Friday, March 31, 2006

How to spot bogus science

There is a great, free article in the Chronicle of Higher Education from back in 2003 that has just come to my attention, about how to spot bogus science. The article is by Bob Park, who writes a weekly "What's New" column that used to grace the APS webpage.

This is an important read, particularly as there seems to be a steady flux these days of news items that seem pretty weird to me. For example, these folks are a bunch of cold fusion advocates, who last week put out a big press release about how happy they are that Martin Fleischmann is joining their product development team. For another example, take this announcement by the European Space Agency that their researchers think they've spotted a funny gravitomagnetic effect near rotating (low Tc) superconductors. The data look pretty marginal to me, and I think it's pretty indicative that on the one hand they put out a big, glossy press release, while on the other hand they submitted the paper to Physica C. I don't want to knock Physica C too badly, but they aren't exactly a high impact journal. At least the ESA researchers are using the peer-reviewed literature, though, and seem to be reasonably careful. They need to be, though - they're claiming big deviations from general relativity, and extraordinary claims require extraordinary evidence.

Tuesday, March 28, 2006

Wow - a really surprising result!

The cover story on the latest issue of Phys. Rev. Letters is quite surprising! A group in Italy have performed what a colleague of mine called a "hero experiment": they've taken linearly polarized light, and passed it through a 3 m long ultrahigh vacuum cavity in a rotating 5.5 T magnetic field. The shocking result is that they observe that the polarization of the light rotates because of the magnetic field. Basically they've measured a magnetic dichroism of vacuum. This is unexpected, and ordinarily it really shouldn't happen - it implies that the photons from their laser are interacting in a very nontrivial way with the (virtual) photons that make up the magnetic field. One way this could happen would be via a two-photon scattering process involving a never-before-seen neutral, spinless, very low mass particle. The paper is also remarkable for being the only PRL I've ever seen that's over the four page length limit of the journal, and for appearing without some enormously overblown marketing in the form of press releases.

This could be a very very big deal if confirmed. There is already one idea for an independent test of this. I would imagine that it would have major astrophysical consequences, too. After all, the hypothesized mechanism would lead to an effect quadratic in magnetic field, and the fields around astrophysical objects like neutron stars can be millions of times bigger than the field used in this experiment....

Saturday, March 25, 2006

This week in cond-mat

Slightly delayed because of the joys of grant proposals, here is this week's installment of my quasi-periodic snippets of things I find interesting on the arxiv preprint server....

cond-mat/0603598 - Siemons et al., Origin of the unusual transport properties observed at hetero-interfaces of LaAlO3 on SrTiO3
This paper is interesting for a couple of reasons. First, the author list includes some luminaries in the field, including Ted Geballe, Mac Beasley, and Walt Harrison. They're all extremely nice guys, and Walt literally wrote the book(s) on electronic structure calculation methods. It's great that these folks are not just still active, but really pushing new ground, at a point in their careers when many full professors decide to kick back. Second, this paper reports data on a relatively new material system, a heterojunction between two oxide materials. Like the GaAs/AlGaAs case, the conduction band offset between the two materials leads to the formation of a potential well right at the interface, so that electrons can be trapped there in a two-dimensional layer. This result studies electronic transport in those layers, and tries to address the question of where the free carriers come from, given that the materials are ideally not doped.

cond-mat/0603482 - Pickett, Design for a room temperature superconductor
Bonus points for the provocative title. This paper (part of a commemorative volume in honor of Vitaly Ginzburg), looks at MgB2, a superconductor that is not a copper oxide, but nonetheless has a transition temperature of nearly 40 K, and tries to argue from that material what would be necessary to have (phonon-mediated) room temperature superconductivity. Thought provoking, and with references to good MgB2 literature for those interested in how that material was discovered to superconduct at the shockingly recent date of 2001.

Thursday, March 16, 2006

APS March Meeting

No cond-mat update this week. I just returned from the March Meeting of the American Physical Society, that annual opportunity to get together with 7000 of my closest condensed matter physics colleagues and stay in over-priced hotels with malfunctioning wireless internet access. The meeting was good - I'll mention just a few observations:
  • There was a particularly nice session on the recent transport experiments in graphene that I've mentioned in previous posts. The talks were interesting, and there were rumors of cool new data not yet in print (i.e. observation of the quantum Hall effect in graphene at room temperature (!!) and 30 Tesla).
  • There was an invited session on topological quantum computation with a couple of talks that were almost utterly incomprehensible to the nonspecialist.
  • The fire marshals kicked a bunch of people out of a ridiculously small room housing a single-molecule electronics talk, and closed the door right in the face of a Nobel laureate, who took that with good grace.
  • Speaking of single-molecule devices, there continues to be lots of interest and lots of effort in that area - a very exciting topic I should write more about later.
  • Apparently, if you're a big enough name in a given field, you can coin new vocabulary and assume that everyone will figure out what you mean.
I'll write more about some science later.

Thursday, March 09, 2006

This week in cond-mat

Here's a couple of preprints that I've found interesting in the last week. Note that I'm not going to be surveying the published literature as much, since there are other resources such as the Virtual Journal of Nanoscale Science that do an excellent job of that (though they miss papers in ACS journals, which increasingly contain results at the border between chemistry and condensed matter physics).

cond-mat/0603173 - Manfra et al., Reentrant anisotropic phases in a two-dimensional hole system
I'm not writing about this one just because Mike Manfra and I used to share an office at Bell Labs. Two-dimensional electron gases (2degs) have been a workhorse physical system over the last 25 years, showing a number of fascinating many-body pieces of physics, including the integer quantum Hall effect (which has led to the definition of the standard Ohm!), the fractional quantum Hall effect (a demonstration of a correlated electronic state that has excitations with fractional quantum numbers, including fractional charge), apparent zero-resistance states under microwave illumination, and interlayer quantum coherence in bilayer electron-hole systems. Another weird effect observed recently is the onset of big anisotropies in the electrical resistance of such 2degs in very clean material at very low temperatures. The explanation for this spontaneous anisotropy is generally thought to involve the electronic system breaking up into some kind of stripes. With the recent development of new high quality two-dimensional hole systems, now one can test this idea. In the new cond-mat paper, Manfra et al. find that the anisotropies are very different in the hole system than the electronic analog, and discuss how details of the single-electron states (like the presence of strong spin-orbit scattering in the hole case that is absent in the electron case) can matter greatly.

cond-mat/0603108 - Badzey and Mohanty, Coherent signal amplification in bistable nanomechanical oscillators by stochastic resonance (also Nature 437, 995 (2005)).
Stochastic resonance is a neat phenomenon, when nonlinear systems can sometimes exhibit improved signal to noise when additional noise is introduced deliberately(!). This paper is a cute implementation of this idea, using bistable nanomechanical resonators as the nonlinear element. When you think about it, bistability (the resonators seem to have two competing, well-defined oscillatory states, one with high amplitude and one with low amplitude) is about as nonlinear a response as you can get. While some of this group's earlier work with these resonators has engendered some controversy, this paper is very pretty.

Sunday, March 05, 2006

HIgh Tc: where are we

As I said in my previous post, Nature Physics has run a fascinating piece surveying a number of theorists about the current state of the high Tc problem. I encourage you to read it, and I'll summarize very briefly for those without access to the journal. Things that everyone seems to agree on:
  • The symmetry of the superconducting pairing is d-wave.
  • The parent compounds of the high Tcs are "Mott Insulators". In the absence of strong electron-electron interactions, these materials would be metals; however, strong on-site repulsions on the coppers (so that no copper site d-orbital can be doubly occupied) lead to insulating behavior, and antiferromagnetic ordering at low temperatures.
  • The normal phase above Tc for the optimally doped compounds is really weird. It appears that the normal concept of quasiparticles fails there. When superconductivity is killed by whopping huge magnetic fields, the weirdness of the normal state persists down to T=0.
  • Understanding the normal phase is probably a good idea for understanding superconductivity.
  • There are signs, even within the superconducting phase, that there can be some kind of charge ordering ("stripe order" is a phrase that is used a lot).
  • In the underdoped compounds, there is a pseudogap in the density of states that exists to temperatures far higher than Tc.
Things that some people agree on:
  • The resonating valence bond picture accurately describes the superconducting phase; there is something called a spin liquid, and the pseudogap essentially corresponds to the formation of some kind of pair-like correlations without global phase coherence.
  • The pairing mechanism is purely electronic (as opposed to phonons in conventional superconductors).
  • The superconductivity is a general feature of doped Mott insulators.
  • There are quasi-2d Mott insulators that do not superconduct at all when doped.
  • There is no quantum phase transition (that is, at T=0 as a function of, say, doping) in these materials.
  • There is a quantum phase transition in these materials, and therefore there is a well-defined (if very hard to detect) breaking of symmetry when going from the strange metal phase to the pseudogap phase.
  • The stripe order is crucial, and competes with superconductivity.
  • The stripe order is incidental and unimportant.
What I think is interesting, as a bystander:
  • Everyone has their favorite handful of experiments that they treasure, and is appreciative that the materials growers have gotten so good at making clean samples of these nasty quaternary compounds.
  • Only Chandra Varma explicitly addresses the reason why copper is special, chemically, in his microscopic picture (which has almost no relation at all to simple concepts of pairing, as far as I can tell).
  • Very few people bother to address the existence of electron-doped superconductivity in these systems.
  • It is clear that the whole field is strongly hampered by the fact that chemical doping is a real bear at these levels - it introduces large amounts of disorder. Field-effect experiments would be great, if only they could really change the charge density by chemically interesting amounts.
The field continues to progress, though it's not for the faint of heart. The holy grail of room temperature superconductivity still beckons, though there are those who make reasonably persuasive arguments that the copper compounds are awfully special, and may be as good as it gets. For me, I'll stick to nanostructures for now.

Thursday, March 02, 2006

20 Years of High Tc

There is a very interesting article in the new issue of Nature Physics regarding the twentieth anniversary of the discovery of high temperature superconductivity. In case you've been living under a rock since 1986, the high temperature superconductors are generally based on perovskite quasi-two-dimensional compounds that have extraordinarily rich (read: so complicated they're hard to understand) phase diagrams. The parent compounds are antiferromagnetic insulators in their ground state. In doped compounds (done by substitutional chemical doping at the ten percent sort of scale, which introduces significant disorder), the superconducting state is well-described by a BCS-like state with spin singlet d-wave Cooper pairs (and just establishing that firmly took years, and an enormous effort at sample growth, and several brilliant experiments).

The normal state of these materials is a real mess. At very high doping levels, the materials seem to be well-described as Fermi liquids, which is the standard picture of ordinary metals. You can think of the electrons as partially filling a band of states that look very much like non-interacting single-particle states. Excitations above the ground state look like well-defined electron quasiparticles, as demonstrated by, e.g., a resistivity that varies like the temperature squared. Near optimal doping for the superconductivity, the normal state is a "strange metal", meaning that the resistivity varies with temperature like T, implying that quasiparticles are not a sensible way to think about excitations of this material. In underdoped materials, the normal state looks like a strange metal with a "pseudogap", vaguely reminiscent of the superconducting gap in the density of states, but persisting up to much higher temperatures than the superconducting state.

The Nature Physics article is a collection of comments by a bunch of big-name condensed matter theorists. Interestingly, and I'll write more about this in a day or two, there still is suprisingly little concensus about what's really going on in these materials. Definitely worth a read!