MIT theorists analyze low-temperature superconducting material, propose using NMR to verify existence of topologically protected Majorana fermions.
|Physicists describe electrons by their energy, momentum, and spin. An electron can occupy a possible energy level and an unoccupied level is called a hole. Here a special electronic state called a Majorana fermion is shown as the sum of an electron and a hole that move freely. MIT Assistant Professor of Physics Liang Fu predicts this special state should occur near absolute zero temperature in a class of superconducting materials. Both electron and hole have the same spin [indicated by downward pointing arrows], a hallmark of Majorana fermions. Illustration courtesy of the researchers.|
A low-temperature superconducting material made up of the elements praseodymium, osmium, and antimony [PrOs4Sb12] promises to host robust Majorana fermions, which are a special particle predicted in 1937, MIT researchers show in a theoretical analysis.
These Majorana fermions are quasiparticles, that is, particles which do not exist as elementary particles in nature but can exist as emergent particles in superconducting materials at very low temperatures. The defining property of Majorana quasiparticles is that they are made up of electrons and holes equally, called a quantum superposition.
The new analysis by graduate student Vladyslav Kozii, postdoctoral associate Jörn Venderbos and Lawrence C. (1944) and Sarah W. Biedenharn Career Development Assistant Professor Liang Fu reveals the likely presence of these quasiparticles in the superconducting material at temperatures close to absolute zero, and they propose that it can be measured from their pair annihilation rate using nuclear magnetic resonance [NMR]. In PrOs4Sb12, these excitations preferably couple to nuclear spins polarized along a particular axis, and it is this directional sensitivity that makes them susceptible to NMR probing.
“We address a certain class of superconductors, show that they have Majorana fermions as itinerant quasiparticles in the bulk, and then look at how they can be detected and what other properties these materials have that one could use in the future for interesting functionality,” says Venderbos. “I think it really very nicely bridges the gap between experiment and theory and it can be used by experimentalists right now.” Their paper is published Dec. 7, 2016, in the journal, Science Advances.
Finding the right material
“Regarding the material that we proposed, actually there is one recent experiment that confirms that time-reversal symmetry is broken in the superconducting state of this material. This reinforces our conclusion that it is indeed a very promising candidate for our theory to apply,” Kozii notes. He adds that the MIT work examines odd-parity superconductors, meaning that paired electrons [Cooper pairs] have a wave function that is odd under another kind of symmetry, inversion symmetry. “That is actually an important requirement and is linked to the topological nature of this superconductor,” he says. The topological nature of a superconductor is revealed on its surfaces and edges, leading to robust features such as dissipation-less heat conductance.
Venderbos notes that Majorana fermions were introduced in 1937 as a special solution to the equation describing relativistic electrons in three dimensions. Princeton researchers reported detection of a zero-dimensional realization of these particles at the end of an atom chain in October 2014. The MIT theorists now show that three-dimensional propagating Majorana fermions, governed by the same equation of motion considered in the original work of Italian physicist Ettore Majorana, ought to exist as emergent particles in the bulk of a certain class of superconducting materials. “The extensive study we have performed shows that this peculiar particle may now find its realization in solid state physics in a real material,” Venderbos says.
A characteristic of this superconducting material revealed through this analysis is that there are some special points in its electronic excitation spectrum where the superconducting gap vanishes, which means that low energy excitations are possible. “However low energy you take, there will be always excitation at this energy. These excitations are exactly these Majorana fermions we were talking about,” Kozii explains. Venderbos adds, “There are some excitations for which you don’t have to put in any energy or just an infinitesimally tiny amount and you can still create the excitation.”
The material analyzed, PrOs4Sb12, has two phases and becomes superconducting at 1.85 kelvin [-456.34 Fahrenheit] with a second transition at 1.7 kelvin. The researchers compared their findings to a known superfluid, helium-3 [3He-A], and found significant differences in the nature of superconductivity. In particular, in the case of helium-3 at low temperatures, its excitations contain both spin-up and spin-down electrons, whereas the PrOs4Sb12 system can host only one spin species, either spin up or spin down. “That actually leads to very important physical consequences that we consider and describe in our paper and that is also one of the key points of our work,” Kozii explains.
Noting that Fu has made “some fantastic predictions in the past,” Princeton University Professor of Chemistry Robert J. Cava, who was not involved in this research, suggests that “Experimentalists should listen to what he has to say. They should process it through the filter of their own experience, and then do what they believe to be best. I am very happy to see that he and his coworkers have presented an analysis of real materials in which their ideas might be embodied.”
|MIT researchers postdoctoral associate Jörn Venderbos, left, and graduate student Vladyslav Kozii co-authored a new paper with Biedenharn Career Development Assistant Professor of Physics Liang Fu proposing that a class of superconducting materials can host Majorana fermions near absolute zero and their existence can be verified using nuclear magnetic resonance (NMR). Photo, Denis Paiste, Materials Processing Center.|
Kozii, Venderbos and Fu analyzed these unconventional superconductors for a year. For Kozii, the work will become part of his doctoral thesis.
The researchers hope their work will inspire experimentalists to look again at some previously studied materials to identify ones that host superconducting states with Majorana fermions. “I think the first step would be just to find a material in which everyone can agree that it has these Majorana fermions. That would be really exciting and constitute the discovery of a new type of superconductor in experiment,” Venderbos says. “The next step would be to think about functionalization of these materials, what could be the specific applications.” Trying to make quantum devices out of these materials is one possible direction. “We hope this research ultimately brings closer efforts from the quantum material and quantum device community in finding out the many facets of Majorana fermions,” Fu adds.
The DOE Office of Basic Energy Sciences, Division of Materials Sciences and Engineering [Award DE-SC0010526] supported Fu’s and Kozii’s work. The Netherlands Organization for Scientific Research supported Venderbos through a Rubicon grant.
– Denis Paiste, Materials Processing Center
Updated December 14, 2016