Researchers detect elusive quantum property known as 'Berry's phase' in a semiconductor

Mar 20, 2014
Figure 2: In BiTeI, electrons display strong Rashba spin–orbit coupling—the energy surface of the system is split into two sheets with opposite spin helicities (indicated by arrows). Credit: Mohammed Saeed Bahramy, Hiroshi Murakawa, RIKEN Center for Emergent Matter Science

In conducting materials, free electrons can move from one point to another, conveying their electrical charge to produce an electrical current. Electrons have another property, known as spin, that could be harnessed and manipulated in 'spintronic' circuits and has the potential to revolutionize the field of conventional electronics, leading to new functionalities and devices with enhanced performance.

Controlling states, however, is not as straightforward as controlling charge, making the development of practical spintronics a significant challenge. Quantum theory predicts that certain exotic energy states produced by the motion of electrons in solid matter could be used to control electron . Yet it is only recently that scientists have even been able to observe such exotic quantum energy states in the lab.

Hiroshi Murakawa and colleagues from the RIKEN Center for Emergent Matter Science, in collaboration with co-workers from the University of Tokyo and the SLAC National Accelerator Laboratory in the United States, have for the first time experimentally detected an elusive quantum property known as Berry's phase in a semiconductor.

Breaking waves to find the phase

"Berry's phase is a ubiquitous and fundamental notion in quantum physics," says Murakawa. Berry's phase gives rise to emergent phenomena such as the quantum version of the classical Hall effect—a transverse electrical current induced by an external magnetic field—as well as novel materials called topological insulators, which act as insulators in their bulk but conduct electricity on their surfaces.

"In quantum mechanics, an electron's motion, behavior and state are described by its wavefunction, which is characterized by an amplitude and a phase," explains Murakawa. "Typically, the phase reflects the time evolution of the electronic state and depends on energy and momentum terms. Berry's phase, on the other hand, is independent of time and reflects the system geometry. Geometrical information can therefore be encoded in the wavefunction and govern the motion of electrons."

It is these geometrical properties, such as the relative orientation of an electron's spin or momentum, that can give rise to Berry's phase. Most commonly, Berry's phase occurs in systems that are subjected to gradually changing cyclic processes that do not involve the transfer of heat or matter between the system and its surroundings. This condition is generally only met when the system has time to adapt its configuration to the changing conditions, such as when particles undergo rotation or translation.

"When the spin Berry's phase exists," says Murakawa, "novel phenomena such as spin-polarized charge flow without enegy dissipation can be realized. Despite its ubiquity and importance, however, experimental observation of Berry's phase stemming from electron spin is challenging."

Finding materials with the right split

An electron's spin can be either 'up' or 'down'. In most materials, electrons in spin-up states have almost identical energy to those in spin-down states. This leads to a phenomenon called energy degeneracy, in which the two states are difficult to distinguish. In their search for real manifestations of Berry's phase, Murakawa and his colleagues had to overcome the complication of energy degeneracy. They did so by focusing their attention on the semiconductor bismuth tellurium iodide (BiTeI)—a material with an unusual atomic structure consisting of stacked layers of its three individual component elements (Fig. 1).

The interfaces between the layers in BiTeI destroy the symmetry of the atomic lattice and give rise to a strong coupling between electron spin and motion, known as spin–orbit interaction. The overall result, known as Rashba splitting, is the removal of energy degeneracy, allowing different states to be easily observed (Fig. 2).

"Rashba semiconductors have not been applied practically until now, mainly due to the lack of an ideal system," says Murakawa. "The three-dimensional crystal structure of BiTeI allows a Rashba spin split to occur on a bulk scale, resulting in a huge number of spin-polarized electrons."

Observing oscillations

The researchers produced a series of single-crystal BiTeI samples with different electron carrier densities by growing the crystals at different temperatures. They then measured the electrical resistance of thin plates of the material. They also measured the Hall effect resistivity of the crystal under a strong magnetic field at temperatures of less than 2 kelvin—close to absolute zero.

The team was looking for the Shubnikov–de Haas (SdH) effect, a telltale sign of quantum behavior, by which behave like simple harmonic oscillators, fluctuating up and down in energy. This can be observed as an oscillation in magnetoresistivity with increasing magnetic field strength in materials at very low temperatures. In BiTeI, two sets of these oscillations corresponding to the two possible spin states could be clearly identified—the result sought after by Murakawa and his team. "The extremely large Rashba energy splitting in BeTeI enabled us to analyze each oscillation peak separately and disclose the spin Berry's phase clearly," he notes.

Murakawa suggests that it might be possible to induce persistent charge flow without energy loss by improving the crystal quality or optimizing the carrier concentration of BiTeI. The researchers are hopeful that they will eventually be able to control non-dissipative transport properties through the manipulation of Berry's .

Explore further: Scientists demonstrate electrical properties of topological insulators

More information: Murakawa, H., Bahramy, M. S., Tokunaga, M., Kohama, Y., Bell, C., Kaneko, Y., Nagaosa, N., Hwang, H. Y. & Tokura, Y. Detection of Berry's phase in a bulk Rashba semiconductor. Science 342, 1490–1493 (2013). dx.doi.org/10.1126/science.1242247

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no fate
2.3 / 5 (3) Mar 21, 2014
This is science. Well done Mr. Murakawa, and kudos to Physorg for supplying a well written interpretation.

The graphic depicitng the electron energy/momentum distribution is consistent with reality. The applied sciences Murakawa and his team employed to formulate a method that would enable them to distinguish between states showed experience, understanding and ingenuity.

Of note: "The team was looking for the Shubnikov–de Haas (SdH) effect, a telltale sign of quantum behavior, by which free electrons behave like simple harmonic oscillators"

Although the SdH effect applies to quantum systems we "hear" similar "P" mode oscillations on the macro scale when we listen to the sun.

http://soi.stanfo...des.html

Lots of system similarities...not a coincidence.
Uncle Ira
1 / 5 (1) Mar 21, 2014
The graphic depicitng the electron energy/momentum distribution is consistent with reality.


Can one of the smart peoples explain to me the graphic? It do not make sense to me and the google is no help no.
Zachia
not rated yet Mar 21, 2014
It's a Fermi surface of topological insulator. The movable electrons inside the lattice of topological insulators like the BiTe do behave like the mercury soaked inside of porous brick: they're expelled from its pores into surface, where they form a conductive layer. The expelled electrons are mutually repulsive and they will form a regular hexagonal lattice at the surface of material. The plane connecting the points of the same potential is called the Fermi surface.
Zachia
not rated yet Mar 21, 2014
The Fermi surfaces of a topological insulator are arranged in so-called the Dirac cone with a vortexlike spin arrangement, which is sign of the presence of time reversal symmetry, but when this symmetry gets broken due to the presence of a external magnetic field, a new gap will open in the Dirac spectrum that disrupts this hexagonal spin-texture (it's 2D analogy of Zeeman effects). The circulating structure of the spins represents so called Berry's phase of electronic wave function, which protects the newly created surface states against backscattering from disorder and impurities. This may lead to unusual electromagnetic and magnetotransport effects in a topological insulator, like the surface quantum Hall effect.
russell_russell
5 / 5 (1) Mar 23, 2014
Uncle Ira
I'll try:

Balance a pencil on a model globe's north pole. The pencil point there can point in any direction. Now move the pencil along any surface longitude of the globe to reach the equator - do not change the direction the pencil point is pointing.

At the equator move the pencil any amount laterally along the equator. Now return the pencil
to the north pole longitudinally - the same way you did to reach the equator in the first place.

At no time did you change the direction the pencil point was pointing. And still the pencil's point points to a different direction when returning the pencil to the north pole.

I am not smart. Smart people can simplify the above simplification even further.
Jizby
Mar 23, 2014
This comment has been removed by a moderator.
russell_russell
3 / 5 (2) Mar 23, 2014
Simplifies visualizing phase space - the pencil point's direction is independent from the pencil's transport of mass and energy over the surface of the globe. The pencil's direction is a classical analogy to the electron spin in QM mentioned in the article here.
antialias_physorg
5 / 5 (1) Mar 23, 2014
explain to me the graphic? It do not make sense to me

I'll give it a go: The way I read it it shows that the coupling of momentum and energy is different for electrons with different spin moving through the crystal.
So when you let electrons with spin 'up' go though the crystal you will get a markedly different quantum Hall effect than with spin 'down' electrons (the two 'cones' aren't perfectly aligned).
Jizby
Mar 23, 2014
This comment has been removed by a moderator.
antialias_physorg
3.7 / 5 (3) Mar 23, 2014
Like I've said,

Remind us again: which of your many sockpuppets said that?
Can you be any more blatant in your disregard for comment guidelines?
Jizby
Mar 23, 2014
This comment has been removed by a moderator.
Jizby
Mar 23, 2014
This comment has been removed by a moderator.
RealityCheck
2 / 5 (4) Mar 24, 2014
@antialias_physorg. :)

Mate, you didn't actually specifically address let alone objectively/scientifically refute Zeph's observations there about the similarity's of 'fluidic' phenomena involved in the current observations and his relevant referenced observations. Is this a double standard? You dismiss other's scientific evidence/references/correlations by using a lame attack on HIM personally while quoting the site rules so cynically while obviously avoiding in this instance YOUR responsibilities under those rules? No wonder the trolls rule the roost. You ENCOURAGE and DEMONSTRATE the TACTICS which those trolls should follow. Some 'role model' scientist/humanist you are deep down, hey? Do better, mate. :)
antialias_physorg
3.7 / 5 (3) Mar 24, 2014
Mate, you didn't actually specifically address let alone objectively/scientifically refute Zeph's observations there about the similarity's of 'fluidic' phenomena involved in the current observations

1) I didn't address it because Zeph (like some others) are on my list of "mostly scroll by" candidates. I didn't read what he said in his first posts. He's a one-trick-pony. I just noticed he' was alluding to soemthing he 'said earlier' without having posted under that name in this thread yet. That screams: 'sockpuppet'.
2) Fluids don't work as a model because the effect observed here is quantized.
YOUR responsibilities under those rules?

Pray tell: what are those repsonsibilities? I fail to find a section that says "thou shalt read every post and answer to it"
Do better, mate.

Coming from you that's proof I'm doing perfectly fine. Thanks. Check-'Mate'.
Jizby
Mar 24, 2014
This comment has been removed by a moderator.
antialias_physorg
5 / 5 (2) Mar 24, 2014
As you probably found already, the forum rules don't prohibit the usage the sockpuppets (after all, most of accounts here are replicas of older, already banned accounts).

And the guidelines also don't explicitly say that people who were told to stay out (read: were banned) should not come back in.
Hint: There's a reason for a ban. And it doesn't mean "change a few characters in your name and you're welcome again!"
Do you really need someone to tell you that? Obviously.

The Zeeman effect analogy at the water surface is quantized too

Then it's not much of an analogy - is it? But IO and others have already shown you a gazillion times that your analogies don't make sense. So there's no real point trying again. Just keep believing what you believe - but don't bother people with it, OK?
RealityCheck
1 / 5 (2) Mar 24, 2014
@a-p. :)

Your responsibilities as a mainstreamer claiming 'authority' to criticize others and their work/posts. So your responsibilities are even MORE than other ordinary members.

So, you use longstanding prejudices to prejudice later/new posts by Zeph/Jizby? How scientific is that? If you post replies without bothering to check for new info which may also be confirmed by latest mainstream work/findings/experiments (some of Zephir's longstanding observations regarding 'vortex features' and effects at the fundamental levels of QM and space-fields/bosonic superconducting/superfluidics etc are being found in latest work), then how can you sit there and pretend you are doing due diligence and being an objective and fair scientist/discourser?

If you don't read/understand latest mainstream discoveries/news, and just react/disparage on your old prejudices because Zeph was ahead of you with his earlier observations/analogies etc, then you are just trolling him with your prejudiced bias. :)
RealityCheck
1 / 5 (1) Mar 24, 2014
@All. :)

My advice is to forget the past and forget all personality old and new stuff and ratings.

Just START FRESH and don't worry who is who and concentrate on what is what OBJECTIVELY.

The 'you're a sockpuppet' and 'downrating gangs' excuse for justifying your personal prejudices and your uninformed dismissals is getting old and stale and a bit pase'.

Be objective scientists/discoursers in spirit and intent, not just 'glorified trolls and pretenders' prejudicing and sabotaging even the occasional new discussion 'gem' of an ideas/observations/confirmations amongst the 'dross'.

Good luck and start fresh, guys! :)
Uncle Ira
3 / 5 (2) Mar 24, 2014
@All. :)

My advice is to forget the past and forget all personality old and new stuff and ratings.

Just START FRESH and don't worry who is who and concentrate on what is what OBJECTIVELY.

The 'you're a sockpuppet' and 'downrating gangs' excuse for justifying your personal prejudices and your uninformed dismissals is getting old and stale and a bit pase'.

Be objective scientists/discoursers in spirit and intent, not just 'glorified trolls and pretenders' prejudicing and sabotaging even the occasional new discussion 'gem' of an ideas/observations/confirmations amongst the 'dross'.

Good luck and start fresh, guys! :)


Really-Skippy you are not the very smart man, eh? You still think you are the Big Chief and can tell the smart peoples who they should talk to, how to say what they say when they talk, how to act like the scientist but still have no science nothing to say yourself? Sonny boy I think these peoples is on to the Really-Skippy, eh?
Jizby
Mar 24, 2014
This comment has been removed by a moderator.
antialias_physorg
5 / 5 (1) Mar 25, 2014
So your responsibilities are even MORE than other ordinary members.

My responsibilities here are:
1) Sticking to guidelines set down by the owners of this site
The End.

I am an ordinary member and in no way special from any other member on this site - as pertains to being on this site and how I should/should not behave.

Just START FRESH and don't worry who is who and concentrate on what is what OBJECTIVELY.

Making a boo-boo is part of the observable data. You don't throw out data just because you don't like it. So no: YOU do not get to start out fresh. Your history is an observable from which conclusions can be drawn.

your uninformed dismissals

With the wealth of data you have provided - how can it be uninformed? It was looked at objectively. But if the data just says: "BS" over and over again then: yeah...at some point it can be dismissed.
Whydening Gyre
5 / 5 (1) Mar 25, 2014
But IO and others have already shown you a gazillion times that your analogies don't make sense
But for their authors they make sense. http://physicswor...-effect.

Y'know, it occurs to me that some people see coherence while others see de-coherence, call it apples or oranges and then proceed to get caught up in arguing for whatever they see.
All the while, forgetting to look to the CENTER (or thereabouts) for the truth. Even thoughts and conversations follow the rules of entropy.
johanfprins
5 / 5 (1) Mar 25, 2014
It is all Berry unlikely!
Jizby
Mar 25, 2014
This comment has been removed by a moderator.
RealityCheck
1 / 5 (2) Mar 25, 2014
@a_p. :)

.I am an ordinary member and in no way special from any other member on this site - as pertains to being on this site and how I should/should not behave.
So is preventing your old personal prejudices biasing new discussions against the rules of assessing objective merits of each new instance/discussion, based on science presented not source, not part of your responsibility as member of this SCIENCE discussion board?

YOU do not get to start out fresh.
I wasn't referring to me. I already said I have ceased detailed discussion here for the reasons stated clearly. It was others whom your personal prejudices do injustice to that I was asking for that fresh start.

But if the data just says: "BS" over and over again ...
Funny, that's almost the same words I posted when I suggested you all do your own due diligence on that mainstream team's 'work'. Scary dejavu'.

So old personal feuds/prejudices die hard with your type of 'objective scientist', hey? Sad.
Uncle Ira
3 / 5 (2) Mar 25, 2014
@ Really-Skippy, ol Ira realize you are not the very smart. So out of my kindness I will try to explain it to you simple like Cher. You can not ask the smart peoples for a fresh start with you keeping that same mealy mouth whiny way you talk acting like you the Big Chief and something special to be here. You ain't one of the smart peoples here and it bothers everyone that you pretend to be the smart person with you dubble up talking and never saying anything.

Laissez les bons temps rouler Really-Skippy if that was to complicated for you to understand you let me know, eh Cher? Maybe this will be easier for you to understand you, If you quit posting up the stupid stuffs, people will quit treating you like you some kind of fool sitting in the corner with a silly looking pointy cap on his head.
RealityCheck
1 / 5 (1) Mar 30, 2014
@Poor poor poor Uncle Ira. :)

Poor poor poor Uncle Ira can't make any sense at all with all that cognitive dissonance floating round in the form of that haze of weed smoke and booze mist he's made all by himself.

Lay off the puff 'n stuff, dude! :(