‘Magnetic Josephson effect’ seen for the first time

Micrographs showing a loop of superconducting material used to demonstrate coherent quantum phase slip. The image on the left shows the loop of superconductor and the image on the right is a magnified section showing how the superconductor narrows to a nanowire. The magnetic field is applied perpendicular to the loop. (Courtesy: RIKEN)

A fundamental prediction of superconductivity theory has been demonstrated in the lab for the first time. An international team of physicists has observed coherent quantum phase slip, a phenomenon similar to the well-known Josephson effect in which magnetic flux takes the place of electric charge. Its discovery has fundamental implications for our understanding of macroscopic quantum systems and could also lead to intriguing applications, including a possible way to produce a qubit in a quantum computer.
In 1962 the British physicist Brian Josephson developed a theory of how superconducting electrons tunnel across a thin insulating layer between two superconductors – a structure now called a Josephson junction. This was quickly verified in the lab and Josephson was awarded the 1973 Nobel Prize for Physics. The Josephson junction has become an important technology in its own right. For example, superconducting quantum interference devices (SQUIDs) that, depending on their design, use either one or two Josephson junctions are among the most sensitive magnetometers to have been invented. The devices have also shown promise as possible quantum bits (qubits) in quantum computers….
Read more: physicsworld.com

Red Wine, Tartaric Acid And The Secret Of Superconductivity

Last year, physicists discovered that red wine can turn certain materials into superconductors. Now they’ve found that Beaujolais works best and think they know why

Last year, a group of Japanese physicists grabbed headlines around the world by announcing that they could induce superconductivity in a sample of iron telluride by soaking it in red wine. They found that other alcoholic drinks also worked–white wine, beer, sake and so on–but red wine was by far the best.

The question, of course, is why. What is it about red wine that does the trick?

Today, these guys provide an answer, at least in part. Keita Deguchi at the National Institute for Materials Science in Tsukuba, Japan, and a few buddies, say the mystery ingredient is tartaric acid and have the experimental data to show that it plays an important role in the process.

First, some background. Iron-based superconductors were discovered in 2008 and have since become the focus of intense interest. Deguchi and co study iron telluride which does not superconduct unless some of the telluride atoms are replaced with sulphur, forming FeTeS.

But even then, FeTeS doesn’t superconduct unless it goes through a final processing stage; heating it in water, for example.

Nobody knows what this process does or how it can convert an ordinary material into a superconductor. But some liquids are better than others, as determined by the fraction of the sample they convert into a superconductor.

This is the stage Deguchi and co have been puzzling over. Their approach is to make a sample of FeTeS, cut it up into slices and then heat each slice in a different liquid.

Water works quite well but whiskey, shochu and beer are all better. And of course, red wine is the best of all.

Now Deguchi and co have repeated the experiment with different types of red wine to see which works best. They’ve used wines made with a single grape variety including gamay, pinot noir, merlot, carbernet sauvignon and sangiovese.

It turns out that the best performer is a wine made from the gamay grape–for the connoisseurs, that’s a 2009 Beajoulais from the Paul Beaudet winery in central France.

They then analysed the wines to see which ingredient correlated best with superconducting performance and settled on tartaric acid as the likely culprit. The Beaujolais has the highest tartaric acid concentration.

Finally, they repeated the experiment using a mixture of water and tartaric acid to find out how well it performed.

Interestingly, they found that the solution performed better than water alone but not as well as the Beaujolais.

So while tartaric acid is clearly part of the answer, there must be another component of red wine that somehow encourages the transition to a superconducting state.

That’s a useful step forward for a team clearly dedicated to unravelling the mysterious powers of alcohol. On that basis alone, the work must be applauded.

However, there are still plenty of unanswered questions here, not least of which is how the superconducting transition process occurs at all in the presence of these liquids.

Corkscrews on standby.

Ref: arxiv.org/abs/1203.4503: Tartaric Acid In Red Wine As One Of The Key Factors To Induce Superconductivity In FeTe0.8S0.2
Read more: www.technologyreview.com

Was a metamaterial lurking in the primordial universe?

An all-sky survey of the cosmic microwave background taken by the ESA's Planck space mission. (Courtesy: ESA)

A scientist in the US is arguing that the vacuum should behave as a metamaterial at high magnetic fields. Such magnetic fields were probably present in the early universe, and therefore he suggests that it may be possible to test the prediction by observing the cosmic microwave background (CMB) radiation – a relic of the early universe that can be observed today.
One of 2011’s strangest predictions in physics was the suggestion by Maxim Chernodub of the French National Centre for Scientific Research that, at incredibly high magnetic fields, superconducting states can emerge from the vacuum. This was particularly interesting because one of the main difficulties facing scientists working on traditional superconductivity is preventing superconducting states disappearing in the presence of even moderate magnetic fields….
Read more: physicsworld.com

How Superconductors Can Detect Gravitational Waves

Superconducting metal bars could revolutionise the detection of gravitational waves, says physicists

Gravitational waves are vibrations in the fabric of spacetime. They are among the most exciting phenomena in the universe because they are generated by exotic processes such as collisions between black holes and even in the moment of creation itself, the Big Bang.

So finding a way to study them is a big deal for astronomers.

But there’s a problem. Gravitational waves squeeze and stretch space as they travel but their effects are tiny. Physicists calculate that the waves passing through Earth are changing the distance between London and New York by about the width of a uranium nucleus.

That makes them tough to spot, although he current generation of gravitational detectors ought to be able to detect this level of change (unless somebody’s got their numbers badly wrong).

Nevertheless, nobody has spotted a gravitational wave directly.

So a new way to find these beasts will surely be of interest. Today Armen Gulian at Chapman University in Maryland and a few pals outline a new type of detector that has the potential to be much smaller than today’s behemoths.

Conventional detectors are giant L-shaped interferometers with each arm being many hundreds of metres long. At the end of each arm is a mirror so a laser beam can bounce back and forth along the arms and then be made to interfere with itself.

Any change in the length of the arms ought to show up in any changes in the resultant interference pattern.

Gulian and co have a different idea. They imagine a bar of superconducting metal being hit by a gravitational wave. The waves act on all masses within the bar but the resulting movement of the metallic lattice, which is bound in place, will be very different from the movement of superconducting electrons, which are entirely unbound and free to move.

“Thus, the wave will tend to accelerate the electrons back and forth, towards and away from the ends of the bar,” they say.

Next, they place another superconducting bar at the end of the first but at right angles to it. While the first bar is squeezed by a gravitational wave, the second will be stretched. So the electrons in this bar will oscillate too, albeit shifted by half a period relative to the first.

Finally, if these bars are connected by a superconducting wire, an oscillating current should flow through it.

There are a few other subtleties to the design, largely to cope with the nature of superconductors, but this is essentially the principle they outline.

They go on to sketch the way a small such detector might work, made of bars just a few tens of centimetres long. A gravitational wave ought to generate a current of a few femtoamperes, a level that could be detectable with off-the shelf equipment.

Noise might be a problem, however. But Gulian and co say that if the frequency of the oscillations are known in advance much of the noise can be filtered out. In addition, the detector could be placed inside a magnetic bottle to screen out magnetic noise.

That’s an interesting idea which looks as if it could be considerably cheaper and simpler than the next generation of laser-based gear now being designed for future space missions such as LISA, (the laser interferometer space antenna). Worth looking at in more detail.

Ref: arxiv.org/abs/1111.2655: : Superconducting Antenna Concept for Gravitational Wave Radiation

www.technologyreview.com

A new kind of superconductivity

Physicists unveil a theory for a new kind of superconductivity

The superfow of two kinds of superconducting electrons (arrows show their velocities) as calculated on supercomputers. Graphic 2 shows superflow of the other subpopulation of electrons on the surface of a vortex cluster. Graphics courtesy of Egor Babaev.

In this 100th anniversary year of the discovery of superconductivity, physicists at the University of Massachusetts Amherst and Sweden’s Royal Institute of Technology have published a fully self-consistent theory of the new kind of superconducting behavior, Type 1.5, this month in the journal Physical Review B.

In three recent papers, the authors report on their detailed investigations to show that a Type 1.5 superconducting state is indeed possible in a class of materials called multiband superconductors.
For years, most physicists believed that superconductors must be either Type I or Type II. Type 1.5 superconductivity is the subject of intense debate because until now there was no theory to connect the physics with micro-scale properties of real materials, say Egor Babaev of UMass Amherst, currently a fellow at the technology institute in Stockholm, with Mikhail Silaev, a postdoctoral researcher there.
Their new papers now provide a theoretical framework to allow scientists to calculate conditions necessary for the appearance of Type 1.5 superconductivity, which exhibits characteristics of Types I and II previously thought to be antagonistic.
Superconductivity is a state where electric charge flows without resistance. In Type I and Type II, charge flow patterns are dramatically different. Type I, discovered in 1911, has two state-defining properties: Lack of electric resistance and the fact that it does not allow an external magnetic field to pass through it. When a magnetic field is applied to these materials, superconducting electrons produce a strong current on the surface which in turn produces a magnetic field in the opposite direction. Inside this type of superconductor, the external magnetic field and the field created by the surface flow of electrons add up to zero. That is, they cancel each other out.
Type II superconductivity was predicted to exist by a Russian theoretical physicist who said there should be superconducting materials where a complicated flow of superconducting electrons can happen deep in the interior. In Type II material, a magnetic field can gradually penetrate, carried by vortices like tiny electronic tornadoes, Babaev explains. The combined works that theoretically described Type I and II superconductivity won the Nobel Prize in 2003.
Classifying superconductors in this way turned out to be very robust: All superconducting materials discovered in the last half-century can be classified as either, Babaev says. But he believed a state must exist that does not fall into either camp: Type 1.5. By working out the theoretical bases for superconducting materials, he had predicted that in some materials, superconducting electrons could be classed as two competing types or subpopulations, one behaving like electrons in Type I material, the other behaving like electrons in a Type II material.
Babaev also said that Type 1.5 superconductors should form something like a super-regular Swiss cheese, with clusters of tightly packed vortex droplets of two kinds of electron: one type bunched together and a second type flowing on the surface of vortex clusters in a way similar to how electrons flow on the exterior of Type I superconductors. These vortex clusters are separated by “voids,” with no vortices, no currents and no magnetic field.
The major objection raised by skeptics, he recalls, is that fundamentally there is only one kind of electron, so it’s difficult to accept that two types of superconducting electron populations could exist with such dramatically different behaviors.
To answer this, Silaev and Babaev developed their theory to explain how real materials can give raise to Type-1.5 superconductivity, taking into account interactions at microscales. In a parallel effort, their colleagues at UMass Amherst and in Sweden including Johan Carlstrom and Julien Garaud, with Babaev, used supercomputers to perform large-scale numerical calculations modeling the behavior of superconducting electrons to better understand the structure of vortex clusters and what they look like in a Type-1.5 superconductor.
They found that under certain conditions they could describe new, additional forces at work between the Type-1.5 vortices, which can give vortex clusters very complicated structure. As more work is done on superconductivity, the team of physicists in Stockholm and at UMass Amherst say the family of multi-band superconducting materials will grow. They expect that some of the newly discovered materials will belong in Type 1.5.
“With the development of theory that works on the microscopic level, as well as our better understanding of inter-vortex interaction, we can now connect the properties of vortex clusters with the properties of electronic structure of concrete materials. This can be useful in establishing whether materials belong in the Type 1.5 superconductivity domain,” says Babaev.
More information: http://prb.aps.org … /i13/e134515
http://prb.aps.org … 4/i9/e094515
http://prb.aps.org … /i13/e134518
http://www.physorg.com

The Challenge of Unconventional Superconductivity

M. R. Norman
During the past few decades, several new classes of superconductors have been discovered. Most of these do not appear to be related to traditional superconductors. As a consequence, it is felt by many that for these materials, superconductivity arises from a different source than the electron-ion interactions that are at the heart of conventional superconductivity. Developing a rigorous theory for any of these classes of materials has proven to be a difficult challenge, and will continue to be one of the major problems in physics in the decades to come….
Read more: http://arxiv.org/ftp/arxiv/papers/1106/1106.1213.pdf

Superconductivity’s third side unmasked

Figure 1: The three types of glue for superconducting electrons: lattice vibrations (top), electron spin (middle), and fluctuations between two electron orbitals (zx and yz) (bottom). The yellow spheres represent Cooper pairs of electrons

The debate over the mechanism that causes superconductivity in a class of materials called the pnictides has been settled by a research team from Japan and China. Superconductivity was discovered in the pnictides only recently, and they belong to the class of so-called ‘high-temperature superconductors’. Despite their name, the temperature at which they function as superconductors is still well below room temperature. Realizing superconductivity at room temperature remains a key challenge in physics; it would revolutionize electronics since electrical devices could operate without losing energy.

Superconductivity in a material arises when two electrons bind together into so-called Cooper pairs. This pairing leads to a gap in the energy spectrum of the , which makes the electrons insensitive to the mechanisms causing . Electrons can bind into Cooper pairs in different ways, leading to different categories of superconductors.

Until the work of Takahiro Shimojima from The University of Tokyo and his colleagues, including researchers from the RIKEN SPring-8 Center in Harima, superconducting materials were classified into two broad categories. In classical superconductors, which function at very low temperatures, vibrations of atoms in the  of the material provide the necessary glue for the pairing. In cuprates, the original high-temperature superconductor compounds,  based on an electron’s spin generate the superconductive pairing (Fig. 1). In the pnictide , physicists assumed that the underlying mechanism was similar to that for the cuprates, but conflicting experimental results meant that the precise mechanism was controversial.

To investigate this debated pairing mechanism of pnictides, the researchers studied the properties of the material’s electronic gap. Thanks to a unique set of high-energy lasers based on very rare laser crystals available to only a few laboratories, their experiments resolved these states with unprecedented detail.

Shimojima and colleagues were surprised to discover that interactions between electron spins do not cause the electrons to form  in the pnictides. Instead, the coupling is mediated by the electron clouds surrounding the atomic cores. Some of these so-called orbitals have the same energy, which causes interactions and electron fluctuations that are sufficiently strong to mediate superconductivity.

This could spur the discovery of new superconductors based on this mechanism. “Our work establishes the electron orbitals as a third kind of pairing glue for electron pairs in superconductors, next to lattice vibrations and electron spins,” explains Shimojima. “We believe that this finding is a step towards the dream of achieving room-temperature superconductivity,” he concludes.

http://www.physorg.com/news/2011-06-superconductivity-side-unmasked.html