Wednesday, August 02, 2006

hot topics and controversies

As was suggested in a recent comment, now that a nonzero number of condensed matter and nano people are (apparently) reading this blog (at least occasionally), this could be a fun opportunity to have a series of discussions about the hot topics and controversies out there in the world of condensed matter and nanoscale science. The idea would be to take maybe one topic a week, give a relatively gentle introduction to the subject, and then have some discussion, just for fun. This only works, of course, if enough people contribute to make the discussion interesting, rather than just me pontificating (though I suppose that would be de rigour for a blog). As a preamble, I suggest trying to generate a list of topics. Here are a few off the top of my head:
  • 2d metal-insulator transition - What is the mechanism for the apparent metal-insulator transition in 2d electron and hole systems at low densities? Is it profound or not?
  • High-Tc - what is the mechanism of high temperature superconductivity? What is the ultimate limit of Tc? What is the "bad metal", and what is the pseudogap, really? How important are stripes and checkerboards? Is the phrase "doped Mott insulator" really a generic description of these systems?
  • Quantum criticality and heavy fermions - Do we really understand these systems? What are the excitations in the "local moment" phase? What is the connection to high-Tc, if any?
  • Manganites - What sets the length scale for inhomogeneities in these materials?
  • Quantum coherence and mesoscopics - Do we really have a complete understanding of mesoscopic physics and decoherence at this point? What about in correlated materials?
  • Quantum Hall systems - Are there really non-Abelian states at certain filling factors? In bilayers, is there excitonic condensation?
  • 1d systems - Is there conclusive evidence of spin-charge separation and Luttinger liquid behavior in semiconductor nanowires? Nanotubes?
  • Mixed valence compounds - Is there or is there not charge ordering at low temperatures in Fe3O4, something that's been argued about for literally 60 years now?
  • Two-channel Kondo physics - Is there firm evidence for the two-channel Kondo effect and non-Fermi liquid behavior in some physical system?
  • Molecular electronics - Is there really improving agreement between experiment and theory? Can novel correlation physics be studied in molecular systems? Can molecules exhibit intrinsic (to the molecule) electronic functionality?
  • Organic semiconductors - What is the ultimate limit of charge mobility in these materials? Are there novel electronic correlation effects to be seen? Can one see a metal-insulator transition in these systems?
  • Nanomechanical systems - Can we demonstrate true "quantum mechanics", in the sense of a mechanical system that acts quantum mechanically?
  • Micro/nano systems to address "fundamental physics" - Can we measure gravity on the 100 nm length scale? Are there experiments with Josephson junctions that can probe "dark energy"?
What am I leaving out? Any other suggestions?


Incoherent Ponderer said...

Supersolid helium and metallic hydrogen are two somewhat "controversial" areas.

Spin glasses/ice is another candidate.

Patrick Shea said...

With respect to organic electronics, some observations of a insulator-metal transition have been observed. The most recent results I've seen are out of UCSB using OFET structures [for example, Shoot et al., Phys. Rev. Lett. 96, 246403 (2006)]

Anonymous said...

First off, thanks for this topic.

As was suggested in a recent comment, now that a nonzero number of condensed matter and nano people are (apparently) reading this blog (at least occasionally)...

Just a note that although I've never commented before (that I can remember), I've been a regular reader for several months now. I'm an ugrad doing research in 1D nano stuff (nanotubes) and molecular electronics right now, but I'm very interested in other cond mat stuff (like mesoscopics/coherence and mems/nems) and would love to do that in grad school (although there's so much more to learn).

I find your blog very, very interesting reading and await every post (especially love the arXiv roundups). Just one sample from your surely extensive demographic.

Could the topics for the upcoming Princeton Summer School in Condensed Matter Physics help with "hot topics"? For example, STM stuff a la Ali Yazdani's recent Nature work on GaMnAs atomic-scale spintronics?


Anonymous said...

A quick, naive question: why isn't the work that has been done to date on Luttinger liquid behavior in NTs (e.g. Bockrath's 1999 work) "conclusive evidence"? What remains to be done?

Sorry for the silly question.


Doug Natelson said...

IP - thanks - don't know how I left out those, or graphene.

Patrick - Yeah, I have a bit of a bone to pick about that result, though I suppose I should be careful how I publicly criticize Heeger's stuff. I'll write more about this soon.

Anon. - thanks for the kind words, and the link. Atomic scale manipulation of matter is definitely popular right now, and one thing that distinguishes Ali's work is the real scientific questions being asked (as opposed to proof-of-concepts demos). As far as Marc et al.'s work on the nanotubes, it's very nice, and it's all certainly consistent with LL properties, but it doesn't explicitly show the most interesting LL prediction (that spin and charge excitations have different velocities, for example). Power law conductances over limited domains are not amazingly satisfying either (though their scaling analysis is considerably moreso). Anyway, this is just the kind of discussion that it's fun to have....

Anonymous said...

Is the second law of thermodynamics immutable?

elvin said...
This comment has been removed by a blog administrator.
elvin said...

just checking in from nanotechbuzz after Rebecca Blood (who writes the outstanding general blog, ) gave me a head's up about this blog. It's a great addition to the nanotech discussion, covering much more technical detail than I can hope to at nanotechbuzz, which is aimed at a much more general audience. Reader participation is obviously excellent. I look forward to participating too, and keeping my readers up-to-date on all the hot topics and controversies.

Uncle Al said...

High Tc supercons: Tc remains low if Cooper pairing involves massive third bodies. William Little proposed exciton high temp supercons, [-C(Ar)=(Ar)C-]n. Linear trans-polyacetylene core with pendant polarizable chromphores Ar whose pi-clouds are dense and tight to the core,

Phys. Rev. 134 A1416-A1424 (1964)

HyperChem models monomer fragment vs. polymer. (Hydrogens and multiple bonds omitted in stereograms)

Synthesis looks easy in 2006. p-Dope product with iodine and test with a magnet.

Acc. Chem. Res. 38(9) 745-754 (2005)
or from scratch,
ArH --> ArCHO + benzoin condensation --> ArCH(OH)-(C=O)Ar --> O=C(Ar)-(Ar)C=O + Tebbe --> H2C=C(Ar)-(Ar)C=CH2 + Schrock or Grubbs --> H2C=CH2 + [=C(Ar)-(Ar)C=]n

That is Little's [-C(Ar)=(Ar)C-]n with the bracket window shifted by one atom. ADMET living polymerization allows block copolymers (molecular diodes, strings of quantum wells) and allows end-capping (sulfur for gold electrode grab).

Somebody should look. A 700 kelvin superconductor that spins filament like any other rigid rod polymer ought to be good for something.

Anonymous said...

Molecular electronics:, Is there agreement between theory and experiment? That's a hard question to address, since there are seemingly so many theories and so many different device platforms. It is not at all surprising to me as an experimentalist that consistent behavior is not observed between evaporative techniques (nanopores, crossbars, etc.), soft contact techniques (conducting AFM, crossed wires), single molecule techniques (break junctions, NJ Tao expt) and STM experiments. Why is this not surprising? 1. Metal evaporation has been shown to either destroy the molecular layer or penetrate through to some extent, thus changing the junction composition. 2. STM expts have one electrode that is spatially (and thus electronically) decoupled from the film, so the electrostatic profile across the junction is much different in STM than in any other testbed. 3. Depending on experimental details, the single molecule experiments tend to stretch a molecule, whereas the other techniques tend to compress the monolayer. Certainly this strain must have some effect on the I-V behavior.

With all that going on just in the experimental side of things, it doesn't surprise me at all that the theorists have such a hard time matching the data sets.

Incoherent Ponderer said...

Re: molecular electronics.
In my primitive understanding of the field, a lot of controversy is caused to subtle variation of quality of metallic contacts for molecules.

A key experiment among many groups seems to be showing how I-V characteristics of a single molecule sandwiched between two metallic junctions depend on the choice of the molecule. The tunelling is mediated by electronic states of the molecule which should in principle allow a very fine tuning of those characteristics. Well, part of the problem in this setup is that the quality, shape, roughness, material, cleanliness etc. of metal contacts - on atomic level are really crucial.

Also, the controversy of manganites, cuprates, nickelates, cobaltites and heavy fermion compounds can be lumped up together in one big theme full of controversy.

Anonymous said...

that's a fantastic list. i'd just like to point out that it's in fact de rigueur rather than de rigour. :)

Doug Natelson said...

Je parle seulement francais un petit peu. :-) (will blog more later.)

Anonymous said...

Can anyone show me experimental evidence for two channel Kondo physics in nonequilibrium? I have seen the Dan Ralph paper from early 90's and since then there is nothing that I know of and they seem to have given up at Cornell as well.

Doug Natelson said...

Last comment before I break out discussion to a new entry.... Regarding two-channel Kondo, there is some nice preliminary work coming out of Goldhaber-Gordon's group at Stanford, based on the theory paper that he and Yuval Gefen wrote. I don't have the reference handy. The new stuff hasn't been written up yet, but the data look pretty....

Pentcho Valev said...


If you insert a SOLID dielectric between two opposite charges, the polarization in the dielectric INCREASES the original force of attraction between the charges.

However if you insert a LIQUID dielectric (e.g. water) that wets the charges, the polarization in the dielectric generates a PRESSURE that pushes the charges apart and so DECREASES the original force of attraction between them. The decrease "cannot be explained by electrical forces alone", as Panofsky and Phillips put it in 1962. In my view, this pressure is the most mysterious force in science - see more in:

Pentcho Valev

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With respect to organic electronics, some observations of a insulator-metal transition have been observed.

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