Majorana fermion: Physicists observe elusive particle that is its own antiparticle

October 2, 2014
Princeton University scientists used scanning-tunneling microscope to show the atomic structure of an one-atom-wide iron wire on a lead surface. The zoomed-in portion of the image depicts the quantum probability of the wire containing an elusive particle called the Majorana fermion. Importantly, the image pinpoints the particle to the end of the wire, which is where it had been predicted to be over years of theoretical calculations. Credit: Yazdani Lab, Princeton University

Princeton University scientists have observed an exotic particle that behaves simultaneously like matter and antimatter, a feat of math and engineering that could yield powerful computers based on quantum mechanics.

Using a two-story-tall microscope floating in an ultralow-vibration lab at Princeton's Jadwin Hall, the scientists captured a glowing image of a particle known as a "Majorana fermion" perched at the end of an atomically thin wire—just where it had been predicted to be after decades of study and calculation dating back to the 1930s.

"This is the most direct way of looking for the Majorana fermion as it is expected to emerge at the edge of certain materials," said Ali Yazdani, a professor of physics who led the research team. "If you want to find this particle within a material you have to use such a microscope, which allows you to see where it actually is."

The hunt for the Majorana fermion began in the earliest days of quantum theory when physicists first realized that their equations implied the existence of "antimatter" counterparts to commonly known particles such as electrons. In 1937, Italian physicist Ettore Majorana predicted that a single, stable particle could be both matter and antimatter. Although many forms of antimatter have since been observed, the Majorana combination remained elusive.

In addition to its implications for fundamental physics, the finding offers scientists a potentially major advance in the pursuit of quantum computing. In quantum computing, electrons are coaxed into representing not only the ones and zeroes of conventional computers but also a strange quantum state that is both a one and a zero. This anomalous property, called quantum superposition, offers vast opportunities for solving previously incalculable systems, but is notoriously prone to collapsing into conventional behavior due to interactions with nearby material.

Despite combining qualities usually thought to annihilate each other—matter and antimatter—the Majorana fermion is surprisingly stable; rather than being destructive, the conflicting properties render the particle neutral so that it interacts very weakly with its environment. This aloofness has spurred scientists to search for ways to engineer the Majorana into materials, which could provide a much more stable way of encoding quantum information, and thus a new basis for quantum computing.

A team led by Yazdani and including colleagues at Princeton and at the University of Texas-Austin published their results in the Oct. 2 issue of the journal Science.

Yazdani noted that the observation of the Majorana fermion bound within a material is different for physicists than the much publicized discovery of particles, such as the Higgs boson, in a vacuum in giant accelerators. In such experiments, scientists collide particles at high speeds, producing a shower of free and ephemeral components. In materials, by contrast, the existence of a particle depends on—or emerges from—the collective properties of atoms and forces surrounding it.

By controlling these interactions, the researchers said, their Majoranas appeared "clean and removed from any spurious particles," which would be unavoidable in high-energy accelerator experiments. "This is more exciting and can actually be practically beneficial," Yazdani said, "because it allows scientists to manipulate exotic particles for potential applications, such as ."

In addition to their potential practical uses, the pursuit of Majoranas has broad implications for other areas of physics. Scientists believe, for example, that another sub-atomic particle called neutrinos, which also interact very weakly and are very hard to detect, could be a type of Majorana—a neutrino and anti-neutrino being the same particle. In addition, scientists regard Majoranas as possible candidates for dark matter, the mysterious substance that is thought to account for most matter in the universe, but which has not been directly observed because it also does not directly interact with other particles.

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Princeton University researchers first deposited iron atoms onto a lead surface to create an atomically thin wire. They then used a scanning-tunneling microscope to create a magnetic field and to map the presence of a neutral signal that indicates the presence of Majorana fermions, which appeared at the ends of the wire. Credit: Ilya Dorzdov, Yazdani Lab, Princeton University

Despite the scientific interest, there was little progress in finding the particle until 2001 when the physicist Alexei Kitaev (now a professor at the University of California-Santa Barbara) predicted that, under the correct conditions, a Majorana fermion would appear at each end of a superconducting wire. Superconductivity is the phenomenon when a material can carry electricity without any resistance. In Kitaev's prediction, inducing some types of superconductivity would cause the formation of Majoranas. These emergent particles are stable and do not annihilate each other (unless the wire is too short) because they are spatially separated. Importantly, Kitaev also outlined how such a particle could be harnessed as a qubit, the basis of a quantum computer, which added significant impetus to the search.

In 2012, a team of researchers at the Delft University of Technology in the Netherlands may have caught a glimpse of Majorana fermions in an experiment that induced superconductivity in a semiconductor known as indium antemonide. They reported very strong evidence for the electrical signal characteristic of a neutral Majorana in nanoscale wires made of these materials.

Scientists, however, have argued that other phenomena could produce the same signal, and Yazdani and his team sought to find more definitive observation of Majorana fermions by capturing an image of them. In 2013, Yazdani and Andrei Bernevig, an associate professor of physics, teamed up to propose a new approach for how the Majorana particle could occur in materials that combine magnetism and superconductivity. They also proposed that such a particle could be directly observed using a device called a scanning-tunneling microscope.

Yazdani and Bernevig received funding to carry out their proposed experiments through the Princeton Center for Complex Materials, an interdisciplinary center on campus funded by the National Science Foundation. Promising results from that work allowed them to partner with the Texas team and win a $3 million grant from the Office of Naval Research under a program called the Majorana Basic Research Challenge.

The setup they created starts with an ultrapure crystal of lead, whose atoms naturally line up in alternating rows that leave atomically thin ridges on the crystal's surface. The researchers then deposited pure iron into one of these ridges to create a wire that is just one atom wide and about three atoms thick. Considering its narrow width, this wire is very long—if it were a pencil it would be five feet from tip to eraser.

The researchers then placed the lead and the embedded iron wire under the scanning-tunneling microscope and cooled the system to -272 degrees Celsius, just a degree above absolute zero. After about two years of painstaking work, they confirmed that superconductivity in the iron wire matched the conditions required for Majorana fermion to be created in their material.

Ultimately, the microscope was also able to detect an electrically neutral signal at the ends of the wires, similar to those seen in the Delft experiment. However, the setup also allowed the researchers to directly visualize how the signal changes along the wire, essentially mapping the quantum probability of finding the Majorana fermion along the wire and pinpointing that it appears at the ends of the wire.

"It shows that this signal lives only at the edge," Yazdani said. "That is the key signature. If you don't have that, then this signal can exist for many other reasons."

Yazdani noted that although the experimental setup used to measure and demonstrate the existence of Majorana particle is very complex, the new approach does not use exotic materials and is straightforward for other scientists to reproduce and use.

"What's very exciting is that it is very simple: it is lead and iron," he said.

In fact, in the course of their experiments the researchers found that their approach is even easier to use than they expected. While they set out to fine-tune the properties of their materials to exact specifications, they found that their system is almost guaranteed to have Majorana fermions, as long as some general features of magnetism and superconductivity are in place. Calculations performed by the Austin team led by Professor Allan MacDonald have confirmed this picture.

"The details turned out to be not that important," Yazdani said.

Bernevig added, "As long as you have a strong magnetic material—like iron but it could be other magnets—in which electrons are magnetically polarized (or electrons feel a very strong magnetic field), the possible range of parameters in which Majoranas appear increases dramatically."

In previous proposals, the appearance of Majoranas would only happen under a narrow range of conditions. Typically, it is hard to have superconductivity and magnetism in the same material—magnetic fields usually kill superconductivity. But in the Princeton team's method, the magnetic field is present only where it is needed, on the wire, so creeps in from the underlying lead unimpeded.

"Once you have that, all you need are some relativistic effects that are easy to induce at the surface of a heavy element like lead, and the Majoranas will appear," Bernevig said. "We expect many more materials will produce these elusive particles."

Explore further: Third research team close to creating Majorana fermion

More information: "Observation of Majorana Fermions in Ferromagnetic Atomic Chains on a Superconductor," by S. Nadj-Perge et al. Science, www.sciencemag.org/lookup/doi/ … 1126/science.1259327

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32 comments

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Rossen
4.3 / 5 (6) Oct 02, 2014
What in fact is this? Particle or quasiparticle?
Tektrix
5 / 5 (10) Oct 02, 2014
What in fact is this? Particle or quasiparticle?


"In condensed matter physics, Majorana fermions exist as quasiparticle excitations in superconductors" - http://en.wikiped..._fermion

Rossen
4.2 / 5 (5) Oct 02, 2014
Thank you, Tektrix.
Goika
Oct 02, 2014
This comment has been removed by a moderator.
Returners
1.7 / 5 (12) Oct 02, 2014
These almost certainly don't exist in nature, as 1 kelvin is colder than any natural object, and even the CMB is warmer than that, which means it's not even possible to reach 1 kelvin except in an intelligently designed microwave chamber.

when the CMB cools to less than 1 kelvin, then this might be possible to exist in nature.

So I'd say nadda when it comes to this as any potential DM/DE candidate. Just doesn't make sense, as it exists at too low of an energy level, while DM adn DE, if they exist, apparently do so at any energy level.
Goika
Oct 02, 2014
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Goika
Oct 02, 2014
This comment has been removed by a moderator.
shavera
3.8 / 5 (10) Oct 02, 2014
Returners: Way to f'ing read the article. Good job on that. Maybe you might have read that it's a majorana fermion quasi particle excitation of a material, and not some sort of fundamental particle like an electron or neutrino.
ogg_ogg
4.3 / 5 (4) Oct 02, 2014
indium antimonide InSb, not (in English at least) indium antemonide.
Goika
Oct 02, 2014
This comment has been removed by a moderator.
saposjoint
3.8 / 5 (13) Oct 02, 2014
Zephyr, you ignorant slut.
Da Schneib
4.3 / 5 (6) Oct 02, 2014
These almost certainly don't exist in nature, as 1 kelvin is colder than any natural object, and even the CMB is warmer than that, which means it's not even possible to reach 1 kelvin except in an intelligently designed microwave chamber.

when the CMB cools to less than 1 kelvin, then this might be possible to exist in nature.

So I'd say nadda when it comes to this as any potential DM/DE candidate. Just doesn't make sense, as it exists at too low of an energy level, while DM adn DE, if they exist, apparently do so at any energy level.
Also, these appear to be solitons of a type that can only exist at the edges of matter. There are no sharp matter edges in intergalactic space no matter what temperature it is. These (Majorana particles) have been mooted as a potential DM candidate, but these limitations to their existence make it unlikely in my mind as well.

indium antimonide InSb, not (in English at least) indium antemonide.
Yeah, you beat me to it.
AmritSorli
1.5 / 5 (8) Oct 03, 2014
antiparticle is diminished energy density of quantum vacuum of particle itself.
MaxC500
5 / 5 (4) Oct 03, 2014
So they copied the work that the Dutch already did to get a piece of the Noble prize action. Clearly the Dutch in their first research and even more so in their following and more advanced research showed that these particles existed without question. As well reported subsequently in the media around the world.

This is really just backing up and advancing that breakthrough. Like finding the Higgs Boson in a different machine than the Large Hadron Collider.
Lex Talonis
3 / 5 (4) Oct 03, 2014
I wonder if they can scale this up?

Like 1Kg of mystical particles from the corner store...

Cleans drains etc.
mytwocts
2 / 5 (4) Oct 03, 2014
I don't like how the importance of this research is hyped with keywords from subatomic physics.
1) Photons and gravitons trivially are their own antiparticles, so what is so exciting about this property? On the contrary, it is mysterious how neutrons and neutrinos are NOT their own antiparticles.
2) "Scientists believe, for example, that another sub-atomic particle called neutrinos, which also interact very weakly and are very hard to detect, could be a type of Majorana—a neutrino and anti-neutrino being the same particle."
Scientists believe that neutrinos are NOT their own antiparticles. The quasiparticles in question are not subatomic, but collective excitations (e.g. phonons) of a system of atoms.
3) "Despite combining qualities usually thought to annihilate each other—matter and antimatter "
Nonsense: this quasiparticle does not combine matter and anti-matter. Inaccurate: matter and anti-matter do not in general annihilate: a positron will only annihilate an electron.
mytwocts
1.8 / 5 (5) Oct 03, 2014
What in fact is this? Particle or quasiparticle?

A quasiparticle since it requires an underlying system of degrees of freedom to exist.
Then again, if you consider the vacuum to hold all degrees of freedom, any particle is a quasiparticle.
Thus, the answer depends on what you choose to be the vacuum. If defined as a chain of iron atoms on a piece of lead, all in the ground state, then this thing would be a particle.
Eseta
Oct 03, 2014
This comment has been removed by a moderator.
Eseta
Oct 03, 2014
This comment has been removed by a moderator.
Eseta
Oct 03, 2014
This comment has been removed by a moderator.
mytwocts
1.5 / 5 (4) Oct 03, 2014
If subatomic particles are themselves quasiparticles of an underlying system of degrees of freedom, then the vacuum is the ground state of this all pervasive system that is at the root of all matter. Then it still is a hype to put this ground state on the same level as that of a line of iron atoms on a lead surface.
mytwocts
3 / 5 (4) Oct 03, 2014
the physics is not about rigid laws and stuffs, but about dependence of observable reality on (the observational perspective of) its observer.

I do not subscribe to this characterization of physics at all.
Note that in my original post I already anticipated this discussion so I am fully aware that the distinction between particle and quasiparticle can be artificial.
Eseta
Oct 03, 2014
This comment has been removed by a moderator.
Eseta
Oct 03, 2014
This comment has been removed by a moderator.
mytwocts
2.3 / 5 (3) Oct 03, 2014
If you were speaking of the notion of quasiparticles and not of physics in general, then I subscribe to your statement. As stated, it depends on what you take as the vacuum.
mytwocts
3.7 / 5 (3) Oct 03, 2014
I already implied in my first post that the quasiparticle issue is a matter of perspective.
In general, physics is not a about perspective, which is what you appeared to say by "the physics is not about rigid laws and stuffs, but about dependence of observable reality on (the observational perspective of) its observer."
Physics is also more and more about fund raising, which is why research is often hyped.
baudrunner
1 / 5 (5) Oct 04, 2014
I think that a correlation can probably be found between the amount of electrolytic activity measured between two different metals and the energy of the so-called Majorana particle measured at the point where you might expect to find the greatest deal of electrolysis, at the ends of a metal wire or bar.

This works better for certain metals under certain conditions..?
Da Schneib
2.3 / 5 (3) Oct 06, 2014
This is really just backing up and advancing that breakthrough. Like finding the Higgs Boson in a different machine than the Large Hadron Collider.
Actually, it's more like finding the Higgs Boson in the CMBR, or in black hole spectroscopy; not merely a different machine, but a different method. Confirmation is important, and this is a second data point that makes this a virtual certainty. It's more like confirming the Aspect Experiment with ions instead of photons.

I make no comment on the political aspects.
dnatwork
3 / 5 (2) Oct 07, 2014
These almost certainly don't exist in nature, as 1 kelvin is colder than any natural object, and even the CMB is warmer than that....

So I'd say nadda when it comes to this as any potential DM/DE candidate.


Also, these appear to be solitons of a type that can only exist at the edges of matter. There are no sharp matter edges in intergalactic space no matter what temperature it is.


Three points:

1) The background temperature is the temperature of the things you can detect. Dark matter/energy are the things you can't detect, so you don't know their temperature.

2a) This is one type of majorana particle. There could be others that exist at different temperatures and not at edges.

2b) Or every hydrogen ion out in space could have one of these attached because it is one-dimensional (very pointy). This experiment shows where we are capable detecting them, which is not the same as limiting where they can exist.
castro
Oct 07, 2014
This comment has been removed by a moderator.
Da Schneib
not rated yet Oct 08, 2014
These almost certainly don't exist in nature, as 1 kelvin is colder than any natural object, and even the CMB is warmer than that... So I'd say nadda when it comes to this as any potential DM/DE candidate.
Also, these appear to be solitons of a type that can only exist at the edges of matter. There are no sharp matter edges in intergalactic space no matter what temperature it is.
Three points:

1) The background temperature is the temperature of the things you can detect. Dark matter/energy are the things you can't detect, so you don't know their temperature.
But we can't detect it, so we know it's got to be under 3K.
(contd)
Da Schneib
not rated yet Oct 08, 2014
2a) This is one type of majorana particle. There could be others that exist at different temperatures and not at edges.
Have any others been proposed that could account for 25-some-odd percent of the mass in the universe? I'd love to see the details.

2b) Or every hydrogen ion out in space could have one of these attached because it is one-dimensional (very pointy).
Have any Majorana particles been proposed that could associate in this manner with hydrogen atoms? Again, I'd love to see the details.

This experiment shows where we are capable detecting them, which is not the same as limiting where they can exist.
But other factors can limit them, so show us where such Majorana particles have been derived from so we can all share your interesting information and understand where they are coming from.

Preferably the source(s) you link to will suggest how they can be detected.

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