With the discovery of the Higgs field, scientists think they have a pretty good handle on the origins of the mass of fundamental particles. Particles that interact with the field have mass, and those that don’t, don’t. However, the neutrino poses an additional mystery. Neutrinos have a very tiny mass, but not zero. Just why this should be is not known. Continue reading Heavy neutrinos
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
In a cavern almost a mile underground in the Black Hills, an experiment called the MAJORANA DEMONSTRATOR, 40 kilograms of pure germanium crystals enclosed in deep-freeze cryostat modules, will soon set out to answer one of the most persistent and momentous questions in physics: are neutrinos their own antiparticles? If the answer is yes, it will require rewriting the Standard Model of Particles and Interactions, our basic understanding of the physical world.
The best way to learn whether neutrinos are their own antiparticles would be to observe a certain kind of radioactive decay, called neutrinoless double-beta decay. It has never been detected conclusively, and if it occurs at all, it’s exceedingly rare,” says Alan Poon of the Nuclear Science Division at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab).
Poon is the current executive-committee chair of the MAJORANA Collaboration, which is comprised of more than 100 researchers from 19 institutions in the United States, Canada, Russia, and Japan, and whose efforts are focused on the experiment now under construction at the Davis Campus of the Sanford Underground Research Facility (SURF) in Lead, South Dakota.
Beta decay, single and double
Ordinary beta decay is a common kind of radioactivity: an atomic nucleus changes into a different kind of element, a neighbor on the periodic table with lower mass, by emitting a beta particle – an electron or positron – plus a neutrino or an antineutrino. For example, carbon-14 transforms to nitrogen-14 when one of its neutrons turns into a proton, emitting an electron and an antineutrino. It was beta decay that led to the proposal that there must be a particle like the neutrino, since an electron alone could not account for all the energy lost in the decay.
“Double-beta decay is also possible and has been observed in a dozen different isotopes since 1986,” says Poon. “But it happens at a really low rate, and not too many nuclei can do it.”
The only conclusive double-beta decays seen so far involve two neutrons that change into two protons while emitting two electrons and two antineutrinos. It’s an uncommon situation, in which single beta decay is blocked because the decaying isotope’s immediate neighbor has a nucleus that’s too heavy, but the nucleus of the neighbor two places away on the periodic table does have lower mass – even though its atomic number is two places higher. Getting there requires double-beta decay….
Read more: http://phys.org
Quest for quirky quantum particles may have struck gold
Evidence for elusive Majorana fermions raises possibilities for quantum computers.
Eugenie Samuel Reich
Getting into nanoscience pioneer Leo Kouwenhoven’s talk at the American Physical Society’s March meeting in Boston, Massachusetts, today was like trying to board a subway train at rush hour. The buzz in the corridor was that Kouwenhoven’s group, based at the Delft University of Technology in the Netherlands, might have beaten several competing teams in solid-state physics — and the community of high-energy physicists — to a long-sought goal, the detection of Majorana fermions, mysterious quantum-mechanical particles that may have applications in quantum computing.
Kouwenhoven didn’t disappoint. “Have we seen Majorana fermions? I’d say it’s a cautious yes,” he concluded at the end of a data-heavy presentation.
Quantum particles come in two types, fermions and bosons. Whereas bosons can be their own antiparticles, which means that they can annihilate each other in a flash of energy, fermions generally have distinct antiparticles; for example, an electron’s antiparticle is the positively charged positron. But in 1937, Italian physicist Ettore Majorana adapted equations that Englishman Paul Dirac had used to describe the behaviour of fermions and bosons to predict the existence of a type of fermion that was its own antiparticle. Over decades, particle physicists have looked for Majorana fermions in nature, and after 2008, condensed-matter physicists began to think of ways in which they could be formed from the collective behaviour of electrons in solid-state materials, specifically, on surfaces placed in contact with superconductors or in one-dimensional wires.
Kouwenhoven’s apparatus is along the latter lines. In his group’s set-up, indium antimonide nanowires are connected to a circuit with a gold contact at one end and a slice of superconductor at the other, and then exposed to a moderately strong magnetic field. Measurements of the electrical conductance of the nanowires showed a peak at zero voltage that is consistent with the formation of a pair of Majorana particles, one at either end of the region of the nanowire in contact with the superconductor. As a sanity check, the group varied the orientation of the magnetic field and checked that the peak came and went as would be expected for Majorana fermions……….
Read more: www.nature.com
Searching for an equation
We review the non-trivial issue of the relativistic description of a quantum mechanical system that, contrary to a common belief, kept theoreticians busy from the end of 1920s to (at least) mid 1940s.
Starting by the well-known works by Klein-Gordon and Dirac, we then give an account of the main results achieved by a variety of different authors, ranging from de Broglie to Proca, Majorana, Fierz-Pauli, Kemmer, Rarita-Schwinger and many others.
A particular interest comes out for the general problem of the description of particles with arbitrary spin, introduced (and solved) by Majorana as early as 1932, and later reconsidered, within a different approach, by Dirac in 1936 and by Fierz-Pauli in 1939.
The final settlement of the problem in 1945 by Bhabha, who came back to the general ideas introduced by Majorana in 1932, is discussed as well, and, by making recourse also to unpublished documents by Majorana, we are able to reconstruct the line of reasoning behind the Majorana and the Bhabha equations, as well as its evolution.
Intriguingly enough, such an evolution was identical in the two authors, the difference being just the period of time required for that: probably few weeks in one case (Majorana), while more than ten years in the other one (Bhabha), with the contribution of several intermediate authors.
Majorana’s paper of 1932, in fact, contrary to the more complicated Dirac-Fierz-Pauli formalism, resulted to be very difficult to fully understand (probably for its pregnant meaning and latent physical and mathematical content): as is clear from his letters, even Pauli (who suggested its reading to Bhabha) took about one year in 1940-1 to understand it.
This just testifies for the difficulty of the problem, and for the depth of Majorana’s reasoning and results…..
Read more: http://arxiv.org