Explainer: what is a superconductor?

Superconducting materials have strange and unusual properties including magnetic levitation. Shutterstock

Materials can be divided into two categories based on their ability to conduct electricity. Metals, such as copper and silver, allow electrons to move freely and carry with them electrical charge. Insulators, such as rubber or wood, hold on to their electrons tightly and will not allow an electrical current to flow.

In the early 20th century physicists developed new laboratory techniques to cool materials to temperatures near absolute zero (-273 °C), and began investigating how the ability to conduct electricity changes in such extreme conditions. In some simple elements such as mercury and lead they noticed something remarkable – below a certain temperature these materials could conduct electricity with no resistance. In the decades since this discovery scientists have found identical behaviour in thousands of compounds, from ceramics to carbon nanotubes.

We now think of this state of matter as neither a metal nor an insulator, but an exotic third category, called a superconductor. A superconductor conducts electricity perfectly, meaning an electrical current in a superconducting wire would continue to flow round in circles for billions of years, never degrading or dissipating.

Electrons in the fast lane

On a microscopic level the electrons in a superconductor behave very differently from those in a normal metal. Superconducting electrons pair together, allowing them to travel with ease from one end of a material to another. The effect is a bit like a priority commuter lane on a busy motorway. Solo electrons get stuck in traffic, bumping into other electrons and obstacles as they make their journey. Paired electrons on the other hand are given a priority pass to travel in the fast lane through a material, able to avoid congestion.

Superconductors have already found applications outside the laboratory in technologies such as Magnetic Resonance Imaging (MRI). MRI machines use superconductors to generate a large magnetic field that gives doctors a non-invasive way to image the inside of a patient’s body. Superconducting magnets also made possible the recent detection of the Higgs Boson at CERN, by bending and focusing beams of colliding particles.

Superconductors are used in medical Magnetic Resonance Imaging. Jan Ainali, CC BY

One interesting and potentially useful property of superconductors arises when they are placed near a strong magnet. The magnetic field causes electrical currents to spontaneously flow on the surface of a superconductor, which then give rise to their own, counteracting, magnetic field. The effect is that the superconductor dramatically levitates above the magnet, suspended in the air by an invisible magnetic force.

What prevents more widespread use of these materials is the fact that the superconductors we know about operate only at very low temperatures. In the simple elements for instance superconductivity dies out at just 10 Kelvin, or -263 °C. In more complicated compounds, such as yttrium barium copper oxide (YBa₂Cu₃O₇), superconductivity may persist to higher temperatures, up to 100 Kelvin (-173 °C). While this is an improvement on the simple elements, it is still much colder than the coldest winter night in Antarctica.

Scientists dream of finding a material where superconductive properties can be used at room temperature, but it’s a challenging task. Turning up the temperature tends to destroy the glue that binds the electrons into superconducting pairs, which then returns a material back to its boring metallic state. One of the great challenges in the field arises from the fact that we don’t yet understand very much about this glue, except in a few limited cases.

From superatom to superconductor

New research from the University of Southern California has taken a novel step towards improving our understanding of how superconductivity arises. Rather than study superconductivity in bulk materials like wires, Vitaly Kresin and his coworkers have managed to isolate and examine small clumps of a few dozen aluminium atoms at a time. These tiny clusters of atoms can act like a “superatom”, sharing electrons in a way that mimics a single, giant atom.

What is surprising is that measurements of these clusters reveal what may be the signature of electron pairing persisting all the way up to 100 kelvin (-173 °C). This is still a frosty temperature of course, but it is 100 times higher than the superconducting temperature of a piece of aluminium wire. Why does a small handful of atoms superconduct at a much higher temperature than the millions of atoms that form a wire? Physicists have some ideas but the effect is largely unexplored, and it might prove an interesting way forward in the quest for superconductivity at higher temperatures.

The need for speed: MagLev trains in Japan use superconductivity to achieve ultra-high speeds. Shutterstock

Hoverboards anyone?

If physicists were able to achieve the goal of room temperature superconductivity in a material that was easy to fashion into wires, important new technologies would soon follow. For starters, devices which use electricity would become considerably more efficient and consume less power.

Transporting electricity over long distances would also become much easier, which is particularly useful for renewable energy applications – and some have proposed giant superconducting cables linking Europe with solar energy farms in North Africa.

The fact that superconductors will levitate above a strong magnet also creates possibilities for efficient, ultra-high speed trains that float above a magnetic track, much like Marty McFly’s hoverboard in Back to the Future. Japanese engineers have experimented with replacing the wheels of a train with large superconductors that hold the carriages a few centimetres above the track. The idea works in principle, but suffers from the fact that the trains need to carry expensive tanks of liquid helium with them in order to keep the superconductors cold.

Many superconducting technologies will probably remain on the drawing board, or too expensive to implement, unless a room temperature superconductor is discovered. It’s just possible however that the advances made by Kresin’s group might mark a milestone on this journey.

There’s no evidence human pheromones exist – no matter what you find for sale online

Whatever the adverts suggest, this isn't going to increase your animal magnetism. Thinglass/Shutterstock

The idea of human pheromones is intuitively appealing, conjuring up the idea of secret signals that make us irresistible to potential partners. But this connection of pheromones with sex may be the wrong way to look at the issue – because despite 45 years of study and various claims over the years there’s still not a lot of evidence that human pheromones exist at all.

The study of pheromones of all kinds is problematic – even the definition is controversial. The word comes from the Greek pherein (to transfer), and hormōn (to excite) and was defined by Karlson and Luscher in 1959 as:

Substances which are secreted by an individual and received by a second individual of the same species, in which they release a specific reaction, for instance a definite behaviour or developmental process.

The snag is that that while many researchers agree on the basic properties of pheromones, there is considerable debate over which olfactory (sense of smell) cues represent pheromones. For example, many species use odours to identify characteristics such as species, sex, relatedness and social status. Many researchers label these odours as pheromones; others feel that by the above definition they’re really just smells.

Similarly not all potential pheromones are secreted externally – some species of salamanders transfer chemical signals to another salamander by directly injecting them into the bloodstream. Some scientists believe that the response to a pheromone should provide an evolutionary advantage to both the sender and the receiver of the signal, and do so unconsciously.

So a lack of consensus on pheromones' definition has led to the over-use of this term. Instead many scientists instead use the term semiochemicals to refer to chemicals that transmit some form of specific message that can influence a recipient’s physiology and behaviour.

That’s right - just two dabs behind the antennae and they’re all over me L. Shyamal, CC BY-SA

Studying smells is hard

Given these problems why are researchers so interested in pheromones at all? Generally olfaction is one of the most crucial forms of communication in the animal world. Odours have the greatest potential range of any method of animal communication, can be transmitted in total darkness and around obstacles. Unlike signals to be seen or heard, odours also remain in the environment for extended periods, providing the opportunity to lay signals – such as when marking territory.

But studying human pheromones is problematic for a number of reasons beyond definition. Olfactory research can be extremely tricky to conduct: smells are invisible and hard to control, there is no real standardised system for labelling and evaluating odours, and a wealth of potentially confounding variables need to be controlled for. Also the problem is that humans can evaluate signals in a variety of quite divergent ways – it’s rare that we show simplistic stimulus–response reaction.

Four pheromone candidates

Four specific substances have been identified as possible human pheromones.

In the 1970s and 1980s, there was a strong focus on testosterone-derived androstenone and androstenol, possible pheromones in pigs also found in human armpits. A number of studies have investigated the effect of these substances on human behaviour, focusing on social interactions and the evaluation of sexual partners. Despite a general pattern for these substances to increase social contact between males and females, findings are extremely inconsistent.

In the 1990s the focus shifted to the similar androstadienone and oestratraenol, an oestrogen-derived substance produced in pregnant women. These were the compounds studied in several experiments that examined the vomeronasal organ (VNO) – a tubular structure located in the nasal cavity which, in some species, is involved in processing pheromones.

Several studies documented finding a VNO in more than 90% of human participants, and reported that stimulating the VNO with artificially-created “putative human pheromones” seemed to stimulate the recipients. This suggested the existence of human pheromones, as a functioning VNO would provide humans the ability to process pheromones.

‘Your powers of magnetic attraction be damned, I’m not kissing a man with nicer braids than mine.’ Frédéric Soulacroix

However, more recent studies cast doubt on this idea, with no evidence that the few VNOs identified in humans have any functional receptor cells to detect anything – the VNO isn’t actually connected to the brain. And those putative “synthetic human pheromones” provided to the studies that claimed to show evidence of their effect on the VNO? It’s been pointed out that they had been provided by EROX – a firm with a commercial interest in patenting and selling them. You’ll find EROX and scores of other firms selling similar products on the internet today.

Research into these four pheromone candidates suffers from all sorts of problems. The substances are used in concentrations between several and millions of times higher than they occur naturally in humans. Experiments tend to be beset with methodological and statistical issues, leading to a contradictory or inconclusive findings. Publication bias leaves it likely that only positive results are published, artificially increasing the amount of supposedly supportive evidence, and findings have often not been independently replicable.

In any case, even if these substances do effect human physiology and behaviour it doesn’t necessarily mean they’re pheromones – there are numerous odours from plants or from industrial chemicals that can produce a behavioural reaction in humans.

The way forward

Tristram Wyatt, in his recent paper for the Royal Society, suggests that we move away from the sexual focus on pheromones. Instead we should focus not just on the substance present but on the range of odours that humans are capable of producing from a variety of sites on the body.

Wyatt’s suggestion is the secretions from the areola of mothers' lactating breasts is a good place to start looking, as smell is very important to suckling behaviour in animals. Any baby, presented with the secretions of any mother, will respond with nipple-searching behaviour, even while asleep.

The search for human pheromones taps into our mysterious sense of smell and appeals to us on an emotional level. Of course, there are also strong commercial motivations to demonstrating their existence and the products that might follow. There is already a well-documented history of this occurring in the field of olfaction research – and these motivations inevitably muddy the water. We need to address these issues with a much more rigorous approach if the science is to progress.

How pigeons bob and weave through obstacles

Pigeons weigh danger, stability and efficiency in the aerial acrobatics they use to avoid obstacles, study shows.

Courtesy of C. David Williams

View the video

To dodge obstacles, pigeons have to know when to hold ‘em and when to fold ‘em. By altering their wing posture, the birds can successfully navigate tight spots, researchers report March 2 in the Proceedings of the National Academy of the

Exposed: the private lives of hairy-chested 'Hoff crabs'

'Hoff' crabs live in the strange realm of deep-sea vents. NERC Chesso Consortium

One-and-a-half miles below the icy surface of the Southern Ocean, some of the world’s coldest seawater meets one of the seafloor’s hottest environments, at undersea hot springs – known as hydrothermal vents – on the ocean floor. Recent research in the Journal of Animal Ecology reveals the private lives of “Hoff crabs” – so-called because of their hairy chest – in this remarkable environment.

At this depth there is no sunlight to support life. But a lost world of deep-sea creatures thrives around the undersea hot springs, ultimately nourished by a process called chemosynthesis. Akin to photosynthesis (the process by which plants use energy from sunlight to make food), chemosynthetic bacteria use chemical energy in warm sulfide-rich fluids gushing from the deep-sea vents to build organic molecules, forming the base of the food chain in these environments.

This bacterial bounty supports abundant animal populations around the deep-sea vents, and different species dominate at different distances from the sources of hot fluids, forming patterns similar in scale to those found among animals on rocky shores.

Unlike rocky shores, however, ecologists cannot stroll around deep-sea volcanic vents with ease, and need to use technology such as remotely operated vehicles (ROVs) to investigate patterns of life in these environments.

In 2010, a British research expedition revealed the marine life at deep-sea vents in the Southern Ocean. The species and patterns of life at these vents were different to those previously found at vents elsewhere, such as in the Pacific and Atlantic Oceans.

The zone closest to where hot fluids jet from the vents is dominated by the Hoff crab (belonging to a genus called Kiwa). These crabs bustle in warm waters of around 20°C, with hundreds of crabs per square metre. To survive, they “farm” chemosynthetic bacteria on their chest hairs for food, using comb-like mouthparts to harvest them.

Small female (left); large male (right) NERC Chesso Consortium

Separation of the sexes

Because of the conflicting demands of feeding and raising young in the conditions at deep-sea vents, male and female crabs lead largely separate lives.

At the base of the mineral spires that form at the vents, males mingle with females in spectacular piles many crabs deep, where they get together to mate. The females then crawl away from the bustling piles of crabs and the warm mineral-rich fluids seeping from the seafloor, which can be toxic to their young.

Away from the mineral spires and warm fluids, the few crabs found are either small juveniles, or females carrying developing offspring under their curled-up tails.

Hundreds of male and female crabs mingle in the warm waters at the base of the chimney NERC Chesso Consortium

Moving away from the warmer waters of the hydrothermal vents takes the females across a gauntlet of predators, such as large sea anemones and seven-arm seastars. Away from the vents, the cold water of the deep Antarctic also slows down the metabolism of the adult female crabs, making them less active than in the warmer waters of the jostling heaps. However, the conditions away from the vents may be more stable and less harmful to their offspring for their early development, making the journey of the females worthwhile.

For the small juveniles, strength may be found in numbers. These crabs appear to be heading back towards the vents, where conditions are better for gardening bacteria on their hairy chests, providing the food they need to grow. Hundreds of these small individuals were found nestled amongst the piles of larger crabs, fighting together en masse for the warmer sulfide-rich waters provided by the vents.

Males, meanwhile, don’t share in child-care arrangements with the females, and instead can climb up the mineral spires of the vents to take advantage of the warmth and conditions best suited for growing bacteria on their hairy chests – growing much larger than the females as a result.

Deep-sea exploration over the last four decades, including our scientific research cruise to the Southern Ocean, has revealed that undersea vents from different regions of the world’s oceans are dominated by different species of animals, and why we find these differences remains an unanswered question in ecology – but every discovery is another piece in the puzzle.

The UK doesn't need net neutrality regulations ... yet

Neutrality in style and substance. mindscanner/Shutterstock

The net neutrality debate in the US has ended, at least for now, with the Federal Communications Commission ruling for stricter regulation of telecoms and internet service providers (ISPs) in order to maintain a level playing field. But why hasn’t the same debate been had in the UK?

There is an idea in game theory (a branch of economics) called the price of anarchy which tries to represent a measure of how badly a system (such as a market) operates due to the selfish behaviour of those involved in it.

This seems like a rather good way to look at net neutrality, the debate around which is about how well the internet as a system is able to function in the presence of competition between the ISPs and telecom firms that provide access to it.

For us customers, think of this in terms of the neutrality of ISPs – these are the firms that connect individuals, homes, businesses, and institutions to the internet. How they act can be seen as a measure of how well they are able to compete.

For example, the degree to which ISPs use traffic management or “middlebox” network appliances to adjust the traffic flowing through their networks might be a measure of how well or poorly they are able to compete with their direct competitors – other ISPs. The same could be said for the degree to which they may try to play the vertical market – their suppliers and customers – by either reducing their costs for carrying bulky (and therefore expensive) content such as video, or by improving performance for a particular group of users who have preference for a particular service or content such as TV-on-demand (and who may be willing to pay for it).

In the US, the FCC has found it necessary to rule in favour of enforcing neutrality with regulation because there was a lot of this sort of gaming of the market going on. Its ruling stated:

The nature of broadband internet access service has … changed [and] broadband providers have even more incentives to interfere with internet openness today.

The regulations therefore dictate that there is to be no blocking, no throttling, and no fast lanes by ISPs.

In the UK’s broadband market, as in Europe, Japan and South Korea, it seems the market is operating properly and so effective competition has reigned in the selfish behaviour of operators – anarchy is working well. In other words, the regulators here have tested the market and found there’s a fairly reasonable collection of price/performance points that customers can choose to buy or rent services at. Despite some bleating to the contrary, there doesn’t appear to be too much in the way of arbitrary interference from ISPs in the way they treat traffic and content travelling over their networks.

Indeed, in the most populated parts of Europe there are quite a few different ways customers can get internet access: ADSL over the telephone network is most common, with internet over the cable television network and more recently the roll-out of fibre optic connections to street cabinets or directly to the home offered in many cities by more than one provider each. Further options such as the increasingly fast 3G and 4G mobile broadband internet on phones and tablets offer further competition to “keep each other honest”.

Having said that, there are certainly aspects to how mobile phone providers treat data on their networks that are not really “neutral” in the same sense, but it would appear most of these are types of traffic management that is aimed at maximising the value of the spectrum to the customers (and therefore the operator), rather than deliberately treating different content providers in different ways.

Of course things could change, so regulators keep a close watch on the operations of the market, using frequent detailed traffic measurement reports to make sure things are not too out-of-kilter. An approach that seems to have worked, for the moment.

Plant growth patterns changing on much of Earth’s surface

More than half of Earth’s land surface has seen major changes in plant growth patterns such as leaf-on date and how much vegetation grows in a season.

K. Travis

Patterns in when and how much plants grow have changed markedly over the past 30 years, scientists report March 2 in Nature Climate Change.

Researchers looked at satellite data of vegetation on the Earth’s surface from 1981 to 2012. They examined 21 markers of plant growth, including the dates when plants start sprouting and losing leaves each year and how much vegetation grows in a season.

Plant growth patterns have shifted noticeably on 95 percent of Earth’s land surface, the scientists found. In the Northern Hemisphere, growing seasons have generally become longer, while in the Southern Hemisphere some areas had more abundant plant growth over time, and others less.

Is iron rain the reason why Earth and the moon are so different?

New experiments show that the asteroids that slammed into Earth and the moon more than 4 billion years ago were vaporised into a mist of iron. The findings, published in Nature Geoscience, suggest that the iron mist thrown up from the high velocity impacts of these asteroids travelled fast enough to escape the moon’s gravity, but stayed gravitationally stuck on more massive Earth. And these results may help explain why the chemistry of the Earth and the moon differ.

When and how Earth’s metallic core formed is uncertain. Clues come from known differences in the preferences of certain elements incorporated in the silicate mantle or the metal core. In a mixture of silicate rock and iron metal, the atoms of certain elements, such as gold and platinum, tend to prefer to enter the metal, while others, such as hafnium, prefer the silicate.

As Earth’s iron-rich core formed it “sucked” the metal-loving elements out of the planet’s rocky mantle. However, measurements of the silicate mantle by James Day have previously shown that there are more of them left in the shallower Earth than would be expected. This has often been attributed to a late veneer of asteroids that delivered an extra dose of metal-loving elements to the rocky mantle.

The Z machine generates electric currents of up to 20 million amps, to shoot aluminium projectiles at iron targets, replicating the impacts of early asteroids. CC BY

One problem with this picture has been that the abundance of the metal-loving elements on Earth is ten to a hundred times greater than that measured on the moon, which should by this argument have the same veneer. The chemical difference between Earth and the moon has been perplexing, and casts a shadow over the prevalent idea that the moon formed from the same stuff as Earth after an impact from a Mars-sized planet early in the history of the Solar System.

Mighty Earth attracts more metal

The new paper seems to reconcile these differences. The experiment relied on Sandia National Laboratory’s “Z-machine”: a huge electromagnetic gun – twice as powerful as the world’s total generating capacity – that can launch projectiles into iron targets at ultra-high velocity.

The impact experiments by Richard Kraus and colleagues show that iron vaporises under the conditions created when an asteroid crashes into Earth or the moon. A cloud of iron mist will have wrapped around the globe after any such collision, falling to Earth as metal rain. These well-mixed droplets will have become incorporated into the mantle, delivering the excess metal-loving chemicals.

The same experiments, however, indicate that the velocity of the iron rain droplets will have been greater than the escape velocity on the moon, but below that of Earth. Earth would therefore have captured the metal cores of colliding asteroids, while the moon will have failed to. William Anderson of Los Alamos National Laboratory, US, said: “The moon may have received, but not retained, a significant portion of the late veneer.”

The results could imply that models for estimating the time scales of Earth’s core formation could be out by as much as a factor of ten, with the core forming much earlier in Earth’s history than previously recognised.