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Discovery of new material state counterintuitive to laws of physics

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Pressure-induced transitions are associated with near 2-fold volume expansions. While an increase in volume with pressure is counterintuitive, the resulting new phases contain large fluid-filled pores, such that the combined solid + fluid volume is reduced and the inefficiencies in space filling by the interpenetrated parent phase are eliminated.

Pressure-induced transitions are associated with near 2-fold volume expansions. While an increase in volume with pressure is counterintuitive, the resulting new phases contain large fluid-filled pores, such that the combined solid + fluid volume is reduced and the inefficiencies in space filling by the interpenetrated parent phase are eliminated.

When you squeeze something, it gets smaller. Unless you’re at Argonne National Laboratory.
At the suburban Chicago laboratory, a group of scientists has seemingly defied the laws of physics and found a way to apply pressure to make a material expand instead of compress/contract.
“It’s like squeezing a stone and forming a giant sponge,” said Karena Chapman, a chemist at the U.S. Department of Energy laboratory. “Materials are supposed to become denser and more compact under pressure. We are seeing the exact opposite. The pressure-treated material has half the density of the original state. This is counterintuitive to the laws of physics.”
Because this behavior seems impossible, Chapman and her colleagues spent several years testing and retesting the material until they believed the unbelievable and understood how the impossible could be possible. For every experiment, they got the same mind-bending results.
“The bonds in the material completely rearrange,” Chapman said. “This just blows my mind.”
This discovery will do more than rewrite the science text books; it could double the variety of porous framework materials available for manufacturing, health care and environmental sustainability.
Scientists use these framework materials, which have sponge-like holes in their structure, to trap, store and filter materials. The shape of the sponge-like holes makes them selectable for specific molecules, allowing their use as water filters, chemical sensors and compressible storage for carbon dioxide sequestration of hydrogen fuel cells. By tailoring release rates, scientists can adapt these frameworks to deliver drugs and initiate chemical reactions for the production of everything from plastics to foods.

“This could not only open up new materials to being porous, but it could also give us access to new structures for selectability and new release rates,” said Peter Chupas, an Argonne chemist who helped discover the new materials.
The team published the details of their work in the May 22 issue of the Journal of the American Chemical Society in an article titled “Exploiting High Pressures to Generate Porosity, Polymorphism, And Lattice Expansion in the Nonporous Molecular Framework Zn(CN)2 .”
The scientists put zinc cyanide, a material used in electroplating, in a diamond-anvil cell at the Advanced Photon Source (APS) at Argonne and applied high pressures of 0.9 to 1.8 gigapascals, or about 9,000 to 18,000 times the pressure of the atmosphere at sea level. This high pressure is within the range affordably reproducible by industry for bulk storage systems. By using different fluids around the material as it was squeezed, the scientists were able to create five new phases of material, two of which retained their new porous ability at normal pressure. The type of fluid used determined the shape of the sponge-like pores. This is the first time that hydrostatic pressure has been able to make dense materials with interpenetrated atomic frameworks into novel porous materials. Several series of in situ high-pressure X-ray powder diffraction experiments were performed at the 1-BM, 11-ID-B, and 17-BM beamlines of the APS to study the material transitions.
“By applying pressure, we were able to transform a normally dense, nonporous material into a range of new porous materials that can hold twice as much stuff,” Chapman said. “This counterintuitive discovery will likely double the amount of available porous framework materials, which will greatly expand their use in pharmaceutical delivery, sequestration, material separation and catalysis.”
The scientists will continue to test the new technique on other materials.
Read more at: http://phys.org/news/2013-06-discovery-material-state-counterintuitive-laws.html#jCp

Written by physicsgg

June 14, 2013 at 12:08 pm

Posted in Materials Science

Video: New Material for Invisibility Cloaks

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invThe new material’s artificial “atoms” are designed to work with a broad range of light frequencies. With adjustments, the researchers believe it could lead to perfect microscope lenses or invisibility cloaks.

One of the exciting possibilities of metamaterials – engineered materials that exhibit properties not found in the natural world – is the potential to control light in ways never before possible. The novel optical properties of such materials could lead to a “perfect lens” that allows direct observation of an individual protein in a light microscope or, conversely, invisibility cloaks that completely hide objects from sight.
Although metamaterials have revolutionized optics in the past decade, their performance so far has been inhibited by their inability to function over broad bandwidths of light. Designing a metamaterial that works across the entire visible spectrum remains a considerable challenge.
Now, Stanford engineers have taken an important step toward this future, by designing a broadband metamaterial that more than doubles the range of wavelengths of light that can be manipulated.
The new material can exhibit a refractive index – the degree to which a material skews light’s path – well below anything found in nature.
“The library of refractive indexes that nature gives us is limited,” said Jennifer Dionne, an assistant professor of materials science and engineering and an affiliate member of the Stanford Institute for Materials and Energy Sciences at SLAC National Accelerator Laboratory. “All natural materials have a positive refractive index.”
For example, air at standard conditions has the lowest refractive index in nature, hovering just a tick above 1. The refractive index of water is 1.33. That of diamond is about 2.4. The higher a material’s refractive index, the more it distorts light from its original path.


All natural materials have a positive index of refraction — the degree to which they refract light. The nanoscale artificial “atoms” that constitute the metamaterial prism shown here, however, were designed to exhibit a negative index of refraction, and skew the light to the left. Technology that manipulates light in such unnatural ways could one day lead to invisibility cloaks.

Really interesting physical phenomena can occur, however, if the refractive index is near-zero or negative.
Picture a drinking straw leaning in a glass of water. If the water’s refractive index were negative, the straw would appear inverted – a straw leaning left to right above the water would appear to slant right to left below the water line.
In order for invisibility cloak technology to obscure an object or for a perfect lens to inhibit refraction, the material must be able to precisely control the path of light in a similar manner. Metamaterials offer this potential.
Unlike a natural material whose optical properties depend on the chemistry of the constituent atoms, a metamaterial derives its optical properties from the geometry of its nanoscale unit cells, or “artificial atoms.” By altering the geometry of these unit cells, one can tune the refractive index of the metamaterial to positive, near-zero or negative values.
One hitch is that any such material needs to interact with both the electric and magnetic fields of light. Most natural materials are blind to the magnetic field of light at visible and infrared wavelengths. Previous metamaterial efforts have created artificial atoms composed of two constituents – one that interacts with the electric field, and one for the magnetic. A drawback to this combination approach is that the individual constituents interact with different colors of light, and it is typically difficult to make them overlap over a broad range of wavelengths.
As detailed in the cover story of the current issue of Advanced Optical Materials, Dionne’s group – which included graduate students Hadiseh Alaeian and Ashwin Atre, and postdoctoral fellow Aitzol Garcia – set about designing a single metamaterial “atom” with characteristics that would allow it to efficiently interact with both the electric and magnetic components of light.
The group arrived at the new shape using complex mathematics known as transformation optics. They began with a two-dimensional, planar structure that had the desired optical properties, but was infinitely extended (and so would not be a good “atom” for a metamaterial).
Then, much like a cartographer transforms a sphere into a flat plane when creating a map, the group “folded” the two-dimensional infinite structure into a three-dimensional nanoscale object, preserving the original optical properties.
The transformed object is shaped like a crescent moon, narrow at the tips and thick in the center; the metamaterial consists of these nanocrescent “atoms” arranged in a periodic array. As currently designed, the metamaterial exhibits a negative refractive index over a wavelength range of roughly 250 nanometers in multiple regions of the visible and near-infrared spectrum. The researchers said that a few tweaks to its structure would make this metamaterial useful across the entire visible spectrum.
“We could tune the geometry of the crescent, or shrink the atom’s size, so that the metamaterial would cover the full visible light range, from 400 to 700 nanometers,” Atre said.
That composite material probably won’t resemble an invisibility cloak like Harry Potter’s anytime soon; while it could be flexible, manufacturing the metamaterial over extremely large areas could be tricky. Nonetheless, the authors are excited about the research opportunities the new material will open.
“Metamaterials will potentially allow us to do many new things with light, things we don’t even know about yet. I can’t even imagine what all the applications might be,” Garcia said. “This is a new tool kit to do things that have never been done before.”

Read more at: http://phys.org/news/2013-05-metamaterial-invisibility-video.html#jCp

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May 7, 2013 at 3:44 pm

Posted in Materials Science

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Spin waves carry energy from cold to hot

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Unidirectional spin-wave heat conveyer

nmat3628-f1
T. An et al
When energy is introduced into a region of matter, it heats up and the local temperature increases.
This energy spontaneously diffuses away from the heated region. In general, heat should flow from warmer to cooler regions and it is not possible to externally change the direction of heat conduction. Here we show a magnetically controllable heat flow caused by a spin-wave current.
The direction of the flow can be switched by applying a magnetic field. When microwave energy is applied to a region of ferrimagnetic Y3Fe5O12, an end of the magnet far from this region is found to be heated in a controlled manner and a negative temperature gradient towards it is formed.
This is due to unidirectional energy transfer by the excitation of spin-wave modes without time-reversal symmetry and to the conversion of spin waves into heat. When a Y3Fe5O12 film with low damping coefficients is used, spin waves are observed to emit heat at the sample end up to 10 mm away from the excitation source.
The magnetically controlled remote heating we observe is directly applicable to the fabrication of a heat-flow controller….
Read more at http://www.nature.com/nmat/journal/vaop/ncurrent/abs/nmat3628.html

Written by physicsgg

April 23, 2013 at 5:34 pm

Posted in Materials Science

Physicists create SQUID-like Bose–Einstein condensate

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Illustration of how the BEC torus is cut by a green laser. The laser is rotated about the axis of the torus such that the cut moves through the torus. (Courtesy: K C Wright et al. 2013 Phys. Rev. Lett.)

Illustration of how the BEC torus is cut by a green laser. The laser is rotated about the axis of the torus such that the cut moves through the torus.

K. C. Wright, R. B. Blakestad, C. J. Lobb‡, W. D. Phillips, and G. K. Campbell
We have observed well-defined phase slips between quantized persistent current states around a toroidal atomic (23Na) Bose-Einstein condensate. These phase slips are induced by a weak link (a localized region of reduced superfluid density) rotated slowly around the ring. This is analogous to the behavior of a superconducting loop with a weak link in the presence of an external magnetic field. When the weak link is rotated more rapidly, well-defined phase slips no longer occur, and vortices enter into the bulk of the condensate. A noteworthy feature of this system is the ability to dynamically vary the current-phase relation of the weak link, a feature which is difficult to implement in superconducting or superfluid helium circuits.
Read more: prl.aps.org and physicsworld.com

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January 19, 2013 at 2:01 pm

The 500 Phases of Matter

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Artist's impression of a string-net of light and electrons. String-nets are a theoretical kind of topologically ordered matter. (Credit: Image courtesy of Perimeter Institute for Theoretical Physics)

Artist’s impression of a string-net of light and electrons. String-nets are a theoretical kind of topologically ordered matter. (Credit: Image courtesy of Perimeter Institute for Theoretical Physics)

Forget solid, liquid, and gas: there are in fact more than 500 phases of matter. In a major paper in a recent issue of Science, Perimeter Faculty member Xiao-Gang Wen reveals a modern reclassification of all of them.

Condensed matter physics — the branch of physics responsible for discovering and describing most of these phases — has traditionally classified phases by the way their fundamental building blocks — usually atoms — are arranged. The key is something called symmetry.
To understand symmetry, imagine flying through liquid water in an impossibly tiny ship: the atoms would swirl randomly around you and every direction — whether up, down, or sideways — would be the same. The technical term for this is “symmetry” — and liquids are highly symmetric. Crystal ice, another phase of water, is less symmetric. If you flew through ice in the same way, you would see the straight rows of crystalline structures passing as regularly as the girders of an unfinished skyscraper. Certain angles would give you different views. Certain paths would be blocked, others wide open. Ice has many symmetries — every “floor” and every “room” would look the same, for instance — but physicists would say that the high symmetry of liquid water is broken.
Classifying the phases of matter by describing their symmetries and where and how those symmetries break is known as the Landau paradigm. More than simply a way of arranging the phases of matter into a chart, Landau’s theory is a powerful tool which both guides scientists in discovering new phases of matter and helps them grapple with the behaviours of the known phases. Physicists were so pleased with Landau’s theory that for a long time they believed that all phases of matter could be described by symmetries. That’s why it was such an eye-opening experience when they discovered a handful of phases that Landau couldn’t describe.
Beginning in the 1980s, condensed matter researchers, including Xiao-Gang Wen — now a faculty member at Perimeter Institute — investigated new quantum systems where numerous ground states existed with the same symmetry. Wen pointed out that those new states contain a new kind of order: topological order. Topological order is a quantum mechanical phenomenon: it is not related to the symmetry of the ground state, but instead to the global properties of the ground state’s wave function. Therefore, it transcends the Landau paradigm, which is based on classical physics concepts.
Topological order is a more general understanding of quantum phases and the transitions between them. In the new framework, the phases of matter were described not by the patterns of symmetry in the ground state, but by the patterns of a decidedly quantum property — entanglement. When two particles are entangled, certain measurements performed on one of them immediately affect the other, no matter how far apart the particles are. The patterns of such quantum effects, unlike the patterns of the atomic positions, could not be described by their symmetries. If you were to describe a city as a topologically ordered state from the cockpit of your impossibly tiny ship, you’d no longer be describing the girders and buildings of the crystals you passed, but rather invisible connections between them — rather like describing a city based on the information flow in its telephone system.
This more general description of matter developed by Wen and collaborators was powerful — but there were still a few phases that didn’t fit. Specifically, there were a set of short-range entangled phases that did not break the symmetry, the so-called symmetry-protected topological phases. Examples of symmetry-protected phases include some topological superconductors and topological insulators, which are of widespread immediate interest because they show promise for use in the coming first generation of quantum electronics.
In the paper featured in Science, Wen and collaborators reveal a new system which can, at last, successfully classify these symmetry-protected phases.
Using modern mathematics — specifically group cohomology theory and group super-cohomology theory — the researchers have constructed and classified the symmetry-protected phases in any number of dimensions and for any symmetries. Their new classification system will provide insight about these quantum phases of matter, which may in turn increase our ability to design states of matter for use in superconductors or quantum computers.
This paper is a revealing look at the intricate and fascinating world of quantum entanglement, and an important step toward a modern reclassification of all phases of matter.
Read more: www.sciencedaily.com

Written by physicsgg

December 22, 2012 at 9:26 pm

Graphene: from a conceptual material to a material of the future

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Costas Galiotis
FORTH/ ICE-HT and Dept. Materials Science, University of Patras

Graphene is a perfect 2D crystal of covalently bonded carbon atoms and forms the basis of all graphitic structures[1,2].

The material was well known as a concept structure (platform) to mathematicians and modellers for many years and to the material physicists and scientists as an individual layer of crystalline graphite.

However, its isolation and characterisation by Geim and Novoselov in 2005[3] has given rise to a dramatic surge in research and potential applications for this material.

Graphene is the best conductor of heat we know, the thinnest material, it conducts electricity much better than silicon, is 100-300 times stronger than steel, has unique optical properties, it is impermeable already as a monolayer, just to mention a few of its characteristics.

Either separately or in combinations, these extreme properties can be exploited in many areas of research; new possibilities are being recognised all the time as the science of graphene and other two-dimensional materials progresses.

Secondly, graphene science and technology relies on carbon one of the most abundant materials on Earth. It is an inherently sustainable and economical technology.

Thirdly, graphene is a planar material and as such compatible with the established production technologies in information and communication technologies.
Regarding mechanical properties which are being examined by our group, graphene is considered one of the stiffest and strongest material in nature combined with high inherent ductility.

In spite of that, very little experimental verification has been provided for its extraordinary mechanical properties. Strain on the other hand has been shown to modulate graphene’s electronic, magnetic and transport properties.

Hence is of outmost importance to understand how the thinnest membrane ever existed in nature can respond to mechanical load.

In general it is expected and already verified experimentally by us and others, that a thin film can withstand relatively large tensile strains in air without early fracture, whereas in compression monolayer graphene is expected to buckle at extremely low strains.

Direct evidence of buckling by means of AFM measurements has already been shown.

Yet axial tensile fracture at the expected strains of over 30% has never been seen or attained.

In fact, the only indication of the high tensile strength and ductility of graphene stems from a number of modelling simulations in which the flake geometry has been largely ignored.

Furthermore, for typical rectangular graphene flakes it is self-evident that when the flake is stretched axially in one direction, Poisson’s contraction in the other direction will immediately induce (lateral) buckling.

This very interesting phenomenon which should be prevalent for any future 2D materials, has not as yet been fully studied, predicted or, even, exploited.

Furthermore, our results show that graphenes embedded in plastic beams exhibit remarkable compression bucking strain compared to that of the suspended ones, due to the effect of the lateral support provided by the polymer matrix, which is indeed dramatic and increases the effective flexural rigidity of graphene by 6 orders of magnitude.

The experimental finding that one atom thick monolayers embedded in polymers can provide reinforcement in compression to high values of strain is very significant for the development of nanocomposites for structural applications[4].

Interface interactions that lead to tensile and/or compression stress transfer from a polymer matrix to a graphene flake is also of extreme interest particularly for composite applications.

Finally, recent attempts to take off graphene research in Greece will be covered. The creation of the FORTH Graphene Centre will be discussed and the possibilities of collaborative work with other institutions and groups will be explored.

References
[1] Q.Z. Zhao, M. B. Nardelli, J. Bernholc, Phys. Rev. B , 65 (2002) 144105
[2] C. Lee, X. D. Wei, J. W. Kysar, J. Hone, Science, 321 (2008) 385
[3] A. K. Geim , K. S. Novoselov , Nat. Mater., 6 (2007)183
[4] O. Frank, G. Tsoukleri, J. Parthenios, K. Papagelis,

Read more: www.demokritos.gr

Written by physicsgg

November 27, 2012 at 1:44 pm

Posted in Materials Science

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Ultracold fermions simulate spin–orbit coupling

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Artist’s impression of the spin diode created by researchers at MIT. Atoms with clockwise spin can only move in one direction, while atoms with anticlockwise spin move in the opposite direction. (Courtesy: Christine Daniloff)

Two independent groups of physicists are the first to use ultracold fermionic atoms to simulate “spin–orbit coupling” – an interaction that plays an important role in the electronic properties of solid materials. Both experiments were done by firing laser beams at the atoms, which caused their momentum to change by an amount that depends on their intrinsic spin. Because the interactions between atoms in such simulations can be adjusted with great precision, the breakthrough could shed further light on a range of physical phenomena, including magnetism, topological insulators and Majorana fermions.

Spin–orbit coupling describes the interaction between the intrinsic spin of an electron in a material and the magnetic field induced by the electron’s movement relative to its surrounding ions. As well as playing a key role in the magnetic properties of materials, spin–orbit coupling also influences the performance of “spintronic” devices – those that exploit the spin, rather than the charge, of electrons and that could one day lead to faster and more energy-efficient computers.

Quantum simulators

Because of its fundamental nature, physicists are therefore very keen to use clouds of ultracold atoms to simulate spin–orbit coupling. Such “quantum simulations” are carried out by subjecting the gas to laser light and magnetic fields, which lets researchers create interactions between atoms that are similar to those experienced by electrons in a solid. The advantage of these simulations is that – unlike in a solid – the strength of these interactions can be easily adjusted, allowing physicists to test theories of condensed-matter physics.

In 2011 Ian Spielman and colleagues at the National Institute of Standards and Technology (NIST) in Maryland were the first to simulate spin–orbit coupling in an ultracold gas of bosonic atoms. Now, two independent groups – one in China led by Hui Zhai of Tsinghua University and Jing Zhang of Shanxi University, and the other in the US headed by Martin Zwierlein and Lawrence Cheuk at the Massachusetts Institute of Technology (MIT) – have extended Spielman’s technique to fermions. As electrons are fermions and not bosons, the new work is much more relevant to electron physics.

Using potassium-40…

The Chinese team began with about two million potassium-40 atoms that are held in an optical trap and cooled to well below the ensemble’s Fermi temperature. This means that nearly all the atoms in the gas are in the lowest possible energy state, like the conduction electrons in a metal. The team focused on two closely spaced magnetic energy states, which are used to simulate the spin of the electron – one state corresponding to spin up and the other to spin down.

The team then fired two laser beams into the gas from opposing directions. The laser light is set to resonate with a transition between the two spin states – a process that involves the atoms continuously absorbing and emitting photons. As these photons carry momentum, if an atom absorbs a photon moving in one direction and then re-emits it in the same direction, the atom’s momentum will not change. However, an atom can also be stimulated by the opposing beam to emit the photon in the opposite direction – thus changing the atom’s momentum. Such an interaction involves a change in the direction of the atom’s spin and is therefore analogous to spin–orbit coupling – albeit in 1D.

The Chinese team used its system to study several aspects of spin–orbit coupling. In one experiment, the researchers began with a state in which all the spins are initially pointing in the same direction. They then turned on the spin–orbit interaction by pulsing the lasers for a very short time – just a few hundred microseconds. They found that the spins began to point in different directions in a process known as “dephasing”. This is expected from fermions because atoms with the same spin cannot have the same momentum and therefore each atom will be affected differently by the spin–orbit interaction.

Understanding dephasing is important because it has a detrimental effect on technological applications of spin such as spintronics and quantum computing. The team also looked at several other effects related to spin–orbit coupling, including its effect on the momentum distribution of the atoms.

…and lithium-6

The MIT physicists, meanwhile, used a gas of lithium-6 atoms, which meant that their realization of spin–orbit coupling was more difficult than for the Chinese team. The problem is that lighter atoms such as lithium are more prone to heating via the resonant absorption of light. So to get round this problem, the MIT team kept most of its atoms in “reservoir states” in which they do not interact with the light and stayed cool – using radio waves to drive a small number of atoms into the spin–orbit coupling states.

The MIT team focused on showing that ultracold atoms can be used to simulate a “spin diode” – a device that is likely to play a key role in the development of spintronic circuits. It allows spin-up atoms to flow forwards but not backwards, and spin-down atoms backwards but not forward. “The gas acts as a quantum diode, a device that regulates the flow of spin currents,” says Cheuk.

Simulating band structure

By applying radio-frequency radiation to the gas, the MIT physicists were also able to simulate a periodic potential similar to that found in a 1D lattice. As expected for real materials, the periodic potential led to the existence of spin-dependent energy bands. According to the team, the ability to create spin-dependent band structures in this way could lead to the simulation of topological insulators.

The spin–orbit coupling simulated by both teams occurs only in 1D and therefore cannot be used to simulate the 2D and 3D systems found in most real-life electronic devices. However, there are several interesting scenarios that can be investigated in a 1D system. For example, it could be used to simulate the behaviour of electrons in semiconductor/superconductor nanowires. Such systems are believed to harbour quasiparticles that resemble Majorana fermions – long sought-after particles that are also their own antiparticle.

Both experiments are described in Physical Review Letters.
Read more at: physicsworld.com

Written by physicsgg

September 5, 2012 at 8:32 am

Physicists see hints of Majorana fermions

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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.”

Read more at: physicsworld.com

Written by physicsgg

August 7, 2012 at 6:45 pm