Tuesday, November 27, 2007

"Unparticles" and condensed matter

At the risk of contributing to what has recently been called the intellectual wasteland that is the physics blogosphere, I want to point out a nice review paper on the arxiv, and its connection to high energy physics. Subir Sachdev at Harvard has put up a relatively pedagogical review about quantum magnetism and criticality. Back when I was a grad student, I didn't appreciate that quantum magnetic systems were so interesting - I thought that they were a zoo or menagerie of semi-random compounds that happened to have effective model Hamiltonians of interest only to rather esoteric theorists. Now I understand the appeal - the relevant Hamiltonians can have some truly bizarre solutions that can be relevant (intellectually if not directly) to whole classes of systems. One class of such systems is the heavy fermion compounds that are non-Fermi liquids, and another comprises some exotic "spin liquids". The low energy excitations of these strongly correlated quantum systems are not readily described as particle-like or wave-like. They don't have simple quantum numbers and simple dispersion relations, and they result from complicated, correlated motion of electrons (or spins, or both). This has been known in condensed matter circles for some time, and is very neat. Much exciting theory work is being done to come up with good ways to treat such systems.

What I don't understand, and perhaps a reader can enlighten me, is how these ideas relate to "unparticles". Howard Georgi, also of Harvard, made a pretty big splash this past year by publishing a PRL (linked above in free form) about the possibility that there may be fundamental excitations of quantum fields (like the ones thought to be relevant in high energy physics) that are not simply described as particles. Since this paper came out, there are now 78 papers on the arxiv that deal with unparticles. So, is this a case of high energy physics reinventing an idea that's been known conceptually for some time in condensed matter? Or is there really a basic underlying difference here? I should point out that at present, while there is experimental evidence for non-particle-like excitations in condensed matter, there is not yet any evidence for such things in high energy experiments as far as I know.

26 comments:

Jacques Distler said...

"Unparticles" are not a terribly exotic idea (at least, for a condensed matter physicist).

Imagine you have a conformally invariant (nontrivial RG fixed point) subsector of your theory. Obviously, if it were completely decoupled, we would never see it. Instead, we imagine that it is coupled to the "ordinary" sector of our theory via terms which are suppressed by inverse powers of some large mass scale.

The corrections to the physics of "ordinary" particles are computable if you know the N-point functions of the CFT.

In beyond-the-Standard-Model physics, the effects of such a conformally-invariant subsector must be suppressed in this fashion.

In other contexts, more interesting couplings are possible.

sylow said...

What sachdev writes in this article is not new at all. His original papers with Nick Read in 1989 and 1991 cover most of this stuff (Ref. 22 here) At that time, it was considered quite a splash but now this is a pretty dead field. Very few people are working on quantum magnetism anymore...

Doug Natelson said...

Sylow, I've never seen you write a positive comment about any area of physics. What would you say is an active, productive area right now?

sylow said...

Graphene?

sylow said...

In late 70's lots of people were fascinated with spin glasses and it is still an interesting problem to work on but most people just moved away from that field partly because it was not fashionable anymore and the problems just got too difficult.
If you want to know what I am learning myself these days, I am trying to grasp Geometric Langlands program. You can take a look at Frenkel's latest book on it if you are curious enough.

Guru said...

Dear Doug,

Your first link (to Distler's post) is actually the URL for trackback and hence does not work.

Doug Natelson said...

Guru - Fixed. Thanks.

Peter Armitage said...

Dear Sylow,

The field of spin glasses is not the field of quantum magnetism.

For instance, the replica trick of spin glasses is in its simplest form a mean field method and is essentially classical statistical mechanics. One can have spin glass behavior in completely non-quantum N--> inf spins.

You would have been correct to say that very few people are working in spin glasses anymore.

In contrast quantum magnetism is a very hot field. As Doug mentions, there are reports (theoretical and experimental) of all kinds of exotic phenomenon in spin systems near T=0 phase transtions (quantum critical points ) and in various spin liquid and topological phases. There is also the work of Sachdev and collaborators on phase transitions "beyond the Landau-Ginzburg-Wilson paradigm" which directly grows out of work on quantum magnets.

All of this is interesting in its own right, but such phenomena may also hold the key to the physics of high-Tc and other novel superconducting systems.

Of course you can consider highly quantum spin 1/2 systems in frustrated configurations that may produce spin glass behavior. But quantum magnet is not a synonym for spin glass.

sylow said...

The hotness of a field is determined by the funding levels, not by hollow statements. You need to feed the researchers so that they can work on a given project. Can you do a search on NSF website and tell us how much research money was awarded to quantum magnetism last year? Even better, how much funding do YOU have to work on it yourself? Sachdev and his collaborators have ZERO funding for quantu mmagnetism.
BTW, where did I say spin glasses=quantu mmagnetism? They are both dead fields anyway..

Doug Natelson said...

To echo Peter's comment, a simple ISI search on the phrase "spin liquid" (for an example of a quantum magnetism concept as I mean it) shows 1500 citations this year alone. The SCES conference, which dealt significantly with this topic, had over 700 abstracts this year. It is unfair and inaccurate to imply that nothing has happened in this field since 1991.

Doug Natelson said...

Sylow, a team of 8 of us at Rice brought in significant funding in this area last year, to form the Keck Program in Quantum Materials. Randy Hulet and his co-PIs pulled in a significant DARPA grant on cold atom physics directly related to this, and two other such teams were also supported elsewhere. I don't know why you persist in telling those of us who are actually faculty that we are clueless.

Doug Natelson said...

Whoops. Broken link. Should read Keck Program in Quantum Materials.

sylow said...

Doug, where is the link to this DARPA grant that have been awarded to you? What is the grant number? DARPA doesn't support university based research anymore.

Doug Natelson said...

The DARPA grant was awarded to Randy, not me. It was in response to this solicitation. Do you think I'm lying to you? I'm tired of your uninformed pronouncements, and won't respond anymore.

sylow said...

The link you provide is a proposal submission site. It just gives some links to papers by the Hulet to describe the context. I also googled this thing. Nothing comes up. You are merely speculating.

Doug Natelson said...

(against my better judgment)...
Sylow, I see Randy every couple of days. My office is upstairs from his. He has apparatus under construction being paid for by this grant. I know you might be shocked to hear this, but there are actually sources of information more reliable than google or unsupported assertion.

Sylow said...

Ok, you can perhaps ask him the grant number next time you see him before sitting in front of your computer and start typing on your keyboard. I did search on
http://www.darpa.mil/SearchAll/darpaSearch.asp
using keywords like "Rice University" and "Hulet" and I get no information indicating that any grant has been awarded to this person. You can try it yourself too. The grants awarded by DARPA are not classified information and would be available here.

Anonymous said...

Sylow,

See http://www.phyast.pitt.edu/~wvliu/cv.pdf, around the middle of p. 2. Although perhaps you think he's lying?

Also, a similarly-sized DOD MURI grant has been awarded for similar research (I think to the Ketterle/MIT collaboration, but I'm not sure). Of course, I suspect you'll think I'm lying about this too...

Why do I feed the trolls?

Doug Natelson said...

On a more general note, for those who don't know, online databases of awards are frequently far behind the times. I just recently was asked to submit an abstract for a grant for such a database, and the grant was awarded over a year ago.

sylow said...

Anonymous and doug, I am not saying such a grant is impossible but the fact that there is no trace of it anywhere is suspicious. DARPA grants are usually multimillion dollar awards and if such a thing happened, there would be press releases and news coverage in popular media. It is not that you can a DARPA grant every year. A typical NSF grant is 100K per year and it usually goes unnoticed for this reason but that is not the case for DARPA. There is also a lot of scrutiny for awards granted by agencies associated with pentagon. There are site visits annually or semiannually to monitor the progress of the project by high profile army people. Long story short, if such a thing happened, it would be visible.

Anonymous said...

Gosh, the entire population of physics faculty all across the US must be engaged in a vast conspiracy to keep our funding information from Sylow. I can't see any other explanation for this string of comments. Other than that he's a dingbat.

sylow said...

Anonymous, why do you say "our funding information"? By the way, you should pick a name for yourself.

Anonymous said...

Dingbat

DanM said...

Ok, I just followed your link to that other blog, and made an earnest attempt to read that blog post and the ensuing string of comments. I ended up with a warm fuzzy feeling inside, the same way I feel when I think about people who volunteer to serve in the armed forces - boy am I glad people are willing to do that, because it means that I don't have to.

Yannick said...

Can someone explain what is a quantum magnet ? Please don't use too much theory as I am a young experimentalist
thank you

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