Fast and curious: Electrons hurtle into the interior of a new class of quantum materials

 

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Scientists at Princeton University have shown that negatively charged particles known as electrons can flow extremely rapidly due to quantum ...

 

As smartphones get smarter and computers compute faster, researchers actively search for ways to speed up the processing of information. Now, scientists at Princeton University have made a step forward in developing a new class of materials that could be used in future technologies.

They have discovered a new quantum effect that enables electrons -- the negative-charge-carrying particles that make today's electronic devices possible -- to dash through the interior of these materials with very little resistance.

The discovery is the latest chapter in the story of a curious material known as a "topological insulator," in which electrons whiz along the surface without penetrating the interior. The newest research indicates that these electrons also can flow through the interior of some of these materials.

"With this discovery, instead of facing the challenge of how to use only the electrons on the surface of a material, now you can just cut the material open and you have light-like electrons flowing in three dimensions inside the materials," said M. Zahid Hasan, a professor of physics at Princeton, who led the discovery.

The finding was conducted by a team of scientists from the United States, Taiwan, Singapore, Germany and Sweden and published in two papers in the journal Nature Communications. The first paper, published May 7, demonstrates that fast electrons can flow in the interior of crystals made from cadmium and arsenic, or cadmium arsenide. The second paper, published May 12, explores fast electrons in a material made from the elements bismuth and selenium.

In most materials, including copper and other metals that conduct electricity, electrons navigate an obstacle course of microscopic outcroppings, ledges and other imperfections that obstruct the tiny particles and send them scattering in the wrong directions. This causes resistance and the conversion of electrical current into heat, which is why electronic appliances become warm during use.

In topological insulators and the new class of materials the Princeton researchers studied, the unique properties of the atoms combine to create quantum effects that coax electrons into acting similar to a light wave instead of like individual particles. These waves can weave around and dodge -- and even move through -- barriers that would typically stop most electrons. These properties were theoretically proposed by Charles Kane and a team at the University of Pennsylvania from 2005 to 2007 and first observed experimentally in solid materials by the Hasan group in 2007 and 2008.

In 2011, the Hasan group detected this fast electron-flow in the interior of a material made from combining several elements -- bismuth, thallium, sulfur and selenium. The results were published in the journal Science.

In the new study in cadmium arsenide, the electrons have an average velocity that is 10,000 times more than that of the previous bismuth-based materials identified by the group. "This is a big deal," Hasan said. "It means the electrons can flow quite easily in the material and many more exotic quantum effects can now be studied. That just wasn't possible in the past."

The most promising application for these materials may be for a proposed "topological quantum computer" based on novel electronics that would use a property of electrons known as "spin" to do calculations and transmit information.

The quantum behavior in this new class of materials has led them to be called "topological Dirac semi-metals" in reference to English quantum physicist and 1933 Nobel Prize winner Paul Dirac, who noted that electrons could behave like light. Semi-metals that are "topological" are ones that retain their spatial electronic properties -- and their speedy electrons -- even when deformed by certain types of stretching and twisting.

The speeds achieved by these electrons have led to comparisons to another novel electronic material, graphene. The new class of materials has the potential to be superior to graphene in some aspects, Hasan said, because graphene is a single layer of atoms in which electrons can flow only in two dimensions. Cadmium arsenide permits electrons to flow in three dimensions. The new study redefines what it means to be a topological material, according to Su-Yang Xu, a graduate student in Hasan's lab and co-first author of the May 7 paper with postdoctoral research associate Madhab Neupane at Princeton and Raman Sankar of National Taiwan University.

"The term topological insulator is now quite famous, and the yet term 'insulator' means that there are no electrons flowing in the bulk of the material," Xu said. "Our study shows that electrons are flowing in the bulk of the material, so clearly cadmium arsenide is not an insulator, but it is still topological in nature, so this is a totally new type of quantum matter," he said.

The team made the discovery using a technique called angle-resolved photoemission spectroscopy. The researchers shined a very powerful X-ray beam -- using a particle accelerator at the Advanced Light Source at Lawrence Berkeley National Laboratory -- onto the surface of the material then monitored the electrons as they were knocked out of the interior.

"When the electron comes out, we measure its energy and velocity, and what we found is that electrons coming out of the cadmium arsenide had measurements that were similar to what is seen in particles that are massless," Neupane said.

In the second paper in Nature Communications, Neupane and co-authors presented a model for controlling the spin direction of the electron particles in a different material, bismuth selenide.

"The Princeton group showed in exquisite details that electrons in certain solids obey the three- dimensional massless Dirac equation," said Patrick Lee, a professor of physics at the Massachusetts Institute of Technology who was not involved in the work. "While predicted by theoretical calculations, this behavior has never been seen before in real materials until this past year. This work adds greatly to the ongoing excitement of how topology can impact electronic states in real materials."

Lighting the way to graphene-based devices

 

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Semiconductors made from graphene and boron nitride can be charge-doped using light. When the GBN heterostructure is exposed to light (green arrows), positive charges move from the graphene layer (purple) to boron nitride layer (blue).

Graphene continues to reign as the next potential superstar material for the electronics industry, a slimmer, stronger and much faster electron conductor than silicon. With no natural energy band-gap, however, graphene's superfast conductance can't be switched off, a serious drawback for transistors and other electronic devices. Various techniques have been deployed to overcome this problem with one of the most promising being the integration of ultrathin layers of graphene and boron nitride into two-dimensional heterostructures. As conductors, these bilayered hybrids are almost as fast as pure graphene, plus they are well-suited for making devices. However, tailoring the electronic properties of graphene boron nitride (GBN) heterostructures has been a tricky affair, involving chemical doping or electrostatic-gating -- until now.

Researchers with Berkeley Lab and the University of California (UC) Berkeley have demonstrated a technique whereby the electronic properties of GBN heterostructures can be modified with visible light. Feng Wang, a condensed matter physicist with Berkeley Lab's Materials Sciences Division and UC Berkeley's Physics Department, as well as an investigator for the Kavli Energy NanoSciences Institute at Berkeley, led a study in which photo-induced doping of GBN heterostructures was used to create p-n junctions and other useful doping profiles while preserving the material's remarkably high electron mobility.

"We've demonstrated that visible light can induce a robust writing and erasing of charge-doping in GBN heterostructures without sacrificing high carrier mobility," Wang says. "The use of visible light gives us incredible flexibility and, unlike electrostatic gating and chemical doping, does not require multi-step fabrication processes that reduce sample quality. Additionally, different patterns can be imparted and erased at will, which was not possible with doping techniques previously used on GBN heterostructures."

Graphene is a single layer of carbon atoms arranged in a hexagonal lattice. Boron nitride is a layered compound that features a similar hexagonal lattice -- in fact hexagonal boron nitride is sometimes referred to as "white graphene." Bound together by the relatively weak intermolecular attraction known as the van der Waals force, GBN heterostructures have shown high potential to serve as platforms not only for high-electron-mobility transistors, but also for optoelectronic applications, including photodetectors and photovoltaic cells. The key to future success will be the ability to dope these materials in a commercially scalable manner. The photo-induced modulation doping technique developed by Wang and a large team of collaborators meets this requirement as it is comparable to the photolithography schemes widely used today for mass production in the semiconductor industry. Illumination of a GBN heterostructure even with just an incandescent lamp can modify electron-transport in the graphene layer by inducing a positive-charge distribution in the boron nitride layer that becomes fixed when the illumination is turned off.

"We've shown show that this photo-induced doping arises from microscopically coupled optical and electrical responses in the GBN heterostructures, including optical excitation of defect transitions in boron nitride, electrical transport in graphene, and charge transfer between boron nitride and graphene," Wang says. "This is analogous to the modulation doping first developed for high-quality semiconductors."

While the photo-induced modulation doping of GBN heterostructures only lasted a few days if the sample was kept in darkness -- further exposure to light erased the effect -- this is not a concern as Wang explains.

"A few days of modulation doping are sufficient for many avenues of scientific inquiry, and for some device applications, the rewritability we can provide is needed more than long term stability," he says. "For the moment, what we have is a simple technique for inhomogeneous doping in a high-mobility graphene material that opens the door to novel scientific studies and applications."

A paper on this research has been published in the journal Nature Nanotechnology entitled "Photoinduced doping in heterostructures of graphene and boron nitride."

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Story Source:

The above story is based on materials provided by DOE/Lawrence Berkeley National Laboratory. The original article was written by Lynn Yarris. Note: Materials may be edited for content and length.

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Journal Reference:

1. L. Ju, J. Velasco, E. Huang, S. Kahn, C. Nosiglia, Hsin-Zon Tsai, W. Yang, T. Taniguchi, K. Watanabe, Y. Zhang, G. Zhang, M. Crommie, A. Zettl, F. Wang. Photoinduced doping in heterostructures of graphene and boron nitride. Nature Nanotechnology, 2014; 9 (5): 348 DOI: 10.1038/nnano.2014.60

How to Train For a 10K

 

Want to tackle a 6.2-mile race? Try this intermediate running plan that helps you build to 60 minutes of continuous running.

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Running just might be the most convenient workout going. You don't need to be a skilled athlete, and there's no fancy equipment involved; just lace up your sneaks and go. It's also one of the most efficient ways to blast fat and burn calories—about 600 an hour.

Sure, walking has its benefits, but research shows that running kicks its butt when it comes to shedding pounds. One recent study of 47,000 runners and walkers, from the Lawrence Berkeley National Laboratory in Berkeley, Calif., found that the runners burned more calories and had a far greater decrease in BMI over a six-year period. The joggers who started out heaviest (those with a BMI over 28) lost up to 90 percent more weight than  the walkers did.

Dropping pounds and toning up are hardly the only benefits of this killer cardio workout: You'll also reduce your risk of heart disease and diabetes, boost your mood, temper stress and build muscle, especially in the lower body and core. You don't even need to dedicate a lot of time to reap these rewards; do 20 to 30 minutes, three to four days a week, and you'll see significant improvement.
Ready to hit the road? Here's a plan for intermediate runners. And it's smart to add in one day of cross-training (think cycling or swimming) to rev up calorie burn and help prevent injury. Soon enough, you'll feel as if you were born to run.
Your stats: You're a "sometimes" runner who does at least three miles without stopping a couple of days a week, most weeks.
The goal: Increase your endurance, run for an hour straight and tackle a 10K by the end of 10 weeks.
Your coach: Jonathan Cane is an exercise physiologist and co-founder of City Coach Multisport in New York City.
The plan: Do three different running workouts every week, on alternate days. In the first run, build speed through intervals; start with a two-minute speed burst at a challenging but sustainable pace, followed by three minutes of easier recovery jogging. Repeat six times for a total of 30 minutes. As the weeks pass, alternate between building up the speed bursts and balancing out the recovery time. For your second weekly workout, which focuses on mixing speed and endurance, begin with running for a couple of miles and build up to 4 ½ miles over the course of the plan. The third day helps you build endurance. Focus on covering the distance, not your pace. Kick off with a 2 ½-mile run. Over 10 weeks, try to work up to running 5 ½ miles.
HERE'S YOUR GUIDE: Intermediate 10K Training Plan
Train smarter!
1. Make three the magic number
If you're used to running twice a week, says Cane, "three times is your sweet spot—you'll get a big bump in both speed and endurance, but it's not so much more that you'll risk getting injured." And if weight loss is a goal, remember that adding just one extra day of running helps you burn an additional 300 to 400 calories, depending on your pace and size.
2. It's OK to hit the treadmill
Some running purists say there's no substitute for the outdoors, but all things being equal, "your heart and lungs don't really know the difference between the road and the treadmill," says Cane. So if it's late in the day, raining or just not a good time to go outside but you really want to keep up your training, feel free to hit the "on" button. To compensate for a lack of wind resistance and natural terrain changes, keep the treadmill deck set at a 1% incline.
3. Turn down the music
Yes, pumping JT through your earbuds can power you up that hill, but don't forget to tune in to how your body feels. "At this stage, you know you can already run for a while," says Cane. "But it's important to be aware of cues: how heavy you are breathing, or if you have a small twinge in your knee and need to slow down. It helps keep you from getting injured and makes you more aware of when you can bump up your pace or give a little more effort."

What to Do If You're Allergic to Your Pet

 

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If you could snap your fingers and make your allergies disappear, youd probably do it in a second. But what if your pet is the cause of your watery eyes, sneezing, and runny nose? Suddenly that oh-so-simple decision becomes a much tougher call. For some, the psychological misery of giving up a pet may outweigh the everyday misery of allergy symptoms.
The problem starts with dander, one of the most stubborn and common allergens. Cats, dogs, and other furry or feathered pets produce dander, which consists of microscopic, dandruff-like flakes of skin and proteins from saliva and urine that can trigger allergies and aggravate asthma. If youre allergic to dander, the easiest route to allergy relief is to find your pet a new home.

In fact, thats what most doctors will tell you to do. Yet this advice is rarely welcomed or followed, even when the pet is causing serious problems. “Some families cant fathom giving away their pet—its almost like giving away one of their children,” says Anne Miranowski, MD, an allergist at the Pediatric Lung Center in Fairfax, Va. “I see some children where exposure to their three cats is clearly making them sick, and the family insists on keeping all three cats.”
Physicians and health organizations recognize the attachment that people have to their pets. If a family is unwilling to remove a pet, experts recommend a host of alternative measures, such as limiting contact between the pet and the allergic person (by keeping pets outdoors or out of bedrooms, for instance) and using air cleaners. (Find out the best ways to reduce pet allergens in your home.)
These measures arent nearly as effective as giving away a pet, however, and going this route will likely have consequences—more symptoms, more medication, and a potential worsening of asthma—that should be weighed against the distress of seeing a cat or dog pitter-patter out of your life forever. And although there may be some breeds touted as better for people with allergies—think the Obamas and their Portuguese water dog, Bo—there are no cats or dogs truly free from dander.
You pet may not be causing your symptoms
Before you think about finding a pet a new home, its important to figure out if you—or your children—are in fact allergic. Though it seems as if pet allergies should be obvious, they are sometimes harder to recognize than you think.
If your eyes start to swell and you sneeze uncontrollably every time you are near a cat, then yes, you are probably allergic to cats. But some people with allergies or asthma who grow up around animals and are in contact with them every day may have more subtle symptoms. Instead of watery eyes and the other classic signs of pet allergies, they may experience chronic, low-level congestion, for instance.
“A lot of times people will say, ‘My dog or cat doesnt bother me, but when somebody is exposed to a pet day in and day out, they dont have the dramatic symptoms every time they see it,” says Andy Nish, MD, an allergist at the Allergy & Asthma Care Center, in Gainesville, Ga. “It may be a more subtle and chronic inflammatory process, and they may not realize that the pet is causing them problems.”
This phenomenon sometimes works in reverse: In some cases, people with asthma may believe that their pet is causing them more problems than it actually is. A respiratory condition in which the lung airways are chronically inflamed, asthma can be triggered by substances other than pet dander, such as dust mites, exhaust, smoke, and cold air, or even allergens from rodents and cockroaches. For some (but certainly not all) asthmatics, pets may actually be a relatively minor contribution to their symptoms, and some asthmatics may not be allergic to pets at all.
“Theres no reason to consider removing a pet unless you can demonstrate that there is a sensitization to that type of animal,” says Gregory Diette, MD, an asthma specialist and associate professor of medicine at the Johns Hopkins University School of Medicine, in Baltimore. “One mistake Ive seen [physicians] make is to generally recommend that asthmatics not have cats, dogs, or other furred pets when they havent done the allergy testing to prove whether theres an abnormal response to that type of animal.”
The easiest way to pinpoint a pet allergy is to visit an allergist and get a series of skin tests, in which the skin is exposed to small samples of the proteins shed by cat, dog, and other allergy-triggering substances, such as pollen or dust. Allergists may also use a blood test known as a RAST as an alternative to or in addition to skin tests.

Pfizer to apply for early U.S. approval of breast cancer drug

 

 

(Reuters) - Pfizer Inc, which is in the process of trying to acquire British rival AstraZeneca for more than $100 billion, on Friday said it planned to seek U.S. approval for palbociclib, its experimental drug for advanced breast cancer, in the third quarter.

The largest U.S. drugmaker said it made the decision to submit its application to the Food and Drug Administration following discussions with the regulatory agency about the results of a midstage clinical trial.

Palbociclib had received the FDA's new breakthrough designation given to help speed up the approval process for potentially important new medicines.

Pfizer presented data from the trial at a cancer meeting last month. Patients in the study who took the experimental Pfizer medicine plus the hormone drug letrozole went 20.2 months on average before their cancer began to worsen, compared with 10.2 months for those that received only letrozole.

Palbociclib is considered to be one of the most important medicines in Pfizer's developmental pipeline. A rival drug is also being developed by Eli Lilly and Co, which several analysts have said appears promising.

Pfizer shares were up 1.2 percent at $29.41 in early afternoon trading on the New York Stock Exchange.

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Coração saudável requer pouco sal

 

Doença cardíaca é uma das principais causas de morte no mundo. O estilo de vida moderno, com muito estresse, ausência de atividade física e alimentação pouco saudável é responsável por aproximadamente 80% das doenças cardiovasculares. Dentre os vários fatores de risco que contribuem para doença cardiovascular, o principal é a pressão alta, responsável por 62% dos derrames e 49% dos ataques cardíacos.

Um dos fatores alimentares que é crítico para a o aumento da pressão arterial e suas consequências indesejáveis é o elevado consumo de sal. Resultados de estudos que avaliaram a redução do consumo de sal por um curto espaço de tempo indicam uma relação direta de dose resposta entre a ingestão de sal e a pressão arterial. A redução de 1 g de sal por dia contribui para a queda de um ponto (medido em milímetros de mercúrio - mmHg) da pressão sistólica (a mais alta das duas).

Agora, uma nova pesquisa, mais longa que as anteriores, realizada na Inglaterra, amplia a análise da relação entre sal e doença cardiovascular. No período de 2003 a 2011 houve uma redução de 15 por cento no consumo de sal. Neste mesmo período, as mortes por doença cardíaca caíram 40 por cento e por derrame 42 por cento. Esta associação expõe o impacto do consumo de sal na saúde da população.

Mesmo que vários outros fatores possam ter uma parcela de contribuição nestas reduções, é indiscutível a importância do sal como um fator isolado no controle da pressão arterial e de suas consequências.

Mesmo que tenha havido uma significativa redução no consumo de sal nos últimos anos, os pesquisadores alertam que o consumo na Inglaterra, continua acima do máximo recomendado por dia (que é 6g de sal). Isto ocorre em vários países ocidentais.

Uma das dificuldades para o controle da ingestão de sal é o crescente uso de alimentos industrializados na dieta do dia-a-dia. Estima-se que 75% do sal ingerido na dieta ocidental provenha da comida industrializada.

Estes resultados dão elementos para reforçar as campanhas institucionais para redução da ingestão de sal, reduzindo o risco de doenças cardiovasculares e promovendo a saúde.

Tabagismo e doença cardíaca nas mulheres

 

ABC da Saúde

O que as mulheres devem fazer para prevenir doença cardíaca

Hoje são conhecidos dezenas de fatores de risco que contribuem para o desenvolvimento de doenças e morte, sendo os principais: hipertensão, tabagismo, dieta pobre em frutas, uso de álcool, poluição ambiental, alto índice de massa corporal (sobrepeso e obesidade), aumento da glicose no sangue, baixa atividade física, dentre tantas outras. A importância específica de cada um destes fatores de risco pode, entretanto, variar conforme a região do planeta, sexo, faixa etária, etc. Um dos problemas das pesquisas na área da saúde - tanto básicas, quanto clínicas e epidemiológicas - é a dificuldade em considerar essas particularidades. Especialmente no que tange à saúde da mulher.

Pois agora, um recente estudo realizado na Austrália revela dados que podem contribuir enormemente para a prevenção de doenças cardíacas em mulheres. Os pesquisadores observaram, desde 1996, mais de 32 mil mulheres subdivididas em 15 diferentes grupos de idades, sendo o subgrupo mais jovem de 22 a 27 anos e o mais velho de 85 a 90 anos. As mulheres nasceram em diferentes períodos, de 1921 a 1926, de 1946 a 1951 e de 1973 a 1978.

O estudo analisou os quatro maiores fatores de risco para doenças cardíacas na Austrália - hipertensão, tabagismo, excesso de peso e sedentarismo - juntos, estes fatores são responsáveis por mais da metade das doenças cardíacas no mundo todo. A análise se concentrou na melhoria dos padrões de saúde com a ausência ou retirada de cada fator de risco, individualmente (nunca ter fumado ou manter peso ideal, por exemplo).

As mulheres do subgrupo mais jovem (22-27 anos) são as que mais fumam (28% do total), e isto aumentou o risco cardíaco em 59%. A proporção de fumantes se reduziu muito nos subgrupos mais velhos, chegando a 5% nas de 73-78 anos. Por sua vez a inatividade física vai aumentando dos 22 aos 90 anos. A conclusão dos pesquisadores é que nas mulheres acima dos 30 anos, até o final do 80s, o sedentarismo, individualmente, é o maior responsável pelas doenças cardíacas. Eles estimam que 150 minutos de atividade física por semana reduziria drasticamente o desenvolvimento de doenças cardiovasculares, poupando milhares de vidas de mulheres anualmente. Nas mais jovens o tabagismo é o principal fator de risco.

Mulheres: Está claro que para viver mais e melhor, segundo as tendências atuais, o mais importante para as com menos de 30 anos é não fumar, e as com mais de 30 ter uma atividade física regular para o resto da vida.

Men and Women Smoke—and Quit—Differently

 

Studies have shown that after six months on nicotine replacement products, women slide back into the habit at higher rates than men.

In 2001, a group of young cigarette smokers was fitted with opaque goggles and nose clips at the University of Pittsburgh School of Medicine. After lighting up, the women in the group collectively rated their cigarettes less "satisfying" than the men did. In another study, women who sampled both standard and low-nicotine cigarettes noted less of a difference in their enjoyment and perceived nicotine intake than male participants did.

These are just a few of the research findings into what has emerged as an apparent gender gap between male and female smokers. This research suggests that men smoke mainly for the nicotine, while women tend to care about the smell and taste, the hand-to-mouth sensation, weight control, and boosting their moods.
Although no one really understands what causes these differences, experts say considering them as you implement your quitting strategy might just give you an edge.

Nicotine replacement doesnt work as well for women

The sexes have about the same rates of success with prescription smoking-cessation drugs, but studies of nicotine replacement therapy (NRT), such as the patch and nicotine gum, reveal differences. NRT seems to help both men and women get through those tricky first few months without cigarettes, but after six months, women slide back into the habit at higher rates than men.
On the other hand, gender differences in smoking addiction may also account for an interesting exception to the NRT gender gap: the inhaler, the small, plastic cigarette-holder-shaped device that provides a dose of vaporized nicotine when you take a determined drag from it. In a 2001 study of 504 smokers, inhalers proved more effective for women than for men (at least in the short term), while men experienced more success with the other three options: spray, patch, and especially gum.
"Women lose both the sensory cues and the nicotine when they quit smoking," Cora Lee Wetherington, PhD, explained in a 2002 article by the National Institute on Drug Abuse (NIDA), where she is the women and gender research coordinator. "Therefore, replacing those cues—something the inhaler can do, but not the patch or gum—and learning ways to avoid or cope with those cues may help more women succeed in quitting."

Women shouldn't sweat weight gain, but should consider their periods

More than half of women smokers say in studies that weight concerns are a major obstacle to quitting. Theres no surprise there, but such research suggests that cognitive behavioral therapy (CBT) might be especially well suited to women trying to quit because it can help them leap that psychological hurdle and accept that, yes indeed, they will probably gain some weight when they quit. A 2001 study following more than 200 women who quit smoking found that after a year, those who received CBT stayed off cigarettes at higher rates—and gained half as much weight, on average—as those who entered a weight-control program.
Women might do well to combine their CBT with a glance at their menstrual calendars as well. Studies show menstruation has a unique effect on tobacco withdrawal symptoms.
"Our work and [the work of] others have shown that women who quit in the luteal phase had more nicotine withdrawal symptoms, and [they] suggest that quitting in the follicular phase may be best," says Michele D. Levine, PhD, an assistant professor of psychiatry and psychology at the University of Pittsburgh Medical Center.
Translation: Women shouldn't exacerbate monthly moodiness (and withdrawal symptoms) by trying to quit smoking in the middle of PMS; it's best to wait until the first day after a period starts to toss those butts.

20 Ways to do a Plank

 

 

Hello abs! Change up your core workout with these fresh variations on the plank.

Cardio Workouts

 

 

5 Fat-burning body weight exercises