The Superconductor That Works at Earth Temperature

Physicists have discovered a material that superconducts at a temperature significantly warmer than the coldest ever measured on the earth. That should herald a new era of superconductivity research.H2S
The world of superconductivity is in uproar. Last year, Mikhail Eremets and a couple of pals from the Max Planck Institute for Chemistry in Mainz, Germany, made the extraordinary claim that they had seen hydrogen sulphide superconducting at -70 °C. That’s some 20 degrees hotter than any other material—a huge increase over the current record….
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Unconventional Superconductivity

Crystal structure of the cuprate Bi2212

Crystal structure of the cuprate Bi2212

M. R. Norman
A brief review of unconventional superconductivity is given, stretching from the halcyon days of helium-3 to the modern world of Majorana fermions. Along the way, we will encounter such strange beasts as heavy fermion superconductors, cuprates, and their iron-based cousins. Emphasis will be put on the fact that in almost all cases, an accepted microscopic theory has yet to emerge. This is attributed to the difficulty of constructing a theory of superconductivity outside the Migdal-Eliashberg framework…
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Mother of Higgs boson found in superconductors

A weird theoretical cousin of the Higgs boson, one that inspired the decades-long hunt for the elusive particle, has been properly observed for the first time. The discovery bookends one of the most exciting eras in modern physics.

The Higgs field, which gives rise to its namesake boson, is credited with giving other particles mass by slowing their movement through the vacuum of space. First proposed in the 1960s, the particle finally appeared at the Large Hadron Collider at CERN near Geneva, Switzerland, in 2012, and some of the theorists behind it received the 2013 Nobel prize in physics.

But the idea was actually borrowed from the behaviour of photons in superconductors, metals that, when cooled to very low temperatures, allow electrons to move without resistance.

Near zero degrees kelvin, vibrations are set up in the superconducting material that slow down pairs of photons travelling through, making light act as though it has a mass.

This effect is closely linked to the idea of the Higgs – “the mother of it actually,” says Raymond Volkas at the University of Melbourne in Australia.

Those vibrations are the mathematical equivalent of Higgs particles, says Ryo Shimano at the University of Tokyo, who led the team that made the new discovery. The superconductor version explains the virtual mass of light in a superconductor, while the particle physics Higgs field explains the mass of W and Z bosons in the vacuum.

Double trouble

Physicists had expected the Higgs-like effect to appear in all superconductors because it is also responsible for their characteristic property – zero electrical resistance. But it had only been seen before by imposing a different kind of vibration on the material.

To find it in a superconductor in its normal state, Shimano and colleagues violently shook the superconductor with a very brief pulse of light. Shimano says it is similar to how particle physicists create the real Higgs boson with energetic particle collisions. They first created the superconducting Higgs last year, and have now studied its properties to show that, mathematically speaking, it behaves almost exactly like the particle physics Higgs.

Noting the similarities between the two systems could be useful in studying the real Higgs boson. “One can prepare various types of ‘vacuum’ in condensed matter systems, which are not able to be realized in particle physics experiments,” Shimano says. “One can really do the experiments in a table-top manner, which would definitely reveal new physics and hopefully provide some useful feedbacks to particle physics.”

Journal reference: Science, DOI: 10.1126/science.1254697

by Michael Slezak –

Metamaterial Superconductor Hints At New Era Of High Temperature Superconductivity

Experiment hints at a new way to engineer high temperature superconductors

Superconductors are among the wonders of modern science. They allow a current to flow with zero resistance in materials cooled below some critical temperature. Superconductors are the crucial ingredients in everything from high-power magnets and MRI machines to highly sensitive magnetometers and magnetic levitation devices.

One problem though is that superconductors work only at very low temperatures. So one of the great challenges in this area of science is to find materials that superconduct at higher temperatures, perhaps even at room temperature. That won’t be easy given that the current record is around 150 kelvin (-120 degrees centigrade).supercondNevertheless, a way of increasing the critical temperature of existing superconducting materials would be hugely useful.

Today, a group of physicists and engineers say they have worked out how to do this. The trick is to think of a superconductor as a special kind of metamaterial and then to manipulate its structure in a way that increases its critical temperature.

Vera Smolyaninova at Towson University in Maryland and colleagues from the University of Maryland and the Naval Research Laboratory in Washington DC, have even demonstrated this idea by increasing the critical superconducting temperature of tin.

First some background about metamaterials. Until relatively recently, physicists had always treated bulk materials as homogeneous lumps of the same stuff. These lumps have bulk properties such as the ability to bend light in a certain way.

But in recent years they have began to think about constructing artificial materials made of periodic patterns of structures that themselves interact with electromagnetic waves, things like wires, c-shaped conductors and so on. If these structures are much smaller than the wavelength of the light passing by, then they act like a homogeneous lump, at least as far as the light is concerned.

By toying with this periodic structure, physicists can create artificial materials with all kinds of exotic properties. The most famous of these is the invisibility cloak, a metamaterial designed to steer light around an object as if it were not there.

Superconductivity can be thought of in a similar way, say Smolyaninova and co. Conventional superconductors made of a single metal are homogeneous lumps of the same stuff that have zero resistance at some critical temperature…..
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X-rays reveal another feature of high-temperature super conductivity

This image shows the result of diffuse scattering on the high-temperature superconductor, which is the first of the two stages in the experiment. The coloured areas enable to identify the wavelength of the phonons where the coupling with the electrons is taking place. Credit: MPI Stuttgart/M. Le Tacon

This image shows the result of diffuse scattering on the high-temperature superconductor, which is the first of the two stages in the experiment. The coloured areas enable to identify the wavelength of the phonons where the coupling with the electrons is taking place. Credit: MPI Stuttgart/M. Le Tacon

Classical and high-temperature superconductors differ hugely in the value of the critical temperatures at which they lose all electrical resistance. Scientists have now used powerful X-rays to establish another big difference: high-temperature superconductivity cannot be accounted for by the mechanism that leads to conventional superconductivity. As this mechanism called “electron-phonon coupling” contributes only marginally to the loss of electrical resistance, other scenarios must now be developed to explain high-temperature superconductivity. The results are published on November 24, 2013 in Nature Physics.

The team of scientists was led by Mathieu Le Tacon and Bernhard Keimer from the Max-Planck-Institute for Solid State Research in Stuttgart (Germany) and comprised scientists from Politecnico di Milano (Italy), Karlsruhe Institute of Technology (KIT) and the European Synchrotron (ESRF) in Grenoble, France.
High-temperature superconductivity was discovered nearly thirty years ago and is beginning to find more and more practical applications. These materials have fascinated scientists since their discovery. For even more practical applications, the origin of their amazing properties must be understood, and ways found to calculate the critical temperature. A key element of this understanding is the process that makes electrons combine into so-called “Cooper pairs” when the material is cooled below the critical temperature. In classical superconductors, these Cooper pairs are formed thanks to electron-phonon coupling, an interaction between electrons carrying the electrical current and collective vibrations of atoms in the material.
To understand the role this interaction plays in high-temperature superconductors, Matthieu Le Tacon and his colleagues took up the challenge to study these atomic vibrations as the material was cooled down below its critical temperature. “Studying electron-phonon coupling in these superconductors is always a delicate task, due to the complex structure of the materials,” says Alexeï Bosak, an ESRF scientist and member of the team. He adds: “This is why we developed a two-level approach to literally find a needle in the hay stack”.
The big surprise came once the electron-phonon coupling had been probed. “In terms of its amplitude, the coupling is actually by far the biggest ever observed in a superconductor, but it occurs in a very narrow region of phonon wavelengths and at a very low energy of vibration of the atoms”, adds Mathieu Le Tacon. “This explains why nobody could see it before the two-level approach of X-ray scattering was developed”.
Because the electron-phonon coupling is in such a narrow wavelength region, it cannot “help” two electrons to bind themselves together into a Cooper pair. The next step will be to make systematic observations in many other high-temperature superconductors. “Although we now know that electron-phonon coupling does not contribute to their superconductivity, the unexpected size of the effect—we call it giant electron-phonon-coupling—happens to be a valuable tool to study the interplay between superconductivity and other competing processes. This will hopefully provide further insight into the origin of high-temperature superconductivity, still one of the big mysteries of science”, concludes Mathieu Le Tacon.

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The tao of modern physics

Shivaji Sondhi
In the bulk of the commentary on the discovery of the Higgs particle at CERN and the recent award of the Nobel prize to Peter Higgs and François Englert, one astonishing aspect has been largely overlooked. This discovery points to one of the most central aspects of postwar physics — its unity across domains at distances (or energies) separated by vast gulfs that have allowed ideas to jump between very different physical problems. In the case of the Higgs particle, its discovery at an energy of one hundred billion electron volts in a complicated special purpose machine is, in a mathematical sense, a precise analogue of a well-understood phenomenon in ordinary metals at an energy of a thousandth of an electron volt — one hundred trillion times lower!

Indeed, this analogy is how the puzzle underlying the Higgs particle was first solved by Philip Anderson in 1963, a year before the papers by Higgs and Englert and Robert Brout that were honoured with the Nobel. Anderson, now 89, is widely regarded as the greatest living condensed matter physicist, a maestro of the part of physics that tries to understand how the small set of subatomic forces and particles can lead to the infinite variety of the matter we see around us. He has led a spectacular career during which he picked up a Nobel in 1977 for completely different work, and could have collected at least two more.

Back in 1963, Anderson had already played a key role in understanding the general phenomenon of “spontaneous symmetry breaking” in condensed matter physics in parallel with important developments in particle physics. An everyday example of this phenomenon is the formation of ice from water. While the molecules in water resemble the crowd in Times Square on a busy day with no clear preference for where they want to be, the molecules in ice are arranged in an array like an honour guard at attention. Their choice of particular positions breaks the symmetry embodied in a lack of positional preference.

More immediately, Anderson had been one of the central players in elucidating the physics of superconductivity, or why metals permit electric current to flow without loss when sufficiently cold. Superconductivity involves an unusual broken symmetry, but with the complication of electromagnetic forces that act over large distances. It was understood by Anderson that a “massless gauge field” (describing ordinary electromagnetic forces) could combine with a “massless Goldstone mode” (a signature of symmetry breaking) to yield purely massive excitations. Roughly, this reflects the dislike that superconductors exhibit for magnetic fields, termed the Meissner effect and often dramatised by levitating magnets above pieces of superconductors.

At this point, Anderson came across particle physicists trying to rescue an appealing potential description of short-ranged forces among the zoo of particles being discovered in accelerators. This description had one key thing wrong — the gauge fields were massless and thus described long-ranged forces. Anderson realised that by introducing a second wrong — a massless Goldstone boson due to symmetry breaking — he could make a right. Today, this magic trick is commonly referred to as the Anderson-Higgs mechanism, to credit Higgs with the subsequent realisation that the mechanism implied a specific additional massive particle Anderson had overlooked. In any event, by staring into a piece of metal, Anderson had divined the solution to a puzzle about fundamental particles.

Now, the energy involved in superconductivity is a thousandth of an electron volt while the energy of the Higgs particle is a hundred trillion times larger, or alternately the size of the Higgs particle is a hundred trillion times smaller than the size of the smallest superconducting unit, the so-called “Cooper pair” of electrons. Why is it that the same mathematics can be used to describe both?

The explanation for this astonishing fact is a central meta-idea in postwar physics, that of the effective field theory. It states that if you don’t look too closely at the spatial details, the mathematics simplifies greatly into a set of “field theories”, which then provide a unifying mathematical framework for a vast range of phenomena. This meta-idea itself has a precise mathematical formulation known as “universality under renormalisation group flows”.

Metals are made of electrons and nuclei, but when we smooth over such detail, we end up with the field theory Anderson considered. In particle physics, the details being smoothed over are unknown — perhaps described by string theory — and we end up with a close cousin of Anderson’s field theory. What Anderson called a mode, Higgs called a particle, but both were describing a disturbance in an underlying medium, one known and the other unknown.

The ubiquity of effective field theories means that the Anderson-Higgs mechanism is by no means the only example of tight analogies between far separated phenomena in modern physics. To take one recent example, the work of particle physicist Edward Witten on topological field theories in the 1980s, for which he won a Fields medal in mathematics, has turned out to be central to our understanding of the quantum Hall effect in semiconductor systems, even though it was designed to do no such thing. Even this writer, also a condensed matter physicist, has had the (far more modest) pleasure of discovering in the same semiconductor systems “skyrmions” 15 orders of magnitude larger than those considered by particle physicist Tony Skyrme as descriptions of protons and neutrons.

So, the discovery of the Higgs particle is a triumph for this syncretic view built into modern physics. It turns out that space devoid of visible particles has something deeply in common with a superconducting metal. Further, it tells us that it was not always so: when the universe was younger and hotter, it resembled more a piece of superconductor heated to the point where the superconductivity vanishes, and thus there was no Higgs particle to speak of.

This brings me to the Nobel prize. I believe the committee missed an opportunity in not including Anderson along with Higgs and Englert. It would have been a more accurate accounting of the credit on this particular discovery and a deserved honour for a man whose contributions are legion. Above all, it would have paid tribute to the remarkable intellectual unity of modern physics.

The writer is a professor of physics at Princeton University, US

Physicists see hints of Majorana fermions

Magnetic diffraction pattern for a Josephson junction with a topological insulator weak link. The horizontal axis is the applied magnetic field, the vertical axis is the current through the junction. The colours represent the differential resistance with white being zero. (Courtesy: Phys. Rev. Lett.)

Evidence for the existence of “Majorana fermions” – theoretically proposed particles that are also their own anti-particles – could be seen in the behaviour of a novel Josephson junction. That is the view of physicists at Stanford University in the US, who have examined the properties of a Josephson junction that incorporates material called a “topological insulator” sandwiched between two superconducting contacts. The researchers found significant deviations from what is seen in conventional Josephson junctions – differences that they believe could be explained in terms of Majorana-like quasiparticles.
First predicted by the Italian physicist Ettore Majorana in 1937 – shortly before he mysteriously disappeared aged just 31 – Majorana fermions are interesting not just because they are their own antiparticles but also because they should be resistant to environmental noise. Majorana fermions, in other words, could be used to store and transmit quantum information without being perturbed by the outside world, which is the bane of anyone trying to build a practical quantum computer.
Although definite proof of the existence of Majorana fermions has not yet been obtained, theorists have calculated that particle-like excitations, or quasiparticles, which look like Majorana fermions could exist at the interface where a topological insulator – a material that only conducts electricity on its surface – is placed next to an ordinary superconductor. These quasiparticles are called “zero-energy modes” because they lie along the Fermi energy of the material.
In the case of a Josephson junction containing a topological insulator as the “weak link” between two superconductors, there are actually two superconductor–topological insulator interfaces back-to-back, and the Majoranas are expected to couple to each other and depart from zero energy. However, if a tiny magnetic field – even as small as half a superconducting flux quantum – is applied to the junction, the two Majorana modes decouple and both reside at zero energy.

The weakest link

David Goldhaber-Gordon and colleagues at Stanford have now studied such junctions and have found some bizarre behaviour, which they have tried to explain in terms of Majorana fermions. When experimentalists plot a graph of the superconducting current flowing across a Josephson junction against the value of an applied magnetic field, they usually see a distinct “magnetic diffraction pattern” (MDP). Normally, the MDP has a strong central peak, but in topological-insulator Josephson junctions, Goldhaber-Gordon and colleagues saw a much more complicated MDP with several unexpected peaks. Indeed, the first minimum occurs at about one-fifth of the magnetic field strength that is expected in a conventional Josephson junction.
According to Goldhaber-Gordon, this more complicated structure could be related to the zero-energy Majorana modes that are expected to occur at specific values of magnetic flux. However, to explain the observed diffraction pattern, Goldhaber-Gordon points out that three – rather than one – zero-energy modes are required. One of these modes could be associated with a Majorana fermion, whereas the other two could be associated with other conventional fermions – something that Goldhaber-Gordon says has been suggested by some theorists.

Smaller critical currents

Another atypical feature seen by the team is the value of the device’s critical current (above which it no longer superconducts) multiplied by its resistance in the normal, non-superconducting state. This product is usually proportional to the superconducting energy gap, but the team measured a value that is much smaller than expected. The value was also found to be inversely proportional to the width of the Josephson-junction device – that is, the distance across the device perpendicular to the flow of the supercurrent.
Building on a theoretical description published in 2008 by Charles Kane and Liang Fu at the University of Pennsylvania in the US, Goldhaber-Gordon and colleagues assume that the Majorana fermions are confined to a 1D wire that runs along the width of the Josephson junction. The result is a series of quantized energy levels that are inversely proportional to the width of the device. The team speculates that the gap between these energy levels provides a new and smaller energy scale above which superconductivity ceases to occur – explaining the smaller measured values.
Although the team analysed its results in the context of Majorana fermions, Goldhaber-Gordon stresses that his team are still only at the early stages of exploring the behaviour of junctions between superconductors and topological insulators. “Many aspects of the materials and junctions are not yet well understood,” he says. “We welcome ideas for the explanation of these data, whether they are Majorana-related, or not.”

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‘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….
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