prin material fără nici o obstrucție.
Acest rezultat surprinzător deschide complet noi posibilități pentru camuflajul activ. Ideea poate fi aplicată diferitelor tipuri de valuri, ar trebui să funcționeze cu valuri de sunet la fel de bine ca și cu unde luminoase. Experimentele se află deja în planificare.Dacă acest model corespunde perfect neregularităților interioare ale materialului care de obicei dispersează lumina, atunci proiecția de deasupra poate opri efectiv împrăștierea și un alt fascicul de lumină care călătorește prin material dintr-o parte poate trece fără nici o obstrucție, împrăștiere sau pierderi.

Posted: Sep 13, 2017

The beam of invisibility

(Nanowerk News) How do we make an object invisible? Researchers from TU Wien (Vienna), together with colleagues from Greece and the USA, have now developed a new idea for a cloaking technology. A completely opaque material is irradiated from above with a specific wave pattern – with the effect that light waves from the left can now pass through the material without any obstruction.
This surprising result opens up completely new possibilities for active camouflage. The idea can be applied to different kinds of waves, it should work with sound waves just as well as with light waves. Experiments are already in the planning.
A material with random irregularities scatters an incident light wave into all directions
A material with random irregularities scatters an incident light wave into all directions.

Outwitting the Scattering of Light
“Complex materials such as a sugar cube are opaque, because light waves inside them are scattered multiple times”, says Professor Stefan Rotter (TU Wien). “A light wave can enter and exit the object, but will never pass through the medium on a straight line. Instead, it is scattered into all possible directions.”
For years many different attempts have been made to outwit this kind of scattering, creating a “cloak of invisibility”. Special materials have been worked out, for example, which are able to guide light waves around an object. Alternatively, also experiments have been performed with objects that can emit light by themselves. When an electronic display sends out exactly the same light as it absorbs in the back, it can appear invisible, at least when looked at in the right angle.
invisibility: The material is irradiated with a specially designed pattern, the wave from the left can pass through the object completely unperturbed
The material is irradiated with a specially designed pattern, the wave from the left can pass through the object completely unperturbed.
At TU Wien a more fundamental approach has now been chosen. “We did not want to reroute the light waves, nor did we want to restore them with additional displays. Our goal was to guide the original light wave through the object, as if the object was not there at all”, says Andre Brandstötter, one of the authors of the study („Wave propagation through disordered media without backscattering and intensity variations”). “This sounds strange, but with certain materials and using our special wave technology, it is indeed possible.”

The Laser Material
The team at TU Wien has spent years working on optically active materials, which are used for building lasers. To make the laser shine, energy has to be supplied by means of a pump beam. Otherwise, the laser material behaves just like any other material – it absorbs part of the incident light.
“The crucial point is to pump energy into the material in a spatially tailored way such that light is amplified in exactly the right places, while allowing for absorption at other parts of the material”, says Professor Konstantinos Makris from the University of Crete (previously TU Wien). “To achieve this, a beam with exactly the right pattern has to be projected onto the material from above – like from a standard video projector, except with much higher resolution.”
If this pattern perfectly corresponds to the inner irregularities of the material which usually scatter the light, then the projection from above can effectively switch off the scattering, and another beam of light travelling through the material from one side can pass without any obstruction, scattering or loss.
“Mathematically, it is not immediately obvious that it is at all possible to find such a pattern”, says Rotter. “Every object we want to make transparent has to be irradiated with its own specific pattern – depending on the microscopic details of the scattering process inside. The method we developed now allows us to calculate the right pattern for any arbitrary scattering medium.”

Light or Sound
Computer simulations have shown that the method works. Now the idea should be confirmed in experiments. Stefan Rotter is confident that this will be successful: “We are already discussing with experimentalists how this could be done. As a first step, we may test this technology with sound instead of light waves. Experimentally, they are easier to handle, and from a mathematical point of view, the difference does not matter significantly.”
Source: Vienna University of Technology