Making magnetic monopoles, and other exotica, in the lab

Feb 05, 2009 By Lauren Schenkman
Physicist Shou-Cheng Zhang. Photo: Lauren Schenkman

Physicist Shou-Cheng Zhang has proposed a way to physically realize the magnetic monopole. In a paper published online in the January 29 issue of Science Express, Zhang and post-doctoral collaborator Xiao-Liang Qi predict the existence of a real-world material that acts as a magic mirror, in which the never-before-observed monopole appears as the image of an ordinary electron. If his prediction is confirmed by experiments, this could mean the opening of condensed matter as a new venue for observing the exotica of high-energy physics.

Zhang is a condensed-matter theorist at the Stanford Institute for Materials and Energy Science (SIMES), a joint institute of SLAC National Accelerator Laboratory and Stanford University. He studies solids that exhibit unusual electromagnetic and quantum behaviors, with an eye towards their use in information storage. But due to his training as a particle physicist, Zhang always keeps the big picture in mind. That’s why it was so easy for him to see that the material he was already working on could behave like what theorists call a magnetic monopole, an isolated north or south magnetic pole.

The monopole is thought of as electric charge’s magnetic cousin, but unlike positive or negative charges, north or south poles always occur together in what’s called a dipole. A lone north or south pole simply doesn’t show up in the real world. Even if you take a bar magnet and cut it in half down the middle, you won’t get a separate north and south pole, but two new dipole magnets instead. For symmetry-minded theorists, however, it’s natural that there should be a magnetic equivalent of charge. String theories and grand unified theories rely on its existence, and its absence undermines the mathematical feng-shui of the otherwise elegant Maxwell’s equations that govern the behavior of electricity and magnetism. What’s more, the existence of a magnetic monopole would explain another mystery of physics: why charge is quantized; that is, why it only seems to come in tidy packets of about 1.602×10-19 coulombs, the charge of an electron or proton.


Join PhysOrg.com on Facebook

For decades, scientists have kept their eyes peeled for the elusive monopole, but perhaps they were looking in the wrong place. “They were literally hoping it would fall from sky,” Zhang says. The notion isn’t as far-fetched as it seems—our world is constantly bombarded by weird particles showering from far-off cosmic events, and magnetic monopoles could very well show up as part of that rain. Some enterprising physicists installed loops of superconducting material on their rooftops. If anything remotely like a magnetic monopole fell through, the loops, being sensitive to magnetic fluctuations, would register it.

But in more than 30 years of searching, no one’s been able to conclusively detect this particle. Accelerator experiments have been no more successful, leading scientists believe existing monopoles must be far too heavy to create in even the Large Hadron Collider.

Interestingly, Zhang’s magnetic monopole didn’t fall from the heavens; instead, it was leading a quiet life on the other side of a mirror, but a mirror made of a very special type of alloy. What’s more, says Zhang, the math to prove the effect is very clear. “You could give the last part of the mathematical derivation as a final exam in a junior or senior year undergraduate physics class.”

To understand how a material can act like a magnetic monopole, it helps to examine first how an ordinary metal acts when a charge—an electron, say—is brought close to the surface. Because like charges repel, the electrons at the surface retreat to the interior, leaving the previously neutral surface positively charged. The resulting electric field looks exactly like that of a particle with positive charge the same distance below the surface—it’s the positive mirror image of the electron. In fact, from an observer’s point of view, it’s impossible to tell the difference.

The concept of an image charge is something undergraduate physics students encounter in their very first electricity and magnetism class, along with the idea that the magnetic monopole doesn’t exist. But Zhang’s “mirror” alloy is no ordinary material. It’s what’s called a topological insulator, a strange breed of solid Zhang specializes in, in which “the laws of electrodynamics are dramatically altered,” he says. In fact, if an electron was brought close to the surface of a topological insulator, Zhang’s paper demonstrates, something truly eerie would happen. Instead of an ordinary positive charge, Zhang says, “You would get what looks like a magnetic monopole in the ‘mirror.’”

To go back to the example of image charges, it’s important to emphasize that there isn’t actually half of a bar magnet somewhere inside this material. Instead, Zhang discovered, due to a peculiarity of the material called strong spin-orbit coupling, the nearby electron would induce a current in the surface that circulates constantly without dying out. This in turn—undergraduate physics majors, get out your pencils—would create a magnetic field that looks like that of a magnetic monopole. Experimentalists have tried to approximate this field before, for instance by arranging permanent magnets in certain ways. But to an outside observer, Zhang’s material would be completely indistinguishable from the monopole particle that physicists were hoping to catch in their superconducting detectors.

“We like to find things that don’t exist,” says Zhang. His work on the monopole has further ramifications; this could be a way to physically realize a number of particles that, until now, have only existed as mathematical loopholes in high-energy physics theories. For instance, Zhang has shown that the electron and image monopole together would act like a so-called “anyon” located at the solid’s surface. “The ‘any,’ in this case, is as in ‘anything,’” Zhang explains—they are particles that only exist in two dimensions, whose properties straddle those of the two classes of three-dimensional particles, fermions and bosons.

Although Zhang works as a theorist, he has close ties to experimental physics. In 2007, his prediction of the quantum spin Hall effect in mercury telluride was confirmed experimentally, earning his work praise in Science as a runner-up breakthrough of that year. “As a theorist you’re always motivated by the math, but it’s a testament to our understanding that we can predict real-world materials,” Zhang says. “Before, new materials were more or less found by accident.” Now other SIMES researchers will be using the Stanford Synchrotron Radiation Lightsource at SLAC to closely study two specific materials, bismuth selenide and bismuth telluride, that Zhang has predicted will exhibit this strange mirror behavior. They hope to confirm the prediction experimentally some time this year.

“Exotic particles such as the magnetic monopole, dyon, anyon, and the axion have played fundamental roles in our theoretical understanding of quantum physics,” Zhang writes in the paper. “Experimental observation of these exotic particles in table-top condensed matter systems could finally reveal their deep mysteries.” Topological insulators could provide a new experimental outlet for high-energy physicists. “You don’t have to look towards the cosmos,” Zhang says. “I think we’ll see more of the beautiful mathematical structures of high-energy physics become realized in condensed matter physics.”

Provided by SLAC National Accelerator Laboratory, By Lauren Schenkman

Explore further: IHEP in China has ambitions for Higgs factory

add to favorites email to friend print save as pdf

Related Stories

Artificial magnetic monopoles discovered

May 31, 2013

A team of researchers from Cologne, Munich and Dresden have managed to create artificial magnetic monopoles. To do this, the scientists merged tiny magnetic whirls, so-called skyrmions. At the point of merging, ...

Recommended for you

IHEP in China has ambitions for Higgs factory

17 hours ago

Who will lay claim to having the world's largest particle smasher?. Could China become the collider capital of the world? Questions tease answers, following a news story in Nature on Tuesday. Proposals for ...

The physics of lead guitar playing

18 hours ago

String bends, tapping, vibrato and whammy bars are all techniques that add to the distinctiveness of a lead guitarist's sound, whether it's Clapton, Hendrix, or BB King.

The birth of topological spintronics

19 hours ago

The discovery of a new material combination that could lead to a more efficient approach to computer memory and logic will be described in the journal Nature on July 24, 2014. The research, led by Penn S ...

The electric slide dance of DNA knots

22 hours ago

DNA has the nasty habit of getting tangled and forming knots. Scientists study these knots to understand their function and learn how to disentangle them (e.g. useful for gene sequencing techniques). Cristian ...

User comments : 20

Adjust slider to filter visible comments by rank

Display comments: newest first

earls
3 / 5 (3) Feb 05, 2009
"I never really got the idea of magnetic monopoles. I studied them for a couple weeks, but they just seem so intuitively wrong. Magnetic fields are caused by electrons moving... but if you have a monopole, that's like saying that you have electrons going somewhere but coming from nowhere. Or coming from somewhere and going to nowhere."
Alexa
1 / 5 (5) Feb 05, 2009
Monopoles can be modelled by unipolar magnetic domains, whose appearance is analogous to point source of electrostatic charge in certain extent.

http://superstrun...pole.jpg

The "sun spots" at magnetar surface could behave like magnetic monopoles due the enourmous field density gradient existing here.
bluehigh
3.6 / 5 (12) Feb 05, 2009
Keep your feet firmly on the ground. This is not a discovery of a magnetic monopole. It's a hypothesis that is yet to be tested by experiment. Remember and lets be quite clear about this - magnetic monopoles DO NOT exist, as far as we know.

So please, go model, extrapolate, devise mathemetical constructs, speculate 'till your hearts content but in the real world - no magentic monopoles. No, not one - none at all, never found, never detected.

OregonWind
3 / 5 (2) Feb 05, 2009
bluehigh

Although you made a good point I would not be surprised that some sort of exotic particle could be found that would present a monopole behavior. As far as I know QFT does not rule that out. We need to research more about it.
holoman
1 / 5 (2) Feb 05, 2009
Ferroelectric / multiferroics
superhuman
3.3 / 5 (7) Feb 05, 2009
The idea of magnetic monopole makes as much sense as a stick with just one end.

To understand how a material can act like a magnetic monopole, it helps to examine first how an ordinary metal acts when a charge%u2014an electron, say%u2014is brought close to the surface. Because like charges repel, the electrons at the surface retreat to the interior, leaving the previously neutral surface positively charged. The resulting electric field looks exactly like that of a particle with positive charge the same distance below the surface%u2014it%u2019s the positive mirror image of the electron. In fact, from an observer%u2019s point of view, it%u2019s impossible to tell the difference.


It looks like a field of a positively charged particle because there *IS* a positively charged particle - a positive metal ion.

The whole idea behind this story is like trying to study positive metal ions in order to better understand electrons - a completely failed attempt, you would get every single thing about electrons wrong!

(and it is possible to tell the difference, ion field originates from three separate sources)
Alizee
Feb 05, 2009
This comment has been removed by a moderator.
brant
2 / 5 (3) Feb 05, 2009
"leading scientists believe existing monopoles must be far too heavy to create in even the Large Hadron Collider."

If they change what a monopole is then they will discover it??
Alexa
1 / 5 (2) Feb 06, 2009
If they change what a monopole is then they will discover it??
Definitelly. For example, things like hidden dimensions or Lorentz symmetry violation exists around us from the very beginning of Universe, it's just a matter of definition, the scientists don't see them. To be able to find something, you should know, what to look for.
Lightfire
1 / 5 (1) Feb 06, 2009
I'd figure that since a minimum of two fundamental particles are necessary to observe a wave traveling through a medium(if only 1 particle is moving you can't tell if a wave is traveling through it) and since two fundamental particles are needed for a dipole, that the wave-particle duality has something to do with the absence of magnetic monopoles. I'm unqualified however.
out7x
1.7 / 5 (3) Feb 06, 2009
....how an ordinary metal acts when a charge%u2014an electron, say%u2014is brought close to the surface.......

More gibberish. Electron configuration is described by bonding.
johanfprins
3 / 5 (6) Feb 06, 2009
QFT allows singularities like point charges in the "real" world. The practitioners of this field then have to "magically" remove infinities in their mathematics by a process called "renormalisation": i.e. I do not get what I believe I should get so I "renormalise" the results to get what I wanted to get from the start. Singularities, like even the centre of mass of a body, are only mathematical tools which can only really exist in Plato's Universe! Show me a concentration of mass at the centre of a hollow ball and I will show you a magnetic monopole.
Alexa
1 / 5 (4) Feb 06, 2009
centre of a hollow ball and I will show you a magnetic monopole
Concept of magnetic monopole doesn't require pinpoint mass. Try to read my posts above. Anyons can be represented by quantum dots with charged particle trapped inside it and such hole can be quite large.

http://superstrun...wave.gif
Adriab
4 / 5 (4) Feb 06, 2009
Still, moving from theory to experiment would be a really interesting feat. I would be happy with a macro-scale magnetic psuedo-monopole. Think of all the new engineering that could be done.
Rossen
1 / 5 (1) Feb 07, 2009
I agree with you Adriab. For example, if we have a magnetic charge, it will rotates around conductor with current (along the magnetic force lines). It can be an ideal DC motor. Or another example: we can produce magneticaly charged UFO-like body flying along the geomagnetic force lines. Etc. But I am some sceptical. This theory don't say us how to make this exotic matherial. In my opinion this is fiction.
johanfprins
1 / 5 (1) Feb 07, 2009
centre of a hollow ball and I will show you a magnetic monopole
Concept of magnetic monopole doesn't require pinpoint mass. Try to read my posts above. Anyons can be represented by quantum dots with charged particle trapped inside it and such hole can be quite large.
http://superstrun...wave.gif


You qoute me out of context: I DID NOT say that a monopole requires a pinpoint mass. All I said is that it is just like the centre of mass of a hollow ball: It is a purely mathematical construct: Not an entity one can physically "see" in isolation.

Has an anyon ever been observed outside of Franck Wlczek's mind?

Alexa
1 / 5 (2) Feb 08, 2009
They appears in thin layers. Fractional quantized Hall states are known to be anyons.
johanfprins
1.5 / 5 (2) Feb 09, 2009
They appears in thin layers. Fractional quantized Hall states are known to be anyons.

Thanks! I am, however, of the opinion that even the integer quantum Hall effect is not yet well understood. But if you want to call the fractional states anyons: So be it!
SomedayIsle
not rated yet Mar 10, 2009
So, are we to take it then, that the folk over at Lockheed Martin are just a bunch of buckaroos who throw out the 'M' word merely for effect??

Search U.S. Patent #5929732 at the USPTO

Hmmm

Guess so.
rasselas21
not rated yet Mar 27, 2009
concerning Lockheed's monopole reference... they're isn't a "monopole" in the sense that there isn't a dipole present... only that the arrangement of magnets produces an amplified (or rather, concentrated) pole, and yet the other pole exists but in a non-focued (or rather, diffused) manner. They're basically saying, "the half of the magnet we are dealing with (amplifying) is a monopole, the other half that we are unconcerned with (the diffused poles) are monopoles as well, except they're all spread out, rather than unified as in the amplified pole. Together they form a dipole... but the amplified pole (that's what we're concerned with) IS monopolic in effect, but is indeed attached to another pole (and thus forms a dipole)."

In other words, you just can approach the Lockeed apparatus from any old direction and repel from it, but only from one (actually smaller approachable region than a normal magnet) direction.

I personally agree that there is no REAL monopole, but as to whether there is an apparent monopole, is indeed another question...
Cittran
not rated yet Jun 18, 2009
The idea of magnetic monopole makes as much sense as a stick with just one end.


...those exist in math; they're called rays.

Also, even if the laws of nature say they do not or cannot exist, who says that humans have a perfect knowledge and grasp of the laws of nature?

Besides, the universe is stranger than we can imagine; humans, regardless of how intelligent we believe ourselves to be, are NOT omniscient, nor are we even close to becoming omniscient.